A Simpleton's Guide to Thermodynamics

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This book waspublishedin2015 by MET220students at Purdue
College of Technology in Columbus, Indiana.
It is a humorous simplification of the following title:
Cengel, Y. (2008). Introduction to Thermodynamics and
Heat Transfer (2nd ed.). New York, NY: McGraw-Hill.

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About the Authors
Brent Byersis a junior at Purdue University majoring in Mechanical Engineering and
Technology. As soon as Brent graduates, he is moving far away from Columbus, IN.
JacobA DeBuskis a senior student at the Purdue College of Technology. He is studying
Mechanical Engineering Technology while also working at Cummins Engine Company.
Casey Jonesis a senior at Purdue University College of Technology in Columbus, IN as a
Mechanical Engineering Technology Major.
Kristina Linvilleis currently a student at Purdue College of Technology in Columbus, IN.
She is currently in her second year of schooling and is studying to get her BS degree in Mechanical
Engineering Technology. Before attending Purdue she graduated from Columbus Signature
Academy - New Tech in the year of 2013.
JordanEzell, was born on May 5, 1994 in Franklin, IN and grew up outside of Shelbyville,
IN where he graduated from high school at Southwestern Jr. / Sr. High School. In 2012 he stared
college at IUPUC, and is currently finishing up his BSMET degree at Purdue College of
Technology in Columbus, IN.
RileyEllisonis a sophomore at Purdue University studying for a Mechanical Engineering
Technology degree. Ellison is working toward a future goal of LEGO engineering.
Doug Weber is a senior studying for his Bachelor’s Degree in Mechanical Engineering
Technology. He is employed as a Quality Manager at a steel forging plant in Columbus, Indiana.
He enjoys playing and watching ice hockey in his spare time.
EricCatlowstudies at the Purdue College of Technology. He enjoys pumping NWA on the
stereo in his car whenever the Fuzz hang around.

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Jalen Ulreyis currently a junior Mechanical Engineering Technology student at the Purdue
University College of Technology in Columbus, IN. He is currently employed at Faurecia
Emission Control Technologies and plans to continue with the company after graduation.
Dr. Tatiana Goris, Clinical Assistant Professor, School of Engineering Technology,
Purdue University
Table of Contents
Chapter 1: Introduction and Overview………………………………………....5
Chapter 2: Introduction and BasicConcepts………………………………….11
Chapter 3: Energy, EnergyTransfer, and General EnergyAnalysis………...20
Chapter 4: Properties of Pure Substances…………………………………….26
Chapter 5: Energy Analysisof Closed Systems…………………………….....32
Chapter 6: Mass and Energy Analysisof Control Volumes………………….37
Chapter 7: The Second Law of Thermodynamics……………………………46
Chapter 8: Mechanismsof Heat Transfer…………………………………….51
Chapter 9: External Forced Convection……………………………………....59
References……………………………………………………………….……..64

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CHAPTER 1: INTRODUCTION AND OVERVIEW
Thermal Sciences
The first concept to understand is the world of thermal sciences. Thermal science is exactly what
it sounds like, the study of heat and how it acts in the world we live in. Heat is actually a form of
energy, and because energy can be converted from one form to another, thermal sciences must
consider the other forms of energy as well.
The world of thermal sciences is significant in the life of the average person. It is what allows for
blankets to keep you warm, occurs when a hair dryer is being used and is the reasoning for why
rubbing your hands together creates heat. Without it cars would not run, television would never
exist and humans would doubtfully have ever made it to the moon. Almost anywhere at any instant,
thermal sciences are taking place in our lives, and have been since the beginning of time.
Thermodynamics
Thermodynamics is the scientific study of energy, which in itself, has no official definition.
Thermal science is technically categorized underneath thermodynamics because of heat being a
form of energy, and that energy then is transferred to some sort of power through thermodynamics.
There are two laws that accompany this concept. The first law states that energy itself cannot be
created nor destroyed, which is the definition of something called the conservation of energy
principle. The second law says that the energy that is being transferred actually has a definable
amount, and a specific quality to it. This law also states that the body with more energy transfers
it to another body or area with less energy.
For example, if you make yourself a tub of ice cream and leave it on the kitchen table, overtime it
will either be eaten or the surrounding warm air will begin to transfer to the cold ice cream. This
will continue until the temperatures are the same in both mediums. In chemistry, it is known that

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substances are made up of things called molecules. These molecules make up particles which are
observed in one type of thermodynamics called statistical thermodynamics. This type deals with
particles in groups and is concerned with the behavior of groups. Classical thermodynamics is the
second type and it only worries about what can be seen or studied without a microscope. Many of
the real world problems people deal with can be achieved using the classical method for analysis.
A major concept that is essential in solving thermodynamics problems is the Conservation of
energy principle. To understand this term, it is imperative to know that energy itself cannot be
produced from nothing. Energy also can never be destroyed, the principle states that it can only be
transferred through different forms. Although the forms of the energy may change, the quantitative
value will remain the same. Say you had a basket of 10 apples you just stole from your local
farmers market and want to place them in plastic bag. By using something like a machete, the form
of the apples can be changed and transferred to another medium (the plastic bag). Even though the
pieces are different and located somewhere else, the amount of apple is still the same as when the
experiment began.
The Transfer of Heat
Heat transfer is something that is interested in the rate of which is heat being moved from one
medium to another in a process. Heat itself is considered to be a type of energy that can only be
transferred between two bodies, or areas, if their individual temperatures are different. It is
important to note that the principle of heat transfer falls within the laws of thermodynamics.
However, thermodynamics’ main focus is on something called equilibrium states and how they
change to another. This means the goal of thermodynamics is essentially to know the amount of
energy (or heat) transferred during a process.
To find the rate of heat transferred with respect to time, the principle of heat transfer is needed.
For heat transfer to take place, it may be obvious to state that a temperature difference between
two systems must exist. One body must be cooler or warmer than the other, or else they are
considered to be in thermal equilibrium and cannot be approached using heat transfer aspects.
Dimensions and Units
The terms associated with dimensions and units are heard in everyday life. When a weather man
reports the temperature, or when you see a speed limit sign, certain values, are attached to those
numbers to define what unit scale is being used. This means that normally in the science and
academic fields, any letters or words that follow a numeric value are considered to be units.
Dimensions are assumed to be anything that has some sort of physical quantity to it. So when you
hear the weather man say, “It’s going to be 125°F today,” the 125 is the dimension and the “°F,”
for Fahrenheit, is the unit. Other things like length, mass (the weight of something without gravity
acting upon it) and time are also dimensions and are considered to be called primary, basic, or
fundamental dimensions. Factors like volume, velocity and energy are called secondary
dimensions.
There are two unit systems that are accepted as the major systems to use in the world. However,
one is much simpler than the other. The first is known to most people as the metric system, but the
industry and academic fields recognize it as the SI. SI is an abbreviation for some fancy French
words meaning International System. This is what most of the world is using because of it is easy

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conversions and obvious scale basses (0°C is when water freezes and 100°C is when water boils).
The other system, English, was created and solely used by the United States of America. The units
within the system are not scaled in a logical manner and there is no definite base. Many of the
conversions to transfer from one scale to another involve ambiguous calculations and conversion
factors. Even though the English system was created in America, many companies produce
products utilizing metric units. The SI is also mainly used by Americans in industrial fields for
ease of use and international consistency. Table 1 provides an example of the length scale for both
systems.
Table 1: US Customary vs. SI
English
12in = 1ft
3ft = 1yd
1760yd = 1mi
SI
10mm = 1cm
100cm = 1m
1000m = 1km
SI and English Units
Each system has their own units for each dimension. In SI, length is measured in meters (m), mass
is a kilogram (kg) and time is in seconds (s). The mass in English is considered to be a pound-mass
(lbm), length is a foot (ft) and time is the same. There are other types of primary dimensions that
require calculations to be resolved. Force (F) is an example of one. As seen in the formula below,
both mass and acceleration values are required. Mass (m) is equal to a kilogram or pound-mass,
and acceleration is equal to meter per second per second or foot per second per second.
𝑭 = ( 𝒎)( 𝒂) (1-1)
Force is also expressed differently for each system. In English, a force is considered to be a pound-
force (lbf). Force in SI is called a Newton (N). The 𝒂 in the equation represents the acceleration.
In many cases, acceleration is considered to be equal to gravity. Although gravity is essentially a
force, it can be observed as the rate at which the gravitational force of the Earth pulls objects close
to its surface. The individual accelerations for gravity are given in table 2.
Table 2: US customary and SI Units

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System Force Mass Acceleration from
Gravity
English lbf lbm 32.174𝑓𝑡/𝑠2
SI N kg 9.81𝑚/𝑠2
Force is also many times used to express weight. Since weight is actually the mass of something
with gravity acting on it, the formula for force is applicable. However, it is normally good practice
to rewrite the force formula when solving for weight as follows:
𝑾 = 𝒎𝒈 (1-2)
Weight is considered to be either pounds or Newtons. From observing the formula, it is clear to
see that gravity has an obvious effect on the weight of an object. This is why astronauts are able
to bounce across the moon’s surface with ease. Mass, however, is something that will remain
constant regardless of its location. Sometimes it is necessary to know what is called the specific
weight (γ) of a substance. This can be thought of as the weight per unit volume given in
𝑘𝑁
𝑚3 and
𝑙𝑏𝑓
𝑓𝑡3 .
For this equation, density (𝒑) is needed along with gravity. Density is considered to be in
𝑘𝑔
𝑚3 .
𝜸 = 𝒑𝒈 (1-3)
The term Work is often thought of as referring to an occupation held by an individual. It is also a
type of energy and is expressed in terms of something called joules (J). A joule is actually equal
to one Newton-meter (N·m). Expressing work in these units allows us to see that work is actually
a force, or energy, applied over some distance. A literal example would be to imagine yourself
carrying laundry from the dryer to the bedroom. The act of physically lifting a load and walking
to the dresser is expending energy during motion.
The British Thermal Unit and Calorie Counting
For some reason, people decided they wanted to know how much energy it would take to make
some water (1lbm) get a little warm (68°F-69°F). The unit was named the Btu (British thermal
unit) and is apparently used in the English system. The SI uses something called a calorie, different
from the kind people lie to you about. This calorie is scaled using similar methods, however it is
acquired using 1g of water heated from 14.5°C to 15.5°C. SI also insists on using the kilojoule
(kJ). They are almost interchangeable depending on the tolerance solution needed. This because
the Btu and kJ have similar values.
1𝐵𝑡𝑢 = 1.0551 𝑘𝐽
Similar Dimensions
There are many different unit scales in thermodynamics and science fields. For many of the
equations to work properly, all of the units associated with their numbers must match up somehow.

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This means every number in the equation must ultimately have the same dimension. The term for
this requirement is called dimensionally homogeneous. If you try to solve an equation without it
being dimensionally sound, then the result will most likely be wrong. Changes to units or
dimensions means changes to the numeric value in most cases. Unity conversion ratios exist to
help in cases where the units themselves are equal to 1.
𝑁
𝑘𝑔 ∙
𝑚
𝑠2
= 1
𝑙𝑏𝑓
32.174𝑙𝑏𝑚 ∙
𝑓𝑡
𝑠2
= 1
These ratios have no actual units, but can be used in cases to replace values within equations.
Thermodynamics Math Methods
Most of the math the average person is required to due on daily basis does not entail much more
than simple addition and subtraction. This is not the case for most critically based science
problems. Much of thermodynamics is consisted of tedious calculations and methods. These
procedures are meant to help determine the solution as accurately and quickly as possible.
Step 1: Problem Statement
It would be wise to insure you completely understand the question being asked to you before you
even look at a pencil. A recommendation is to read the problem several times out loud. Even during
a test, it is always good to just make sure everyone knows you’re three problems ahead of them.
Once you have a solid understanding, write down a summarized sentence of what is being asked.
Step 2: Doodle Time
Time to bust out that 64 pack of Crayolas and draw some systems. Everything that is moving, or
is some sort of fixed plane or has mass needs to be drawn and annotated. This means units and
arrows showing directions of action. Any calculations or other information should also be listed
somewhere in the vicinity. This is also a section to list any specific details as well (the type of steel
being used).
Step 3: Educated Guessing
Normally a problem will state if something about the environment is a little wacky. Factors such
as gravitational force or pressure in certain areas may vary and need to be considered when
approaching a problem analytically. Some values may be obviously assumed (the surface on which
the problem is taking place could be the zero point relating to heights). It is not very often that
there is a trick value not easily established. Information such as being on the moon is typically
given.

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Step 4: Physics
Asses the entire situation and observe any physical principles that need to be applied. For instance,
if the problem includes a car on an inclined plane, it is obvious to think that gravity would cause
the car to roll down the hill. It is also good to keep in mind the medium in which the problem is
taking place. Look for any areas where significant friction may occur and analyze forces.
Step 5: Properties of Substances/Materials
All materials, gasses, fluids and vapors act differently in the world. Values like yield strength or
specific heat may be needed find solutions. Many material properties are listed in tables in
scholarly sources and other online references. Normally, information such as pressure and
temperature are required to find such property values.
Step 6: Math
By this step, all other missing quantities should be fulfilled other than the final solutions. It is
good practice to write out equations using their alphabetic symbol, then substitute in the numeric
values. Always make sure the units for every value match up or will cancel out somehow.
Step 7: Talking Time
Do not assume the answer is correct. Perform a logic check to insure everything makes sense. If
you are working in group, discuss the results with your colleagues for a more accurate analysis. Is
the power output of the Honda really 500kW, or is your thumb too big for the division key? If
possible, try to utilize different equations known to achieve the same values. Use this time to insure
that everything pending grading is legible. This aids not only the instructor, but also yourself from
creating algebraic mistakes.
SigFigs
Significant digits are something that is usually taught in grade school. In the engineering fields,
normally three digits are used. The point of this method is to attempt to prevent false dimensions
from being calculated. In a situation where the final answer comes out to something like 7.47897
lbs, the final answer should be rounded to 7.48 lbs. This prevents any errors in calculations or
unwanted dimensions.

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CHAPTER 2: BASIC CONCEPTS
In this chapter we will introduce the following terminology as well as many more concepts.
Boundary- The real or imaginary surface that separates the system from its surroundings.
Closed system- Consists of a fixed amount of mass, and no mass can cross its boundary.
Continuum imaginary – disregard atomic nature of substance and view it as a homogenous
matter. Assume that properties of system vary with no jump discontinuities despite the large gaps
between molecules, a substance can be treated as continuum because of the very large number of
molecules even in an extremely small volume.
Density- Mass per unit volume ρ(density) = m(mass) / V(volume)
Equilibrium- State of a balance; there are no driving forces within the system, and no changes in
time. A system is not in thermodynamic equilibrium unless the conditions of all relevant types of
equilibrium are satisfied.
Extensive properties- Those that depend on the size of the system.
In general, density depends on temperature and pressure.
Intensive properties- Those that are independent of the mass.
Isolated system- Energy is not allowed to cross the boundary.
Open system- Properly selected region in space.

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Property- Any characteristic of a system (volume, temperature, mass, etc.)
Property- characteristics of the system
Specific Gravity (relative density)- Is a ratio of a substance to the density of some standard
substance (usually water at 4 ◦C)
Specific volume- Volume per unit mass which is the reciprocal to density. v = V/m = 1/ρ
Specific weight = density*gravity
Surroundings- The mass or region outside the system.
System- A quantity of matter or a chosen region in space.
In the current moment in the United States, we use four major temperature scales that can be
transferable from one to another using the following calculations:
Kelvin to the Celsius
T(K) = T (C) + 273.15
Rankine to Fahrenheit
T(R) = T (F) + 459.67
Rankine to Kelvin
T(R) = 1.8 T(K)
Fahrenheit to Celsius
T (F) = 1.8 T (C) + 32
Different types of pressures are used throughout thermodynamics. These will help you remember
which means what, and how it is calculated.
Absolute pressure- The actual pressure at a given position and measured relatively to absolute
vacuum.
Gage pressure- Is the difference between the absolute pressure and the local atmospheric pressure.
Pgage =Pabs – Patm
Vacuum Pressure- Is the difference between atmospheric pressure and the absolute pressure.
Pvac = Patm – Pabs

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Pascal's law- Pressure exerted anywhere in a confined incompressible fluid is transmitted equally
in all directions throughout the fluid such that the pressure variations (initial differences) remain
the same.
Pressure (atm) = density*gravity*height
Closed system
Piston- Cylinder Device: What happens to the gas when it heated? The gas expands. Below is a
closed system which means no mass is crossing the boundary; only energy can cross the boundary.
Figure 2.1
In a closed system, no mass may be transferred in or out of the system boundaries. Figure 2.1
shows the system always contains the same amount of matter, but heat and work can be exchanged
across the boundary of the system. Whether a system can exchange heat, work, or both is
dependent on the property of its boundary.
 Adiabatic boundary – not allowing any heat exchange: A thermally isolated system
 Rigid boundary – not allowing exchange of work: A mechanically isolated system
Open System (controlled volume)

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This is a specifically selected space or region usually enclosed by a device that involves mass flow
(compressor or turbine) like in Figure 2.2.
In an open system, mass and energy can both cross the boundaries. In general, any random region
in space can be selected as a controlled volume open system.
Control surfaces have boundaries of control volume, which can be real or imaginary.
Most control volumes have fixed boundaries.
Figure 2.2
In an open system, matter may flow in and out of some segments of the system boundaries. There
may be other segments of the system boundaries that pass heat or work but not matter. Respective
account is kept of the transfers of energy across those and any other several boundary segments.
A hot water heater is an example of an open system. Hot water leaves the tank and replaced by
cold water; while the interior surfaces of the tank make up the control surface.
Using Figure 2.3, you can get a better understanding of what the following terms are actually
expressing.
Thermal equilibrium- The temperature is the same through the entire system.
Mechanical equilibrium- There is no change in pressure at any point in system.
Phase equilibrium- The mass of each phase reaches equilibrium level.
Chemical Equilibrium- Chemical composition does not change in time.

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Figure 2.3
Equilibrium thermodynamics is the systematic study of transformations of matter and energy in
systems as they approach equilibrium. The word equilibrium implies a state of balance.
Equilibrium thermodynamics, in origins, derives from analysis of the Carnot cycle.
Simple Compressible System- In the absence of electrical, magnetic, and gravitational motion
there is a need in 2 independent properties to fix the state. Two properties are independent if one
property can be varied while the other one is held constant. Temperature and specific volume are
always independent properties; together they can fix the state of simple compressible system.
Figure 2.4 shows temperature and specific volume.

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Figure 2.4
Process- Any change that system undergoes from one equilibrium state to another.
Path- A series of states through which a system passes during a process.
Figure 2.5 represents a path between states 1 and 2 and the process pass.
Quasi-Equilibrium process- A sufficiently slow process that allows the system to adjust itself
internally, so that properties in one part of the system do not change faster than those at other parts.

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Piston moved slowly (Figure 2.6, A) The molecules have sufficient time to re-distribute. Pressure
inside of the cylinder will rise at the same rate in all locations.
Gas is compressed suddenly (Figure 2.6, B) Molecules near the piston do not have enough time to
escape; creating of a high pressure region there.
Figure 2.6
Figure 2.7

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Cycle- A system returns to its initial state at the end of the process. Figure 2.7 shows this using a
graph overhead a cylinder.
 Isothermal Process: T –constant
 Isobaric process: P-constant
 Steady process- Means no change in time.
Steady-flow process – Fluid flows through
a control volume steadily. Figure 2.8
During a steady-flow process, fluid properties
within the control volume may change with
position but not with time. Figure 2.9
Under steady flow conditions, the mass and
energy contents of a control volume remain
constant. Figure 2.10
The zeroth law of thermodynamics- If two bodies are in thermal equilibrium with a third body
(often it is just a thermometer), they are also in thermal equilibrium with each other.
Figure 2.11 shows that when a body is brought into contact with another body that is at different
temperature, heat is transferred from the body at higher temperature to the one at lower temperature

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until both bodies attain the same temperature. The diagram below states that if you place a piece
of iron that is 150 degrees on top of a piece of copper that is 20 degrees; then the iron will transfer
heat until both are at equilibrium.
Figure 2.11

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CHAPTER 3: ENERGY,
ENERGY TRANSFER,
AND GENERAL ENERGY ANALYSIS
This Chapter will highlight the general importance of energy in life, some of the instances of
transfer as well as what “work” is.
Introduction
Energy is conserved, this is a phrase we heard as a child, it’s something that makes sense; that is
until you apply the real world. Energy conserved, is an expression of the 1st law of
Thermodynamics. This law stands true.
A great example of this is putting a refrigerator in a sealed, insulated room. The room has no other
energy entering in expect for the electrical energy. This is to say the electrical energy is transferred
to thermal energy. Since no energy can escape the room, the energy would be put into the air,
causing the temperature to rise.
(Just don’t let this hot air get to your head)
Forms of Energy
Energy can take many forms; many people think that when things are inefficient, the energy not
directly involved in the intended action (motion in the engine) just goes away. The energy simply
goes into other forms.
An example of this can be illustrated with a gasoline engine. Think, some of the energy is lost in
friction of the internal parts; Noise is a form of energy. Energy is also converted into motion, as
intended. If someone was able to capture all the energy released in every form, it would equal the
energy input, the gasoline.

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Macroscopic energy relates to the energy acting on an object, such as kinetic energy of a moving
ball or the potential energy of an object on top of a tall building. These along with internal energy
of an object, make up the system analyzed.
Nuclear reactions show internal energy released, instead of chemical reactions (most often
illustrated as combustion). Nuclear energy works by releasing the internal energy of a molecule.
This reaction is related to𝐸 = 𝑀𝑐2
. This equation shows the relationship of energy to mass. “E”
is energy, measured in joules. “M” is mass measured in kilograms and “C” is the speed of light in
𝑀/𝑆 (8000000
The amount of energy released in nuclear reactions is massive. 1kg of uranium can release enough
energy as 3000 tons of burned coal.
This is fantastic for humanity and energy usage, however nuclear energy has its own set of
problems. The reaction also produces several other elements that are harmful.
Flow rates of energy are energies with one dimension of time. Flow rates, can show not only the
motion of energy, but how it moves.

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Energy Transfer by Heat
Heat is a form of energy that is transferred between two systems and its surroundings by virtue of
temperature difference.
This is to say that energy can be transferred through heat. Something we all know implicitly, but
have difficulty applying as a stated law. Heat flow is like energy flow, specifically associated with
the movement of energy through heat. A process where there is no heat transfer is an adiabatic
process.
Heat Transfer by Work
Work, like heat is and energy interaction between a system and its surroundings. It is the other
form energy can take when crossing a boundary, which is not heat. Work is force multiplied by a
distance. Power is a rate of work, meaning that it is work with a unit of time.
Mechanical Forms of Work
Work is the integration of a function of work with regard to distance from point start to finish.
The fact that work is utilizing an integration, means that it is a path function. This is to say that the
function of force can change at any point within the start and finish points and we’re looking for
the area underneath the curve.
Shaft work is energy transmission that is very common in engineering. Torque equals force
multiplied than radius of the rotating shaft. 𝑇 = 𝐹𝑟

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Spring Work utilizes
𝐹 = 𝑘𝑥
(k= spring constant) (x is the distance of extension)
𝑊 =
𝑘𝑥2
2
(this looks familiar to the kinetic energy equation)
This is to say that the force of a spring increases linearly as the distance of the spring is increased
or it is stretched, but the work done to achieve the stretched distance increases exponentially and
is the integration of the force equation with respect to distance.
Work needed to raise or to accelerate a body.

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Work is needed to raise a body, because by raising an object you are increasing the total
macroscopic energy in the system, by giving the object a potential energy. Power just applies a
dimension of time which can show the movement of energy.
A great example is in the book, here
Example 1
Notes on the Example: As we see here, 𝑊 =
𝑚𝑣2
2
(w is work in joules, m is mass in kilograms, v
is velocity in m/s) This gives us the amount of work done, in a certain amount of time. This allows
for us to solve for power.
3-6 First Law of Thermodynamics.

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This part of the chapter refers to the transfer of energy into different forms. A good example of
this is to imagine a rock on top of a cliff. As the rock sits on top it has 10 units of potential energy,
however when the rock starts to fall the energy transfers from potential to kinetic. The total energy
however is still equal at all points in time.

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CHAPTER 4: PROPERTIES OF PURE SUBSTANCES
Pure Substances: A pure substance is a substance that is made out of the same material throughout
and has the same properties throughout. A pure substance cannot be separated into other
substances. The reason that it cannot be separated is simply because it is only made out of one
material. All matter can be separated into two categories. These categories are pure substances and
mixtures. Some examples of pure substances are water and oil. If these two pure substances were
mixed together, they would no longer be pure substances. They would now be considered a
mixture. A few more examples of substances with a fixed chemical composition are salt, nitrogen,
helium, and carbon dioxide. Sometimes a mixture can be considered a pure substance, but only if
it is a homogeneous mixture. A homogeneous mixture is a mixture where the components of the
mixture are evenly distributed throughout the mixture. For example, the reason that water and oil
cannot be a pure substance when they are mixed together is because they want to separate from
each other. Oil goes to the top and water goes to the bottom and they are not evenly mixed.
Mixtures of two things that can be a pure substance are salt and sand. If they were mixed together
very well, they would be evenly distributed and would not separate like oil and water. Salt and
sand mixed together is an example of a homogeneous mixture. Matter has three phases, gas, liquid,
and solid. A mixture of two of more phases of a pure substance can still be a pure substance as
long as the substances still have the same composition throughout. A mixture of ice and water is a
great example of this. The ice will still float to the top, but the ice has the same chemical
composition as the water, so it is still a pure substance.
Phases of a Pure Substance: There are three main phases of a pure substance, liquid, solid, and
gas. To better understand these I will use water as an example. When water is frozen into ice, this
is its solid state. When water is at room temperature, and we are able to drink it, this is its liquid
state. When water is boiled, it evaporates. This vapor that it is transformed to is its gaseous state.
An intermolecular force is the force of attraction or repulsion that acts between neighboring
particles in a substance. This is simply the force that holds them all together. The intermolecular
bonds are the weakest when the substance is in its gaseous state and the strongest when a substance
is in its solid state. This explains why solids have molecules that are closely packed together and
gases have particles that are pretty far apart. The molecules in a solid substance are arranged in a
three-dimensional pattern that is evenly repeated throughout it (As shown in figure 2-1). Since the
particles are so closely packed together, they are almost in fixed positions. The intermolecular
forces that were explained earlier are attraction forces until the particles get too close to each other,
then the force turns to a repulsion force to make sure that the molecules stay where needed and
don’t pile up on each other.

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Figure 4-1
The little area that each particle is moving around in is its equilibrium position. The temperature
of these particles controls how fast they are moving around in their equilibrium positions. When
the temperature gets too high and they start moving too fast, they can overcome the intermolecular
force holding them together. This will allow the particles to start breaking away, which is also
called the melting process.
The difference between a solid state and a liquid state is that, in a liquid state the particles aren’t
in fixed positions relative to one another anymore. The intermolecular forces are weaker in liquids,
causing the molecules to group together in small groups and move freely about one another.
In a gaseous state, the intermolecular forces are even smaller than those in a liquid, causing the
particles to move around at random. The only time the particles come in contact with each other
is when they randomly collide. Particles in a gas phase have a decently higher amount of energy
that those in a solid or liquid state. Therefore, when a gas is going to be converted back to a solid
or liquid, it will need to release a decent amount of energy before it does so.
Section 3- Phase-Change Process of Pure Substances: The phase change processes of pure
substances are very important to understand in this chapter. We will go over a few terms to get
started. A compressed liquid, also called a subcooled liquid, is a liquid that is not about to vaporize.
The opposite of a compressed liquid is a saturated liquid. A saturated liquid is a liquid that is about
to vaporize. An example of this is water that is slowly being heated up. The temperature that it
starts out at is 20 degrees Celsius and 1 atm pressure. At this point, the water still fully exists in
the liquid phase and is not even trying to vaporize, so it is a compressed liquid. When the water is
heated up to 100 degrees Celsius, it is not yet vaporizing but is extremely close to vaporizing. In
this case the water is a saturated liquid. If we now pretend that this water that is about to vaporize
is in an expandable container, and is being heated up even more. When it is heated up and it
vaporizes, all the vapor will raise to the top and the water will stay at the bottom until it all
vaporizes too. Now, the heat will be taken away from the container and the vapor will start to cool.
As the temperature gets lower, the vapor will get closer to condensing. This means that it will be
changing from a vapor back to a liquid soon. Right before it does this, it is a saturated vapor. When
this water starts condensing, but not all of it is condensed, it is considered a saturated liquid-vapor
mixture. This means the water is part liquid and part vapor. If the heat were not taken off of this
water after it vaporized, and the vapor continued to heat up, it would be considered a superheated
vapor. A superheated vapor is a vapor that is not about to condense, meaning the temperature is
extremely high and would have to drop a decent amount for the vapor to get even close to
condensing.

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When the last example was completed, the pressure stayed at 1 atm the whole time. Conditions are
not always ideal and stay at a constant pressure. When the pressure raises and lowers, the
temperature at which a liquid will boil changes too. At a given pressure, the temperature at which
a pure substance changes phase is called the saturation temperature. The opposite of the saturation
temperature is the saturation pressure. Saturation pressure is defined as the pressure at which a
pure substance changes phase when at a given temperature. When you put both of these terms
together on a graph, you get a liquid-vapor saturation curve. The curve will change for every pure
substance because they are all different. The example below is a liquid-vapor saturation curve for
water.
Figure 4.2
It is easy to see, as the saturation pressure increases, so does the saturation temperature.
The amount of energy it takes to melt a solid or vaporize a liquid is decently high. For example, to
turn water into vapor, the water has to be 100 degrees Celsius. To get water up to 100 degrees
Celsius, it is going to take quite a bit of energy. The amount of energy that is absorbed or released
by this water as it changes from one phase to another is called latent heat. When water, or any
other pure substance, is being changed from one phase to another, the amount of energy needed to
change it and change it back is going to be the same. For example, the amount of energy it takes
for water to melt is equal to the amount of energy it takes for water to freeze. A few terms that are
related to this are latent heat of fusion and latent heat of vaporization. Latent heat of fusion is
simply the amount of energy absorbed when a substance melts, and latent heat of vaporization is
the amount of energy absorbed when a substance vaporizes.
Property Diagrams for Phase-Change Processes: In this section we will discuss three different
diagrams for pure substances. These include the T-v diagram, the P-v diagram and the P-T
diagram. The T-v diagram shown below is for the heating process of water at a constant

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temperature. This diagram only has one line so that it is easier to understand at first. When more
pressures are added to the graph, more lines are created. The saturated mixture line is the horizontal
line in the center. When the pressure gets higher, the saturated mixture line gets shorter and shorter.
When the pressure gets high enough, this line eventually disappears. When it disappears, it
becomes a point. This point is called the critical point, and is defined as the point at which the
saturated liquid and saturated vapor states are identical.
Figure 4-3
The next diagram is the P-v diagram. The P-v diagram is extremely similar to the T-v diagram,
except the superheated vapor region is going down instead of up. This is because the pressure
inside the container that we talked about earlier is being decreased slowly, causing the volume of
the water to slightly increase. When this volume increases, the vapor region line is slowly sifted
to the right as the temperature drops. The critical point that we talked about earlier is on the P-v
diagram too. Nothing has changed with this point. It still represents the same thing.
Figure 4-4

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The third diagram that needs to be discussed is the P-T diagram. The P-T diagram is also called a
phase diagram, because it displays all three phases of a pure substance on one diagram. The phase
diagram below is a phase diagram of water. It still has the critical point which still means the same
thing it has in the last two graphs. Something that the phase diagram has that the T-v diagram and
the P-v diagram doesn’t, is a triple point. The horizontal lines on the P-v and the T-v diagrams are
often called the triple line. On the P-T diagrams, this is a point instead of a line; therefore it is
called the triple point. All phases that are on the triple line have the same pressure and temperature
but have different specific volumes. This is the same concept for the triple points. All three phases
are separated by lines to let one know at what pressure and temperature they will be changing from
one phase to another.
Figure 4-5
Property Tables: Throughout all the examples that have been discussed this chapter, water has
been used for every one of them. It has been used, because it is one of the easiest to understand.
Many thermodynamic properties are too much of a hassle to be presented by equations, so tables
were made to make things a bit easier. There are many of these tables in the back of
thermodynamics texts books to help work through problems faster and with ease. An example of
a saturated ammonia table is shown below. Most tables include the temperature, pressure, specific
volume, internal energy and the enthalpy of different substances.

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Equations: There are tons of equations that relate to this topic to help find different numbers about
pure substances. Not all of the equations are listed, but the ones that are listed are very important.
The first equation (Equation 1) is Pv=RT . This equation is known as the equation of state and is
fairly simple. If one has three of the four variables, the fourth one can easily be solved with basic
algebra. In this equation, P is the absolute pressure, T is the absolute temperature, v is the specific
volume and R is the gas constant (8.31447 kJ/kmol*K).
The second equation (Equation 2) is Pv=ZRT. This equation is the compressibility factor equation.
Just like the previous formula, this one needs all but one value to be able to solve the unknown
variable. In this compressibility factor equation, Z is the compressibility value of a given
substance, P is the absolute pressure, T is the absolute temperature, v is the specific volume and R
is the gas constant (8.31447 kJ/kmol*K).

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CHAPTER 5: ENERGY ANALYSIS OF CLOSED SYSTEMS
Moving Boundary Work
One common form of mechanical work is associated with the expansion and compression of gas
in a piston-cylinder device. This expansion and compression work is often called moving boundary
work (Figure 5-1). Boundary work is the primary form of work involved in the car engine. During
the expansion or power cycle in an automobile engine, ignited combustion gases force the piston
to move, which forces the engines crankshaft to rotate. In reality, when a piston is moving at high
speed, it is hard for the gas to remain stable because the boundary is moving rapidly. When a
system remains stable nearly at all times is when the piston moves at a low speed. This process of
a system remaining stable at all times is called a quasi-equilibrium or quasi-static process.
A gas does a different amount of work as it forces a piston to move (Figure 5-2). The work can be
calculated by multiplying the absolute pressure and the change in volume.
Figure 5-1
The work associated with a moving boundary is
called boundary work.

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The quasi-equilibrium expansion process is shown on a P-V diagram in Figure 5-3. On the P-V
diagram shown, the area under the curve represents the boundary work done per unit mass.
Figure 5-3
The area under the curve on a P-V diagram
represents the boundary work. As the volume
increases, the pressure decreases, and as the
volume decreases, the pressure increases.
Figure 5-2
A gas does a differential amount of work as it
forces the piston to move a certain distance.

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Example 5-1 shows how to calculate the boundary work using a pressure and specific volume
graph.
Gas can follow several different paths as it expands from state 1 to state 2. The boundary work is
related to the path followed. The net work done during a cycle is the difference between the work
done by a system and the work done on the system (Figure 5-4).
Figure 5-4
The net work done during a cycle is the
difference between the work done by the
system and the work done on the system.
Example 5-1

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Energy Balance for Closed Systems
Energy balance for any system is a difference between energy in and energy out. For a closed
system undergoing a cycle, the initial and final states are identical. Therefore, the heat to be
transferred to the system is equal to the work to be done by the system (Figure 5-5).
Various forms of the first-law and energy balance relationship can be found in Figure 5-6. These
relationships furthermore validate the first law of thermodynamics.
Figure 5-5
For a cycle, the change in energy = 0,
therefore Q = W.
Figure 5-6
Various forms of the first-law relation for
closed systems.

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Specific Heats
As discussed in earlier chapters, it takes different amounts of energy to raise the temperature of
identical masses of different substances by one degree. Specific heat is a property that allows the
comparison of the energy storage capabilities of various substances. Specific heat is defined as the
energy required to raise the temperature of a unit mass of a substance by one degree. There are
two certain types of specific heats we will deal with; specific heat at constant volume Cv ad specific
heat at constant pressure Cp. Cv is a measure of the variation of internal energy of a substance. Cp
is a measure of the variation of enthalpy of a substance.
Internal Energy, Specific Heats of Ideal Gases, and Incompressible Substances
An ideal gas is defined as a gas whose temperature, pressure, and specific volume are related by
Equation 5-4.
A substance whose specific volume (or density) does not change is called an incompressible
substance. The specific volumes of solids and liquids remain constant during a process (Figure 5-
7). Math shows that the constant-volume and constant-pressure specific heats are identical for
incompressible liquids. Both specific heats for an incompressible substance can be represented by
the symbol c (Equation 5-5).
Equation 5-4
Ideal Gas Law
P is pressure, v is specific volume, R is
ideal gas constant, and T is temperature.
Equation 5-5
Specific Heat for Incompressible
Liquid

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CHAPTER 6: MASS AND ENERGY
ANAYLSIS OF CONTROL VOLUMES
Understanding Mass and Energy of an Area (Control Volume) in Space
Conservation of Mass Principle (Keep the Mass Safe!)
Mass (m): not the same as weight! This is a measure of how much “stuff” is in something. Figure
2 defines mass and shows a fun way to understand it.
Weight (w): is a force and can be calculated by multiplying the mass by the acceleration of gravity
(at standard sea level of the earth this equals 9.81 meters per second squared.)
Mass and energy are conserved properties. Their systems do not interact with the environment
and certain mechanical properties of the system cannot
change.
An object would have the same mass on the Moon as it does
on Earth, however, the same object would weigh six-times as
much on Earth as it would on the Moon.
Mass cannot be created or destroyed!
If mass is not added or taken away, the amount of mass in a
closed area must stay the same over time.
Figure 2: Mass, defined
Energy (E): The strength needed to continue an activity and
the ability to do work. This can come in many forms such as
heat, light, and electrical to name a few. Mass is a form of
energy and mass and energy are related according to physicist
Albert Einstein’s (1879-1955.) mass-energy equivalence
formula shown in Figure 1. To understand this, the formula
he suggested was E = mc2 (Formula 1) where E is energy, m is mass, and c is the speed of light in
a vacuum (an empty place where air has been removed.) The value c is always the same, it is a
constant, and since light moves at 186,282 miles per second or 2.9979 x 108 meters per second,
this is used as c in the equation. The “2” means that this value is multiplied by itself. This equation
as a whole simply means that when mass changes, energy also changes.
Mass flow rate (ṁ): the amount of mass traveling through something in a certain amount of time.
The dot above the m just means that time is involved. You can think of a pipe with water going
through it to better understand this. Figure 3 describes mass flow rate with a picture and also
shows the equation of continuity and volume flow rate formula. The formula describing mass flow
rate looks like this:

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δṁ = ρVndAc (Formula 2)
δ is usually used for amounts of heat, work, and mass transfer
δṁ is mass flow rate of fluid traveling over a small area
ρ is density which is the amount of mass something has divided by its volume
Volume: the amount of space that something occupies
Vn is the velocity (speed) of the flow
perpendicular to dAc
dAc is the small area
Since water slows down near the pipe
walls or boundaries, the velocity is
never the same at every location inside
a pipe.
Figure
3: Understanding mass flow rate
The average velocity (Vavg) is the
average speed of Vn over the entire cross-
section of a pipe. Figure 4 shows what a
cross-section is.
Velocity is never uniform over a cross section of a pipe
because of the fluid sticking to the surface and therefore
having zero velocity at the wall.
Vavg = ṁ/ρAc (Formula 3)
Figure 4: Cross-section example
Volume flow rate (ὐ): the volume of fluid that is going through a cross section per unit of time.
Mass flow rate and volume flow rate are related when the specific volume is included. Specific
volume (v) is the ratio of the substance's volume to its mass. It is the reciprocal of density (1/ρ)
and is a basic property of matter. It can be defined as the number of cubic meters occupied by one
kilogram of a particular substance. ṁ = ρὐ = ὐ/v (Formula 4)
As described before, the conservation of mass principle for closed systems requires that the mass
of the system remain constant during a process. For control volumes, however, mass can cross the

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boundaries, so we need to keep track of the amount of mass entering and leaving the control
volume.
Figure 5: Conservation of Mass Principle
This can be done with the following formula which basically states that the total mass entering
minus the total mass leaving equals the net change, or what is left over during a time change.
Figure 5 explains the definition and what is happening in words. Figure 6 is the same formula
except it is represented with symbols. Figure 7 shows a graphic representation of this principle.
Figure 6: Conservation of Mass
∑ means the sum of all, therefore, the sum of all the mass entering (in) minus the sum of all mass
exiting. ∆ means the change in, therefore, the change in mass in the control volume.
Figure 7: Graphic describing a control volume and conservation of mass
Mass Balance for Steady-Flow Process
The steady flow process occurs when the total amount of mass inside a control volume (mcv) does
not change with time. The total amount of mass entering a control volume must equal the total
amount of mass leaving it. For a steady flow process we are interested in the amount of mass that

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flows per unit of time (mass flow rate.) The picture below in Figure 8 shows the conservation of
mass principle for a two-inlet/one-outlet steady flow system.
ṁ1 = ṁ2 (Formula 5) ρ1V1A1 = ρ2V2A2 (Formula 6)
Where: m is mass, ρ is density, V is volume, and A is area
Special Case: Incompressible Flow
The conservation of mass relationship can be simplified even more
when the fluid is incompressible (the density of the fluid does not
change.) For a steady flow of liquids, the density cancels from both
sides of the equation showing that the sum of volume flow rates in
equals the sum of volume flow rates out. ∑ὐin = ∑ὐout
(Formula 7)
Figure 8: Steady flow system
Since there is no “conservation of volume principle,” the volume
flow rates going in and out of a steady-flow device may be different even though the mass flow
rates are the same. An air compressor is a good example of this case. The
volume flow rate at the outlet of a compressor is much less than that at the
inlet even though the mass flow rate of air through the compressor stays
constant.
Figure 9: Air compressor example
Flow Work and the Energy of a Flowing Fluid (Work is required!)
Control volumes involve mass flow across boundaries and work is required
to push the mass in or out of the control volume. This type of work is called
flow work, or flow energy. This work is necessary for keeping the flow continuous through a
control volume. To understand look at Figures 10 and 11 below, which show imaginary pistons.
The fluid element has a certain volume (V) and the fluid to the right forces the fluid in the element
to enter the control volume (CV.) If the fluid pressure (P) and the cross-sectional area of the fluid
element is (A), the force applied on the fluid element by the imaginary piston is: F = PA (Formula
8)

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To push the entire fluid element into the control volume, the force must act along a distance (L.)
Wflow = FL = PAL = PV (Formula 9)
The flow work per unit of mass is:
Wflow = PV (Formula 10)
If there is no acceleration then the force applied on a fluid
by the piston is equal to the force applied on the piston by
the fluid. Figure 10: Imaginary piston
Figure 11: Force
on piston
Total Energy of a Flowing Fluid
The total energy of a simple compressible system
consists of three parts: internal, kinetic, and potential
energies. Figure 11 helps explain different types of
energy.
Figure 11: Energy explanation
This energy (e) can be expressed by:
(Formula 11)
In this equation, u is internal energy, ke is kinetic energy, and pe is potential energy. Kinetic
energy is expressed by the velocity squared divided by two and potential energy is expressed by
gravitational acceleration multiplied by the elevation of the system as compared to an external
reference point. If the fluid is flowing, the fourth part, flow energy (Pv) is added to the formula. θ
is a total energy per unit mass. It is defined as the total energy of a flowing fluid. Figure 12 shows
examples of these formulas.
Figure 12: Energy equation for flowing vs. nonflowing fluid

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Energy Analysis of Steady-Flow Systems (No change with time!)
Many engineering devices such as turbines, compressors, and nozzles operate for long periods of
time under the same conditions once the start-up period is complete. These devices are called
steady-flow devices. The process that is involved in these devices is called the steady-flow
process. This is a process during which a fluid flows through a control volume steadily (it means
there is no change with time.) The fluid properties can change from point to point within the
control volume, but at any point, they remain constant during the entire process. The volume (V),
mass (m), and total energy content (E) of the control volume remain constant. Since these
properties remain constant, the boundary work is zero for steady-flow system.
Boundary work is a form of mechanical
work that is usually found when gas
expands or compresses in a piston-
cylinder device. During this process, part
of the boundary (the inner face of the
piston) moves back and forth leading to
expansion and compression work. The
fluid properties at an inlet or exit also
remain constant during a steady-flow
process. The properties may be different
at different inlets and exits and may vary
over the cross section of an inlet or exit but
the properties, including velocity and
elevation, must remain constant with time
at a fixed point at an inlet or exit.
Figure 13: Energy analysis of steady-flow system

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The heat and work interactions between a steady-flow system and the surroundings also do not
change with time meaning the power delivered by a system and the rate of heat transfer remain
constant as well. Figure 13 explains that power plants operate under steady systems and shows
what happens in these conditions. Noting that energy can be transferred by heat, work, and mass
only, the energy balance for a general steady-flow system can be written: Ėin = Ėout (or)
Qin + Win + ∑ṁθ = Qout + Wout + ∑ṁθ (Formula 12)
● Q is the rate of heat transfer between the control
volume and its surroundings (when the control
volume is losing heat Q is negative. If the control
volume is well insulated then Q is zero.
Figure 14: Hot-water tank control volume
● W is power. For steady-flow devices, the control
volume is constant so there is no boundary work
involved. Since many steady-flow devices such as turbines, compressors, and pumps move power
through a shaft, W is the shaft power for the devices. W can also represent electrical work done
per unit of time. If neither are present then W is zero. Figure 14 shows a hot-water tank control
volume with cold water in and hot water out.
Some Steady-Flow Engineering Devices
Nozzle: a device that increases the velocity of a fluid at the expense of pressure
Diffuser: a device that increases the pressure of a fluid by slowing it down
Nozzles and diffusers are commonly used in jet engines, rockets, spacecraft, and garden hoses.
Nozzles and diffusers perform opposite tasks. The rate of heat transfer between the fluid flowing
through a nozzle or a diffuser and the surroundings is very small (Q = 0) since the fluid has such
high velocities and therefore does not spend enough time in the device for any significant heat
transfer to take place. Nozzles and diffusers usually do not involve work (W = 0) but do involve
very high velocities.
Turbine: in steam, gas, or hydroelectric power plants, this device drives the electric generator
The turbine produces work when fluid passes through it and the shaft rotates. Blades are attached
to the shaft and the work is done against the blades.

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Compressors: along with pumps and fans, these devices are used to increase the pressure of a
fluid
Work is supplied to these devices from an external source through a rotating shaft. These devices
function similarly but do different tasks. A fan increases the pressure of a gas slightly and is
mainly used to move a gas. A compressor is capable of compressing the gas to very high pressures.
Pumps work very much like compressors except that they handle liquids instead of gases.
Figure 15: Engineering Devices
Mixing chambers: the section where the mixing process of two streams of fluids takes place
These are usually well insulated and do not involve any kind of work.
Heat exchangers: devices where two moving fluid streams exchange heat without mixing
These are widely used in various industries and come in many designs. Heat exchangers also
involve no work interactions most of the time.
Let’s Review!
The conservation of mass principle states that the net mass transfer to or from a system during a
process is equal to the net change (increase or decrease) in the total mass of the system during that
process.

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The amount of mass flowing through a cross section per unit time is called the mass flow rate.
The volume of the fluid flowing through a cross section per unit time is called the volume flow
rate.
The work required to push a unit mass of fluid into or out of a control volume is called flow work
or flow energy.
Thermodynamic processes involving control volumes can be considered in two groups: steady-
flow processes and unsteady-flow processes. During a steady-flow process, the fluid flows
through the control volume steadily, experiencing no change with time at a fixed position. The
mass and energy remain constant also.
There are several types of steady-flow engineering devices.

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CHAPTER 7: THE SECOND LAW OF THERMODYNAMICS
In order for any process to occur, both the first and second laws of thermodynamics must be
satisfied. The first law tells us that energy cannot be created nor destroyed. Energy can, however,
change forms and can flow from one place to another. The first law does not control the order in
which a process takes place. Any naturally occurring process occurs in only one direction. Think
of an ice cube in a glass of water. The ice cube melts over time as the warmer water transfers its
heat to the ice cube. This causes the water to become colder until the liquid and the ice are the
same temperature. The ice cube does not transfer its cold to the rest of the liquid, instead it absorbs
the warmth in the liquid and melts. This process will continue until the liquid and ice are the same
temperature. This is where the second law comes into play. The second law of thermodynamics
tells us that processes occur in a specific direction. Processes always occur in the direction from
a higher temperature to a lower temperature. The second law also states that energy has quality
connected to it, not just quantity. The quality of energy is a big concern of engineers and the
second law gives the tools necessary to measure the quality and how much it drops during a
process.
Thermal energy reservoirs are used in helping to understand the second law. These reservoirs are
bodies with a large size to take in or provide a certain amount of heat with no change in its own
temperature. Many different bodies can be considered reservoirs, such as rivers, lakes, oceans, or
the air in the atmosphere. There are two kinds of thermal energy reservoirs and they both deal
with energy in the form of heat. A reservoir is a source if it provides energy in the form of heat.
On the other hand, a reservoir that absorbs energy is called a sink. It is easiest to remember which
type of reservoir does what by thinking about a kitchen faucet and sink. Think of the water the
flows from the faucet as heat. The faucet, in this case, can be thought of as the source. The sink
takes the rejected water and disposes of it. An important part of thermal energy reservoirs to
understand is that size does not necessarily mean something is a reservoir. The size relative to the
heat being supplied or absorbed is the key. Let us use the ice cube as an example again. While a
river or lake would be considered a thermal energy reservoir if an ice cube were added to it, a five
gallon bucket of water would be one as well. As the ice melts, it would not change the temperature
of the water in the bucket. The temperature of a glass of water, on the other hand, would change
from the same ice cube. When thermal energy reservoirs are used in industrial applications, heat
transfer is something to be concerned about. When these parts of the environment are used as
sinks, the heat being transferred must not be so great that it changes the temperature of the
reservoir. A rise in this temperature could result in marine life being disrupted. This is what is
known as thermal pollution and must be carefully considered.
Work can be changed to heat directly and completely, however changing heat to work requires
something called a heat engine. All heat engines operate in the same way and this process is a
relatively simple one. Work is pulled out of heat that flows from a hot object to a cooler object.
The components of heat engines are a pump, a condenser, a boiler and a turbine which produces
work. There are many different heat engines today, but they all have four basic characteristics:
1) A high temperature source provides them with heat.
2) Part of this heat is converted to work.
3) Remaining waste heat is sent to a low-temperature sink.
4) Operation occurs on a cycle.

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Two methods can be used to determine the amount of work output from a heat engine. Let’s start
by defining a few variables for these equations:
Qin = amount of heat supplied from a high-temperature source.
Qout = amount of heat rejected to a low-temperature sink.
Wout = amount of work delivered that operates the turbine.
Win = amount of work needed to compress water to boiler pressure.
Heat engines are open systems that can be studied as closed systems which means the amount of
useful work, or net work, they provide can be determined by looking at the work in and work out,
or by the amount of heat supplied and the amount of heat rejected.
𝑊𝑛𝑒𝑡,𝑜𝑢𝑡 = 𝑊𝑜𝑢𝑡 − 𝑊𝑖𝑛 Equation 1
𝑊𝑛𝑒𝑡,𝑜𝑢𝑡 = 𝑄𝑖𝑛 − 𝑄 𝑜𝑢𝑡 Equation 2
The thermal efficiency of heat engines is something that is a concern of engineers. The Kelvin-
Planck Statement of the second law tells us that it is impossible for any device that operates on a
cycle to receive heat from a single reservoir and produce an amount of work. This statement can
be further understood to mean that no heat engine can turn all of the energy it gets into work. Some
of this energy is rejected. A heat engine must exchange heat with a low-temperature reservoir as
well as a high-temperature source in order for it to operate. There are many different ways energy
will be lost while a heat engine is running. Heat could leak into the surrounding area from the heat
source, or from the piping used to transfer heat not being properly protected. There will also be
some loss of energy through friction in the bearings that are in the turbine. Lastly, some of the
energy will be passed on to the cooling water, or low-temperature sink. With heat engines, only a
small amount of the heat they get is used as work. This is called their thermal efficiency. Thermal
efficiency is determined by the net work output divided by the total heat input, that is:
𝑇ℎ𝑒𝑟𝑚𝑎𝑙 𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦, 𝜂𝑡ℎ =
𝑁𝑒𝑡 𝑤𝑜𝑟𝑘 𝑜𝑢𝑡𝑝𝑢𝑡
𝑇𝑜𝑡𝑎 𝑙 ℎ𝑒𝑎𝑡 𝑖𝑛𝑝𝑢𝑡
=
𝑊𝑛𝑒𝑡 ,𝑜𝑢𝑡
𝑄𝑖𝑛
Equation 3
Another equation for thermal efficiency is:
𝜂𝑡ℎ = 1 −
𝑄 𝐿
𝑄 𝐻
Equation 4
Where QL is the amount of heat transfer between the device and the low-temperature sink. On the
other hand, QH is the amount of heat transfer between the device and the high-temperature source.
The automobile gasoline engine is an example of a heat engine. The high-temperature source is
the burning fuel while the exhaust into the environment is the low-temperature sink. (Heat Engine
Cycle) The automobile gasoline engine, only operates at about 25 percent efficiency. That is, 25
percent of the burning fuel provided to it is converted to work that can be used to make the vehicle
move. One of the most efficient heat engines used today is the combined gas-steam power plant
which operates at an efficiency of about 60 percent. Therefore, about half of the energy received
is sent out into the environment with no work resulting from this lost energy. The efficiency of
heat engines is directly related to the temperature drop across them. The larger the temperature
difference between the heat coming in and the heat exiting, the more efficient the heat engine is.

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Since heat engines reject heat, the question then becomes, can the rejected heat be recycled and
used again in the same engine? Unfortunately, the answer to this question is no. Heat engines run
on cycles and have to complete one cycle before the next one can start. Without some kind of heat
exiting the engine, a cycle cannot be completed. This rejected heat will be at a lower temperature
than the reservoir. We know that heat is always transferred from the higher temperature to the
lower, so no energy will be gained by feeding rejected, lower temperature, heat back into the
engine. This waste heat can be used for other applications. Thomas Edison was one of the first to
use energy recycling. The world’s first commercial power plant, Pearl Street Station built in 1882,
was a combined heat and power plant. It produced both electricity and thermal energy and used
the waste heat from its process to warm neighboring buildings. Recycling of waste allowed this
plant to achieve approximately 50 percent efficiency. (Cogeneration)
Heat transfer occurs naturally from higher temperatures to lower temperatures and no device is
required to enable this process. For the opposite process to occur, a common household appliance
is required. Refrigerators transfer heat from a lower temperature to a higher one. There are five
basic items involved in a refrigeration cycle; refrigerant, a compressor, a condenser, an expansion
valve, and an evaporator. The compressor does just that, it compresses the gas refrigerant which
raises its temperature. As this occurs, the refrigerant is pushed into the condenser coils where the
hot gas transfers its heat to the cooler air temperature and becomes liquid. Now in liquid form, at
high pressure, the refrigerant cools down as it flows through the coils inside the refrigerator. At
this point, the refrigerant absorbs the heat inside the refrigerator which cools the air inside. Finally,
the refrigerant evaporates into gas in the evaporator and flows back into the compressor, where the
cycle starts again. (Sforza, n.d.) When a refrigerator is running, warm air can be felt behind it.
This is from energy in the form of heat being pulled from the refrigerated space, then exhausted
into the room. The energy source, in this case, is the refrigerated space and the sink is the room,
or kitchen where the refrigerator is located. If a refrigerator were to be left open for a long period
of time, it would begin to get very warm due to the refrigerator constantly operating and trying to
cool down the open refrigerated space.
Heat Pumps are another device that works opposite of the normal direction of heat flow. Heat
pumps absorb heat from a cold space and release it to a warmer one. These are commonly used in
heating homes during the cold winter months. There are two types of heat pumps, air-source and
ground-source (geothermal). Air-source heat pumps get heat from cold air outside. These kind of
heat pumps do not work well in colder climates. Their efficiency drops significantly when
temperatures are below freezing. For colder climates, ground-source pumps are used. This kind
of pump costs more to install because they require underground piping. They are about 45 percent
more efficient because they get their heat from the ground. The ground is warmer than the air,
therefore geothermal pumps get their heat from a warmer source.
The efficiency of both refrigerators and heat pumps can be found by the coefficient of performance
(COPR and COPHP). Both of these coefficients use the same calculation, but they use slightly
different variables. All of these variables have been presented previously.
𝐶𝑂𝑃 𝑅 =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑜𝑢𝑡𝑝𝑢𝑡
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐼𝑛𝑝𝑢𝑡
=
𝑄 𝐿
𝑄 𝑛𝑒𝑡 ,𝑖𝑛
Equation 5

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49
𝐶𝑂𝑃 𝐻𝑃 =
𝐷𝑒𝑠𝑖𝑟𝑒𝑑 𝑜𝑢𝑡𝑝𝑢𝑡
𝑅𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝐼𝑛𝑝𝑢𝑡
=
𝑄 𝐻
𝑄 𝑛𝑒𝑡 ,𝑖𝑛
Equation 6
An air conditioner works a lot like a refrigerator. It takes heat from a room and discharges it to
the outside. If this same air conditioner were turned around in cold weather, it would work like a
heat pump. It would pull the heat from the outside cold air and discharge it into the room as warm
air. From this comparison, it is easy to see that there is not much difference between refrigerators
and heat pumps. They basically function in opposite ways to one another. The Clausius Statement
says that it is impossible to build a device that operates in a cycle that produces no effect other
than to transfer heat from a low-temperature body to a higher-temperature body. What this
suggests is that a refrigerator can be built and work as it is intended, but only if it operates with
some help. We use electricity as the help to make it operate.
In thermodynamics, processes can be completed either reversibly or irreversibly. The goal of
reversible processes is to get most of their work from energy rather than from heat. Since heat
cannot be fully converted to work, it is lost, usually to the surroundings where the process is taking
place. (Reversible process (thermodynamics)) Reversible processes do not leave any trace on
their surroundings. For this to occur, the net heat and network between the system and its
surroundings must be zero. These types of processes do not occur naturally. They may be models
of actual devices, however a truly reversible process cannot exist. Reversible processes are used
to find the theoretical limits of irreversible processes.
Irreversible processes occur naturally. The system and its surroundings cannot be brought back to
their original states prior to the process occurring. Although the system itself might be able to be
brought back to its original state, the environment will have had some kind of effect from the
process that will not allow it return to its original state. There are many things which cause a
process to be irreversible. These things are called irreversibilities. A few examples are friction,
mixing of two fluids, and heat transfer across a certain temperature difference, and solids that are
deformed such that they will not return to their original state. (Cengel, 2008) Consider a car
stopped on a street. There are four tires all in a certain position to start. The car then begins to
move down the street. The car could very easily return to the original position, however some
changes will have occurred to both the tires and the environment due to friction. Depending on
how far and fast the vehicle travelled, the tires will have lost some amount of their tread. As well,
both the tires and the environment will have gained some heat. This would be considered an
irreversible process since neither the tires nor the environment will return to their original state.
Another example of an irreversible process could be a tube of toothpaste. When the tube is
squeezed, toothpaste comes out and the tube is deformed. The tube will not go back to how it was
before it was squeezed, nor will the toothpaste be replaced unless some amount of work is done to
add more toothpaste to the tube.
The quality of energy is very important due to the fact that higher quality energy results in more
work that can be made from it. As can be seen in efficiency equations which were shown earlier,
this results in a more efficient device. When considering the quality of energy, the higher its
temperature, the greater its quality. Whenever energy is used, the quality of it is reduced. Low
quality energy is expensive, difficult, and sometimes cannot be used. It cannot be used in nature.
Consider the sun constantly shining its light and providing its heat to Earth. Without this heat

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being provided, no plants would be able to survive and neither could humans or animals. Thinking
about the universe as a whole, energy is constantly being used. Stars are constantly using energy
in the form of hydrogen. As more and more time passes, the quality of energy in the universe
worsens. As we know from the first law of thermodynamics, energy cannot be created. Therefore,
the universe is always working towards a balance of energy. When this point is reached,
everything will be the same temperature. Atoms and molecules will still be moving and colliding,
but they will all have the same average level of energy. Heat transfer will no longer occur because
there will no longer be any difference in the concentrations of energy from one body to another.
(Watson, n.d.) This will take a very long time and will not occur in our lifetimes. However, based
on our understanding of thermodynamics and physics, this is the direction the universe is going.

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CHAPTER 8: MECHANISMS OF HEAT TRANSFER
Introduction
We will talk about the different types heat can be transferred to; conduction, convection, and
radiation. As seen in Figure 1 below, a pot of boiling water produces three types of heat transfer.
The heat from the fire produces radiation which is what heats up the bottom of the pot. When cold
water is added to the pot, the cold water “sinks” down and as the cold water is warmed up, the
warmer water rises to the surface and pushes more cold water down to the bottom. This process of
warm water rises and cold water sinking is called convection. Finally, the heat from the surface of
the water is transferred to the handle of the pot. This is known as conduction.
If you can recall, heat is the thermal energy transported from one system to another due to a
temperature difference. For example, say you have two identical systems with equal temperatures,
energy will not flow. On the flip side, if you have two systems with different temperatures, the
energy will start to flow. Heat can do anything: move from one area to another, get atoms excited,
and even increase energy. You might ask where energy comes into play,
well really heat is energy. So when you increase the heat in a system,
you’re really increasing the amount of energy in the system. Energy is
transferred on an atomic level, which is just fancy term for saying a really
small scale. The unit of measure for heat is typically given in a calorie.
You might be thinking, “Calories, like the calories that are in food?”
Yeah just about, except these calories are measured as the amount of
energy needed to raise the temperature of one gram of water by one
degree Celsius. This knowledge becomes handy when measuring the
specific heat capacity. The specific heat capacity is also the amount of
energy required to raise the temperature of one gram of a substance by
one degree Celsius. Because it is energy, scientists use the units of Joules
to measure it. Get this; one calorie equals 4.186 Joules which also equals
4.186 Watt-seconds (Ws). So wait a minute, does that mean you can measure the amount of energy
you make in your body in one second and express that in terms of an electric value? Yes! The rate
Figure 1: Three Mechanisms of Heat Transfer

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at which energy is created and used in your body can actually be expressed as electrical power.
Talk about mind boggling!
Conduction
Conduction is the transfer of heat within a system or between two systems that are touching. Say
you put a perfectly good Popsicle on the table to sit and you
came back to what’s seen in Figure 2. Well according to
conduction, your Popsicle started melting due to the change
of temperatures. If you were to put your finger into the liquid
of your melted Popsicle, you’ll feel the liquid at the same
temperature that is in the room. Conduction takes place in
solids, liquids, or gases. When trying to solve problems with
conduction, you are typically trying to calculate the rate of
heat conduction. The rate of heat conduction depends on the
geometry of what you’re looking at. In order to solve for
heat conduction, you will need the thermal conductivity which is either a given variable or can be
found online or in a thermodynamic textbook. Thermal conductivity is basically a materials ability
to conduct heat. You will also need to know the thickness of the material, the area, and the
temperature difference. When solving problems, the variable 𝑄̇ 𝑐𝑜𝑛𝑑 stands for the rate of heat
conduction. As mentioned earlier, the variable k stands for thermal conductivity and the variable
A is used for area. The formula will look something like this: 𝑄̇ 𝑐𝑜𝑛𝑑 =
𝑘𝐴
∆𝑇 (𝐶ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 )
𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠
.
Thermal Conductivity
Thermal conductivity is the ability of materials to conduct heat. Specific heat capacity is
represented by the symbol 𝐶 𝑝, which is a measure of material to store thermal energy. Again,
thermal conductivity is denoted by k. It is also the rate of heat transfer through a unit of thickness
of the material per unit area per unit temperature difference (See formula). 𝑘 =
𝐿 (𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠)
𝐴𝑟𝑒𝑎(𝑇1− 𝑇2)
∗ 𝑄̇.
Where k is the thermal conductivity of a material, L is the thickness of the material, A is the area
of the material, 𝑄̇ is the rate of heat, and T is the temperature of the material. In the case of thermal
Figure 2: Melting Popsicle Example

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conductivity, you will have two temperatures per geometry you are looking at. If you get a value
of k that is high, it indicates that the material is a good heat conductor and if the value is low, the
material is an insulator. The range of thermal conductivity varies based of the materials at room
temperature. The heat capacity of the material can also be given as 𝜌𝐶 𝑝. This is useful when solving
for thermal diffusivity. Where α represents the thermal diffusivity, k is the heat that is conducted
in a material, and 𝜌𝐶 𝑝 is the heat stored. α =
𝐻𝑒𝑎𝑡 𝐶𝑜𝑛𝑑𝑢𝑐𝑡𝑒𝑑
𝐻𝑒𝑎𝑡 𝑆𝑡𝑜𝑟𝑒𝑑
=
𝑘
𝜌𝐶 𝑝
.
Convection
Convectionis the way heat is transferred from one area to another when there is a “bulk movement
of matter.” It is the movement of huge amounts of material, taking the heat from one area and
placing it in another. An example of this is when warm air rises and cold air replaces it. The heat
has moved, this is known as the transfer of heat by motion of objects. The faster the fluid motion,
the greater the convection heat transfer. In the absence of any bulk fluid motion, heat transfer
between a solid surface and the adjacent fluid is by pure conduction. The presence of bulk motion
of the fluid enhances the heat transfer between solid surface and the fluid, but it also complicates
the determination of heat transfer rates. The rate of convection heat transfer is observed to be
proportional to the temperature difference, and is conveniently expressed by Newton’s law of
cooling. Newton's law of cooling is expressed as 𝑄̇ 𝑐𝑜𝑛𝑣 = ℎ𝐴 𝑠(𝑇𝑠 − 𝑇∞)where h is the convection
heat transfer coefficient, As is the surface area through which convection heat transfer takes place,
Ts is the surface temperature and T is the temperature of the fluid sufficiently far from the surface.
Convection can be split up into two types of convection, forced convection and natural (or free)
convection.
Forced Convection
Forced convection is when the fluid is forced to flow over the surface by external means such as
a fan, pump, or the wind. Forced convection should be considered one of the main methods of
useful heat transfer as significant amounts of heat energy can be transported very efficiently.
Figure 3 below shows a pan that is being heated up and the fan is blowing over the pan. This is an
example of forced convection, because the fan is forcing the fluid to flow over the pan.
Figure 3: Forced Convection Example

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Natural Convection
Natural convection is when the fluid motion is not generated by an external source and only by
density differences in the fluid occurring due to temperature rises. An example of this is in the
absence of a fan, heat transfer from the surface of the hot block is by natural convection since any
motion in the air in this case is due to the rise of the warmer air near the surface and the fall of the
cooler air to fill its place. Figure 4 shows that heat transfer happens without any external forces,
such as a fan, to force the fluid over unlike forced convection.
Radiation
Radiation is when the transfer of energy
happens when there is no conductive medium
such as space. That lack of medium means
there is no matter there for heat to pass
through. Radiation is the energy carried by
electromagnetic waves (light). Those waves
could be radio waves, infrared, visible light,
UV or Gamma rays. Heat radiation is usually found in the infrared sections of the electromagnetic
spectrum. Figure 5 shows how the radiation is going from the fire to the person’s hands. This
merely means that the heat from the fire is being transferred to the hands, so that the hands start to
warm up. Scientists have discovered that objects that are good at giving off thermal radiation are
also good at absorbing the same energy. Usually the amount of radiation given off by an object
depends on the energy of the objects and molecules surrounding you. Thermal radiation is
interesting in the studies of heat transfer, because the form of radiation is emitted by bodies because
of their temperature. It differs from other forms of electromagnetic radiation such as x-rays, gamma
Figure 4: Natural Convection Example

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rays, microwaves, radio eaves, and television waves that are not related to temperature. Radiation
is expressed by Stefan-Boltzman law as:
𝑄̇ 𝑟𝑎𝑑 = 𝜀𝜎𝐴 𝑠 (𝑇𝑠
4
− 𝑇𝑠𝑢𝑟𝑟
4
) where 𝜀 is the emissivity of surface, As is the surface area, Ts is the
surface temperature, Tsurr is the average surrounding surface temperature and 𝜎 =
5.67𝑥10−8
𝑊
𝑚2 ∗°𝐾4 is
the Stefan-Boltzman constant.
Simultaneous Heat Transfer Mechanisms
As mentioned earlier in the chapter, there are three mechanisms of
heat transfer, but not all three can exist at the same time in a medium.
For example, heat transfer is only by conduction in opaque solids, but
by conduction and radiation in semitransparent solids. Thus, a solid
may involve conduction and radiation but not convection. However,
a solid may involve heat transfer by convection and/or radiation on
its surfaces that are exposed to a fluid or other surfaces. For example,
the outer surfaces of a cold piece of rock will warm up in a warmer
environment as a result of heat gained by convection from the air and
the radiation from the sun or the warmer surrounding surfaces, but the
inner parts of the rock will warm up as the heat is transferred to the
inner region of the rock by conduction. As a summary, heat can be
transferred by conduction and/or radiation in a still fluid (no bulk
fluid motion) and by convection and/or radiation in a flowing fluid.
When heat is transferred through a fluid you will either have
conduction or convection, but not both. Gases are practically
transparent to radiation, except for those gases that are known to
absorb radiation at certain wavelengths. Finally, heat transfer through
a vacuum is by radiation only since conduction or convection requires
the presence of a material medium.
Practice Problems
Problem 1: An aluminum pan whose thermal conductivity is 237 W/m* ºC has a flat bottom with
diameter 15 cm and thickness 0.4cm. Heat is transferred steadily to boiling water in the pan through
its bottom at a rate of 800 W. If the inner surface of the bottom of the pan is at 105ºC, determine
the temperature of the outer surface of the bottom pan.
Given:
𝑘 = 237
𝑊
𝑚∗°𝐶
𝐷𝑖𝑎𝑚𝑒𝑡𝑒𝑟 = 15𝑐𝑚 = 0.15𝑚
𝑄̇ 𝑐𝑜𝑛𝑑 = 800𝑊 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠 = 0.4𝑐𝑚 = 0.004𝑚
𝑇2 = 105°𝐶
105C
800 W
0.4 cm

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Heat transfer area:
𝐴𝑟𝑒𝑎 𝑐𝑖𝑟𝑐𝑙𝑒 = 𝛱𝑟2
= 𝜋(0.075𝑚)2
= 0.018𝑚2
Conduction heat transfer through bottom pan:
𝑄̇ 𝑐𝑜𝑛𝑑 = 𝑘𝐴
∆𝑇
𝐿
= 𝑘𝐴
𝑇2 − 𝑇1
𝐿
800 𝑊 = (237
𝑊
𝑚 ∗ °𝐶
)(0.0177 𝑚2
)
𝑇2 − 105°𝐶
0.004𝑚
Answer: 𝑇1 = 105.76°𝐶
Problem 2: Hot air at 80°C is blown over a 2m x 4m flat surface at 30°C. If the average convection
heat transfer coefficient is 55
𝑊
𝑚2 ∗°𝐶
, determine the rate of heat transfer from the air to the plate, in
kW.
Given:
ℎ =
55𝑊
𝑚2 ∗°𝐶
𝐴 𝑠 = 2𝑥4 = 8𝑚2
∆𝑇 = (80°𝐶 − 30°𝐶) = 50°𝐶
Convection heat transfer:
𝑄̇ 𝑐𝑜𝑛𝑣 = ℎ𝐴 𝑠 ∆𝑇
𝑄̇ 𝑐𝑜𝑛𝑣 = (
55𝑊
𝑚2 ∗ °𝐶
)(8𝑚2
)(50°𝐶)
Answer: 𝑄̇ 𝑐𝑜𝑛𝑣 = 22,000𝑊 = 22𝑘𝑊
80C
Air
30C

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Problem 3: Consider a person standing in a room maintained at 20°C at all times. The inner
surfaces of the walls, floors, and ceiling of the house are observed to be at an average temperature
of 12°C in winter and 23°C in summer. Determine the rates of radiation heat transfer between this
person and the surrounding surfaces in both summer and winter if the exposed surface area,
emissivity, and the average outer surface temperature of the person are 1.6𝑚2
, 0.95, and 32°C
respectively.
Given:
𝜀 = 0.95 𝐴 𝑠 = 1.6𝑚2
𝑇𝑠 = 32°𝐶 + 273 = 305°𝐾
Summer: 𝑇𝑠𝑢𝑟𝑟 = 23°𝐶 + 273 = 296°𝐾
Winter: 𝑇𝑠𝑢𝑟𝑟 = 12°𝐶 + 273 = 285°𝐾
Stefan-Boltzman law: 𝜎 =
5.67𝑥10−8
𝑊
𝑚2 ∗°𝐾4
Radiation heat transfer:
Summer: 𝑄̇ 𝑟𝑎𝑑 = 𝜀𝜎𝐴 𝑠(𝑇𝑠
4
4
)
𝑄̇ 𝑟𝑎𝑑 = (0.95)(
5.67𝑥10−8
𝑊
𝑚2 ∗ °𝐾4
)(1.6𝑚2)[(305°𝐾)4
− (296°𝐾)4
]
Answer: 𝑄̇ 𝑟𝑎𝑑 = 84.2 𝑊
Winter: 𝑄̇ 𝑟𝑎𝑑 = 𝜀𝜎𝐴 𝑠(𝑇𝑠
4
4
)
𝑄̇ 𝑟𝑎𝑑 = (0.95)(
5.67𝑥10−8
𝑊
𝑚2 ∗ °𝐾4
)(1.6𝑚2)[(305°𝐾)4
− (285°𝐾)4
]
Answer: 𝑄̇ 𝑟𝑎𝑑 = 177.2 𝑊
Tsurr
Qrad

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Summary
In conclusion, there are three mechanisms of heat transfer. One of them is conduction which is the
heat that is transferred within a system or between two systems that are touching. Another one was
convection. Convection can be split into two subcategories; natural convection and forced
convection. Forced convection is where the fluid is being forced to flow over a surface externally
by something like a fan or pump. Natural convection is when the fluid motion is caused by
resistance forces that are induced by density differences due to the variation of temperature in the
fluid. The last mechanism of heat transfer is radiation. Radiation is when the transfer of energy
happens and there is no conductive medium such as space. That lack of medium means there is no
matter there for heat to pass through.

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CHAPTER 9: EXTERNAL FORCED CONVECTION
As you have probably surmised, or if you haven’t bothered to read the title yet, this chapter covers
the topic of convection, particularly forced convection. Unfortunately, we will also be discussing
some math and equations about convection. Just remember the equations are more scared of you
than you are of them.
Physical Mechanism of Convection
Earlier in this guide to learning thermodynamics, we discussed that there are three basic types of
heat transfer mechanisms: conduction, convection and radiation. Conduction and convection are
alike in the fact that they both need a physical object in order to transfer, but convection also
requires fluid motion. You have probably stopped reading to think of different forms of
convection. You have probably thought hard enough to go on a ten minute day dream, so let me
give a few examples to prevent this. Turning on an outdoor garden hose lying in the hot sun is one
form. As water flows through, the hose slowly cools down. This is a form of forced convection,
where an outside force causes some form of fluid transfer across the object in question. Forced
convection could even be as simple as blowing on your bowl of soup. By forcing air across the
soup’s surface, water molecules in the air are forced along with the air. Yes, even vapor forms of
fluid can count as fluids.
Convection can be difficult to wrap your head around, but the equations that come with it are fun
and simple (usually). For this section, we are only going to look at two equations. Below is the
first of which, Newton’s law of cooling (9-1). It is the rate of convection heat transfer (qconv)
proportional to, or equal to, the difference in temperature (Ts and T∾). When the section of the
fluid being observed is far from the surface being observed, its temperatures, velocities, etc. are
considered infinite, hence the infinity symbol subscribing the temperature. The (h) represents the
convection heat transfer coefficient. You will be seeing it often in this chapter.
𝑞 𝑐𝑜𝑛𝑣 = ℎ(𝑇𝑠 − 𝑇∾) (9-1)
The second equation we are going to look at is something called the Nusselt number (9-2). This
number will come out dimensionless. Usually, like in this case, the equations will be
nondimensionalized to make things easier further down the road to solving the problem.
𝑁𝑢 =
ℎ𝐿 𝑐
𝑘
(9-2)
The (h) is the same value as in the previous equation. I told you it would come back. Next we have
the characteristic length (Lc) and the thermal conductivity (k). Be careful with (Lc) in equations of
fluid-related mechanics. Sometimes you will use it for length and other times you will use it for
width.

60
60
Classification of Fluid Flow
Convection transfer goes hand in hand with fluid mechanics, the science dealing with static and
dynamic fluid behaviors. A multitude of problems can occur with fluid flow. These problems are
typically categorized by a common quality. Below are a few of these categories:
 Viscosity: This is basically friction in fluid terms. A slow layer of fluid causes friction with
a fast layer of fluid. There is one key aspect to understanding viscosity; no fluid has zero
viscosity. All fluids have some form of internal resistance to flow. There are two major
types of frictional effects in flow: viscous flows and inviscid flow regions. Viscous flows
are flows with significant frictional effects while inviscid flow regions have considerably
small friction effects when compared to the other inertial forces or pressures. Inviscid flows
are often neglected to help simplify the analysis while staying fairly accurate.
 Internal/External Flow: These two types of flows are exactly what they sound like. External
flows are unbound and travel across a surface. Internal flows are forced through confined
spaces and channels. Viscosity greatly affects internal flows while external flows are only
affected by the viscous effects of boundary layers (see below for boundary layers).
 Compressibility: This is a dependent value of a fluid depends on the level of varying
density while the fluid flows. If the flow is incompressible, this means the density is
relatively the same throughout the system. This also means the volume is unchanged as it
flows throughout the system. In general, since density is relatively constant, fluid flow is
determined incompressible. This is why most fluids are called incompressible substances.
 Laminar/Turbulent Flow: Fluid flow can be orderly or chaotic. These changes in flow are
described as being laminar or turbulent. Laminar flows are smooth and streamlined with
low velocities. Turbulent flows, on the other hand, move at fluctuating, high velocities. If
a flow changes from one flow type to another, it is considered to be transitional. Think of
the way smoke billows from a fire. The smoke’s movements begin smooth and gentle, but
they quickly turn violent and random. This is a perfect example of the changes of flow.
The smoke begins in laminar flow, slowly becomes transitional, and finally ends up as
turbulent flow.
 Forced Flow: Depending on how a fluid’s motion began, it can be determined natural flow
or forced flow. Natural flow occurs when natural forces takes effect on fluids. For example,
the buoyancy effect causes warm fluid to rise and cool fluid to fall. Forced flow is the
opposite of natural flow; the fluid flows from an outside influence. An easy example is the
circulatory system of nearly every living creature on Earth. The heart pumps blood
throughout our bodies through veins. This is no different from any machine containing a
hydraulic system.
 Steady Flow: Steady and uniform are terms often used in technical applications, even the
classification of devices and machines. A steady flow device can be almost any device that
runs at constant conditions for a long time. Some of these devices start up as with transient,
or unsteady, flow. This is simply the opposite of steady flow; unsteady flows change
throughout the operation. A perfect example of this is a rocket engine. As it starts up, the
engine builds pressure and the flow accelerates. Once the rocket gets high enough, the
engine slows down to a uniform flow.

61
61
Boundary Layers
Boundary layers are the flow regions next to the wall where significant viscous flows are present.
Think of cholesterol in your arteries. As the outermost layer of cholesterol begins causing friction
with the interior wall of you arteries, it slowly begins to harden and build up. This is why fluid
powered machines malfunction; they have high cholesterol. In reality, all boundary layers are
simply the outside layer of fluid. Here, we are going to discuss two types of boundary layers:
velocity and thermal layers.
Velocity Boundary Layers
Inside a pipe, when the outermost layer is at an assumed velocity, it will slow down with friction.
This creates a domino effect as it slows down the adjacent layer, which slows down the next layer
and so on. If a flow area with this situation feels the effects of shearing viscous forces, the area is
called a velocity boundary layer. In Fig. 1, you can see a flow area that changes from laminar flow
to transitional flow and then to turbulent flow throughout the diagram. The entire pink/red region
is the velocity boundary layer. At the bottom, you can see individually labeled layers. These small
layers are those that create the domino effect throughout the entire layer.
The friction happening between the boundary and the outermost layer is called friction force. For
readers who don’t know much about forces, stress is equal to force (F) over area (A) (9-3). To
clarify, a force spread out over an area will create some form of force or pressure. That is why you
have to distribute your weight on ice.
Standing straight up in one small spot creates
a lot of pressure, but by spreading yourself out
you create a wider area with less force or
pressure. In this scenario, friction force (Ff)
over unit mass (A) produces a different type
of stress: shear stress (9-4). This shear stress
is given the variable (τ), the Greek latter Tau
(pronounced tauw).
Figure 1: Velocity
Boundary Layer
Figure 2: Icy Accident
http://blog.cheaperthandirt.com/survive-falling-ice/

A Simpleton's Guide to Thermodynamics

A Simpleton's Guide to Thermodynamics

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