HEAT TRANSFER

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HEAT TRANSFER
‫االسم‬
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‫حمزة‬ ‫مرزة‬ ‫حاتم‬ ‫عدي‬
A ‫الشعبة‬
:
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‫المرحلة‬
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‫الثالث‬

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1.1-Introduction
Heat transfer is the science that seeks to predict the energy transfer that may
take place between material bodies as a result of a temperature difference.
Thermodynamic teaches that this energy transfer is defined as heat. The science
of heat transfer seeks not merely to explain how heat energy may be transferred,
but also to predict the rate at which the exchange will take place under certain
specified conditions. The fact that a heat-transfer rate is the desired objective of
an analysis points out the difference between heat transfer and thermodynamics.
Thermodynamics deals with systems in equilibrium; it may be used to predict the
amount of energy required to change a system from one equilibrium state to
another; it may not be used to predict how fast a change will take place since the
system is not in equilibrium during the process. Heat transfer supplements the
first and second principles of thermodynamics by providing additional
experimental rules that may be used to establish energy-transfer rates. As in the
science of thermodynamics, the experimental rules used as a basis of the subject
of heat transfer are rather simple and easily expanded to encompass a variety of
practical situations. As an example of the different kinds of problems that are
treated by thermodynamics and heat transfer, consider the cooling of a hot steel
bar that is placed in a pail of water. Thermodynamics may be used to predict the
final equilibrium temperature of the steel bar–water combination.
Thermodynamics will not tell us how long it takes to reach this equilibrium
condition or what the temperature of the bar will be after a certain length of time
before the equilibrium condition is attained. Heat transfer may be used to predict
the temperature of both the bar and the water as a function of time. Most
readers will be familiar with the terms used to denote the three modes of heat
transfer: conduction, convection, and radiation. In this chapter we seek to explain
the mechanism of these modes qualitatively so that each may be considered in
its proper perspective. Subsequent chapters treat the three types of heat transfer
in detail.
1-Heat transfer

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1.2-Types of heat transfer
a- CONDUCTION HEAT TRANSFER
b- CONVECTION HEAT TRANSFER
c- RADIATION HEAT TRANSFER
1-Objective
The objective of this report is to study heat conduction in fluids by using Fourier's
law. In this report the thermal conductivity of water will be determined and the
results will be compared with known values.
2- Introduction;-
Conduction does take place in fluids as well as solids. Usually the most common
mode of heat transferred in a fluid is convection because of the bulk motion
created by buoyancy forces due to density gradients throughout the liquid.
However, as the space occupied by the fluid becomes very small, density gradients
become negligible. Since there is negligible bulk motion, heat transfer is primarily
due to conduction. In this experiment we will use an apparatus, shown in Figure
that will enable us to neglect density gradients and allow us to study conduction in
the fluids air and water.
3- Theory:-
To find the thermal conduction coefficient we must use Fourier's Law. Solving for k we get,
(1)
For radial heat conduction in a cylinder, dx becomes dr and area A is the cross
sectional area of the conducting path. Now for measurements made at steady state
Report 1
Heat conduction in fluids

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conditions across the small radial gap, dr becomes dr, and dT becomes dT so we
can obtain,
(2)
In order to find the heat by conduction (qc) we must make use of conservation of
energy. When
applied to this system we get,
(3)
The symbols V and R are the voltage and resistance of the heater element which
generates
electrical heat . In this mechanism there are heat transfers other than that
transferred by conduction through the fluid under test. These heat "losses" are
defined as
incidental heat transfer. The heat losses can be a result of:
1. Heat conduction through the O-ring seals
2. Heat radiated from the plug
3. Heat losses to the surroundings by radiation and convection from the exposed
ends of the plug.
From a simple understanding of heat transfer, we may assume qlost to be
proportional to
the temperature difference between the plug and the jacket. qlost can be estimated
from the
calibration graph of incidental heat transfer versus the plug and jacket temperature
difference
. For this analysis you will use the known thermal conductivity of air . The thermal
conduction coefficient can then be calculated for other fluids by the
temperature difference across the fluid.In other words, the data from the air
calibration test is used to calculate qc fromEquation 2, using the known tabulated
thermal conductivity of air, kair. The value forqgen isthen calculated and Equation

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3 solved for qlost. The graph of the three qlost vs. dT values is thecalibration
curve, from which the values of qlost for the water test will be found (it is assumed
that heat loss is directly related to temperature difference). The qlost from the
graph and the calculated value of qgen for the water tests are then put into
Equation 3 to find qc. Finally, the value of qc is put into Equation 2 to give the
experimental thermal conductivity of water.
4-Apparatus
H470 Heat Conduction Unit, the apparatus used in this report consists of three items.
4.1 The first item
Is the transformer used to convert 110 to 220 volts.
4.2 The second item
Is the plug jacket assembly, which consists of two cylinders. The smaller cylinder
or plug is machined from aluminum (to reduce thermal inertial and temperature
variation) and contains a cylindrical heating element whose resistance at the
working temperature is accurately measured. A thermocouple is inserted into the
plug close to its external surface, and the plug also has ports for the introduction
and venting of the fluid under test. The second cylinder or water cooled jacket fits
concentricity around the plug. The fluid whose thermal conductivity is to be
determined fills the small radial clearance between the heated plug and the water
cooled jacket. The clearance is small enough to prevent natural convection in the
fluid. Due to the positioning of the thermocouples and the high thermal
conductivities of the materials involved, the temperatures measured are effectively
the temperatures of the hot and cold faces of the fluid surface.
4.3 The Third Item
The console, which is connected by flexible cables to the plug/jacket assembly and
provides for the control of the voltage supplied to the heating element. An analog
voltmeter enables the power input to be determined and a digital temperature
indicator with 0.1K resolution displays the temperatures of the plug and jacket
surfaces. The features of the console are shown in Figure .
The following are specifications of the apparatus needed for calculations.
1. Nominal resistance of heating element, R = 55 Ω
2. Nominal radial clearance between plug and jacket, ∆r = 0.30 mm *
3. Effective area of conducting path through fluid, A = 0.0133 m2
*The values to be used are engraved on the head of the plug!

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Schematic of Console Unit
Schematic of Plug/Jacket Assembly
Cross-Sectional View of Plug/Jacket Assembly

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5-Procedure:-
1. Ensure the main switch is off.
2. Connect the thermocouple from the jacket to the hand held thermocouple
measuring device. Connect the main power cord from the test apparatus to the
main control box. Note that the core temperature can be taken from the main
control box, and the water jacket temperature is taken from the hand held
thermocouple meter. Make sure that thetoggle switch is in the down (T2) position.
Please leave the switch alone during the remainder of the experiment.
3. Pass water through the jacket at about 3 liters per minute (the actual quantity is
unimportant but a copious supply is necessary so the jacket will operate at a
sensibly constant temperature). The space between the plug and the jacket will
remain occupied by air.
6-Discussion:-
liquid as well as gases are considered as fluids. Mechanism of conduction in
liquids is happens by transfer of vibrational forces between molecules.as we
increase the temperature of the liquid there increasing in kinetic energy of
molecules respectively .this lead to increase in vibrational forces .Incase of gases
,there is two type of modes of heat conduction which are forced molecules nearer
to hot surface get heated increases it temperature at the time decreases its density
.so molecules nearer to surface move upward respectively.
1-AIM:-
To determine the natural convection heat transfer coefficient for the vertical
cylindrical tube which is exposed to the atmospheric air and losing heat by natural
convection.
2-OBJECTIVE:-
The purpose of this experiment is to study experimentally the natural convection
pipe flows at different heating level.
3-INTRODUCTION:-
There are certain situations in which the fluid motion is produced due to change in
density resulting from temperature gradients, which is the heat transfer mechanism
Report 2
HEAT TRANSFER IN NATURAL CONVECTION

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called as free or natural convection. Natural convection is the principal mode of heat
transfer from pipes, refrigerating coils, hot radiators etc. The movement of fluid in
free convection is due to the fact that the fluid particles in the immediate vicinity of
the hot object become warmer than the surrounding fluid resulting in a local change
of density. The warmer fluid would be replaced by the colder fluid creating
convection currents. These currents originate when a body force (gravitational,
centrifugal, electrostatic etc.) acts on a fluid in which there are density gradients.
The force which induces these convection currents is called a buoyancy force which
is due to the presence of a density gradient within the fluid and a body force.
Grashoff number (Gr) plays a very important role in natural convection. In contrast
to the forced convection, natural convection phenomenon is due to the temperature
difference between the surface and the fluid is not created by any external agency.
Vertical plate
The test section is a vertical, open ended cylindrical pipe dissipating heat from the
internal surface. The test section is electrically heated imposing the
circumferentially and axially constant wall heat flux. As a result of the heat
transfer to air from the internal surface of the pipe, the temperature of the air
increases. The resulting density non-uniformity causes the air in the pipe to rise.
The present experimental setup is designed and fabricated to study the natural
convection phenomenon from a vertical cylinder in terms of the variation of the
local heat transfer coefficient and its comparison with the value which is obtained
by using an appropriate correlation.

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4-THEORY:-
When a hot body is kept in a still atmosphere, heat is transferred to the surrounding
fluid by natural convection. The fluid layer in contact with the hot body gets heated,
rises up due to the decrease in its density and the cold surrounding fluid rushes in to
take its place. The process is continuous and heat transfer takes place due to the
relative motion of hot and cold particles. The heat transfer coefficient is given by:
(1)
Here,
h = Average surface heat transfer coefficient.
q = Heat transfer rate.
As
= Area of heat transferring surface
Ts
= Average surface temperature (°C),
Where ,
(2)
Ta
= Ambient temperature in the duct (°C) = T8
The surface heat transfer coefficient of a system transferring heat by natural
convection depends on the shape, dimensions and orientation of the body, the
temperature difference between the hot body and the surrounding fluid and fluid
properties like κ, μ, ρ etc. The dependence of ‘h’ on all the above mentioned
parameters is generally expressed in terms of non-dimensional groups, as follows:
(3)
hl/k is called the Nusselt Number (Nu),
is called the Grashoff Number (Gr) and,

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A and n are constants depending on the shape and orientation of the heat transferring
surface.
L is a characteristic dimension of the surface,
κ is the thermal conductivity of the fluid,
ν is the kinematic viscosity of the fluid,
μ is the dynamic viscosity of the fluid,
Cp
is the specific heat of the fluid,
β is the coefficient of volumetric expansion of the fluid,
g is the acceleration due to gravity at the place of experiment,
∆T=Ts-Ta
For gases,
For a vertical cylinder losing heat by natural convection, the constants A and n
of equation (3) have been determined and the following empirical correlations
have been obtained:
(4)
(5)
5-APPARATUS:-
The apparatus consists of a stainless steel tube fitted in a rectangular duct in a
vertical fashion. The control panel for the natural convection apparatus . The heat
input to the heater is measured by an ammeter and a voltmeter and is varied by a
dimmerstat. The temperatures of the vertical tube are measured by seven
thermocouples and are marked on the Temperature Indicator Switch of the
instrument panel . One more thermocouple is used to measure ambient
temperature.

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Control panel for natural convection apparatus
6-PROCEDURE:-
1-Switch on the supply and adjust the dimmerstat to obtain the required heat
input (say 40 W, 60 W, 70 W).
2-Monitor the temperature T1
to T8
every five minutes till steady state is reached.
3- Wait till the steady state is reached. This is confirmed from temperature
readings (T1
to T7). If they remain steady and do not register a change of more
than 1 C per hour.
4-Measure the surface temperature at various points (T1
to T7
).
5-Note the ambient temperature, T8
.
6- Repeat the experiment for different heat inputs (say 40 W, 60 W, 70 W) by
varying dimmerstat position.
7-Discussion:-
Natural convection has attracted a great deal of attention from researchers because
of its presence both in nature and engineering applications. In nature, convection
cells formed from air raising above sunlight-warmed land or water are a major
feature of all weather systems. Convection is also seen in the rising plume of hot
air from fire, plate tectonics, oceanic currents (thermohaline circulation) and sea-
wind formation (where upward convection is also modified by Coriolis forces). In

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engineering applications, convection is commonly visualized in the formation of
microstructures during the cooling of molten metals, and fluid flows around
shrouded heat-dissipation fins, and solar ponds. A very common industrial
application of natural convection is free air cooling without the aid of fans: this can
happen on small scales (computer chips) to large scale process equipment.
1-Introduction:-
Radiant heat transfer involves the transfer of heat by electromagnetic
radiation that arises due to the temperature of a body. Most energy of
this type is in the infra-red region of the electromagnetic spectrum
although some of it is in the visible region. The term thermal radiation is
frequently used to distinguish this form of electromagnetic radiation from
other forms, such as radio waves, x-rays, or gamma rays. The transfer of
heat from a fireplace across a room in the line of sight is an example of
radiant heat transfer.
Radiant heat transfer does not need a medium, such as air or metal, to
take place. Any material that has a temperature above absolute zero
gives off some radiant energy. When a cloud covers the sun, both its heat
and light diminish. This is one of the most familiar examples of heat
transfer by thermal radiation
REPORT 3
RADIATION HEAT TRANSFER

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2-Black Body Radiation:-
A body that emits the maximum amount of heat for its absolute
temperature is called a black body. Radiant heat transfer rate from a
black body to its surroundings can be expressed by the following
equation.
Q =𝜎AT⁴
where
Q = heat transfer (W) (Btu/hr)
𝜎 = Stefan-Boltzman constant 5.6703 10⁻⁸ (W/m².K⁴) (0.174 Btu/hr-ft²-
°R⁴)
A = surface area (m²) (ft²)
T = temperature (K) (°R)
Two black bodies that radiate toward each other have a net heat flux
between them. The net flow rate of heat between them is given by an
adaptation of Equation
Q =𝜎A(T₁⁴-T₂⁴)
where
A = surface area of the first body (m²) (ft²)
T₁ = temperature of the first body (K) (°R)
T₂ = temperature of the second body (K) (°R)

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3-Emissivity Coefficient:-
Real objects do not radiate as much heat as a perfect black body. They
radiate less heat than a black body and are called gray bodies. To take
into account the fact that real objects are gray bodies
Q =𝓔𝜎AT⁴
where:
𝓔 = emissivity of the gray body (dimensionless)
Emissivity is simply a factor by which we multiply the black body heat
transfer to take into account that the black body is the ideal case.
Emissivity is a dimensionless number and has a maximum value of 1.0.
NOTE : The emissivity lies in the range 0 < ε < 1 and depends on the type
of material and the temperature of the surface. The emissivity of some
common materials are:
1-oxidized Iron at 390 o
F (199 o
C) - ε = 0.64
2-polished Copper at 100 o
F (38 o
C) - ε = 0.03
4-Radiation Configuration Factor:-
Radiative heat transfer rate between two gray bodies can be calculated
by the equation stated below.
Q = fa fe 𝜎 A(T₁⁴- T₂⁴ )
where:
fa = is the shape factor, which depends on the spatial arrangement of the
two objects (dimensionless)

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fe = is the emissivity factor, which depends on the emissivities of both
objects (dimensionless)
The two separate terms fa and fe can be combined and given the symbol
f. The heat flow between two gray bodies can now be determined by the
following equation:
Q = f 𝜎 A(T₁⁴- T₂⁴ )
The symbol (f) is a dimensionless factor sometimes called the radiation
configuration factor,
which takes into account the emissivity of both bodies and their relative
geometry.
Once the configuration factor is obtained, the overall net heat flux can
be determined. Radiant heat flux should only be included in a problem
when it is greater than 20% of the problem.

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5-JDiscussion:-
1- Black body radiation is the maximum amount of heat that can be
transferred from an ideal object.
2- Emissivity is a measure of the departure of a body from the ideal black
body.
3-Radiation configuration factor takes into account the emittance and
relative geometry of two objects.

HEAT TRANSFER

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