Lectures on Heat Transfer - Introduction - Applications - Fundamentals - Gove...tmuliya
This file contains Introduction to Heat Transfer and Fundamental laws governing heat transfer.
The slides were prepared while teaching Heat Transfer course to the M.Tech. students in Mechanical Engineering Dept. of St. Joseph Engineering College, Vamanjoor, Mangalore, India.
Recognize numerous types of heat exchangers, and classify them.
Develop an awareness of fouling on surfaces, and determine the overall heat transfer coefficient for a heat exchanger.
Perform a general energy analysis on heat exchangers.
Obtain a relation for the logarithmic mean temperature difference for use in the LMTD method, and modify it for different types of heat exchangers using the correction factor.
Develop relations for effectiveness, and analyze heat exchangers when outlet temperatures are not known using the effectiveness-NTU method.
Know the primary considerations in the selection of heat exchangers.
Definition and Requirements
Types of Heat Exchangers
The Overall Heat Transfer Coefficient
The Convection Heat Transfer Coefficients—Forced Convection
Heat Exchanger Analysis
Heat Exchanger Design and Performance Analysis
Engineering Thermodynamics-second law of thermodynamics Mani Vannan M
This file consists of content which covers the basics of second law of thermodynamics,heat reservoir,heat source ,heat sink,refrigerator, heat pump,heat engine,carnot theorem,carnot cycle and reversed carnot cycle
This file contains slides on One-dimensional, steady state heat conduction without heat generation. The slides were prepared while teaching Heat Transfer course to the M.Tech. students.
Topics covered: Plane slab - composite slabs – contact resistance – cylindrical Systems – composite cylinders - spherical systems – composite spheres - critical thickness of insulation – optimum thickness – systems with variable thermal conductivity
This file contains slides on Transient Heat conduction: Part-I
The slides were prepared while teaching Heat Transfer course to the M.Tech. students in Mechanical Engineering Dept. of St. Joseph Engineering College, Vamanjoor, Mangalore, India, during Sept. – Dec. 2010. Contents: Lumped system analysis – criteria for lumped system analysis – Biot and Fourier Numbers – Response time of a thermocouple - One-dimensional transient conduction in large plane walls, long cylinders and spheres when Bi > 0.1 – one-term approximation - Heisler and Grober charts- Problems
Understand the physical mechanism of convection and its classification.
Visualize the development of velocity and thermal boundary layers during flow over surfaces.
Gain a working knowledge of the dimensionless Reynolds, Prandtl, and Nusselt numbers.
Distinguish between laminar and turbulent flows, and gain an understanding of the mechanisms of momentum and heat transfer in turbulent flow.
Derive the differential equations that govern convection on the basis of mass, momentum, and energy balances, and solve these equations for some simple cases such as laminar flow over a flat plate.
Non dimensionalize the convection equations and obtain the functional forms of friction and heat transfer coefficients.
Use analogies between momentum and heat transfer, and determine heat transfer coefficient from knowledge of friction coefficient.
Lectures on Heat Transfer - Introduction - Applications - Fundamentals - Gove...tmuliya
This file contains Introduction to Heat Transfer and Fundamental laws governing heat transfer.
The slides were prepared while teaching Heat Transfer course to the M.Tech. students in Mechanical Engineering Dept. of St. Joseph Engineering College, Vamanjoor, Mangalore, India.
Recognize numerous types of heat exchangers, and classify them.
Develop an awareness of fouling on surfaces, and determine the overall heat transfer coefficient for a heat exchanger.
Perform a general energy analysis on heat exchangers.
Obtain a relation for the logarithmic mean temperature difference for use in the LMTD method, and modify it for different types of heat exchangers using the correction factor.
Develop relations for effectiveness, and analyze heat exchangers when outlet temperatures are not known using the effectiveness-NTU method.
Know the primary considerations in the selection of heat exchangers.
Definition and Requirements
Types of Heat Exchangers
The Overall Heat Transfer Coefficient
The Convection Heat Transfer Coefficients—Forced Convection
Heat Exchanger Analysis
Heat Exchanger Design and Performance Analysis
Engineering Thermodynamics-second law of thermodynamics Mani Vannan M
This file consists of content which covers the basics of second law of thermodynamics,heat reservoir,heat source ,heat sink,refrigerator, heat pump,heat engine,carnot theorem,carnot cycle and reversed carnot cycle
This file contains slides on One-dimensional, steady state heat conduction without heat generation. The slides were prepared while teaching Heat Transfer course to the M.Tech. students.
Topics covered: Plane slab - composite slabs – contact resistance – cylindrical Systems – composite cylinders - spherical systems – composite spheres - critical thickness of insulation – optimum thickness – systems with variable thermal conductivity
This file contains slides on Transient Heat conduction: Part-I
The slides were prepared while teaching Heat Transfer course to the M.Tech. students in Mechanical Engineering Dept. of St. Joseph Engineering College, Vamanjoor, Mangalore, India, during Sept. – Dec. 2010. Contents: Lumped system analysis – criteria for lumped system analysis – Biot and Fourier Numbers – Response time of a thermocouple - One-dimensional transient conduction in large plane walls, long cylinders and spheres when Bi > 0.1 – one-term approximation - Heisler and Grober charts- Problems
Understand the physical mechanism of convection and its classification.
Visualize the development of velocity and thermal boundary layers during flow over surfaces.
Gain a working knowledge of the dimensionless Reynolds, Prandtl, and Nusselt numbers.
Distinguish between laminar and turbulent flows, and gain an understanding of the mechanisms of momentum and heat transfer in turbulent flow.
Derive the differential equations that govern convection on the basis of mass, momentum, and energy balances, and solve these equations for some simple cases such as laminar flow over a flat plate.
Non dimensionalize the convection equations and obtain the functional forms of friction and heat transfer coefficients.
Use analogies between momentum and heat transfer, and determine heat transfer coefficient from knowledge of friction coefficient.
Connector Corner: Automate dynamic content and events by pushing a buttonDianaGray10
Here is something new! In our next Connector Corner webinar, we will demonstrate how you can use a single workflow to:
Create a campaign using Mailchimp with merge tags/fields
Send an interactive Slack channel message (using buttons)
Have the message received by managers and peers along with a test email for review
But there’s more:
In a second workflow supporting the same use case, you’ll see:
Your campaign sent to target colleagues for approval
If the “Approve” button is clicked, a Jira/Zendesk ticket is created for the marketing design team
But—if the “Reject” button is pushed, colleagues will be alerted via Slack message
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Charlie Greenberg, Host
State of ICS and IoT Cyber Threat Landscape Report 2024 previewPrayukth K V
The IoT and OT threat landscape report has been prepared by the Threat Research Team at Sectrio using data from Sectrio, cyber threat intelligence farming facilities spread across over 85 cities around the world. In addition, Sectrio also runs AI-based advanced threat and payload engagement facilities that serve as sinks to attract and engage sophisticated threat actors, and newer malware including new variants and latent threats that are at an earlier stage of development.
The latest edition of the OT/ICS and IoT security Threat Landscape Report 2024 also covers:
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Cyber risk predictions
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https://sectrio.com/resources/ot-threat-landscape-reports/sectrio-releases-ot-ics-and-iot-security-threat-landscape-report-2024/
Dev Dives: Train smarter, not harder – active learning and UiPath LLMs for do...UiPathCommunity
💥 Speed, accuracy, and scaling – discover the superpowers of GenAI in action with UiPath Document Understanding and Communications Mining™:
See how to accelerate model training and optimize model performance with active learning
Learn about the latest enhancements to out-of-the-box document processing – with little to no training required
Get an exclusive demo of the new family of UiPath LLMs – GenAI models specialized for processing different types of documents and messages
This is a hands-on session specifically designed for automation developers and AI enthusiasts seeking to enhance their knowledge in leveraging the latest intelligent document processing capabilities offered by UiPath.
Speakers:
👨🏫 Andras Palfi, Senior Product Manager, UiPath
👩🏫 Lenka Dulovicova, Product Program Manager, UiPath
GDG Cloud Southlake #33: Boule & Rebala: Effective AppSec in SDLC using Deplo...James Anderson
Effective Application Security in Software Delivery lifecycle using Deployment Firewall and DBOM
The modern software delivery process (or the CI/CD process) includes many tools, distributed teams, open-source code, and cloud platforms. Constant focus on speed to release software to market, along with the traditional slow and manual security checks has caused gaps in continuous security as an important piece in the software supply chain. Today organizations feel more susceptible to external and internal cyber threats due to the vast attack surface in their applications supply chain and the lack of end-to-end governance and risk management.
The software team must secure its software delivery process to avoid vulnerability and security breaches. This needs to be achieved with existing tool chains and without extensive rework of the delivery processes. This talk will present strategies and techniques for providing visibility into the true risk of the existing vulnerabilities, preventing the introduction of security issues in the software, resolving vulnerabilities in production environments quickly, and capturing the deployment bill of materials (DBOM).
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Gopinath Rebala
Gopinath Rebala is the CTO of OpsMx, where he has overall responsibility for the machine learning and data processing architectures for Secure Software Delivery. Gopi also has a strong connection with our customers, leading design and architecture for strategic implementations. Gopi is a frequent speaker and well-known leader in continuous delivery and integrating security into software delivery.
LF Energy Webinar: Electrical Grid Modelling and Simulation Through PowSyBl -...DanBrown980551
Do you want to learn how to model and simulate an electrical network from scratch in under an hour?
Then welcome to this PowSyBl workshop, hosted by Rte, the French Transmission System Operator (TSO)!
During the webinar, you will discover the PowSyBl ecosystem as well as handle and study an electrical network through an interactive Python notebook.
PowSyBl is an open source project hosted by LF Energy, which offers a comprehensive set of features for electrical grid modelling and simulation. Among other advanced features, PowSyBl provides:
- A fully editable and extendable library for grid component modelling;
- Visualization tools to display your network;
- Grid simulation tools, such as power flows, security analyses (with or without remedial actions) and sensitivity analyses;
The framework is mostly written in Java, with a Python binding so that Python developers can access PowSyBl functionalities as well.
What you will learn during the webinar:
- For beginners: discover PowSyBl's functionalities through a quick general presentation and the notebook, without needing any expert coding skills;
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Slack (or Teams) Automation for Bonterra Impact Management (fka Social Soluti...Jeffrey Haguewood
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We believe integration and automation are essential to user experience and the promise of efficient work through technology. Automation is the critical ingredient to realizing that full vision. We develop integration products and services for Bonterra Case Management software to support the deployment of automations for a variety of use cases.
This video focuses on the notifications, alerts, and approval requests using Slack for Bonterra Impact Management. The solutions covered in this webinar can also be deployed for Microsoft Teams.
Interested in deploying notification automations for Bonterra Impact Management? Contact us at sales@sidekicksolutionsllc.com to discuss next steps.
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Participants will gain insights into the responsibilities, challenges, and best practices associated with test management in SAP projects. Additionally, the webinar delves into the significance of heatmaps as a visual aid for identifying testing priorities, areas of risk, and resource allocation within SAP landscapes. Through this session, attendees can expect to enhance their understanding of test management principles while learning practical approaches to optimize testing processes in SAP environments using heatmap visualization techniques
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2. Heatmap utilization for testing
3. Optimization of testing processes
4. Demo
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Are you looking to streamline your workflows and boost your projects’ efficiency? Do you find yourself searching for ways to add flexibility and control over your FME workflows? If so, you’re in the right place.
Join us for an insightful dive into the world of FME parameters, a critical element in optimizing workflow efficiency. This webinar marks the beginning of our three-part “Essentials of Automation” series. This first webinar is designed to equip you with the knowledge and skills to utilize parameters effectively: enhancing the flexibility, maintainability, and user control of your FME projects.
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- Essentials of FME Parameters: Understand the pivotal role of parameters, including Reader/Writer, Transformer, User, and FME Flow categories. Discover how they are the key to unlocking automation and optimization within your workflows.
- Practical Applications in FME Form: Delve into key user parameter types including choice, connections, and file URLs. Allow users to control how a workflow runs, making your workflows more reusable. Learn to import values and deliver the best user experience for your workflows while enhancing accuracy.
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Chapter 1INTRODUCTION AND BASIC CONCEPTS
1. Chapter 1
INTRODUCTION AND BASIC
CONCEPTS
Heat and Mass Transfer: Fundamentals & Applications
Fourth Edition
Yunus A. Cengel, Afshin J. Ghajar
McGraw-Hill, 2011
Abdul moiz Dota
Dept.Food Engineering
University of Agriculture Faisalabad
2. 2
Objectives
• Understand how thermodynamics and heat transfer are related
to each other
• Distinguish thermal energy from other forms of energy, and heat
transfer from other forms of energy transfer
• Perform general energy balances as well as surface energy
balances
• Understand the basic mechanisms of heat transfer, which are
conduction, convection, and radiation, and Fourier's law of heat
conduction, Newton's law of cooling, and the Stefan–Boltzmann
law of radiation
• Identify the mechanisms of heat transfer that occur
simultaneously in practice
• Develop an awareness of the cost associated with heat losses
• Solve various heat transfer problems encountered in practice
3. 3
THERMODYNAMICS AND HEAT TRANSFER
• Heat: The form of energy that can be transferred from one
system to another as a result of temperature difference.
• Thermodynamics is concerned with the amount of heat
transfer as a system undergoes a process from one
equilibrium state to another.
• Heat Transfer deals with the determination of the rates of
such energy transfers as well as variation of temperature.
• The transfer of energy as heat is always from the higher-
temperature medium to the lower-temperature one.
• Heat transfer stops when the two mediums reach the same
temperature.
• Heat can be transferred in three different modes:
conduction, convection, radiation
6. 6
Historical Background Kinetic theory: Treats molecules as
tiny balls that are in motion and thus
possess kinetic energy.
Heat: The energy associated with the
random motion of atoms and
molecules.
Caloric theory: Heat is a fluidlike
substance called the caloric that is a
massless, colorless, odorless, and
tasteless substance that can be
poured from one body into another
It was only in the middle of the
nineteenth century that we had a true
physical understanding of the nature of
heat.
Careful experiments of the Englishman
James P. Joule published in 1843
convinced the skeptics that heat was
not a substance after all, and thus put
the caloric theory to rest.
8. 8
ENGINEERING HEAT TRANSFER
Heat transfer equipment such as heat exchangers, boilers, condensers, radiators,
heaters, furnaces, refrigerators, and solar collectors are designed primarily on the
basis of heat transfer analysis.
The heat transfer problems encountered in practice can be considered in two groups:
(1) rating and (2) sizing problems.
The rating problems deal with the determination of the heat transfer rate for an
existing system at a specified temperature difference.
The sizing problems deal with the determination of the size of a system in order to
transfer heat at a specified rate for a specified temperature difference.
An engineering device or process can be studied either experimentally (testing and
taking measurements) or analytically (by analysis or calculations).
The experimental approach has the advantage that we deal with the actual physical
system, and the desired quantity is determined by measurement, within the limits of
experimental error. However, this approach is expensive, timeconsuming, and often
impractical.
The analytical approach (including the numerical approach) has the advantage that it
is fast and inexpensive, but the results obtained are subject to the accuracy of the
assumptions, approximations, and idealizations made in the analysis.
10. 10
• Energy can exist in numerous forms such as:
thermal,
mechanical,
kinetic,
potential,
electrical,
magnetic,
chemical,
nuclear.
• Their sum constitutes the total energy E (or e on a unit
mass basis) of a system.
• The sum of all microscopic forms of energy is called the
internal energy of a system.
HEAT AND OTHER FORMS OF ENERGY
11. 11
• Internal energy: May be viewed as the sum of the kinetic and
potential energies of the molecules.
• Sensible heat: The kinetic energy of the molecules.
• Latent heat: The internal energy associated with the phase of a
system.
• Chemical (bond) energy: The internal energy associated with
the atomic bonds in a molecule.
• Nuclear energy: The internal energy associated with the bonds
within the nucleus of the atom itself.
What is thermal energy?
What is the difference between thermal
energy and heat?
12. 12
Internal Energy and Enthalpy
• In the analysis of systems
that involve fluid flow, we
frequently encounter the
combination of properties u
and Pv.
• The combination is defined
as enthalpy (h = u + Pv).
• The term Pv represents the
flow energy of the fluid (also
called the flow work).
13. 13
Specific Heats of Gases, Liquids, and Solids
• Specific heat: The energy required to
raise the temperature of a unit mass of a
substance by one degree.
• Two kinds of specific heats:
specific heat at constant volume cv
specific heat at constant pressure cp
• The specific heats of a substance, in
general, depend on two independent
properties such as temperature and
pressure.
• At low pressures all real gases approach
ideal gas behavior, and therefore their
specific heats depend on temperature
only.
14. 14
• Incompressible substance: A
substance whose specific volume (or
density) does not change with
temperature or pressure.
• The constant-volume and constant-
pressure specific heats are identical
for incompressible substances.
• The specific heats of incompressible
substances depend on temperature
only.
15. 15
Energy Transfer
Energy can be transferred to or from a given
mass by two mechanisms:
heat transfer and work.
Heat transfer rate: The amount of heat
transferred per unit time.
Heat flux: The rate of heat transfer per unit
area normal to the direction of heat transfer.
when is constant:
Power: The work
done per unit time.
16. 16
THE FIRST LAW OF THERMODYNAMICS
The energy balance for any
system undergoing any process
in the rate form
The first law of thermodynamics (conservation of energy
principle) states that energy can neither be created nor destroyed
during a process; it can only change forms.
The net change (increase or
decrease) in the total energy of
the system during a process is
equal to the difference between
the total energy entering and the
total energy leaving the system
during that process.
17. 17
In heat transfer problems it is convenient to
write a heat balance and to treat the
conversion of nuclear, chemical,
mechanical, and electrical energies into
thermal energy as heat generation.
18. 18
Energy Balance for Closed Systems (Fixed Mass)
A closed system consists of a fixed mass.
The total energy E for most systems
encountered in practice consists of the
internal energy U.
This is especially the case for stationary
systems since they don’t involve any
changes in their velocity or elevation during
a process.
19. 19
Energy Balance for
Steady-Flow Systems
A large number of engineering devices such as
water heaters and car radiators involve mass flow
in and out of a system, and are modeled as
control volumes.
Most control volumes are analyzed under steady
operating conditions.
The term steady means no change with time at a
specified location.
Mass flow rate: The amount of mass flowing
through a cross section of a flow device per unit
time.
Volume flow rate: The volume of a fluid flowing
through a pipe or duct per unit time.
20. 20
Surface Energy Balance
This relation is valid for both steady and
transient conditions, and the surface
energy balance does not involve heat
generation since a surface does not
have a volume.
A surface contains no volume or mass,
and thus no energy. Thereore, a surface
can be viewed as a fictitious system
whose energy content remains constant
during a process.
21. 21
HEAT TRANSFER MECHANISMS
• Heat as the form of energy that can be transferred from one
system to another as a result of temperature difference.
• A thermodynamic analysis is concerned with the amount of heat
transfer as a system undergoes a process from one equilibrium
state to another.
• The science that deals with the determination of the rates of such
energy transfers is the heat transfer.
• The transfer of energy as heat is always from the higher-
temperature medium to the lower-temperature one, and heat
transfer stops when the two mediums reach the same temperature.
• Heat can be transferred in three basic modes:
conduction
convection
radiation
• All modes of heat transfer require the existence of a temperature
difference.
22. 22
Heat conduction
through a large plane
wall of thickness ∆x
and area A.
CONDUCTION
Conduction: The transfer of energy from the more
energetic particles of a substance to the adjacent less
energetic ones as a result of interactions between the
particles.
In gases and liquids, conduction is due to the
collisions and diffusion of the molecules during their
random motion.
In solids, it is due to the combination of vibrations of
the molecules in a lattice and the energy transport by
free electrons.
The rate of heat conduction through a plane layer is
proportional to the temperature difference across the
layer and the heat transfer area, but is inversely
proportional to the thickness of the layer.
23. 23
When x → 0 Fourier’s law of
heat conduction
Thermal conductivity, k: A measure of the ability
of a material to conduct heat.
Temperature gradient dT/dx: The slope of the
temperature curve on a T-x diagram.
Heat is conducted in the direction of decreasing
temperature, and the temperature gradient becomes
negative when temperature decreases with
increasing x. The negative sign in the equation
ensures that heat transfer in the positive x direction
is a positive quantity.
The rate of heat conduction
through a solid is directly
proportional to its thermal
conductivity.
In heat conduction
analysis, A represents
the area normal to the
direction of heat
transfer.
25. 25
Thermal
Conductivity
Thermal conductivity:
The rate of heat transfer
through a unit thickness
of the material per unit
area per unit
temperature difference.
The thermal conductivity
of a material is a
measure of the ability of
the material to conduct
heat.
A high value for thermal
conductivity indicates
that the material is a
good heat conductor,
and a low value indicates
that the material is a
poor heat conductor or
insulator.
A simple experimental setup
to determine the thermal
conductivity of a material.
27. 27
The mechanisms of heat
conduction in different
phases of a substance.
The thermal conductivities of gases such
as air vary by a factor of 104
from those of
pure metals such as copper.
Pure crystals and metals have the highest
thermal conductivities, and gases and
insulating materials the lowest.
28. 28
The variation of
the thermal
conductivity of
various solids,
liquids, and gases
with temperature.
29. 29
Thermal Diffusivity
cp Specific heat, J/kg · °C: Heat capacity
per unit mass
ρcp Heat capacity, J/m3
·°C: Heat capacity
per unit volume
α Thermal diffusivity, m2
/s: Represents
how fast heat diffuses through a material
A material that has a high thermal
conductivity or a low heat capacity will
obviously have a large thermal diffusivity.
The larger the thermal diffusivity, the faster
the propagation of heat into the medium.
A small value of thermal diffusivity means
that heat is mostly absorbed by the
material and a small amount of heat is
conducted further.
30. 30
CONVECTION
Convection: The mode of
energy transfer between a
solid surface and the
adjacent liquid or gas that is
in motion, and it involves
the combined effects of
conduction and fluid motion.
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.
Heat transfer from a hot surface to air
by convection.
31. 31
Forced convection: If
the fluid is forced to flow
over the surface by
external means such as
a fan, pump, or the wind.
Natural (or free)
convection: If the fluid
motion is caused by
buoyancy forces that are
induced by density
differences due to the
variation of temperature
in the fluid.
The cooling of a boiled egg by
forced and natural convection.
Heat transfer processes that involve change of phase of a fluid are also
considered to be convection because of the fluid motion induced during
the process, such as the rise of the vapor bubbles during boiling or the
fall of the liquid droplets during condensation.
32. 32
Newton’s law of cooling
h convection heat transfer coefficient, W/m2
· °C
As the surface area through which convection heat transfer takes place
Ts the surface temperature
T∞ the temperature of the fluid sufficiently far from the surface.
The convection heat transfer
coefficient h is not a
property of the fluid.
It is an experimentally
determined parameter
whose value depends on all
the variables influencing
convection such as
- the surface geometry
- the nature of fluid motion
- the properties of the fluid
- the bulk fluid velocity
34. 34
RADIATION
• Radiation: The energy emitted by matter in the form of electromagnetic
waves (or photons) as a result of the changes in the electronic
configurations of the atoms or molecules.
• Unlike conduction and convection, the transfer of heat by radiation does
not require the presence of an intervening medium.
• In fact, heat transfer by radiation is fastest (at the speed of light) and it
suffers no attenuation in a vacuum. This is how the energy of the sun
reaches the earth.
• In heat transfer studies we are interested in thermal radiation, which is
the form of radiation emitted by bodies because of their temperature.
• All bodies at a temperature above absolute zero emit thermal radiation.
• Radiation is a volumetric phenomenon, and all solids, liquids, and
gases emit, absorb, or transmit radiation to varying degrees.
• However, radiation is usually considered to be a surface phenomenon
for solids.
35. 35
Stefan–Boltzmann law
σ = 5.670 × 10−8
W/m2
· K4
Stefan–Boltzmann constant
Blackbody: The idealized surface that emits radiation at the maximum rate.
Blackbody radiation represents the maximum
amount of radiation that can be emitted from
a surface at a specified temperature.
Emissivity ε : A measure of how closely
a surface approximates a blackbody for
which ε = 1 of the surface. 0≤ ε ≤ 1.
Radiation emitted
by real surfaces
36. 36
Absorptivity α: The fraction of the radiation energy incident on a
surface that is absorbed by the surface. 0≤ α ≤ 1
A blackbody absorbs the entire radiation incident on it (α = 1).
Kirchhoff’s law: The emissivity and the absorptivity of a surface at
a given temperature and wavelength are equal.
The absorption of radiation incident on
an opaque surface of absorptivity .
37. 37
Radiation heat transfer between a surface
and the surfaces surrounding it.
Net radiation heat transfer:
The difference between the
rates of radiation emitted by the
surface and the radiation
absorbed.
The determination of the net
rate of heat transfer by radiation
between two surfaces is a
complicated matter since it
depends on
• the properties of the surfaces
• their orientation relative to
each other
• the interaction of the medium
between the surfaces with
radiation
Radiation is usually
significant relative to
conduction or natural
convection, but
negligible relative to
forced convection.
When a surface is completely enclosed by a
much larger (or black) surface at temperature
Tsurr separated by a gas (such as air) that
does not intervene with radiation, the net rate
of radiation heat transfer between these
two surfaces is given by
38. 38
Combined heat transfer coefficient hcombined
Includes the effects of both convection and radiation
When radiation and convection occur
simultaneously between a surface and a gas:
39. 39
SIMULTANEOUS HEAT
TRANSFER MECHANISMS
Although there are three mechanisms
of heat transfer, a medium may involve
only two of them simultaneously.
Heat transfer is only by conduction in opaque solids,
but by conduction and radiation in semitransparent
solids.
A solid may involve conduction and radiation but not
convection. A solid may involve convection and/or
radiation on its surfaces exposed to a fluid or other
surfaces.
Heat transfer is by conduction and possibly by
radiation in a still fluid (no bulk fluid motion) and by
convection and radiation in a flowing fluid.
In the absence of radiation, heat transfer through a
fluid is either by conduction or convection, depending
on the presence of any bulk fluid motion.
Convection = Conduction + Fluid motion
Heat transfer through a vacuum is by radiation.
Most gases between two solid surfaces
do not interfere with radiation.
Liquids are usually strong absorbers of
radiation.
43. 43
EES (Engineering Equation Solver)
(Pronounced as ease): EES is a program that
solves systems of linear or nonlinear
algebraic or differential equations
numerically. It has a large library of built-in
thermodynamic property functions as well as
mathematical functions. Unlike some
software packages, EES does not solve
engineering problems; it only solves the
equations supplied by the user.
Engineering Software Packages
Thinking that a person who can use the
engineering software packages without
proper training on fundamentals can
practice engineering is like thinking that a
person who can use a wrench can work as
a car mechanic.
44. 44
A Remark on Significant Digits
In engineering calculations, the
information given is not known to
more than a certain number of
significant digits, usually three
digits.
Consequently, the results
obtained cannot possibly be
accurate to more significant
digits.
Reporting results in more
significant digits implies greater
accuracy than exists, and it
should be avoided.
A result with more significant
digits than that of given data
falsely implies more accuracy.
45. 45
Summary
• Thermodynamics and Heat Transfer
Application areas of heat transfer
Historical background
• Engineering Heat Transfer
Modeling in engineering
• Heat and Other Forms of Energy
Specific heats of gases, liquids, and solids
Energy transfer
• The First Law of Thermodynamics
Energy balance for closed systems (Fixed Mass)
Energy balance for steady-flow systems
Surface energy balance
46. 46
• Heat Transfer Mechanisms
• Conduction
Fourier’s law of heat conduction
Thermal Conductivity
Thermal Diffusivity
• Convection
Newton’s law of cooling
• Radiation
Stefan–Boltzmann law
• Simultaneous Heat Transfer Mechanisms
• Problem Solving Technique
Engineering software packages
Engineering Equation Solver (EES)
A remark on significant digits