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7/29/2019 Unit1 Conduction
1/20
Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
UNITI
DEFINITION OF HEAT TRANSFER
Heat transfer is energy in transit due to temperature difference. Whenever thereexists a temperature difference in a medium or between media, heat transfer must
occur. The basic requirement for heat transfer is the presence of temperature
difference. There can be no net heat transfer between two mediums that are at the
same temperature. The temperature difference is the driving force for heat transfer,
ust as the voltage difference is the driving force for electric current flow and
pressure difference is the driving force for fluid flow. The rate of heat transfer in a
certain direction depends on the magnitude of the temperature gradient (the
temperature difference per unit length or the rate of change of temperature) in that
direction. The larger the temperature gradient, the higher the rate of heat transfer.
Figure1:Applications of Heat Transfer
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Basics of Heat Transfer
In the simplest of terms, the discipline of heat transfer is concerned with only two
things: temperature, and the flow of heat. Temperature represents the amount of
thermal energy available, whereas heat flow represents the movement of thermalenergy from place to place.
1.1)Difference between heat and temperatureIn describing heat transfer problems, we often make the mistake of interchangeably
using the terms heat and temperature. Actually, there is a distinct difference
between the two. Temperature is a measure of the amount of energy possessed by
the molecules of a substance. It is a relative measure of how hot or cold a substance
is and can be used to predict the direction of heat transfer. The usual symbol for
temperature is T. The scales for measuring temperature in SI units are the Celsius
and Kelvin temperature scales. On the other hand, heat is energy in transit. The
transfer of energy as heat occurs at the molecular level as a result of a temperature
difference. The usual symbol for heat is Q. Common units for measuring heat are
the Joule and calorie in the SI system.
1.2)Difference between thermodynamics and heat transferThermodynamics tells us:
how much heat is transferred (Q)
how much work is done (W)
final state of the system
Heat transfertells us:
how (with what modes) Q is transferred
at what rate Q is transferred
temperature distribution inside the body
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
1.3) Heat FluxHeat transfer always occurs from a higher-temperature object to a cooler
temperature one as described by thesecond law of thermodynamics or the Clausius
statement. Where there is a temperature difference between objects in proximity,
heat transfer between them can never be stopped; it can only be slowed.
Figure2: Heat Flux
http://en.wikipedia.org/wiki/Second_law_of_thermodynamicshttp://en.wikipedia.org/wiki/Second_law_of_thermodynamicshttp://en.wikipedia.org/wiki/Second_law_of_thermodynamics7/29/2019 Unit1 Conduction
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
1.4) Different Heat Transfer ModesHeat transfer modes are classified into three types
Figure3:Heat Conduction and its Mechanisms
Conduction: Conduction refers to the heat transfer that occurs across the medium.
Medium can be solid or a fluid, as transfer of heat occurring through interveningmatter without bulk motion of the matter.
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Convection: Convection is the transfer of heat energy between a solid surface and
the nearby liquid or gas in motion. As fluid motion goes more quickly the
convective heat transfer increases. The presence of bulk motion of fluid enhances
the heat transfer between the solid surface and the fluid.
There are two types of Convective Heat Transfer:
Figure4:Types of Convective Heat Transfer
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Radiation: Radiation takes place in the absence of intervening medium;
there is net heat transfer between two surfaces at different temperatures in the
form of electromagnetic waves.
Figure5: Radiation Heat Transfer
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Figure6: One Dimensional Heat Conduction
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
7/29/2019 Unit1 Conduction
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
7/29/2019 Unit1 Conduction
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Figure7: Heat Conduction in long cylinder
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
7/29/2019 Unit1 Conduction
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Figure8: Heat Generation in Solids
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Consider a Cylindrical Shell
Figure9: Heat Generation in Cylindrical Shell
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Figure10: Maximum Temperature in a Symmetrical Solid
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Figure11: Unsteady Heat Conduction (Temperature distribution)
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Figure12: Conduction Resistance
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Figure13: Conduction Resistance in Two Walls
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Heat and Mass Transfer Mechanical Engineering
Ajai S | Lecturer/MECH
Pictorial Representations
References
1.Heat Transfer - A Practical Approach by Yugnus A Cengel.2.
Sachdeva R C, Fundamentals of Engineering Heat and Mass TransferNew Age International, 1995.
3. www.efunda.com/formulae/heat_transfer/index.cfm4. nptel.iitm.ac.in/Heat%20and%20Mass%20Transfer/Student_Slides_M1.pdf5. images.google.co.in/heat conduction images.
http://www.efunda.com/formulae/heat_transfer/index.cfmhttp://www.efunda.com/formulae/heat_transfer/index.cfmhttp://www.efunda.com/formulae/heat_transfer/index.cfmhttp://www.efunda.com/formulae/heat_transfer/index.cfmhttp://www.efunda.com/formulae/heat_transfer/index.cfmhttp://www.efunda.com/formulae/heat_transfer/index.cfmhttp://www.efunda.com/formulae/heat_transfer/index.cfm