Unit1 Conduction

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


2/20



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


3/20



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_thermodynamics


4/20



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.


5/20



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


6/20



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


7/20



Figure6: One Dimensional Heat Conduction


8/20




9/20




10/20



Figure7: Heat Conduction in long cylinder


11/20




12/20




13/20



Figure8: Heat Generation in Solids


14/20




15/20



Consider a Cylindrical Shell

Figure9: Heat Generation in Cylindrical Shell


16/20



Figure10: Maximum Temperature in a Symmetrical Solid


17/20



Figure11: Unsteady Heat Conduction (Temperature distribution)


18/20



Figure12: Conduction Resistance


19/20



Figure13: Conduction Resistance in Two Walls


20/20



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

Documents

Unit1 Conduction