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Fundamentals of

Heat Transfer

(Basic Concepts)

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Topics to be covered

• History

• Thermodynamics

• Heat transfer

• Thermodynamics versus Heat Transfer

• Areas and Applications of Heat Transfer

• Heat Transfer problems

Historical Background

Heat is fluid like substance called caloric that is

mass-less, colorless, odorless, tasteless that can be

poured from one body into another : Caloric Theory

James P. Joule:

Published in 1843 that

heat was not a substance

Heat: Perceived to be something that produces in us

a sensation of warmth

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Historical Background

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Thermodynamics

and Heat Transfer

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ThermodynamicsThermodynamics deals with the science of “motion”

(dynamics) and/or the transformation of “heat’

(thermo) and energy into various other energy

containing forms.

System: It is the subject of the investigation.

• open system, closed system, isolated system

Surroundings: Everything external to the system is

the surroundings.

Boundary: It is a closed surface surrounding a

system through which energy and mass may enter

or leave the system.

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Thermodynamics

• Science of thermodynamics deals with the

amount of heat transfer as system undergoes a

process from one equilibrium state to another

without any information concerning the nature of

interaction or the time rate at which it occurs.

• Study of the relationship between heat, work

and energy.

• Deals with the equilibrium and feasibility of

process and properties of system, direction of

change etc.

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Energy

The capacity to do work or transfer heat.

Work

The energy required to move an object against a

force.

Work = force × distance

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Energy can exist in various forms such as

thermal

mechanical

kinetic

potential

electrical

magnetic

chemical

nuclear

Their sum constitutes the

total energy E of a

system.

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Internal energy is the energy stored in a body.

Internal energy is the sum of kinetic and potential

energy of all particles in the body.

Unit of internal energy: joule (J)

It increases when the temperature of the body

rises or when the body changes from solid to liquid

or from liquid to gas.

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Temperature is a measure of velocity and hence

kinetic energy of the molecule of the system.

When all the molecular motion ceases and there is

no motion energy within the molecules, the

temperature of the molecules is absolute zero.

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Heat

According to the modern or dynamic theory of heat:

“ Heat is a form of energy.

The energy in transit is termed as heat.

Energy flow due to temperature difference is called

heat.

The mean kinetic energy per molecule of the

substance is proportional to its absolute

temperature.

Unit of heat: joule (J)

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Facts• Whenever there is an exchange of heat, heat is

consumed (heat lost by the hot body is always

equal to heat gained by the cold body).

• The heat flow takes place from high to lower

temperature.

• The substances expand on heating.

• In order to change the state of a body from solid

to liquid to gas, certain amount of heat is

required.

• When a body is heated or cooled, its weight

does not change.

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Whenever there exists a temperature difference in

mediums or within medium, heat transfer must occur

(always from the high temperature medium to the

lower temperature one)

Heat transfer stops when the two mediums reach the

same temperature (thermal equilibrium).

Heat transfer is

transmission of energy

from one region to another

region as a temperaturedifference between them.

Heat Transfer

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When two bodies of different temperatures touch

each other, energy is transferred from the hot body

to the cold body until they reach the

same temperature. (The bodies are then said to be

in thermal equilibrium.)

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Heat Transfer: Driving potential: Temperature

difference

Mass transfer: Driving potential: Concentration

difference (concentrate on mass motion which

result in changes in composition and are caused by

variations in concentration of the various constituent

species. This transfer is also known as “diffusion”)

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Heat Transfer

Science of thermodynamics and fluid mechanics

Heat transfer is a branch of thermal science which

deals with analysis of rate of heat transfer and

temperature distribution taking place in a system

as well as the nature of heat transfer.

Heat transfer cannot be measure directly but the

effects produced by it can be observed and

measured.

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Note: Heat cannot be measured directly by an

instrument as temperature is by a thermometer.

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Heat Temperature

It is a form of energy called

thermal energy.

It is a thermal state of a

body which distinguishes a

hot body from a cold body

It is measure of degree of

hotness or coldness of

system

Measured in kcal, BTU etc. Measured in 0C, 0F, K etc.

Colorimeter is used for

measurement.

Thermometer is used for

measurement.

Extensive property Intensive property

Extensive properties (i.e. mass) are dependent upon

the amount of a substance, while intensive

properties (i.e. density) are independent of quantity.

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Difference between

Thermodynamics

and Heat Transfer

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Interested in how long it

takes for the hot coffee in a

thermos bottle to cool to a

certain temperature which

can not be determined from

a thermodynamic analysis

alone

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Consider the cooling of a hot steel bar placed in

water bath.

Thermodynamic analysis will predict the final

equilibrium temperature of the composite system

comprised by steel bar-water combination but it

will not predict how long it takes to reach this

equilibrium condition or what the temperature of

bar will be after a certain length of time beforethe equilibrium condition is attained.

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But heat transfer study will predict the temperature

of both the bar and water as a function of time.

That is the temperature at all points of interest

within the bar or temperature at any specific point

(such as at the center of the bar where it is the

highest) at any time can be predicated. Also the

instantaneous heat transfer rate can be predicated

from all or from any part of the surface of the bar at

any time.

Heat transfer studies mainly require the

knowledge of thermodynamics, fluid

mechanics, physics and mathematics.

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Thermodynamics Heat Transfer

It is concerned with

equilibrium states and

precludes existence of

temperature gradient

It is concern with non-

equilibrium states.

Temperature gradient must

exist for exchange of heat

It helps to determine quantity

of heat and work interactions

when a system changes from

one equilibrium state to

another but do not provide

information on nature of

interaction and time rate at

which interaction occurs

∆u = ∆Q ± ∆W

It helps to predict rate at

which energy is exchanged

and also predicts temperature

distribution as a function of

coordinates and time within

regions of matter

We can not obtain

temperature profile

We can obtain temperature

profile

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Summary:

1. In thermodynamics, no consideration is given to time or

temperature difference required to bring about the

transfer of heat energy and whether or not there is

uniform temperature within the thermodynamic system.

2. The subject of heat transfer seeks to provide answer to

the question such as

• Possibility of removal or addition of heat at a

desire rate

• Temperature distribution existing within the system

• Amount of heat to be transferred

• Time taken (duration of heating and cooling) for a

certain duty and surface area required to

accomplish that duty.

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Thermodynamic entails four laws

1. Zeroth Law of thermodynamics

2. First Law of thermodynamics

3. Second Law of thermodynamics

4. Third Law of thermodynamics

Law of Thermodynamics

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The zeroth law of thermodynamics states that if

two thermodynamic systems are each in thermal

equilibrium with a third, then they are in thermal

equilibrium with each other.

Zeroth Law of Thermodynamics

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The first law, also known as Law of Conservation of

Energy, states that energy cannot be created or

destroyed in an isolated system.

First Law of Thermodynamics

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The second law of thermodynamics states that the

entropy of any isolated system always increases.

Second Law of Thermodynamics

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• Second law of Thermodynamics indicates the

limit of converting heat into work and introduces

the principle of increase of entropy.

OR

It states that "Heat will flow naturally from one

reservoir to another at a lower temperature but

not in opposite direction without assistance.” (This

law established the direction of energy transport).

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The third law of thermodynamics is

sometimes stated as follows, regarding the

properties of systems in equilibrium at absolute

zero temperature: The entropy of a perfect

crystal at absolute zero is exactly equal to zero

Third Law of Thermodynamics

• Zeroth law of Thermodynamics deals with

thermal equilibrium and establishes a concept of

Temperature.

• First law of Thermodynamics throws light on

concept of internal energy.

• Second law of Thermodynamics

Thermal energy is transferred whenever a

temperature gradient exists and the free flow of

heat energy is always from a higher temperature

to a lower temperature in accordance with the

second law of thermodynamics.

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Third law of Thermodynamics defines

absolute zero of entropy.

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Importance of Heat Transfer

Heat transfer study is carried out for the follows

purpose:

1. To estimate the rate of flow of energy as heat

through the boundary of a system under study

(both under steady and transient conditions).

2. To determine the temperature field under

steady and transient conditions

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Application Areas of Heat Transfer

Energy production and Conversion:

For thermal design of boilers, steam turbines,

condensers, gas turbines, cooling towers, feed

heaters, internal combustion engine

Have to make a detailed heat transfer analysis(amount of heat to be transmitted as well as the rate at which

heat is to be transferred)

Refrigeration and Air Conditioning:

The thermal design of compressors,

evaporators, incinerators involves an intensive

study of heat transfer.

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Application Areas of Heat Transfer

Electric Machines:

Insulations provide on electric wires need a

heat transfer analysis to avoid conditions

which will cause overheating.

heat transfer analysis must also be accounted

in the design of electronic components,

electric machines, transformers and

bearings to avoid overheating and damage ofequipment

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Application Areas of Heat Transfer

Civil Engineering:

Design of buildings for the minimization of

heat losses need heat transfer study.

Construction of dams and other heavy

structures, calculation of thermal expansion of

suspension bridge and railway tracks.

Manufacturing process:

The casting of metals, extrusion, metal cutting

and heat treatment of metals involve heat

transfer study.

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Application Areas of Heat Transfer

Chemical and Petrochemical operations

Environmental Engineering

Earth Sciences

Astronomy

Agriculture and Food processing

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Areas covered under discipline of Heat Transfer

1. Design of thermal and nuclear power plants

including heat engines, steam generators,

condensers and other heat exchange

equipment, catalytic converters, heat shields

for space vehicles, furnaces, electronic

equipment etc.

2. Internal combustion engine

3. Refrigeration and air conditioning units

4. Design of cooling systems for electric motors,

generators and transformers

5. Heating and cooling of fluids etc. in chemical

operations

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Areas covered under discipline of Heat Transfer

6. Construction of dams and structures;

minimization of building-heat losses using

improved insulation techniques

7. Thermal control of space vehicles

8. Heat treatment of metals

9. Dispersion of atmospheric pollutants

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Application Areas of Heat Transfer

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Importance of Study of Heat Transfer

NPTEL Video Lecture - 1 Introduction on Heat

and Mass Transfer (Time: 6.57 to 30.36)

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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.

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Engineering Heat Transfer

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, time

consuming, and often impractical.

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Questions1. The capacity to do work is _____________.

2. Heat is measured in _________.

3. The form of energy that produced feeling of hotness is

called________.

4. __________is neither created nor destroyed it can only

change one form to another.

5. What is the driving force for a heat transfer?

6. Heat transfer takes place according to __________law of

thermodynamics.

7. How does the science of heat transfer differ from the

science of thermodynamics?

8. How do rating problems in heat transfer differ from the

sizing problems?