HEAT TRANSFER BY CONDUCTION,CONVECTION AND RADIATION

As we all know energy exists in various forms. Heat is also a form of energy. In this section our prime importance is to study about the transfer of heat energy. Temperature is function of heat, which gives us idea of degree of coldness or hotness of a body. Heat can flow from one point to another as a result of temperature difference. Always remember that heat flows from higher temperature to lower temperature.

So we can say that heat transfer is branch of science and engineering that deals with rate of heat transfer between bodies.

MODES OF HEAT TRANSFER:-

In this context we will know the modes of heat transfer. There are three known modes of heat transfer. These are conduction, convection and radiation. All these three forms require temperature difference. When two bodies are at same temperature no heat transfer takes place.

CONDUCTION:-

It is mode of heat transfer from one point to another in the same body or from one body to another body when they are in direct contact with each other. It is transfer of energy from more energetic particle to subsequently less energetic particle due to direct interaction.

Solids has compact structure and the atoms and molecules are tightly bound. When these atoms gain energy they start vibrating about their mean position and subsequently other atoms in contact with it also starts vibrating at their mean position. In this way heat is propagated in solids. Hence conduction is mode of heat transfer without appreciable movement of molecules rather only molecular interaction is needed.

The various factors that affect the rate of heat transfer by conduction is:-

• Temperature difference across the medium
• Medium material
• Geometry of medium
• Thickness of material

Heat flow through single homogeneous solid is directly proportional to area of section at right angle to heat flow and change in temperature with respect to length of path of flow of heat.

         Q  =  -kA(THOT-TCOLD)

                -------------------------------------------------
                     L 

K=Coefficient of thermal conductivity.

A=Area of section.
L=Length

Unit of rate of heat flow=Watt

Unit of K=watt/meter*Kelvin

Some important points:-

1. Conduction of heat occurs fastest in metal and slowest in insulator. Conductivity of non-metal lies between metal and insulator.

2. Thermal conductivity (K) is measure of material ability to conduct heat. Iron's thermal conductivity is 120 times more than water.

3. With increase in density of gas its conductivity decreases.

4. Gases having higher molecular weight has low thermal conductivity (K).

CONVECTION HEAT TRANSFER

Convection is another mode of heat transfer which takes place between solid and adjacent liquid or gas which is in motion. The rate of heat transfer depends on the speed of fluid flow. In other words it can be said that greater is the speed of fluid greater is the convection heat transfer. If the fluid motion is not high then the heat transfer between the solid body and the fluid as result of temperature difference will be by pure conduction.

     
Convection heat transfer is given by

                               Q = hA ΔT

Where h is convection heat transfer coefficient
A is area
ΔT is change in temperature

Convection is broadly classified as under

1.     Free natural convection

2.     Forced convection

Free natural convection:-

When there is temperature difference between the fluid layers, the density will also differ depending on the temperature of layers. As a result fluid motion is caused by buoyancy.

The denser portion will experience greater force of gravity hence it will tend to move down as compared to less denser layer which will experience less force.

Forced convection:-

Forced convection is situation in which the fluid is forced to flow over the surface by external means like blowers, pumps and fans. Also when the fluid phase change occurs it is characterized under convection. This is due to the fact that vapour bubbles rise during boiling and during condensation it falls under gravity.

RADIATION HEAT TRANSFER

Apart from conduction and convection, radiation is another mode of heat transfer which does not require any intervening medium for transfer of heat. All bodies whose temperature is above absolute zero temperature emit thermal radiations. It is surface phenomenon in which electromagnetic waves are emitted from the body which is caused by change in electronic configuration of atoms and molecules of the body. Radiation heat transfer is fastest heat transfer among all the three modes. 

Numerical problems.

1) Heat transfer occurs from?

1) Cold body to hot body
2) Hot body to cold body
3) Bigger body to small body
4) In any direction.

2) In conduction, greater the coefficient of thermal conductivity depicts __________ conduction.

1) Lesser
2) Greater
3) Constant

4) Radiation mode of heat transfer requires

1) Some intervening medium for transfer of heat.
2) No medium for heat transfer.
3) Both, depending on situation.

5) How many mode of heat transfer is possible?

1) Two
2) Three
3) Four
4) One

  

Be first to answer the question via comment.
Thankyou.