1. State Fourier’s Law of Heat Conduction.

The rate of heat conduction is proportional to the area measured normal to the direction of heat flow and to the temperature gradient in that direction.

Q α –A (dT/dx)

Q= -KA (dT/dx)

Where A-Area in m²

(dT/dx)-Temperature gradient in K/m

 k- Thermal conductivity in W/mK

2. Define thermal conductivity and list out the factors affecting the thermal conductivity.

Thermal Conductivity:

It is defined as the ability of a substance to conduct the heat.

Factors affecting the thermal conductivity:

(i) Moisture

(ii) Density of material

(iii) Pressure

(iv)Temperature

(v) Structure of material

3. State Newton’s law of cooling or State Newton’s law of convection.

Heat transfer by convection is given by Newton’s law of cooling

                           Q= hA (T_s-T_∞ )

Where A-Area exposed to heat transfer in m²

            h- Heat transfer coefficient in W/m²K

             T_s - Temperature of the surface in K

             T_∞ - Temperature of the fluid in K

4. Define Overall heat transfer coefficient.

The overall heat transfer by combined mode is usually expressed in terms of an overall conductance or overall heat transfer coefficient “U”

                           Q = UA ΔT

Q- Heat transfer in Watts

U- Overall heat transfer coefficient in W/m²K

ΔT – Temperature difference in K

5. What are the modes of heat transfer?

(i) Conduction

Heat transfer within the same medium (Solid to Solid)

Ex: Heating a steel rod

(ii) Convection

Heat transfer from one medium to another medium (Solid to liquid)

Ex: Heating of a pan which is full of water

(iii) Radiation

Heat transfer from one medium to another without any transmitting medium.

Ex: Sun light direct to the earth

6. What is critical radius of insulation or critical thickness?

Addition of insulating material on a surface does not reduce the amount of heat transfer rate always.

Infact under certain circumstances it actually increases the heat loss up to certain thickness of insulation.

“The radius of insulation for which the heat transfer is maximum is called critical radius of insulation” and the corresponding thickness is called “critical thickness”.

7. Define fins or extended surfaces.

It is impossible to increase the heat transfer rate by increasing the surface of heat transfer.

The surfaces used for increasing heat transfer are called “Extended surfaces” or sometimes known as “fins”.

8. State the applications of fins.

            (i) Cooling of electronic components

            (ii) Cooling of motor cycle engines

            (iii) Cooling of transformers

            (iv) Cooling of small capacity compressors

9. Define fin efficiency and fin effectiveness.

Fin efficiency:

It is the ratio of actual heat transferred to the maximum possible heat transferred by the fin.

              Ŋ _fin = Q_{fin /} Q _max

Fin effectiveness:

It is the ratio of heat transfer with fin to the heat transfer without fin.

               € _fin = Q_{with fin /} Q_{without fin}

10. Differentiate steady state and unsteady (Transient) heat conduction.

Steady state heat conduction:

If the temperature of a body does not vary with time, it is said to be in a steady state and that type of conduction is known as “steady state heat conduction”.

Unsteady state heat conduction:

If the temperature of a body varies with time, it is said to be in a Unsteady state and that type of conduction is known as “Unsteady state heat conduction or transient heat conduction”.