Important Questions and Answers - Heat and Mass Transfer - Conduction

1. Define Heat Transfer.

Heat transfer can be defined as transmission of energy from one region to another region due to temperature difference.

2. What are the modes of heat transfer?

Conduction,

Convection,Radiation.

3.State Fourier law of 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 = -kAdT / dx

4. Define Thermal Conductivity.

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

5. Write down the three dimensional heat conduction equations in cylindrical coordinates.

The general three dimensional heat conduction equation in cylindrical coordinates

6. List down the three types of boundary conditions.

1. Prescribed temperature.

2. Prescribed heat flux.

3. Convection boundary conditions.

7. State Newtons law of cooling.

Heat transfer by convection is given by Newtons law of cooling

Q= h A ( T_s -T_inf )

Where A- Area exposed to heat transfer in

h- Heat transfer coefficient in W/      K

T- Temperature of the surface and fluid in K.

8. What is meant by lumped heat analysis?

In a Newtonian heating or cooling process the temperature throughout the solid is considered to be uniform at a given time. Such an analysis is called lumped heat capacity analysis.

9.Define fin efficiency and fin effectiveness.

The efficiency of a fin is defined as the ratio of actual heat transfer to the maximum possible heat transferred by the fin.

Ƞfin = Q fin / Q max.

Fin effectiveness is the ratio of heat transfer with fin to that without fin.

Fin effectiveness = Q with fin/ Q without fin.

10. What is critical radius ofinsulation?( AU 2010)

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

11.Write the Poisson’sequationfor heat conduction.

When the temperature is not varying with respect to time, then the conduction is called as steady state condition.

12.What is heat generation in solids? Give examples.

The rate of energy generation per unit volume is known as internal heat generation in solids.

Examples: 1.Electric coils 2. Resistanceheater3. Nuclearreactor.

13. A 3 mm wire of thermal conductivity 19 W / m K at a steady heat generation of 500

Solution;

Critical / Centre temperature,

T _c= T_inf +   qr²/4K        

          = 298 + 500×X(0.015) ²

          =298+14.8 

Tc  = 312.8 K.

14. What are the factors affecting the thermal conductivity?

Moisture,

Density of material,

Pressure,

Temperatures.

15. What are Heislercharts ?

In Heisler chart, the solutions for temperature distributions and heat flow in plane walls, long cylinders and spheres with finite internal and surface resistance are presented. Heisler chart nothing but a analytical solution in the form of graphs.