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Mechanical - 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} + qr^{2}/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.

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Mechanical : Heat and Mass Transfer : Conduction : Important Questions and Answers - Heat and Mass Transfer - Conduction |

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