KONGU ENGINEERING COLLEGE

(Autonomous)
DEPARTMENT OF MECHANICAL ENGINEERING

HEAT AND MASS TRANSFER

Two-mark Question and Answers
UNIT-I
Sl.No. QUESTION AND ANSWERS
1. Define Heat transfer?
Heat transfer can be defined as the transmission of energy from one region to
another region to temperature difference. `
2. What are the modes of heat transfer?
1. Conduction
2. Convection
3. Radiation
3. What is conduction?
Heat conduction is a mechanism of heat transfer from a region of high temperature to
a region of low temperature with in a medium (Solid, liquid or Gases) or different
medium in direct physical contact. In conduction, energy exchange takes place by the
kinematics motion or direct impact of molecules. Pure conduction is found only in
solids.
4. Define convection.
Convection is a process of heat transfer that will occur between solid surface and a
fluid medium when they are at different temperatures. Convection is possible only in
the presence of fluid medium.
5. Define Radiation
The heat transfer from one body to another without any transmitting medium is
known as radiation. It is an electromagnetic wave phenomenon.
6. State Fourier’s 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.

7. Write down the equation for conduction of heat through a slab or plane wall.

8. Write down the equation for conduction of heat through a hollow cylinder.

1 Prepared by
K.V.Satheesh Kumar,
Department of Mechanical Engineering,
Kongu Engineering College.
9. Write down the general equation for one dimensional steady state heat transfer
in slab or plane wall with and without heat generation.

With heat generation:-

Without heat generation:-

10. Write down the equation for heat transfer through composite pipes or cylinder.

11. Define Thermal conductivity.

Thermal conductivity refers to the ability of a given material to conduct/transfer heat.
It is generally denoted by the symbol ‘k’ but can also be denoted by ‘λ’. The reciprocal
of this quantity is known as thermal resistivity. Materials with high thermal
conductivity are used in heat sinks whereas materials with low values of k are used as
thermal insulators.
12. What is meant by 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.
13. What is meant by transient heat conduction or unsteady state conduction?
If the temperature of a body varies with time, it is said to be in a transient state and
that type of conduction is known as transient heat conduction or unsteady state
conduction
14. State Newton’s law of cooling or convection law.
Newton's law states that the rate of heat loss of a body is proportional to the difference
in temperatures between the body and its surroundings while under the effects of a
breeze. The constant of proportionality is the heat transfer coefficient.

15. Define overall heat transfer co-efficient.

The overall heat transfer by combined modes is usually expressed in terms of
an overall conductance or overall heat transfer co-efficient

16. Define critical thickness of insulation with its significance.

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, and the corresponding thickness is
called critical thickness.
For cylinder, Critical radius = rc = k/h,
Where k- Thermal conductivity of insulating material,

2 Prepared by
K.V.Satheesh Kumar,
Department of Mechanical Engineering,
Kongu Engineering College.
 h- heat transfer coefficient of surrounding fluid.
Significance: electric wire insulation may be smaller than critical radius. Therefore,
the plastic insulation may actually enhance the heat transfer from wires and thus
keep their steady operating temperature at safer levels.
17. Write any two examples of heat conduction with heat generation.
i) Resistance heater wires - resistance heating in wires is conversion of electrical
energy to heat and heat energy is conducted along the wire
ii)A nuclear fuel rod - Heat is generated in rod and conducted along it
18. How does transient heat transfer differ from steady heat transfer?
The term steady implies no change with time at any point within the medium while
transient implies variation with time or time dependence. Therefore, the temperature
or heat flux remains unchanged with time during steady heat transfer through a
medium at any location although both quantities may vary from one location to
another. During transient heat transfer, the temperature and heat flux may vary with
time as well as location.
19. What is heat generation in a solid?
In heat conduction analysis, the conversion of electrical, chemical, or nuclear energy
into heat (or thermal) energy in solids is called heat generation.
20. Write down the one-dimensional transient heat conduction equation for a plane
wall with constant thermal conductivity and heat generation in its simplest form,
and indicate what each variable represents.
The one-dimensional transient heat conduction equation for a plane wall with constant
thermal conductivity and heat generation is

Define fins or extended surfaces.

21. It is possible to increase the heat transfer rate by increasing the surface of heat
transfer. The surfaces used for increasing heat transfer are called extended surfaces
sometimes known as fins.
22. State the applications of fins.
1. Cooling of electronic components.
2. Cooling of motor cycle engines.
3. Cooling of small capacity compressors
4. Cooling of transformers.
23. Define fin efficiency.
The efficiency of a fin is defined as the ratio of actual heat transferred to the maximum
possible to heat transferred by the fin.

24. Define Fin effectiveness.

The effectiveness of the fin is the dimensionless parameter which can be measured the
ratio of the heat transfer with fin to the heat transfer occupied without fin

25. What is the difference between the fin effectiveness and the fin efficiency? The
fins attached to a surface are determined to have an effectiveness of 0.9. Do you
think the rate of heat transfer from the surface has increased or decreased as a
result of the addition of these fins?

3 Prepared by
K.V.Satheesh Kumar,
Department of Mechanical Engineering,
Kongu Engineering College.
 The fin efficiency is defined as the ratio of actual heat transfer rate from the fin to the
ideal heat transfer rate from the fin if the entire fin were at base temperature, and its
value is between 0 and 1. Fin effectiveness is defined as the ratio of heat transfer rate
from a finned surface to the heat transfer rate from the same surface if there were no
fins, and its value is expected to be greater than 1.Heat transfer rate will decrease since
a fin effectiveness smaller than 1 indicates that the fin acts as insulation.
26. Consider one-dimensional heat conduction through a cylindrical rod of diameter
D and length L. What is the heat transfer area of the rod if (a) the lateral
surfaces of the rod are insulated and (b) the top and bottom surfaces of the rod
are insulated?

27. What does the thermal resistance of a medium represent? and Why are the
convection and the radiation resistances at a surface in parallel instead of being
in series?
The thermal resistance of a medium represents the resistance of that medium against
heat transfer. The convection and the radiation resistances at a surface are parallel
since both the convection and radiation heat transfers occur simultaneously.
28. What is Periodic heat flow?
In Periodic heat flow, the temperature varies on a regular basis.
Example;
1. Cylinder of an IC engine.
2. Surface of earth during a period of 24 hours
29. What is non Periodic heat flow?
In non Periodic heat flow, the temperature at any point within the system varies non
linearly with time.
Example:
1. Heating of an ingot in furnace.
2. Cooling of bars.
30. What is meant by Newtonian heating or cooling process?
The process in which the internal resistance is assumed as negligible in comparison
with its surface resistance is known as Newtonian heating or cooling process.
31. What is lumped system analysis? When is it applicable?
In heat transfer analysis, some bodies are observed to behave like a "lump" whose
entire body temperature remains essentially uniform at all times during a heat transfer
process. The temperature of such bodies can be taken to be a function of time only.
Heat transfer analysis which utilizes this idealization is known as the lumped system
analysis. It is applicable when the Biot number (the ratio of conduction resistance
within the body to convection resistance at the surface of the body) is less than or
equal to 0.1.
32. In what medium is the lumped system analysis more likely to be applicable: in
water or in air? why?
The lumped system analysis is more likely to be applicable in air than in water since
the convection heat transfer coefficient and thus the Biot number is much smaller in
air.
33. What is meant by semi-infinite solids?

4 Prepared by
K.V.Satheesh Kumar,
Department of Mechanical Engineering,
Kongu Engineering College.
 In semi-infinite solids, at any instant of time ,there is always a point where the effect
of heating or cooling at one of its boundaries is not felt at all .At this point the
temperature remains unchanged .
34. What is meant by infinite solid?
A solid which extends itself infinitely in all directions of space is known as infinite
solid. In infinite solids, the biot number value is in between 0.1 < Bi < 100
35. What is a semi-infinite medium? Give examples of solid bodies that can be
treated as semi infinite mediums for heat transfer purposes.
A semi-infinite medium is an idealized body which has a single exposed plane surface
and extends to infinity in all directions. The earth and thick walls can be considered to
be semi-infinite media.
36. Define Biot number.
Biot number is the ratio of internal conductive resistance within the body to the external
convective resistance at the surface of the body.

37. What is the physical significance of the Biot number? Is the Biot number more
likely to be larger for highly conducting solids or poorly conducting ones?
Biot number represents the ratio of conduction resistance within the body to convection
resistance at the surface of the body. The Biot number is more likely to be larger for
poorly conducting solids since such bodies have larger resistances against heat
conduction.
38. What are the factors affecting the thermal conductivity?
1. Moisture
2. Density of material
3. Pressure
4. Temperature
5. Structural of material
39. Explain the significance of thermal diffusivity.
The physical significance of thermal diffusivity is that it tells us how fast heat is
propagated or it diffuses through a material during changes of temperature with time
40. What are Heislers charts?
Heisler charts are a graphical analysis tool for the evaluation of one-dimensional
transient conductive heat transfer in thermal engineering. Heisler charts permit
evaluation of the central temperature for transient heat conduction through an
infinitely long plane wall of thickness 2L, an infinitely long cylinder of radius ro,
and a sphere of radius ro.

5 Prepared by
K.V.Satheesh Kumar,
Department of Mechanical Engineering,
Kongu Engineering College.