ME8693 Heat and Mass Transfer

UNIT: I
PART A - 2 Marks (Questions and Answers)
1. State Fourier’s Law of conduction. (April/May 2011, Nov/Dec 14 ,
Nov/Dec 16 & Nov/Dec 19 )
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.

Where, A are in m2

is temperature gradient in K/m

K is Thermal Conductivity W/mk

2. State Newton’s law of cooling or convection law. (May/June 2009 ,
Nov/Dec 19 )
Heat transfer by convection is given by Newton’s law of cooling
Q = hA (Ts - T∞)
Where
A – Area exposed to heat transfer in m2 , h - heat transfer coefficient in
W/m2K
Ts – Temperature of the surface in K, T∞ - Temperature of the fluid in K.
3. Define overall heat transfer co-efficient. (May/June 2007)
The overall heat transfer by combined modes is usually expressed in
terms of an overall conductance or overall heat transfer co-efficient ‘U’.
Heat transfer Q = UA ∆T.
4. Write down the equation for heat transfer through composite pipes
or cylinder. (April/May 2008)

Heat transfer where

5. What is critical radius of insulation (or) critical thickness?

(May/June 2014) (Nov/Dec 2008)
ME8693 Heat and Mass Transfer

Critical radius = rc Critical thickness = rc – r1

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.
6. Define Fin efficiency and Fin effectiveness. (Nov/Dec 2015&
Nov/Dec 2010 & Nov/Dec 2018)
The efficiency of a fin is defined as the ratio of actual heat
transfer by the fin to the maximum possible heat transferred by the fin.

Fin effectiveness is the ratio of heat transfer with fin to that

without fin

7. Define critical thickness of insulation with its significance. [MAY-JUN

14]
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, 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.
8. What is lumped system analysis? When is it applicable? [Nov/Dec 14
& April/May 2016]
ME8693 Heat and Mass Transfer

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.
9.Write the three dimensional heat transfer poisson and laplace
equation in Cartesian coordinates(May/June 2012)(April/May
2010)
Poisson equation:

Laplace equation:

10. A 3 mm wire of thermal conductivity 19 W/mK at a steady heat

generation of 500 MW/m3 .Determine the center temperature if
the outside temperature is maintained at 250C ( May 2012)

Critical temperature

11. List down the three types of boundary conditions. (Nov/Dec 2005)
1. Prescribed temperature
2. Prescribed heat flux
3. Convection Boundary Conditions.
12. Define fins (or) extended surfaces.
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 or sometimes known as fins.
13. How thermodynamics differ from heat transfer? (May/June 2016)
ME8693 Heat and Mass Transfer

 Thermodynamics doesn’t deals with rate of heat transfer

 Thermodynamics doesn’t tell how long it will occur
 Thermodynamics doesn’t tell about the method of heat transfer
14. What are the three modes of heat transfer ? (Nov/Dec 2018)
 Conduction
 Convection
 Radiation
15. What is the thermal time constant? (April/May 2019)
The time required for a thermistor to change 63.2 % of the total
difference between its intial and final temperature of a body .
16. What are the factors affecting thermal conductivity of material
(Nov/Dec 2017)
Free electrons, Purity of material, effect of forming, density, crystalline
structure .
17. What are Heisler charsts? (Nov/Dec 2017)/ (Nov/Dec 2018)
It is a graphical analysis tool for the evaluation of heat transfer
18. Difference between conduction and convection? (April/May 2017)
Conduction
Heat transfer between two solid surfaces (Solid medium)
Convection
Heat transfer between two liquid to solid surfaces (Different
medium)
19. State practical application of transient heat conduction?
(April/May 2017)
 Heat transfer in automobile engine’
 Power plant boiler temperature
20. What are the conditions needed to check for solving transient heat
conduction problem using heisler chart (April/May 2018 )
 Biot number <1 – Lumped heat analysis
 Biot number –Infinitive – Infinite solids
 Biot number =0.1<Bi<100 –Semi infinite solids
21. State the applications of fins.
 ME8693 Heat and Mass Transfer

1. Cooling of electronic components

2. Cooling of motor cycle engines
3. Cooling of transformers
4. Cooling of small capacity compressors.
22. Under what circumstance from the heat transfer point of view, will the
use of finned walls be better. (Nov/dec 2016)
 To increase the heat transfer rate.
23. Will the thermal resistance be greater for smooth or rough plane
surface?
In rough surface the area is increase, so the resistance is less.
24. What Thermal diffusivity? (April/May 2015)
How fast the heat is propagated or it diffuses through a material during
changes of temperature with time.

UNIT: II
PART A - 2 Marks (Questions and Answers)
1. Define critical Reynolds number. What is its typical value for flow
over a flat plate and flow through a pipe? (May 2013)
The critical Reynolds number refers to the transition from laminar to
turbulent flow.
The critical Reynolds number for flow over a flat plate is 5*105; the critical
Reynolds number for flow through a pipe is 4000.
2. How does or Distinguish laminar flow differ from turbulent flow?
(May 2013 & May 2015)
Laminar flow: Laminar flow is sometimes called stream line flow. In this
type of flow, the fluid moves in layers and each fluid particle follows a smooth
continuous path. The fluid particles in each layer remain in an orderly sequence
without mixing with each other.
Turbulent flow: In addition to the laminar type of flow, a distinct irregular
flow is frequently observed in nature. This type of flow is called turbulent flow.
The path of any individual particle is zig-zag and irregular.

Vel
o
c Turbulent
i flow
Laminar
t
flow
y
Ti
m
 ME8693 Heat and Mass Transfer

3. Differentiate viscous sublayer and buffer layer. (May 2014)

In the turbulent boundary layer, a very thin layer next to the wall where
viscous effect is dominant called the viscous sublayer. The velocity profile in this
layer is very nearly linear and the flow is streamlined.
In the turbulent boundary layer, next to viscous sublayer, a layer called
buffer layer in which turbulent effects are becoming significant, but
the flow is still dominated by viscous effects.
4. Define grashoff number and prandtl number. Write its significance.
(May 2014 & Nov 2014 & Nov 2015-Reg 2008)(Nov 2015)
(April/May 2017 & Nov/Dec 2017)
Grashoff number is defined as the ratio of product of inertia force and
buoyancy force to the square of viscous force.
Gr = Inertia Force * Buoyancy Force [HMT Data Book, P.No 112]
(Viscous Force)2
Significance: Grashoff number has a role in free convection similar to
that played by Reynolds number in forced convection.
Prandtl number is the ratio of the momentum diffusivity of the thermal
diffusivity.
Pr = Momentum Diffusivity [HMT Data Book, P.No. 112]
Thermal Diffusivity
Significance: Prandtl number provides a measure of the relative
effectiveness of the momentum and energy transport by diffusion.
5. Define velocity boundary layer thickness. (May 2015)
The region of the flow in which the effects of the viscous shearing
forces caused by fluid viscosity are felt is called velocity boundary layer.
The velocity boundary layer thickness, δ, is defined as the distance from
the surface at which velocity, u = 0.99V
6. Air at 27OC and 1 atmospheric flow over a flat plate at a speed of
2m/s. Calculate boundary layer thickness at a distance 40 cm from
leading edge of plate. At 27OC viscosity (air) = 1.85 *10-5 kg/ms.
(Nov 2012)
Given Data:
ME8693 Heat and Mass Transfer

T = 27OC = 27+273 = 300K

P = 1 atm = 1 bar = 1.01325 * 105 N/m2
U = 2 m/s
µ = 1.85 *10-5 kg/ms. (At 27OC)
R = 287 (Gas constant)
To Find: δ at X = 40 cm = 0.4 m
Solution:
Step: 1 Density ρ = P/RT
= 1.01325 * 105
(287*300)
= 1.177 Kg/m3
(Note: If Surface temperature (Tw) is given, then properties to be taken for Tf
Value.)
Step: 2 Reynolds Number Re = ρUX/ µ [HMT Data Book, P.No. 112]

= 1.177*2*0.4
1.85 *10-5
= 55160. (Re < 5*105, flow is laminar)
Step: 3 Boundary layer thickness δ = 5* X * (Re)-0.5
[HMT Data Book, P.No.113]
= 5 * 0.4 * (55160)-0.5
= 0.0085 m

Boundary layer thickness δ at X (0.4m) = 0.0085

m
7. A square plate 40*40 cm maintained at 400K is suspended vertically
in atmospheric air at 300 K. Determine the boundary layer
thickness at trailing edge of the plate. (Nov 2012)
Given Data:
Length of horizontal plate X = 40 cm = 0.4m
Wide W = 40 cm = 0.40 m
Plate temperature Tw = 400K = 1270C
Fluid temperature Tα = 300K = 270C
ΔT = (Tw- Tα) = 400-300 = 100
To Find: δ at X = 40 cm = 0.4 m
Solution:
ME8693 Heat and Mass Transfer

Step: 1 Film Temperature (Tf) = Tw + Tα

2
= 127+27 = 770C = 350K
2
Step: 2 Properties of air at 770C (apprx 750C)
[HMT Data Book, P.No.34]
ν = 20.56 * 10-6 m2/s
Pr = 0.693
Step: 3 Find β = 1 / Tf in K
= 1 / 350
= 2.857 * 10-3 K-1
Step: 4 For free Convection (Note: As Velocity not given)
Gr = g* β*X3*ΔT [HMT Data Book, P.No.135]

ν2
= 9.81 * 2.857 * 10-3 * (0.4)3 * (400-300)
(20.56 * 10-6)2
= 4.24 * 108
Step: 5 Boundary layer thickness δ = 3.93 * X * (Pr)-0.5 * (0.952+Pr)0.25 * Gr-
0.25

[HMT Data Book, P.No.135]

= 3.93 * 0.4 * (0.693) -0.5 * (0.952+0.693)0.25 * (4.24*108)-0.25
= 0.0155 m

Boundary layer thickness δ at X (0.4m) = 0.0155

m
8. Define the term thermal boundary layer thickness. (Nov 2013)
The thickness of the thermal boundary layer δt at any location
along the surface is defined as the distance from the surface at which
the temperature difference equals to 0.99(Tα-Ts), in general T=0.99Tα
9. Why heat transfer coefficient for natural convection is much lesser
than that for forced convection? (Nov 2013 & May 2016)
Heat transfer coefficient depends on the fluid velocity.
In natural convection, the fluid motion occurs by natural means
such as buoyancy. Since the fluid velocity associated with natural
ME8693 Heat and Mass Transfer

convection is relatively low, the heat transfer coefficient

encountered in natural convection is low.
The reason for higher heat transfer rates in forced convection is
because the hot air surrounding the hot body is immediately removed
by the flow of air around it. This is why forced convection heat transfer
coefficient is greater than natural convection heat transfer coefficient.
10. Name four dimensions used for dimensional analysis. (Nov 2014)
1. Velocity
2. Density
3. Heat transfer coefficient
4. Thermal conductivity

11. Mention the significance of boundary layer. (Nov 2015) (Nov/Dec

2017)
Boundary layer is the layer of fluid in the immediate vicinity of a
bounding surface where the effects of viscosity are significant.
12. What is Dittus Boelter equation? When does it apply? (Nov 2015)
Dittus-Boelter equation (for fully developed internal flow -
turbulent flow) is an explicit function for calculating the Nusselt
number. It is easy to solve but is less accurate when there is a large
temperature difference across the fluid. It is tailored to smooth tubes,
so use for rough tubes (most commercial applications) is cautioned.
The Dittus-Boelter equation is:
NuD=0.023 ReD 0.8 Prn [HMT Data Book, P.No.126]

13. What is the difference between friction factor and friction

coefficient? (May 2016)
Friction factor, a dimensionless quantity used in the Darcy–
Weisbach equation, for the description of friction losses in pipe flow as
well as open-channel flow. Friction coefficient applied at the value of x
(x=x-Local friction coefficient, x=L – Average friction coefficient)
14. Differentiate free and forced convection. (May 2016)(Nov /Dec
2016)
ME8693 Heat and Mass Transfer

Natural convection, or free convection, occurs due to

temperature differences which affect the density, and thus relative
buoyancy, of the fluid. Free convection is governed by Grashoff number
and Prandtl number.
Example: Rise of smoke from a fire.
In forced convection, fluid movement results from external
forces such as a fan or pump. Forced convection is typically used to
increase the rate of heat exchange. It is governed by the value of the
Reynolds number.
Example: Cooling of IC engines with fan in a radiator.
15. Differentiate hydrodynamic and thermal boundary layer. (May
2016)
The hydrodynamic boundary layer is a region of a fluid flow, near
a solid surface, where the flow patterns (velocity) are directly
influenced by viscous drag from the surface wall. The velocity of the
fluid is less than 99% of free stream velocity.
The thermal boundary layer is a region of a fluid flow, near a
solid surface, where the fluid temperatures are directly influenced by
heating or cooling from the surface wall. The temperature of the fluid is
less than 99% of free stream temperature.
16. What is critical Reynolds number for the flow over flat plate
(Nov/Dec 2016)
For flow past a flat plate, the transition from laminar to turbulent
begins when the critical Reynolds number reaches 5 x 105.The
boundary layer changes from laminar to turbulent at this point.

17. What is the difference between Reynolds number over colburn

analogies (April/May 2017)

 Reynolds number defines heat transfer and momentum .

 Colburn analogies defines momentum and mass transfer

18. A horizontal plate is insulated at one side and other side is not
.Assume the plate temperature is lesser than the surrounding air
 ME8693 Heat and Mass Transfer

temperature .To support heat transfer, suggest the orientation of

plate (facing up or down ) with valid reason (April/May 2018)

A plate should be place in facing up. Because at the bottom the is

insulated so there is no heat transfer at bottom

19. What is the usefulness of Rayleigh number in free convection

(April/May 2019?)
It is used to characterize the laminar to turbulence transition of free
convection boundary layer flow.
20. For a heated horizontal plate in quiescent air .larger for top or
bottom surface? Why (April/May 2019)
For a hot horizontal plate in quiescent air ,the heat transfer in upper
surface is maximum ,because the warm fluid is flows over the flat plate.
21. Sketch the temperature and velocity profile in free convection on a heated
vertical plate (Nov/Dec. 2019)

22. Recall the term boundary layer thickness. (Nov/Dec. 2019)

The thickness of the boundary layer has been defined as the distance from the
surface at which the local velocity or temperature reaches 99% of the
external velocity or temperature.
UNIT-III PHASE CHANGE HEAT TRANSFER AND HEAT EXCHANGER
PART-A
1. Distinguish between the two basic types of condensation. (Nov/Dec 2019 –
R13)
Film wise
The liquid condensate wets the solid surface, spreads out and forms a continuous
film over the entire surface is known as film wise condensation.
 ME8693 Heat and Mass Transfer

Drop wise condensation

In drop wise condensation the vapour condenses into small liquid droplets of
various sizes which fall down the surface in a random fashion.
2. How do you define the effectiveness of heat exchanger? (Nov/Dec 2019 – R13)
Heat exchanger effectiveness is defined as the ratio of the actual amount of heat
transferred to the maximum possible amount of heat that could be transferred with an
infinite area.
3. Consider film condensation on a vertical plate. Will the heat flux be higher at
the top or at the bottom of the plate? Why? (Apr/May 2019 – R13)
During film condensation on a vertical plate, heat flux at the top will be higher
since the thickness of the film at the top, and thus its thermal resistance, is lower.
4. Briefly explain why in steam condensers the LMTD is independent of flow
arrangement? (Apr/May 2019 – R13)
In steam condensers, the steam temperature doesn’t change during the heat
exchanging process so the LMTD is independent of flow arrangement.
5. Two fluids A and B exchanger heat in a counter flow heat exchanger. Fluid A
enters at 420 °C and has a mass flow rate of 1 kg/s. Fluid B enters at 20 °C and has
a mass flow rate of 1 kg/s. The effectiveness of heat exchanger is 75%. Determine
the exit temperature of fluid B. (Nov/Dec 2018 – R13)
 ME8693 Heat and Mass Transfer

6. What is meant by drop wise condensation? (Nov/Dec 2018 – R13)

In drop wise condensation the vapour condenses into small liquid droplets of
various sizes which fall down the surface in a random fashion.
7. What are the effects of non condensable gases in condenser? (April/ May 2018 –
R13)
The presence of non-condensable gases inside a compression vapour
refrigerating circuit introduces an additional thermal resistance at the condenser, which
can significantly decrease the energy efficiency of the system.
8. What is micro heat exchanger? (April/ May 2018 – R13)
The Micro-Heat exchangers are heat exchangers in which fluids flows in very
confined area such as tubes or small cavities whose dimensions are below 1mm in size.
9. What are the factors on which overall heat transfer coefficient depends? (Nov/
Dec 2017 – R13)
 Physiochemical properties of fluids (both cold and hot ) such a viscosity , density,
specific heat, thermal conductivity.
 Geometry of the exchanger ( equivalent length and heat exchanging area )
 Velocity of flowing fluids.
10. Differentiate pool boiling and forced convection boiling. (Nov/ Dec 2017 –
R13)
 ME8693 Heat and Mass Transfer

Boiling is called pool boiling in the absence of bulk fluid flow, and flow boiling (or
forced convection boiling) in the presence of it.

In pool boiling, the fluid is stationary, and any motion of the fluid is due to natural
convection currents and the motion of the bubbles due to the influence of buoyancy.
11. Sketch the temperature variation of condenser and evaporator. (April/ May
2017 – R13)

12. Draw the pool boiling curve for water. (April/ May 2017 – R13)
 ME8693 Heat and Mass Transfer

13. What are the fouling factors? (Nov/ Dec 2016 – R13)
The fouling factor represents the theoretical resistance to heat flow due to such
build up of a layer of dirt or other fouling substance on the tube surfaces and in the
tubes of the heat exchanger .
14. Give examples for pool boiling and flow boiling (Nov/ Dec 2016 – R13)
pool boiling
 Boling of water in a pan on stove
flow boiling
 Diabatic flow are to be found in the riser tubes of steam generators and boiler
tubes in power plants
15. What are the assumptions made in Nusselt theory of condensation (May/June
2016 – R13)
1. The plate is maintained at a uniform temperature which is less that the saturation
temperature of vapour
2. Fluid properties are constant.
3. The shear stress at the liquid vapour interface is negligible
4. The heat transfer across the condensate layer is by pure conduction and the
temperature distribution is linear.
16. What is fouling and how does it affect the rate of heat transfer? (May/June
2016 – R13)
Fouling is any kind of deposit of extraneous material that appears upon the heat
transfer surface during the life time of the heat exchanger.
This fouling will cause an additional resistance to heat transfer is introduced and the
 ME8693 Heat and Mass Transfer

operational capability of the heat exchanger is correspondingly reduced. In many cases,

the deposit is heavy enough to significantly interfere with fluid flow and increase the
pressure drop required to maintain the flow rate through the exchanger.
17. Define effectiveness and NTU of a heat exchanger. (May/June 2016 – R8)
The Number of Transfer Units (NTU) Method is used to calculate the rate of heat
transfer in heat exchangers. When there is insufficient information to calculate the Log-
Mean Temperature Difference (LMTD). In heat exchanger analysis, if the fluid inlet and
outlet temperatures are specified or can be determined by simple energy balance, the
LMTD method can be used; but when these temperatures are not available The NTU or
The Effectiveness method is used.
18. What is a compact heat exchanger? Give the applications. (May/June 2016 –
R8)
Special purpose heat exchangers called compact heat exchangers. They are
generally employed when convective heat transfer coefficient associated with one of the
fluids is much smaller than that associated with the other fluid. In variety of
applications including,
Compressed Gas / Water coolers
Condensers and evaporators for chemical and technical processes of all kinds.
Oil and water coolers for power machines
Refrigeration and air-conditioning units
19. What is meant by sub cooled and saturated boiling? (Nov/ Dec 2015 – R8)
The subcooled boiling or saturated boiling, depending on the bulk liquid
temperature.
Sub cooled boiling:
There is sharp increase in temperature near to the surface but through most of the
liquid, temperature remains close to saturation temperature ( Ta < Ts)
Saturated boiling:
When the temperature of the liquid equals to the saturation temperature (Tu=Ts)
20. Discuss the advantage of NTU method over the LMTD method. (April/ May
2015 – R8, Nov/ Dec 2015 – R8)
The LMTD method cannot be used for the determination of heat transfer rate and outlet temperature
of the hot and cold fluids for prescribed fluid mass flow rates and inlet temperatures when the type and
size of heat exchanger are specified.
Effectiveness NTU is superior for the above case because LMTD requires tedious iterations for the
same.
 ME8693 Heat and Mass Transfer

21. How heat exchangers are classified? (April/ May 2015 – R8)
The heat exchangers are classified as follows
1. Direct contact heat exchangers
2. Indirect contact heat exchangers
3. Surface heat exchangers
4. Parallel flow heat exchangers
5. Counter flow heat exchangers
6. Cross flow heat exchangers
7. Shell and tube heat exchangers
8. Compact heatexchangers.

UNIT-IV RADIATION

PART-A

1. How do you define the black body and emissivity of surface? (Nov/Dec 2019 –
R13)
Black body is an ideal surface having the following properties.
A black body absorbs all incident radiation, regardless of wave length and direction. For
a prescribed temperature and wave length, no surface can emit more energy than black
body.
Emissivity
It is defined as the ability of the surface of a body to radiate heat. It is also defined as the
ratio of emissive power of any body to the emissive power of a black body of equal
temperature.
2. Define the term absorptivity, transmissivity and reflectivity? (Nov/Dec 2019 –
R13)
Absorptivity is defined as the ratio between radiation absorbed and incident radiation.
Reflectivity is defined as the ratio of radiation reflected to the incident radiation.
Transmissivity is defined as the ratio of radiation transmitted to the incident radiation.
3. What is the crossed strings method? (Apr/May 2019 – R13)
For any two non-touching infinitely-long bands, 1 and 2 one can also find all the view
factors from simple algebraic relations as in the triangular enclosure before, extending
the result (8) to what is known as crossed-string method
4. What is thermal radiation and how does it differ from the other forms of
electromagnetic radiation? (Apr/May 2019 – R13)
Thermal radiation has high frequency than other forms of electromagnetic radiation
 ME8693 Heat and Mass Transfer

5. The effective temperature of a body having an area of 0.12 m2 is 527 °C.

Calculate the wave length of the maximum monochromatic emissive power. .
(Nov/Dec 2018 – R13)
From Wien’s displacement law
λmaxT = 0.0029mK
λmax = 0.0029/T
=3.625μm
6. What are the properties of black body? (Nov/Dec 2018 – R13), (Nov/ Dec 2017
– R13)
Black body is an ideal surface having the following properties.
A black body absorbs all incident radiation, regardless of wave length and direction. For
a prescribed temperature and wave length, no surface can emit more energy than black
body.
7. Two black square plates of size 1.0 m by 1.0 m are kept parallel to each other at the
depth of 0.4 m. one plate is maintained at a temperature of 800°C and the other at
420°C. Find the net exchange of energy due to radiation between the two plates. ?
(April/ May 2018 – R13)
8. In gas radiation, what is Beers law? (April/ May 2018 – R13)
The Beer–Lambert law, also known as Beer's law, the Lambert–Beer law, or the Beer–
Lambert–Bouguer law relates the attenuation of light to the properties of the material
through which the light is travelling. This expression is
A= Ɛlc
Where

 Ɛ is the molar attenuation coefficient or absorptivity of the attenuating species

 I is the optical path length
 c is the concentration of the attenuating species
9. Define radiosity. (Nov/ Dec 2017 – R13), (April/ May 2015 – R8)
It is used to indicate the total radiation leaving a surface per unit time per unit area. It is
expressed in W/m2.
10 State lamberts cosine law for radiation. (April/ May 2017 – R13)
It states that the total emissive power Eb from a radiating plane surface in any
direction proportional to the cosine of the angle of emission
Eb cos 

11. What are the applications of radiation shields? (April/ May 2017 – R13)
Radiation shields are used in spacecrafts for protection against thermal radiation that
can potentially damage equipment on board.
 ME8693 Heat and Mass Transfer

The multi-layer construction prevents radiant heat being transmitted both

by radiation - using reflecting surfaces - and convection - by evacuation of gases
between layers.
12. Define monochromatic emissive and emissive power? (Nov/ Dec 2016 – R13)
(May/June 2016 – R13)
The energy emitted by the surface at a given length per unit time per unit area in all
directions is known as monochromatic emissive power.
The emissive power is defined as the total amount of radiation emitted by a body per
unit time and unit area. It is expressed in W/m2.
13. What do you mean by infrared and ultraviolet radiation (Nov/ Dec 2016 – R13)
Infrared radiation (IR), sometimes called infrared light, is electromagnetic
radiation (EMR) with wavelengths longer than those of visible light. It is therefore
generally invisible to the human eye,
Ultraviolet (UV) is electromagnetic radiation with wavelength from 10 nm to 400 nm,
shorter than that of visible light but longer than X-rays. UV radiation is present
in sunlight, and constitutes about 10% of the total electromagnetic radiation output
from the Sun.
14. What is black body radiation? (May/June 2016 – R13)
A black body absorbs all incident radiation, regardless of wave length and
direction. For a prescribed temperature and wave length, no surface can emit more
energy than black body.
15. Define radiation intensity (May/June 2016 – R8)
It is defined as the rate of energy leaving a space in a given direction per unit solid
angle
per unit area of the emitting surface normal to the mean direction in space.
In = Eb/π
16. Differentiate black body and grey body. (May/June 2016 – R8)

Black body is an object that absorbs all the radiant energy reaching its surface with all
the wavelengths. While a gray body is defined as a body whose absorption does not
change with variation in temperature & wavelength of the incident radiation
17. Define irradiation and radiosity (Nov/ Dec 2015 – R13)
It is defined as the total radiation incident upon a surface per unit time per unit
area. It is expressed in W/m2.
It is used to indicate the total radiation leaving a surface per unit time per unit area. It
is expressed in W/m2.
18. What is the greenhouse effect? Why is it a matter of great concern among
atmospheric scientists? (Nov/ Dec 2015 – R13)
Its warming effect occurs even when the sky is clear and dry.
Climate scientists are so concerned about carbon dioxide because the more carbon
dioxide in the atmosphere, the hotter the earth will become, changing the Earth's
climate.
 ME8693 Heat and Mass Transfer

19. State kirchhoff’s law? (April/ May 2015 – R8)

This law states that, for any node (junction) in an electrical circuit, the sum
of currents flowing into that node is equal to the sum of currents flowing out of that
node; or equivalently: The algebraic sum of currents in a network of conductors
meeting at a point is zero.
UNIT-V MASS TRANSFER
1. What is mass transfer?
The process of transfer of mass as a result of the species concentration difference in a
mixture is known as mass transfer.
2. Give the examples of mass transfer.
Some examples of mass transfer.
1. Humidification of air in cooling tower
2. Evaporation of petrol in the carburetor of an IC engine.
3. The transfer of water vapour into dry air.
3. What are the modes of mass transfer? (Nov/Dec 2017)
There are basically two modes of mass transfer,
1. Diffusion mass transfer
2. Convective mass transfer
4. What is molecular diffusion?
The transport of water on a microscopic level as a result of diffusion from a region of
higher concentration to a region of lower concentration in a mixture of liquids or gases
is known as molecular diffusion.
5. What is Eddy diffusion?
When one of the diffusion fluids is in turbulent motion, eddy diffusion takes place.
6. What is convective mass transfer?
Convective mass transfer is a process of mass transfer that will occur between surface
and a fluid medium when they are at different concentration.
7. State Fick’s law of diffusion. (Nov/Dec 2019)
The diffusion rate is given by the Fick’s law, which states that molar flux of an element
per unit area is directly proportional to concentration gradient.
8. What is free convective mass transfer?
If the fluid motion is produced due to change in density resulting from concentration
gradients, the mode of mass transfer is said to be free or natural convective mass
transfer.
Example : Evaporation of alcohol.
9. Define forced convective mass transfer.
 ME8693 Heat and Mass Transfer

If the fluid motion is artificially created by means of an external force like a blower or
fan, that type of mass transfer is known as convective mass transfer.
Example: The evaluation if water from an ocean when air blows over it.
10. Define Schmidt Number. (Nov/Dec 2016)
It is defined as the ratio of the molecular diffusivity of momentum to the molecular
diffusivity of mass.
11. Define Scherwood Number.
It is defined as the ratio of concentration gradients at the boundary.
12. Give two examples of convective mass transfer.
· Evaporation of alcohol
· Evaporation of water from an ocean when air blows over it.
13. Define Mass concentration.
Mass of a component per unit volume of the mixture. It is expressed in kg/m3.
14. What do you understand by Molar concentration?
Number of molecules of a component per unit volume of the mixture. It is expressed in
kgmole/m3.
15. Define Mass fraction.
The mass fraction is defined as the ratio of mass concentration of species to the total
mass density of the mixture.
16. Write the expression for determining the rate of mass convection. (Nov/Dec
2019)
Similar to Newton’s law for convective heat transfer, the convective mass transfer
equation can be written as: m = hm A ΔC
where, hm is the convective mass transfer coefficient and
ΔC is the difference between the boundary surface concentration and the average
concentration of fluid stream of the diffusing species A. Similar to convective heat
transfer, convective mass transfer coefficient depends on the type of flow, i.e., laminar
or turbulent and forced or free.
17. What is mass diffusivity? (Apr/May 2019)
Diffusivity, mass diffusivity or diffusion coefficient is a proportionality constant
between the molar flux due to molecular diffusion and the gradient in the concentration
of the species.
18. State the reason for development for concentration boundary layer. (Apr/May
2019)
 ME8693 Heat and Mass Transfer

The concentration boundary layer develops when there is a difference in concentration

of a component between the free stream and the surface. A concentration profile
develops, and the thickness of the concentration boundary layer is defined as that point
at which the difference in concentration between the fluid and the surface is 99% of the
difference in concentration between the free stream fluid and the surface.
19. Give any two examples of mass transfer in day to day life. (Nov/Dec 2018)
1. The evaporation of water from a pond to the atmosphere,
2. The purification of blood in the kidneys and liver,
3. The distillation of alcohol.
20. What do you mean by equimolar counter diffusion? (Nov/Dec 2018)
Equimolar counterdiffusion is an instance of molecular diffusion in a binary mixture,
and occurs when equal numbers of molecules of the two substances are moving in
opposite directions.
21. What is the significance of Lewis number? (Apr/May 2018)
The dimensionless Lewis number is defined as the ratio of thermal diffusivity to mass
diffusivity. Le = α / DAB and it represents the relative magnitudes of heat and mass
diffusion at molecular level in the thermal and concentration boundary layers,
respectively.
A Lewis number of unity indicates that heat and mass diffuse at the same rate, and the
thermal and concentration boundary layers coincide.
22. Compare the Reynolds analogy of heat and mass transfer. (Apr/May 2018)
The Reynolds Analogy is popularly known to relate turbulent momentum
and heat transfer. That is because in a turbulent flow (in a pipe or in a boundary layer)
the transport of momentum and the transport of heat largely depends on the same
turbulent eddies: the velocity and the temperature profiles have the same shape.
23. Define mass transfer coefficient. (Nov/Dec 2017)
The mass transfer coefficient is a diffusion rate constant that relates the mass transfer
rate, mass transfer area, and concentration change as driving force

24. Distinguish between mass concentration and molar concentration. (Apr/May

2017)
Molar concentartion ( Active mass) is defined as number of moles per volume litre.
For example, consider gas A. Its molar concentartion is represented by [ A] or (nA/V)
 ME8693 Heat and Mass Transfer

where nA= no. Of moles of gas A and V is measured in litre. It signifies us about the
concentration of moles of any element per volume litre.
mass concentration
The mass concentration ρi (or γi) is defined as the mass of a constituent mi divided by
the volume of the mixture V.
25. Give examples for free and forced convection mass transfers. (Apr/May 2017)
Forced convection: In this type the fluid moves under the influence of an external force
(pressure difference) as in the case of transfer of liquids by pumps and gases by
compressors.
Natural convection: Natural convection currents develop if there is any variation in
density within the fluid phase. The density variation may be due to temperature
differences or to relatively large concentration differences.
26. Define molar concentration (April/May 2015)
Molar concentration (also called molarity, amount concentration or
substance concentration) is a measure of the concentration of a chemical species, in
particular of a solute in a solution, in terms of amount of substance per unit volume of
solution.
27. Define convective mass transfer.(Nov/Dec 2019)
Convective mass transfer is a process of mass transfer that will occur between a
surface and a fluid medium when they are at different concentrations.

28.Identify any two examples of convective and diffusion mass transfer.

convective mass transfer
Evaporation of alcohol
Evaporation of water from an ocean when air blows over it
diffusion mass transfer
The noxious smell of ammonia gas spreads in air.
30. What is driving force for heat and mass transfer?
1. The temperature gradient is Driving force for heat transfer.
2. The concentration gradient is driving force for mass transfer
31. Write down anlalogous terms in heat and mass transfer
ME8693 Heat and Mass Transfer