Download GTU (Gujarat Technological University) BE/BTech (Bachelor of Engineering / Bachelor of Technology) 2019 Summer 5th Sem New 2151909 Heat Transfer Previous Question Paper
Seat No.: ________ Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER ?V (NEW) EXAMINATION ? SUMMER 2019
Subject Code: 2151909 Date: 06/06/2019
Subject Name:Heat Transfer
Time: 02:30 PM TO 05:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
4. Use of air, water properties chart is allowed.
Q.1 (a) Distinguish between the conduction, convection and radiation modes of
heat transfer with suitable example.
03
(b) Define Biot and Fourier numbers, and point out their physical
significance.
04
(c) Derive general heat conduction equation in cylindrical coordinate
system
2 2 2
2 2 2 2
1 1 1
g
q
t t t t t
r r r r z k ? ? ?
?? ? ? ? ? ?
? ? ? ? ?
??
? ? ? ? ?
??
07
Q.2 (a) What is critical radius of insulation? Explain its importance in
electrical and thermal system.
03
(b) State the difference between the fin effectiveness and fin efficiency,
and setup the relation between their performance parameters.
04
(c) Steam at 350
o
C flowing in a pipe (k=80 W/mK) 5 cm i.d., 5.6 cm o.d.
is covered with 3 cm thick insulation (k=0.05 W/mK). Heat is lost to
the surroundings at 5
o
C by natural convection and radiation with
combined h=20 W/m
2
K and hi=60 W/m
2
K. Find: (i) the rate of heat
loss from the pipe per unit length (ii) the temperature drops across the
pipe and the insulation.
07
OR
(c) Explain with neat sketch, the various regimes in boiling and explain the
condition for the growth of bubbles. What is the effect of bubble size
on boiling?
07
Q.3 (a) Explain the phenomenon of heat transfer by forced convection. What
forces control the fluid motion? Cite suitable examples to illustrate
your answer.
03
(b) How does film-wise condensation differ from drop-wise condensation?
Which type has a higher film coefficient and point out the reason
thereof.
04
(c) The temperature of a gas stream is to be measured by a thermocouple
whose junction can be approximated as 1 mm diameter sphere. The
properties of a junction are k=35 W/mK, density = 8500 kg/m
3
,
Cp=320 J/kg K and h=210 W/m
2
k. Determine how long it will take for
thermocouple to read 99% ( ? /?i=0.01) of the initial temperature
difference.
07
OR
Q.3 (a) What are the fundamental dimensions? Explore thermal resistance,
thermal diffusivity and convective heat transfer coefficient in
fundamental dimensions.
03
(b) What is meant by thermal boundary layer? State the relation between
thermal and hydrodynamic boundary layer thickness.
04
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Seat No.: ________ Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER ?V (NEW) EXAMINATION ? SUMMER 2019
Subject Code: 2151909 Date: 06/06/2019
Subject Name:Heat Transfer
Time: 02:30 PM TO 05:00 PM Total Marks: 70
Instructions:
1. Attempt all questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
4. Use of air, water properties chart is allowed.
Q.1 (a) Distinguish between the conduction, convection and radiation modes of
heat transfer with suitable example.
03
(b) Define Biot and Fourier numbers, and point out their physical
significance.
04
(c) Derive general heat conduction equation in cylindrical coordinate
system
2 2 2
2 2 2 2
1 1 1
g
q
t t t t t
r r r r z k ? ? ?
?? ? ? ? ? ?
? ? ? ? ?
??
? ? ? ? ?
??
07
Q.2 (a) What is critical radius of insulation? Explain its importance in
electrical and thermal system.
03
(b) State the difference between the fin effectiveness and fin efficiency,
and setup the relation between their performance parameters.
04
(c) Steam at 350
o
C flowing in a pipe (k=80 W/mK) 5 cm i.d., 5.6 cm o.d.
is covered with 3 cm thick insulation (k=0.05 W/mK). Heat is lost to
the surroundings at 5
o
C by natural convection and radiation with
combined h=20 W/m
2
K and hi=60 W/m
2
K. Find: (i) the rate of heat
loss from the pipe per unit length (ii) the temperature drops across the
pipe and the insulation.
07
OR
(c) Explain with neat sketch, the various regimes in boiling and explain the
condition for the growth of bubbles. What is the effect of bubble size
on boiling?
07
Q.3 (a) Explain the phenomenon of heat transfer by forced convection. What
forces control the fluid motion? Cite suitable examples to illustrate
your answer.
03
(b) How does film-wise condensation differ from drop-wise condensation?
Which type has a higher film coefficient and point out the reason
thereof.
04
(c) The temperature of a gas stream is to be measured by a thermocouple
whose junction can be approximated as 1 mm diameter sphere. The
properties of a junction are k=35 W/mK, density = 8500 kg/m
3
,
Cp=320 J/kg K and h=210 W/m
2
k. Determine how long it will take for
thermocouple to read 99% ( ? /?i=0.01) of the initial temperature
difference.
07
OR
Q.3 (a) What are the fundamental dimensions? Explore thermal resistance,
thermal diffusivity and convective heat transfer coefficient in
fundamental dimensions.
03
(b) What is meant by thermal boundary layer? State the relation between
thermal and hydrodynamic boundary layer thickness.
04
2
(c) Which of the following arrangement of pin fins will give higher heat
transfer rate from a hot surface? (i) 6 fins of 10 cm length (ii) 12 fins of
5 cm length. The base temperature of the fin is maintained at 200
o
C
and the fin is exposed to a convection environment at 15
o
C with h=25
W/m
2
C. Each fin has cross sectional area 2.5 cm
2
, perimeter 5 cm and
is made of a material having thermal conductivity 250 W/mC. Neglect
the heat loss from the tip of fin.
07
Q.4 (a) Define a black body. Give examples of some surfaces which don?t
appear black but have high value of absorptivity.
03
(b) What do you mean by thermal capacity and thermal diffusivity of
material? Explain with example.
04
(c) A thermo flask has a double walled bottle and the space between the
walls is evacuated so as to reduce the heat flow. The bottle surfaces are
silver plated, and the emissivity of each surface is 0.025. If the contents
of the bottle are at 375 K, find the rate of heat loss from the thermos
bottle to ambient air at 300 K. What thickness of cork (k=0.03 W/mK)
would be requited if the same insulating effect is to be achieved by the
use of cork?
07
OR
Q.4 (a) Explain the meaning of the terms radiosity and irradiation. 03
(b) Making use of Plank?s law of distribution, establish the relation for the
Wien?s displacement law.
04
(c) A square room 4 m x 4 m and height 3 m has all its walls perfectly
insulated. The floor and ceiling are maintained at 300 K and 280 K
respectively. Assuming an emissivity value 0.75 for all the surfaces,
make calculation for the wall temperature and net heat interchange
between the floor and ceiling. Take floor to ceiling shape factor as
0.28.
07
Q.5 (a) Explain the operation of a heat pipe. Why is it called a superconductor? 03
(b) Write the energy equation for a fluid element. What is dissipation
function? When is its effect significance and when can it be neglected?
04
(c) Water (Cp=4.187 kJ/kg K) is heated at the rate of 1.4 kg/s from 40
o
C to
70
o
C by an oil (Cp=1.9 kJ/kg K) entering at 110
o
C and leaving at 60
o
C in a counter flow heat exchanger. If Uo=350 W/m
2
K, calculate the
surface area required.
Using the same entering fluid temperature and the same oil flow rate,
calculate the exit temperature of oil and water and the rate of heat
transfer, when the water flow rate is halved.
07
OR
Q.5 (a) Draw the simple electrical circuit which is analogous to transient heat
transfer from a body at uniform temperature.
03
(b) Show that for parallel flow heat exchanger
? ? 1 exp 1
1
NTU R
R
?
? ? ? ??
??
?
?
04
(c) What is heat exchanger? Classify the heat exchanger types with
example.
07
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This post was last modified on 20 February 2020