Download GTU (Gujarat Technological University Ahmedabad) B.Tech/BE (Bachelor of Technology/ Bachelor of Engineering) 2020 Winter 5th Sem 2151909 Heat Transfer Previous Question Paper
Enrolment No.___________
GUJARAT TECHNOLOGICAL UNIVERSITY
BE- SEMESTER?V (NEW) EXAMINATION ? WINTER 2020
Subject Code:2151909 Date:22/01/2021
Subject Name:Heat Transfer
Time:10:30 AM TO 12:30 PM Total Marks: 56
Instructions:
1. Attempt any FOUR questions out of EIGHT questions.
2. Make suitable assumptions wherever necessary.
3. Figures to the right indicate full marks.
MARKS
Q.1 (a) Define :
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1) Fin efficiency
2) Critical radius and
3) Fourier number.
(b) Define time constant for the thermocouple." Diameter of thermocouple wire
04
should be as low as possible" Justify above statement analytically
(c) Define Boiling and Write different modes of it. Draw boiling curve and show
07
different regimes on that. Explain in brief 1) Film boiling 2) Critical heat flux
and 3) Leidenfrost Point
Q.2 (a) Write the equation of fin effectiveness for the infinitely long fin. Based on this
03
state two conclusions for selection of fin.
(b) Write the differential equation for steady state heat conduction in radial
04
direction for sphere without heat generation in it. Solve the equation and
represent the solution graphically.
(c) A steam pipe 10cm in outer diameter is covered with two layers of insulation
07
material each is 2.5 cm thick. Ratio of their thermal conductivity is 3. Compare
the effective thermal conductivity of two layers when better insulation material
is placed inside layer than when it is on the outside.
Q.3 (a) Differentiate mean film temperature and bulk mean temperature.
03
(b) Write equation for hydraulic diameter. Calculate its value for 1) two parallel
04
plates and 2) rectangular duct.
(c) Atmospheric air at 275K and free stream velocity of 20m/s flows over 1.5m
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long flat plate maintained at uniform temperature of 325K.
1) Average heat transfer co-efficient over entire length of 1.5m
2) Heat transfer rate from the plate over 1.5m and 1m wide.
3) Average heat transfer co-efficient over laminar boundary layer..
1
For turbulent flow :
0.8
3
Nu 0.037(Re
871)Pr
L
L
1
1
For laminar flow :
2
3
Nu 0.664Re
Pr
L
L
For calculation use : Kair = 0.026 W/mK, Pr = 0.708,
Kinematic viscosity = 16.8 * 10-6 m2/s,
Dynamic viscosity = 1.98*10-5 kg/ms
Assume transition occurs at Re = 2 * 105 .
Q.4 (a) Define hydrodynamic and thermal boundary layer. Represent their respective
03
thickness for the liquid metal.
(b) Air at temperature 300K and 1 atm flows flows over a flat plate at a speed of 2
04
m/s. Calculate the mass flow rate enters the boundary layer between 0.2m and
0.4 m from the leading edge. Assume unit depth in z- direction.
Take : viscosity of air = 1.85 * 10-5 kg/m-s.
1
Density of air is 1.177 kg/m3.
(c) Using Buckingham ? theorem show that
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Nu = f ( Gr, Pr)
Q.5 (a) Do as directed :
03
1) Write Wien's displacement law.
2) Define monochromatic emissive power.
3) Define absorptivity
(b) Calculate the shape factor of the conical hole whose semi cone angle is .
04
(c) Two large parallel plates at temperature 1000K and 600K have emissivity of
07
0.5 and 0.8 respectively. A radiation shield of emissivity 0.1 on one side and
0.05 on other side is placed between these plates. Calculate
1) Heat transfer rate by radiation with and without radiation shield.
2) % reduction in heat transfer rate by radiation shield.
Q.6 (a) Do as directed:
03
1) Define : Radiosity and
2) Define : Grey body
3) State Kirchoff's Law for radiation.
(b) Explain summation rule and superposition rules for shape factor in brief.
04
(c) A spherical liquid oxygen tank 0.3m in diameter is enclosed concentrically in
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a spherical container of 0.4m diameter and space between them is evacuated.
Tank surface is at -1830C and has emissivity of 0.2. Container is exposed to 15
0C and has emissivity of 0.25.
Calculate :
1) Net radiant heat transfer rate
2) Rate of evaporation of Oxygen if latent heat is 220kJ/kg.
Q.7 (a) Classify heat exchanger based on
03
1) Constructional features
2) Flow arrangement.
(b) "Flow arrangement doesn't affect performance of the heat exchanger which
04
involves phase change process" Justify the statement analytically.
(c) A chemical having specific heat of 3.3kJ/kg-K at a rate of 20000kg/h enters a
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parallel flow heat exchanger at 120?C. Cooling water flow rate is 50000kg/h
with an inlet temperature of 20?C. Heat transfer area is 10m2 and overall heat
transfer co-efficient is 1050W/m2-K.
Calculate :
1) Effectiveness of heat exchanger
2) Outlet temperature of hot and cold fluids.
Take specific heat of water is 4.2 kJ/kg-K
Q.8 (a) Explain in brief fouling. Write the equation for fouling factor.
03
(b) Determine the area required for a shell and tube heat exchanger with two tube
04
passes to cool oil at rate of 10 kg/s from 60?C to 30?C flowing in the shell using
water at 20?C passing through the tubes and heated up to 26?C. The specific
heat of the oil is 2200 J/kg K.
The value of overall heat transfer coefficient is 300 W/m2 K. Take correction
factor F = 0.875.
(c) Define: effectiveness. Develop its equation for the counter flow heat exchanger.
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This post was last modified on 04 March 2021