GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER- VII (New) EXAMINATION — WINTER 2019
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Subject Code: 2170102 Date: 26/11/2019Subject Name: Theory of Heat Transfer
Time: 10:30 AM TO 01:00 PM Total Marks: 70
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
- Tables for properties of air and water are permitted.
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Q.1 (a) Explain the following terms: 03
- Thermal diffusivity
- Thermal Conductivity
- Thermal contact resistance
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(b) Define fin effectiveness. When the use of fins is not justified 04
(c) An air stream at 0 °C is flowing along a heated plate at 90 °C at a speed of 07
75 m/s. The plate is 45 cm long and 60 cm wide. Assuming the transition
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of boundary layer takes place at Recr= 5 x 105. Calculate the average valueof friction coefficient and heat transfer coefficient for full length of the
plate. Also calculate the heat dissipation from the plate.
The thermo-physical properties at of air are as follows:
p=1.113 kg/m3, k = 0.0276 W/m-K, Pr=0.693, n = 1.928 x 10-5 kg/ms
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Q.2 (a) What is transient heat conduction? How does transient heat conduction 03
differ from steady state heat conduction?
(b) What is the “critical radius” of insulation? Derive an expression for the 04
same for cylinders.
(c) A steel pipe is covered with two layers of insulation, first layer being 3 07
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cm thick and second 5 cm. The pipe is made of steel (k=58 W/m-K)having internal diameter of 160 mm and Outside diameter of 170 mm. The
inside and outside film coefficients are 30 and 5.5 W/m2-K, respectively.
Calculate the heat lost per metre of pipe, if the steam temperature is 300°C
and air temperature is 50°C. The thermal conductivity of two insulating
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material are 0.17 and 0.093 W/m-K, respectively.OR
(c) Derive an expression for heat transfer for an adequately long of 07
Rectangular fin with insulated tip.
Q.3 (a) Differentiate: 03
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- Mean film temp and bulk mean temp
- Velocity and thermal boundary layer
(b) Explain lumped heat capacity method of heat transfer and state its 04
assumptions.
(c) Using dimensional analysis, obtain a general form of equation for forced 07
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Convective heat transfer.OR
Q.3 (a) Distinguish between natural and forced convection heat transfer. 03
(b) Show that for laminar flow along a flat plate, 6/6 = 3.0, where 6 and 6* are the 04
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boundary layer flow with negligible pressure gradient, 6*u/y°= 0 at y =0.
(c) Discuss the concept of thermal boundary layer in case of flow over the 07
plates. How it differ from velocity boundary?
Q.4 (a) What is a heat exchanger and where are they used? 03
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(b) Explain mechanism of nucleate boiling? 04
(c) Obtain an expression for overall heat transfer coefficient for tubular heat 07
exchanger, subjected to fouling on its two sides of heat transfer surface.
OR
Q.4 (a) Why is counter-flow Heat Exchanger more effective than a parallel flow 03
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heat exchanger.(b) What are the fouling factors? Explain their effect in Heat Exchanger 04
design.
(c) Prove that the effectiveness of parallel flow heat exchanger is given by 07
(1-exp[-NTU(1+C)])/(1+C)
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Q.5 (a) Explain Kirchhoff’s law of radiation? 03
(b) Explain dropwise and filmwise condensation. 04
(c) Derive a general relation for the radiation shape factor in case of radiation 07
between two surfaces. Explain Wein’s displacement law of radiation.
OR
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Q.5 (a) What does the view factor represent? When is the view factor from a 03
surface to itself not zero?
(b) Define following: 04
- Absorptivity
- Reflectivity
- Transmissivity
- Total Emissivity
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(c) Define Radiation Intensity? State & Explain the Wien Displacement Law. 07
Show that Ep. will be maximum, when ?max. T =2900 µK
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