GUJARAT TECHNOLOGICAL UNIVERSITY
BE - SEMESTER-VII(NEW) EXAMINATION — SUMMER 2019
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Subject Code:2170102 Date:14/05/2019Subject Name:Theory of Heat Transfer
Time:02:30 PM TO 05:00 PM Total Marks: 70
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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Q.1 (a) What are the modes of Heat Transfer? Also mention their governing laws. 03
(b) Differentiate Conduction & Convection heat transfer. 04
(c) Derive general heat conduction equation in Cartesian coordinate system. 07
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Q.2 (a) Differentiate Natural & Forced Convection. 03
(b) Compare Electrical Analogy & Thermal Analogy for Plane Wall in terms of Voltage and Temperature Difference respectively. 04
(c) 0.5cm thick glass (k= 0.78 W/mK) is exposed to air at 25° C (inner side) with convection heat transfer coefficient of 15 W/m2K. The outside air at -15° C with convection coefficient of 50 W/m2K. Determine:
(1) Temperature at inner and outer glass surfaces
(2) Heat transfer rate 07
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OR
(c) An outer wall of a house may be approximated by a 0.1m layer of common brick (k= 0.7 W/m°C) followed by a 0.04m layer of Gypsum plaster (k= 0.48 W/m°C). What thickness of loosely packed rockwool insulation (k= 0.065 W/m°C) should be added to reduce the heat loss or gain the wall by 80% ? 07
Q.3 (a) What are the common applications of Extended surfaces? 03
(b) What are the different types of Extended surfaces? 04
(c) Derive an expression for heat flow through Rectangular fins. 07
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OR
Q.3 (a) What is (1) Prandtl Number (2) Grashoff Number & (3) Nusselt Number? 03
(b) Briefly discuss the concept of Hydrodynamic boundary layer. 04
(c) Using Rayleigh’s Method, Prove that when a sphere of diameter ‘d’, moves with a uniform velocity “V’, through a fluid of density ‘p’ and dynamic viscosity ‘µ’, the resistance ‘R’ will be
R = pd2v2 [µ/pvd] 07
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Q.4 (a) Define Emissive Power & Emissivity. 03
(b) Define Absorptivity, Reflectivity & Trasnmissivity. 04
(c) A black body of total area 0.045 m2 is completely enclosed in a space bounded by 5 cm thick walls. the walls have a surface area 0.5 m2 and thermal conductivity 1.07 W/m°C. If the inner surface of the wall is maintained at 30°C, calculate the temperature of the black body. Neglect the difference between inner and outer surface areas of enveloping material. 07
OR
Q.4 (a) What is Overall heat transfer coefficient? 03
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(b) Classify Heat Exchangers on the basis of its Mechanical Design. 04(c) Derive an expression for Radiation between two non-black parallel surfaces. 07
Q.5 (a) What do you mean by Heat Exchanger effectiveness? 03
(b) Differentiate Filmwise and Dropwise condensation. 04
(c) Derive LMTD Method. 07
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OR
Q.5 (a) Define Boiling, Condensation & Evaporation? 03
(b) Discuss Fouling/Scale deposition phenomenon in Heat Exchangers. Also state it’s Disadvantages. 04
(c) Write a short note on Boiling Phenomenon. 07
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