GUJARAT TECHNOLOGICAL UNIVERSITY
BE- SEMESTER-VII (NEW) EXAMINATION - WINTER 2020
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Subject Code:2170102 Date:21/01/2021
Subject Name:Theory of Heat Transfer
Time:10:30 AM TO 12:30 PM Total Marks: 56
Instructions:
- Attempt any FOUR questions out of EIGHT 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) What do you understand by fin effectiveness and fin efficiency? 04
(c) A flat plate, 1 m wide and 1.5 m long is to be maintained at 90°C in air 07 with a free stream temperature of 10°C . Determine the velocity with which air must flow over flat plate along 1.5 m side so that the rate of energy dissipation from the plate is 3.75 KW. Take the following thermo- physical properties 50°C. p=1.09 kg/m³, k= 0.028 W/m°C, Pr=0.7, µ = 2.03 x 10?5 kg/m-s , Cp=1.007 KJ/Kg°C.
Q.2 (a) What are Fourier and Biot numbers? Write their significance. 03
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(b) What is the “critical radius” of insulation? Derive an expression for the 04 same for cylinders.
(c) The composite wall of a furnace is made up with 120 mm of fire clay ( k= 07 0.25 (1 +0.0009 t) W/m°C and 600 mm of red brick (k=0.8 W/m°C). The inside surface temperature is 1250°C and the outside air temperature is 40°C. Determine :
- The temperature at the layer interface, and
- The heat loss for 1 m² of furnace wall.
Q.3 (a) Differentiate between velocity and thermal boundary layer. 03
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(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 free 07 Convective heat transfer.
Q.4 (a) Distinguish between natural and forced convection heat transfer. 03
(b) Show using momentum equation that in the case of incompressible boundary 04 layer flow with negligible pressure gradient, ?³u/?y³= 0 at y = 0.
(c) Discuss the concept of thermal boundary layer in case of flow over the plates. 07 How it differ from velocity boundary?
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Q.5 (a) Define heat exchanger and classify in detail. 03
(b) Write a note on Nucleate boiling? 04
(c) Derive expression for logarithmic mean temperature difference (LMTD) 07 in the case of counter-flow heat exchanger.
Q.6 (a) Write assumptions used when we drive expression for LMTD for various 03 types of heat exchangers.
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(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)
Q.7 (a) State and Prove Kirchoff’s law of radiation? 03
(b) Differentiate between 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.
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Q.8 (a) Enumerate the factors on which the rates of emission of radiation by a 03 body depend.
(b) Define following: 04
- Gray Body
- Lambert’s Law of radiation
- Transmissivity
- Total Emissivity
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(c) Define Radiation Intensity? Prove that the intensity of radiation is given 07 by Ib=Eb/p
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