Subject Code: 2153502
GUJARAT TECHNOLOGICAL UNIVERSITY
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BE - SEMESTER- V (New) EXAMINATION - WINTER 2019Subject Name: Introduction to Heat Transfer
Time: 10:30 AM TO 01:00 PM
Instructions:
1. Attempt all questions.
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2. Make suitable assumptions wherever necessary.3. Figures to the right indicate full marks.
Q.1 (a) State Fourier’s law of heat conduction? Mention the significance of negative sign in heat conduction equation. [03]
(b) Differentiate Critical thickness of Insulation and Optimum thickness of Insulation [04]
(c) Describe the different kind of Boundary conditions used in heat transfer studies [07]
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Q.2 (a) An electrically heated plate dissipates heat by convection at a rate of q = 8000 W/m² into ambient air at Tr= 25 °C. If the surface of hot plate is at Tw = 125 °C. Calculate heat transfer coefficient for convection between plate and air. [03]
(b) What is lumped System analysis? Mention the importance of Biot number in lumped system analysis. [04]
(c) Develop expression for one dimensional steady state temperature distribution T(r) in a sphere where inner surface at r = a and outer surface of r = b of hollow sphere are maintained at temperatures of Ti and To respectively. Consider that thermal conductivity (k) of sphere is constant [07]
OR
Q.2 (a) Atmospheric air at T = 400 K with a velocity us = 1.5 m/s flows over a flat plate L = 2 m long maintained at uniform temperature of Tw = 300K. Calculate average value of heat transfer coefficient and average heat transfer rate from airstream to plate over entire length of L =2m. [07]
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Q.3 (a) Write the significance of following dimensionless numbers used in heat transfer studies (i) Reynolds number (ii) Prandtl number (iii) Nusselt number [03]
(b) Differentiate velocity boundary layer and thermal boundary layer [04]
(c) Estimate the total heat loss by convection and radiation from an unlagged steam pipe, 50 mm o.d. at 415 K to air at 290K (17 °C). Emissivity e = 0.90 Film coefficient for calculation of heat loss by natural convection is given by hc = 1.18 (?T/D)0.25 W/m²K [07]
OR
Q.3 (a) Mention the factors governing the rate of heat transfer in forced and natural convection. [03]
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(b) Define Absorptivity, Reflectivity and Transmissivity [04](c) Calculate heat transfer area of 1 – 2 heat exchanger from the following data. Inlet and outlet temperatures of cold fluid are 303 K and 318 K respectively. Overall heat transfer coefficient = 4100 W/m² K. Heat loss =407 kW. LMTD correction factor = 0.84. [07]
Date: 04/12/2019
Total Marks: 70
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Q.4 (a) Why is drop wise condensation better for heat transfer than film wise condensation? [03]
(b) With neat diagrams explain classification of heat exchangers according to flow arrangement. [04]
(c) A heat exchanger is required to cool 20 kg/s of water from 360K to 340K by means of 25kg/s of water entering at 300K. If the overall heat transfer coefficient is 2000 W/m²K. calculate the surface area required in
a) A counter current concentric heat exchanger
b) A co-current flow concentric tube heat exchanger. (Take Cp of water =4.187 kJ/kg K) [07]
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OR
Q.4 (a) Discuss the phenomena of hysteresis in a boiling curve. [03]
(b) Why saturated steam is used as heating medium in industries? [04]
(c) Describe the significance of € - NTU method for heat exchange analysis. [07]
Q.5 (a) What is understood by boiling point elevation in evaporators [03]
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(b) What is Vapor recompression? Mention different methods in which vapor recompression can be done. [04](c) With help of boiling curve mention different regimes in a pool boiling. [07]
OR
Q.5 (a) Define the terms of Capacity & Economy in Evaporators [03]
(b) Explain the different methods of feeding of multiple effect evaporators. [04]
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(c) An evaporator operating at atmospheric pressure is designed to concentrate 5% solute to 20 % solute by weight at a rate of 5000 kg/h. Dry saturated steam at a pressure corresponding to saturation temperature of 399K is used. The feed is at 298 K and boiling point rise is 5 K. Overall heat transfer coefficient is 2350 W/m² K. Calculate economy of evaporator and area of heat transfer to be provided assuming that there is no boiling point elevation for the solvent.Data: Latent heat of Condensation of steam = 2185 kJ/kg
Latent heat of vaporization of water at 101.325 kPa and 373 K = 2257 kJ/kg
Specific heat of feed =4.187 kJ/ kg K [07]
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