Subject Code: 161906
GUJARAT TECHNOLOGICAL UNIVERSITY
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BE - SEMESTER-VI (OLD) EXAMINATION - WINTER 2018
Subject Name: Heat And Mass Transfer
Time: 02:30 PM TO 05:00 PM
Instructions:
- Attempt all questions.
- Make suitable assumptions wherever necessary.
- Figures to the right indicate full marks.
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Q1 (a) Derive the expression for heat conduction through composite wall using electrical analogy. [07]
(b) Find the shape factor F1-1, F1-2 and F2-1 for the following geometries. [07]
- A black body inside a black enclosure
- A tube whose section is equilateral triangle
- Radiation exchange between a hemisphere and a plane surface.
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Q2 (a) 0.5 cm thick glass (k=0.78W/m. K) is exposed to air at 25° C (inner side) with convection heat transfer coefficient of 15 W/m2 K. The outside air is at -15°C with convection coefficient of 50 W/m2 K. Determine (1) temperatures at inner and outer glass surfaces (2) heat transfer rate. [07]
(b) What is critical radius of insulation? Derive the expression for condition for minimum resistance for maximum heat flow rate for solid cylinder under one dimensional steady state heat conduction. [07]
OR
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Q2 (a) A circular shaft (k= 50W/m K) 60 mm diameter having 60° C rise in temperature due to friction. The heat transfer coefficient is 6.5W/m2 K. Determine (1) the expression for temperature distribution (2) the amount of heat dissipated through the shaft. Assume the shaft is a rod of infinite length. [07]
(b) A polished metal pipe 5 cm outside diameter and 370 K temperature at the outer surface is exposed to ambient conditions at 295 K temperature. The emissivity of the surface is 0.2 and the convection coefficient of heat transfer is 11.35 W/m2 -deg. Calculate the heat transfer by radiation and natural convection per meter length of pipe. Take thermal radiation constant s = 5.67 x 10-8 W/m2 K4. What would be the overall coefficient of heat transfer by combined mode of convection and radiation? [07]
Q3 (a) Derive an expression for LMTD for counterflow heat exchanger. [07]
(b) OR A heat treated steel plate measures 3 m x 1 m and is initially at 30° C. it is cooled by blowing air parallel to 1 edge at 9 kmph. If the air is at 10°C, calculate the convective heat transfer from both sides of the plate. Take the correlation Nu= 0.664 (Re)0.5 (Pr)0.33 [07]
Q3 (a) Derive the expression for effectiveness of parallel flow heat exchanger. [07]
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FirstRanker.com [07]
Date: 13/12/2018
Total Marks: 70
Q4 (a) State and explain Wien’s displacement law and define Lambert’s cosine law of radiation [07]
(b) Discuss the concept of thermal boundary layer in case of flow over the plates. How it differs from velocity boundary? [07]
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OR
Q4 (a) What is meant by a lumped capacity? What are the physical dimensions necessary for a lumped unsteady state analysis to apply? [07]
(b) State and explain Fick’s law of diffusion and compare it with Fourier’s law of heat conduction [07]
Q5 (a) A steam condenser is transferring 250 kW of thermal energy at a condensing temperature of 65°C. The cooling water enters the condenser at 20°C with a flow rate of 7500kg/hr. calculate the log mean temperature difference. If the overall heat transfer coefficient for the condenser surface is 1250 W/ m2-deg, what surface area is required to handle this load? [07]
(b) By dimensional analysis show that for forced convection heat transfer the Nusselt number can be expressed as a function of Prandtl number and Reynolds number. [07]
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OR
Q5 (a) A counterflow heat concentric tube heat exchanger is used to cool the lubricating oil of a large industrial gas turbine engine. The oil flows through the tube at 0.19 kg/s (Cp = 2.18 kJ/kg K), and the coolant water flows in the annulus in the opposite direction at a rate of 0.15 kg/s (Cp =4.18 kJ/kg K). The oil enters the coolant at 425K and leaves at 345 K while the coolant enters at 285 K. How long must the tube be made to perform this duty if the heat transfer coefficient from oil to tube surface is 2250 W/m2 K and form tube surface to water is 5650W/m2? The tube has a mean diameter of 12.5 mm and its wall presents negligible resistance to heat transfer [07]
(b) Define pool boiling. Draw pool boiling curve for water and explain various regimes of the curve. [07]
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