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Posted By: kishore Member Level: Gold Posted Date: 06 Jun 2008
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2007 Jawaharlal Nehru Technological University B.Tech Mechanical Engineering HEAT TRANSFER seno4 Question paper
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1. (a) Define thermal conductivity, thermal resistance and thermal conductance.
What is the approximate range of thermal conductivity of solids, liquids and
gases?
(b) List some good conductors of heat, and some poor conductors. [8+8]
2. Consider a slab of thickness L and constant thermal conductivity k in which energy
is generated at a constant rate of qW/m
2. The boundary surface at x=0 is insulated
and that at x=L dissipates heat by convection with a heat transfer coefficient into
a fluid at temperature of T . Derive expression for the temperature and heat flux
in the slab. Calculate the temperatures at the surfaces x=0 and x=L under the
following conditions. L=10mm, k=20W/mK,q=8×107 W/m
3, h = 4000 W/m
2K
and T = 1000C. [16]
3. A wall of thickness 100 mm is insulated on one side and other side is exposed to 00C.
Determine the wall temperature insulated surface if the internal heat generation in
the wall is at the rate of 106 W/m
3. Take k = 40 W/mK. [16]
4. (a) Give a general equation for the rate of heat transfer by convection.
(b) List the various factors on which the value of this coefficient depends. [10+6]
5. (a) A square plate, 40 cm x 40 cm, is at 1200C and is exposed to air at 200C. Find
the heat loss from the plate if
i. the plate is kept vertical
ii. the plate is placed horizontally. Find the percentage change in heat flow
due to the change in position.
(b) Explain the various regimes of a velocity boundary layer over a flat plate.[8+8]
6. (a) Water at 5 bar flows inside a 25mm dia tube under local boiling conditions
where the tube wall temperature is 150C above the saturation temperature.
Estimate the heat transfer in a 0.6m length of tube.
(b) A platinum wire is submerged in saturated water at 5.5MPa. What is the
heat flux for a temperature excess of 120C? [10+6]
7. (a) Define the terms
i. absorptivity
ii. reflectivity and
iii. transmissivity.
(b) Differentiate between specular and diffuse reflections.
(c) Derive Stefan-Boltzmann’s law from Plank’s law. [6+4+6]
8. A heat exchanger (17.2 m
2) is used to cool oil at 2000C by water available at
200C . The mass flow and sp.heat of oil are 10000 kg/hr and 1. 9 kJ/kg-K and
corresponding values for water are 300W/m
2-K. Find the outlet temperature of oil
and water for parallel flow and counter flow arrangements using LMTD method
and (2) NTU method. [16]
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