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Reg No.:Name:

APJ ABDUL KALAM TECHNOLOGICAL UNIVERSITY
SIXTH SEMESTER B.TECH DEGREE EXAMINATION, APRIL 2018

course Code: ME 302
Course Name: HEAT AND MASS TRANSFER (MA, ME, AU)

Max. Marks: IOO Duration: 3 Hours
(Use ofheat and mass transfer data book permiited.)
PART A
Answer any threefull questions, each carries 10 marks. Marks

I a) Derive the general heat conduction equation in cylindrical coordinates. (6) b) Explain the
significance of critical radius of insulation. (4)

2 A hollow aluminium sphere, with an electrical heater in the centre, is used in (10) tests to
determine the thermal conductivity of insulating materials. The inner and outer radii of the
sphere are 0.15 and 0.18 m, respectively, and testing is done under steady-state conditions
with inner surface of the aluminium maintained at 2500C In a particular test, a spherical shell
of insulation is cast on the outer surface of the sphere to a thickness om.12 m. The system is
in a room for which the air temperature is 20°C and the convection coefficient at the outer
surface of the insulation is 30 Wm2 K. If 80 W are dissipated by the heater under steady-state
conditions, what is the thermal conductivity of the insulation?

3 a) Discuss the importance of non-dimensional numbers in heat transfer problems. (3) b) A
square (10 nun x 10 mm) silicon chip is insulated on one side and cooled on (7) the opposite
side by atmospheric air in parallel flow at = 20 m/s and Tto= 24°C. When in use, electrical
power dissipation within the chip maintains a uniform heat flux at the cooled surface. If the
chip temperature may not exceed 80° C at any point on its surface, what is the maximum
allowable power?.

4 A vertical plate 1m wide and 20 cm high is at a temperature of 100 °c and (10) rests in still
air at I bar and 20 °C. Determine (i) the boundary layer thickness at 10 cm from the leading
edge. (11) the local heat transfer coefficient at 100/11 from the leading edge and (iii) the average

heat coefficient over the entire length of the plate.

PART 8
Answer any threefull questions, each carries ZO marks.

5 Steel balls 12 mm in diameter are annealed by heating tol 150 K and then (10) slowly
cooling to 400 K in an air environment for which temperature Tw = 325 K and heat
transfer coefficient, h —20 W/m? K. Assume the properties of the steel to be the
following. Thermal conductivity as 40W/mK, density is 7800 kg/m’, and specific heat
is 600 .Vkg K, estimate the time required for the cooling process.

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