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B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2010
ME2251 — HEAT AND MASS TRANSFER
Time: Three hours Maximum: 100 Marks
Heat and mass transfer tables, steam tables are permitted for use.
Answer ALL Questions
PART A — (10 × 2 = 20 Marks)
1. Write the Poisson's equation for heat conduction.
2. What is lumped heat capacity analysis?
3. Define thermal boundary layer thickness.
4. What do you understand by free and forced convection?
5. What is effectiveness of a heat exchange?
6. Give the expression for NTU.
7. Find the temperature of the sun assuming as a black body, if the intensity of
radiation is maximum at the wave length of 0.5µ .
8. State Kirchhoff's law.
9. Define molar concentration.
10. What is mass average velocity?
PART B — (5 × 16 = 80 Marks)
11. (a) Derive the general heat conduction equation in cylindrical coordinates. (Marks 16)
(b) Derive the general heat conduction equation for a hollow cylinder. (Marks 16)
12. (a) Air at 20°C at 3m/s flows over a thin plate of 2m long and 1m wide. Estimate the boundary layer thickness at the trailing edge, total drag force, mass flow of air between x =30cm and x =80cm. Take 6 ? = 15 ×10 and 3 ? =1.17kg/m . (Marks 16)
(b) Calculate the convective heat transfer from a radiator 0.5m wide and 1m high at 84°C in a room at 20°C. Treat the radiator as a vertical plate. (Marks 16)
13. (a) Dry steam at 2.45 bar condenses on a vertical tube of height of 1m at 117°C. Estimate the thickness of the condensate film and the local heat transfer coefficient at a distance 0.2 m from the upper end of the plate. (Marks 16)
(b) Derive the LMTD for a parallel flow heat exchanger stating the assumptions. (Marks 16)
14. (a) Derive the radiation exchange between
(i) Large parallel gray surfaces and
(ii) Small gray bodies. (Marks 16)
(b) Two large parallel plates of 1m×1m spaced 0.5m apart in a very large room whose walls are at 27°C. The plates are at 900°C and 400°C with emissivities 0.2 and 0.5 respectively. Find the net heat transfer to each plate and to the room. (Marks 16)
15. (a) The temperature recorded by a thermometer whose bulb covered by a wet wick in dry air at atmospheric pressure is 22°C. Estimate the true air temperature. (Marks 16)
(b) Dry air at 27°C and 1 bar flows over a wet plate of 50cm at 50m/s. Calculate the mass transfer coefficient of water vapor in air at the end of the plate. (Marks 16)