B.Tech IV (Fourth) Semester Examination 2012-13
Course Code: ECH401 Paper ID: 0964103
Heat Transfer
Time: 3 Hours Max. Marks: 70 Max Marks: 75
Note: Attempt six questions in all. Q. No. 1 is compulsory.
1. Answer any five of the following (limit your answer to 50 words). (4x5=20)
a) Why is heat transfer coefficient in nucleate boiling higher than is film boiling?
b) Why does a drop of water put on a hot surface bounce up and down several times before being completely vaporized?
c) How does radiation play an important role in boiling heat transfer?
d) Can a bubble collapse in a superheated liquid? Explain.
e) How does the presence of non condensable gases affect the rate of condensation of a vapor?
f) What is critical thickness of insulation? Explain.
g) What is contact resistance? Explain.
h) What approach is used if the inlet and outlet temperature of fluid in heat exchanges in not known? Why can’t we evaluate these temperatures and use LMTD approach? What is the difficulty involved?
2. The walls of a house in a cold region consist of three layers an outer Brick of 15 cm thickness and an inner wooden panel of 1.2 cm thickness. The intermediate layer is made of an insulating material 07 cm thick. The thermal conductivities of brick and wood used are 0.70 W/M0C and 0.18 W/M0C respectively. The inside and outside temperatures of the composite wall are 210C and -150C respectively. The layer of insulation offers twice the thermal resistence of the brick wall. Calculate (a) the rate of the heat loss per unit area of the wall. (b) Thermal conductivity of insulating material. (10)
3. An aluminum sphere weighing 5.5 kg and initially at a temperature of 290 0C is suddenly immersed in a fluid at 15 0C. The convective heat transfer coefficient is 58 W/M2K. Estimate the time required to cool the aluminium to 95 0C., using lumped Capacity method of analysis is considering internal resistance of sphere is negligible. (10)
4. Define the following Dimensionless Numbers and explain their significance in forced convection: (10)
a) Nusselt Number
b) Reynold Number
c) Prandtl Number
5. A vertical square plate, 30 by 30 is exposed to steam at atmospheric pressure. The plate temperature is 98oC. Calculate the heat transfer and the mass of steam condensed. (10)
6. What are the different boiling regimes? Define critical heat flux. (10)
7. a) Explain the concept of mean temperature difference in heat exchange calculation. (5)
b) How are heat exchangers classified? Sketch the temperature variation in parallel and counter flow heat exchangers. (5)
8. a) Write a short notes on extended surface heat exchange equipments. (5)
b) How is L.M.T.D? Defined, Derive its equation. What are the basic concepts in deriving the L.M.T.D. equation? (5)