JNTU III B.Tech II Semester Supplimentary Examinations, Aug/Sep 2008
( Common to Mechanical Engineering and Automobile Engineering)
1. (a) Discuss the different modes by which heat be transferred. Give suitable example to illustrate your answer.
(b) Write the fourier rate equation for heat transfer by conduction. Give the units and physical significance of each term appearing in this equation.
2. A composite slab consists of 250 mm fire clay brick (k=1.09 W/mK) inside, 100 mm fired earth brick(0.26 W/mK) and outer layer of common brick (0.6 W/mK) of thickness 50 mm. If inside surface is at 1200 0C and outside surface is at 100 0C,
(a) heat flux,
(b) the temperature of the junctions and
(c) the temperature at 200 mm from the outer surface of the wall.
3. (a) Define the term overall heat transfer coefficient? And explain its significance.
(b) An aluminium fin (200 W/mK) of 3 mm thick and 75 mm long protrudes from a wall at 300 0C. The ambient temperature is 50 0C with heat transfer coefficient of 10 W/m2K. Calculate the heat loss from the pin for unit depth of material. Also calculate its effectiveness and efficiency?
4. (a) A flat electrical heater of 0.4 m × 0.4 m size is placed vertically in still air at 20 0C. The heat generated is 1200 w/m2. Determine the value of convective heat transfer co-efficient and the average plate temperature.
(b) Explain Grashoff number significance in natural convective heat transfer.
5. (a) Describe the relation between fluid friction and heat transfer. (b) A flat plate of length 2m, with width 1.5 m was placed along the path of air stream at velocity of 5.6 m/s. Determine the type of flow along the plate. Temperature of air was 25 0C.
6. (a) Distinguish between filmwise and dropwise condensation. Which of the two gives a higher heat transfer coefficient? Why?
(b) Dry saturated steam at a pressure of 2.5 bar condenses on the surface of a vertical tube of height 1.5m. The tube surface temperature is 120 0C. Estimate the thickness of the condensate film and the local heat transfer coefficient at a distance of 0.3m from the upper end of the tube.
7. A vertical plate 0.6 m high and 0.3 m vide is maintained at a temperature of 93 0C in a room where the air is 20 0C and 1 bar. The walls of the rooms are also at 200C. Assume that ? = 0.7 for the plate. How much heat is lost by the pipe through radiation?
8. A hot liquid of specific heat of 0.86 flowing at the rate of 0.833 kg/sec enters a parallel flow heat exchanger at 100 0C Water at the rate of 1.4 kg/sec is circulated for cooling of the hot liquid. The inlet temperature of water is 10m2 and overall heat exchanger coefficient is 1163 W/m-k. calculate Outlet temperature of water and hot liquid. Effectiveness of heat exchanger. If the flow quantities and temperature drops for water and hot liquid are maintained with a counter flow heat exchanger, Then find the area required for counter flow. Find also the maximum drop in temperature of heat fluid possible with parallel flow arrangement.