JNTU III B.Tech II Semester Supplimentary Examinations, Aug/Sep 2008

HEAT TRANSFER

(Common to Mechanical Engineering and Automobile Engineering)

SET-I

1. (a) Explain the scope of the study of heat transfer.

(b) A cubical tank of water of volume 1 m3 is kept at a steady temperature of 650C by a 1 kW heater. The heater us switched off. How long does the tank take to cool to 50 0C if the room temperatue is 15 0C.

2. (a) Sketch various types of fin configurations?

(b) Aluminum fins of rectangular profile are attached on a plane wall with 5 mm spacing. The fins have thickness 1 mm, length = 10 mm and the normal conductivity K = 200 W/mk. The wall is maintained at a temperature of 2000C and the fins dissipate heat by convection into ambient air at 400C, with heat transfer coefficient = 50 W/m2k. Find the heat loss.

3. (a) A long 20cm diameter cylindrical shaft made of stainless steel 304 comes out

of an oven at a uniform temperature of 6000C. The shaft is then allowed to cool lowly in an environment chamber at 2000C with an average heat transfer coefficient of h = 80 W/m2 0C. Determine the temperature at he centre of the shaft 45 min. after the start of the cooling process. Also, determine the heat transfer per unit length of the shaft during this time period.

(b) What are Biot number, Fourier number and Heisler charts?

4. (a) Define the local and average skin friction (drag) coefficient for a flat smooth plate at zero incidence.

(b) A thin flat plate has been placed longitudinally in a steam of air at 200C and while flows with undisturbed velocity of 7.5 m/s. The surface of plate is maintained at a uniform temperature of 1200C.

i. calculate the heat transfer coefficient 0.8m from the leading edge of the plate,

ii. Also calculate the rate of heat transfer from one side of the plate to the air over the first 0.8 m length. Assume unit width of the plate.

5. (a) Derive an expression for the maximum velocity in the free convection boundary

layer on a vertical plate. At what position in the boundary layer does this maximum velocity occur?

(b) A horizontal pipe of 6 cm diameter is located in a room, whose temperature of air is 200C. The surface temperature of the pipe is 1400C. Calculate the free convection heat loss per meter length of the pipe.

6. (a) Explain how the condensate film thickness on a vertical plate is influenced by

different parameters?

(b) Saturated steam at 54oC condenses as a film on the outside surface of a 25mm

OD 3m long vertical tube. The tube surface is maintained at temperature of 43oC. Calculate the average condensation heat transfer coefficient and the rate of steam condensation at the bottom of the tube.

7. (a) Give comparison of parallel flow and counter flow heat exchangers. Why are

counter flow heat exchangers mostly used?

(b) A heat exchanger is required to cool 55000 kg/h of alcohol from 66oC to 40Oc using 40000 kg/h of water entering at 5oC. Calculate the surface area required for

i. parallel flow

ii. counter flow mode. Take U = 580 W/m2K, cp for water = 4180 J/kgK, cp for alcohol = 3760 J/kgK.

8. (a) What are the factors that influence the radiant heat exchange between two bodies?

(b) Two equal discs of diameter 200 mm each are arranged in two parallel planes 400 m apart. The temperature of the first disc is 500oC and that of the second disc is 300oC. Determine the radiating heat flux between them, if these are

i. Black

ii. Gray with emissivities 0.3 and 0.5 respectively.