# JNTU Question Papers-BE-III-year-Heat Transfer-Supplementary Exam-Set-III-Aug/Sep-2008

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

HEAT TRANSFER

( Common to Mechanical Engineering and Automobile Engineering)

SET-III

1. (a) The surface of steel plate measuring 0.9m long × 0.6m wide × 0.025m thick is maintained at a uniform temperature of 300 0C, and the plate loses 250 watt by radiation. If air at 15 0C temperature and 20 w/m2-deg convective heat transfer coefficient blows over the plate, calculate the temperature on inside surface of the plate. Take thermal conductivity of plate as 45w/m-deg.

(b) Derive expressions for temperature distribution during steady sate heat conduction in a solid sphere with internal heat generation.

2. (a) The rear window of an automobile is defogging by passing warm air at 40 0C over its inner surface and associated heat transfer coefficient is 30 W/m2K. The out side ambient temperature is -10 0C and the associated heat transfer coefficient 65W/m2K. Estimate the inner and outer surface temperatures of the window, if window glass (0.2 W/mK) is 4 mm thick.

(b) A solid cylinder rod of diameter 10 mm and length 150 mm is the insulated on its cylindrical surfaces. Determine the heat flow rate through the rod if k=0.78 W/mK. The temperatures of the ends of the rods are 0 0C and 100 0C respectively.

3. (a) The thermal conductivity of plane slab varies with temperature with the following relation: k = k0(1+bT+cT2). If the wall thickness is L and the surface temperatures are T1 and T2, Estimate the heat flux through the wall?

(b) Determine the heat loss per meter length through a thin walled tube of stainless steel (k=19 W/mK) with 20 mm ID and 40 mm OD which is covered with a 30 mm layer of asbestos insulation(0.2 W/mK). The inside and outside temperatures of this configuration are 600 0C and 100 0C respectively.

4. forced convection from a flat plate, the local Nusselt number is given by the follow-

ing correlations for Laminar and turbulent flow:

Laminar: NUx=0.331 Re1/2x pr1/3 Turbulent: NUx=0.0288 Re0.8 x pr1/3 The fins on an Air-cooled motor cycle Engine may be considered as individual flat plates of Length L=0.2m owing to disturbances in the free stream, transition occurs at Rex,Transition=2 × 105. Calculate, for a road speed of 140 kmph, the average heat transfer co-efficient from the fin surface allowing for separate laminar and turbulent sections and compare this with the result obtained assuming purely Turbulent flow.

Comment the result. Take ? =1.1 kg/m3; ?=1.7 × 10?5 kg/ms, K=0.026 w/mk and pr=0.7.

5. What do you understand by the hydrodynamics and thermal boundary layers.

Illustrate with reference to flow over a flat heated plate.

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) Define the terms:

i. absorptivity

ii. reflectivity and

iii. transmissivity.

(b) Differentiate between specular and diffuse reflections.

(c) Derive Stefan-Boltzmann’s law from Plank’s law.

8. (a) What is meant by “fouling” in heat exchangers mean? List the factors responsible for fouling.

(b) Water flows through a copper pipe (k=380 w/m-K) of 18mm diameter. It is surrounded by another steel pipe of 21mm and oil flow through the annular passage between copper and steel pipe. On the water side, the film coefficient is 4500 w/m2-K and the fouling factor of .00032 m2-K/W. The corresponding values for the soil side are 1250 W/m2-K and 00082 m2-K/W. Find the overall heat transfer coefficient between water and oil.