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

HEAT TRANSFER

( Common to Mechanical Engineering and Automobile Engineering)

SET-III

1. (a) What is Fourier‘s law of heat conduction? Explain.

(b) A brick (k=1.2 W/m K) wall 0.15 m thick separates hot combusition gases of a furnace from the outside ambient air which is at 25 0C. The outer surface temperature of the brick wall is found to be 100 0C. If the natural convection heat heat transfer coefficient on the brick wall is 20 W/m2 K and its emissivity is 0.8, calculate the inner surface temperature of the brick wall.

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) What are Biot and Fourier numbers? Explain their physical significance.

(b) A slab of Aluminum 10cm thick is originally at a temperature of 5000C. It is suddenly immersed in a liquid at 1000C resulting it a heat transfer coefficient of 1200 W/m2k. Determine the temperature at the centerline and the surface 1 min after the immersion. Also the total thermal energy removal per unit area slab during this period. The properties of aluminum for the given condition are: ? = 8.4 × 10?5m2 /s, K=215 W/mk, ? = 2700 kg/m3, Cp= 0.9 kJ/kg.

4. (a) Explain for fluid flow along a flat plate.

i. Velocity distribution in hydrodynamic boundary layer.

ii. Temperature distribution in thermal boundary layer.

iii. Variation of local heat transfer co-efficient along the flow.

(b) Under forced flow conditions how does the prandtl number affect relative thickness of thermal Boundary layer and velocity Boundary layer. Show that the velocity and temperature distributions within the Boundary layer are going to be similar in nature.

5. (a) Why is an analytical solution of a free convection problem more involved than its forced convection counterpart?

(b) A vertical plate is at 96oC in an atmosphere of air at 20oC. Estimate the local heat transfer coefficient at a distance of 20 cm from the lower edge and the average value over the 20 cm length.

6. (a) What is nucleate boiling? Why is it important?

(b) Saturated water at 100oC is boiled with a copper heating element having a heating surface of area 0.04 m2 which is maintained at 15oC. Calculate the surface heat flux and the rate of evapoaration.

7. (a) What do you mean by fouling factor? What are the causes of fouling?

(b) A double pipe heat exchanger is constructed of a stainless steel ( k = 15.1W/mK) inner tube of Di = 15 mm and Do = 19 mm and the outer tube of diameter 32 mm. The convective heat transfer coefficient is given to be hi = 800W/m2K and ho = 1200W/m2K. For a fouling factor of Rfi =0.0004m2K/W on the tube side and Rfo = 0.0001m2K/W on the shell side, determine

i. The total thermal resistance

ii. Ui and

iii. Uo of the heat exchanger.

8. A spherical satellite of 1 m diameter encircles the earth at an altitude of 483 km. Estimate the shape factor of the earth from the satellite and hence calculate the equilibrium temperature of the satellite on the dark side and on the bright side of the earth. Assume that the diameter of the earth is 12880 km and its black body temperature is 20oC. The temperature of outer space may be taken as 0 K and the satellite is irradiated with a heat flux of 1.3 kW/m2 from the Sun when on the bright side.