JNTU Question papers-Mechanical Engineering-III-Year-Heat transfer-Set-IV-Aug/Sep-2008

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

HEAT TRANSFER

( Common to Mechanical Engineering and Automobile Engineering)

SET-IV

 

1. (a) Distinguish between conduction, convection and radiation modes of heat transfer.

(b) A solar pane, 1 m × 1.25m receives solar radiation 1500 watts, Calculate surface temperature of the pane if the ambient temperature is 25 0C and the convective heat transfer coefficient of the air film over the surface of pane is

12.5 W/m2-deg.

 

2. Consider a slab of thickness L and constant thermal conductivity k in which energy

is generated at a constant rate of qW/m2. The boundary surface at x=0 is insulated and that at x=L dissipates heat by convection with a heat transfer coefficient into a fluid at temperature of T_. Derive expression for the temperature and heat flux in the slab. Calculate the temperatures at the surfaces x=0 and x=L under the following conditions. L=10mm, k=20W/mK,q=8×107 W/m3, h = 4000 W/m2K and T_ = 100 0C.

 

3. (a) Derive the expression for temperature distribution with solid slab with heat generation of 1. Both surface temperatures of the slab are Tw K and at the center is T0 K.

(b) A long cylinder rod of radius 50 cm with thermal conductivity of 10 W/Mk contains radioactive material, which generates heat uniformly within the cylinder at rate of 3×105 W/m3. The rod is cooled by convection from its cylindrical surface into the ambient air at T_ = 50 0C with a heat transfer coefficient of 60 W/m2K. Determine the temperature at the end center and at the outer surface of the cylindrical rod?

 

4. (a) Give a general equation for the rate of heat transfer by convection.

(b) List the various factors on which the value of this coefficient depends.

 

5. (a) A thin 80 cm long and 8 cm wide horizontal plate is maintained at a temperature of 130 0C in a large tank full of water at 70 0C. Estimate the rate of heat input into the plate necessary to maintain the temperature of 130 0C.

(b) Differentiate velocity and thermal Boundary layers by a neat diagram.

 

6. (a) Distinguish between:

i. Subcooled boiling & Saturated boiling

ii. Nucleate boiling & film boiling.

(b) Water at atmospheric pressure is boiled in a Kettle made of copper. The bottom of the Kettle is flat, 35 cm in diameter and maintained at a temperature of 1150C by an electric heater. Calculate the rate of heat required to boil water. Also estimate the rate of evaporation of water from the Kettle.

 

7. (a) A black body is kept at a temperature of 1000k. Determine the fraction of thermal radiation emitted by the surface in the wavelength band 1.0 to 6.0?.

(b) Estimate the rate of solar radiation on a plate normal to the sun rays. Assume the sun to be a black body at a temperature of 5527 0C. The diameter of the sun is 1.39 × 106 km and its distance from the earth is 1.5 × 108 km.

 

8. Cold water at 1495 kg/hr enters at 25 0C through a parallel flow heat exchanger to cool 605 kg/hr of hot water entering at 70 0C and leaving at 50 0C. Find the area of heat exchanger the individual heat transfer coefficients on both sides are 1590W/m2-K. Use LMTD and NTU methods. Find also the exit temperature of cold and hot streams if the flow of hot water is doubled. Assume the individual heat transfer coefficient are proportional to 0.8 th power of flow rate. For water Cp =4180 J/kg-K.

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