Download JNTUA M.Tech 1st Sem 2018 Aug-Sept 9D17102 Conduction And Radiation Heat Transfer Question Paper

M.Tech I Semester Supplementary Examinations August/September 2018

CONDUCTION & RADIATION HEAT TRANSFER
(Refrigeration & Air Conditioning)

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Time: 3 hours
Answer any FIVE questions
All questions carry equal marks
Max. Marks: 60

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1. (a) Differentiate between steady and transient heat conduction.
   (b) Calculate the rate of heat loss for a red brick wall of length 5 m, height 4 m and thickness 0.25 m. The temperature of the inner surface is 110°C and that of the outer surface is 40°C. The thermal conductivity of the red brick, k = 0.70 W/mK. Calculate also the temperature at an interior point of the wall at a distance of 20 cm from the inner wall.
2. (a) Drive an expression for heat transfer through a plane and composite walls.
   

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   (b) A 40 x 40 cm copper slab 5 mm thick at a uniform temperature of 250°C suddenly has its surface temperature lowered at 30°C. Find the time at which the slab temperature becomes 90°C. Take p = 9000 kg/m³, C = 0.38 kJ/kg K, K = 370 W/mK and h = 90 W/m² K.
3. (a) What is meant by lumped heat analysis?
   (b) Aluminum sphere weighing 5.5 kg and initially at a temperature of 290°C is suddenly immersed in a fluid at 15°C. The convective heat transfer coefficient is 58 W/m²K. Estimate the time required to cool the aluminum to 95°C, using lumped capacity method of analysis.
4. (a) What is Heisler chart?
   (b) A surface wall consists of 23 cm of fire brick and 11.5 cm of insulating brick having thermal conductivities of 0.72 W/mK and 0.27 W/mK respectively. Calculate the rate of heat lost per square meter when the temperature difference between inner and outer surface is 650 K.

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5. (a) What are Biot and Fourier numbers? Explain their physical significance.
   (b) The inner surface temperature of an annealing oven varies according to a sine function from 800°C to 200°C. Each complete cycle requires 12 hours, compute:
   (i) Time lag of the temperature wave at a depth of 10 cm from the inner surface.
   (ii) The heat flow through a surface located at a distance of 10 cm from the surface during the first six hours interval while the temperature is above the mean value. Assumea = 0.02 m²/h; K = 1.8 W/mK.
6. (a) Derive the expression for radiation exchange between small gray bodies.
   

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   (b) Emissivities of two large parallel planes maintained at 800°C and 300°C are 0.3 and 0.5 respectively. Find the net radiant heat exchange per square meter for these plates.
7. (a) Distinguish between the black body and grey body.
   (b) Two parallel plates of size 0.5 m x 1 m are placed parallel to each other at a distance of 0.5 m. One plate is maintained at a temperature of 1000°C and other at 500°C and the emissivities are 0.2 and 0.5 respectively. The plates are located in a large room whose walls are at 27°C. If the plates exchange heat with each other and with the room, but only plate surface facing each other are to be consider in the analysis. Calculate: (i) Heat lost by the plates. (ii) Heat gain by the room.
8. (a) Determine the radiation heat transfer per unit length between two long concentric cylinders of radius 0.5 m and 0.6 m with emissivity values of 0.6 and 0.4 respectively. The inner cylinder is at 600°C while the outer is at 300°C.
   (b) Derive the radiation heat transfer coefficient

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