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Heat Transfer

(125101 / 215101)P. Pages : 3

Time : Three Hours Max. Marks : 80

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Instructions to Candidates :1. Do not write anything on question paper except Seat No.2. Graph or diagram should be drawn with the black ink pen being

used for writing paper or black HB pencil.3. Students should note, no supplement will be provided.4. All questions are compulsory and solve any twoout of a, b, c in

each question.5. Use of heat transfer data book and non-programmable calculator

is allowed.6. Assume suitable data if required.7. Neat diagrams must be drawn whenever necessary.

1. a) Derive the equation for steady state heat conduction throughcylinder of uniform conductivity without heat generation.

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b) Calculate the rate of heat flow per m2through a furnace wall

consisting of 200 mm thick inner layer of chrome brick, a centrelayer of Kaoline brick 100 mm thick and an outer layer of masonrybrick 100 mm thick. The unit surface conductance at the innersurface is 74 w/m2C and the outer surface temperature is 70C.The temperature of the gases inside the furnace is 1670C whattemperature prevail at the inner and outer surfaces of the centre

layer ? Take, ,Cm/w25.1K brickchrome Cm/w074.0K brickkaolin

Cm/w555.0K brickmansory Assume steady heat flow.

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c) Explain significance of critical radius of insulation ? A pipe of outerdiameter is 20 mm is to be insulated with asbestos which has mean

thermal conductivity of Cm/w1.0 . The convective heat transfer

coefficient is Cm/w5 20 comment upon the suitabilit y of asbestos as

the insulating material. What should be the maximum value ofthermal conductivity of insulating material.

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*5780*Seat Number

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2. a) Prove that the maximum temperature for a cylindrical surface is.

2ggamax R

k4

qR

h2

qTT

The meat rolls of 25 mm diameter having Cm/w1K are heated up

with the help of microwave heating for roasting. The centretemperature of the rolls in maintained at 100C when thesurrounding temperature is 30C. The heat transfer coefficient on

the surface of the meat roll is .Cm/w20 20 Find the microwave

heating capacity required in 3m/w .

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b) Derive an expression for temperature distribution & heatdissipation in fin insulated at tip of rectangular profile.

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c) A steel tube carries steam at a temperature of 320C. A thermometer

pocket of iron [K=52.3 W/mC] of inside diameter 15 mm & 1 mmthick is used to measure the temperature. The error to be toleratedis 1.5% of maximum. Estimate the length of the pocket necessary tomeasure the temperature within this error. The diameter of steel

tube is 95 mm. Assume Cm/w93h 20 and tube wall temperature is

120C. Suggest a suitable method of locating the thermometerpocket.

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3. a) Show by dimensional analysis for forced convection Pr][Re,Nu . 8

b) A cylindrical body of 300 mm diameter and 1.6 m height ismaintained at a constant temperature of 36.5C. The surroundingtemperature is 13.5C. Find out the amount of heat to be

generated by the body per hour if ,Ckg/kJ96.0C,m/kg025.1 p3

126 k298

1&Chm/kJ0892.0k,s/m1006.15 . Assume

31Pr]Gr[12.0Nu .

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c) Discuss :

i) Filmwise and dropwise condensation .

ii) Pool boiling phenomenon.

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4. a) Define :

i) The stefan-Boltzmann law.

ii) Kirchhoff's law.

iii) Planck's law.

iv) Wiens displacement law.

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b) Define Radiation shield & Derive.

]Q[1h

1Q Shieldwithoutshieldsnwith

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c) Net radiation from the surfaces of two large plates maintained attemperature of 73C & 23C is to be reduced by 105 times. Calculatethe number of radiation screens to be placed between them to

achieve this effect. Assume that the emissivity of screen 05.0S &

that of surface 8.021 After this if one of the screen is removed

how many times will the net radiation be reduced ?

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5. a) Prove that effectiveness of counter flow heat exchanger is

NTU1

NTU

.

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b) A shell and tube heat exchanger is to be provided with tubes 4 mlong, 3.1 cm. 0.D and 2.7 cm I. D. The heat exchanger is requiredfor heating water from 22C to 45C with the help of condensingsteam at 100C on the outside of tubes. Calculate the number oftubes required if the water flow rate through tubes is 10 kg/s andthe heat transfer coefficients on vapour & water side are 5500 & 850

w/m2k resp. Neglect all other resistances Assume kkg/J4186cwp

Also ca lculate rate o f steam consumption if latent heat is 2200kJ/kg.

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c) Write short note on:

i) Design Aspects of heat exchanger.

ii) Requirements of good heat exchanger.

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