Heat Transfer

361. The Fourier number (defined as a.t/L2) is used in the analysis of problem involving heat transfer by

1. Forced convection
2. Natural convection
3. Transient conduction
4. Steady state conduction

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362. The Graetz number is concerned with the

1. Mass transfer between a gas and a liquid
2. Absorption with chemical reaction
3. Heat transfer in turbulent flow
4. Heat transfer in laminar flow

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363. The heat flux (from outside to inside) across an insulating wall with thermal conductivity, K = 0.04 W/m.°K and thickness 0.16m is 10 W/m². The temperature of the inside wall is - 5°C. The outside wall temperature is

1. 25°C
2. 30°C
3. 35°C
4. 40°C

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364. The heat flux in the nucleate boiling regimes is proportional to (where, ΔT = excess temperature)

1. (ΔT)²
2. (ΔT)⁴
3. (ΔT)³
4. √(ΔT)

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365. The heat transfer by radiation from a mild steel surface is to be reduced by reducing the emissivity of the surface. This can be best achieved by

1. Painting the surface black
2. Painting the surface white (with aluminium paint)
3. Giving the surface a mirror finish
4. Roughening the surface

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366. The heat transfer co-efficient in film type condensation is _________ that for dropwise condensation.

1. Greater than
2. Lower than
3. Is same as
4. Half

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367. The inner wall of a furnace is at a temperature of 700°C. The composite wall is made of two substances, 10 and 20 cm thick with thermal conductivities of 0.05 and 0.1 W.m^-1.°C- 1 respectively. The ambient air is at 30°C and the heat transfer co-efficient between the outer surface of wall and air is 20 W.m^-2.°C^-1. The rate of heat loss from the outer surface in W.m^-2is

1. 165.4
2. 167.5
3. 172.5
4. 175

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368. The inside heat transfer co-efficient in case of turbulent flow of liquid in the tube side in a 1-2 shell and tube heat exchanger is increased by _________ times, when the number of tube passes is increased to 8.

1. 2^0.8
2. 4^0.8
3. 4^0.4
4. 2^0.4

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369. The interchange factor for radiation heat transfer from surface 'x' to surface 'y' in case of an infinite parallel planes with emissivities ε_x & ε_y is given by

1. ε_x + ε_y
2. ε_x . ε_y
3. 1/ε_x + 1/ε_y
4. (ε_x + ε_y)/( ε_x + ε_y - ε_x . ε_y)

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370. The left face of a one dimensional slab of thickness 0.2 m is maintained at 80°C and the right face is exposed to air at 30°C. The thermal conductivity of the slab is 1.2 W/m.K and the heat transfer co-efficient from the right face is 10 W/m².K. At steady state, the temperature of the right face in °C is

1. 77.2
2. 71.2
3. 63.8
4. 48.7

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