Heat Transfer

171. Heat transfer co-efficient equation for forced convection, Nu = 0.023R_e ^0.8. P_r ⁿ, is not valid, if the value of

1. n = 0.4 is used for heating
2. n = 0.3 is used for cooling
3. Reynolds number for the flow involved is > 10000
4. Reynolds number for the flow involved is < 2100

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172. Heat transfer efficiency leading of energy conservation in a heat exchanger can be achieved by

1. Keeping the heat transfer surface clean
2. Enhancing the fluid pumping rate
3. Increasing the tube length
4. None of these

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173. Heat transfer in the laminar sub-layer in case of a liquid flowing through a pipe, is mostly by

1. Eddies current
2. Conduction
3. Convection
4. None of these

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174. Heat transfer occurs by natural convection because change in temperature causes difference in

1. Viscosity
2. Density
3. Thermal conductivity
4. Heat capacity

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175. Heat transfer rate described by Fourier's law will decrease, if the _________ increases.

1. Thermal conductivity
2. Thickness
3. Temperature difference
4. Heat transfer area

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176. Heat transfer rate per unit area is called

1. Thermal conductivity
2. Heat flux
3. Heat transfer co-efficient
4. Thermal diffusivity

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177. Heat waves

1. Cannot pass through vacuum
2. Travel in straight line
3. Can be reflected by a mirror
4. Both (B) and (C)

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178. Hot water (0.01 m³ /min) enters the tube side of a counter current shell and tube heat exchanger at 80°C and leaves at 50°C. Cold oil (0.05 m³/min) of density 800 kg/m³ and specific heat of 2 kJ/kg.K enters at 20°C. The log mean temperature difference in °C is approximately

1. 32
2. 37
3. 45
4. 50

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179. If a single tube pass heat exchanger is converted to two pass, then for the same flow rate, the pressure drop per unit length in tube side will

1. Increase by 1.8 times
2. Decrease by 22
3. Increase by 216
4. Remain unchanged

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180. If air (a non-condensing gas) is present in a condensing vapor stream, it will _________ the condensation rate of vapor.

1. Increase
2. Decrease
3. Not affect
4. Increase the condensing film co-efficient as well as

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