Heat Transfer

121. For a given ambient air temperature with increase in the thickness of insulation of a hot cylindrical pipe, the rate of heat loss from the surface would

  1. Decrease
  2. Increase
  3. First decrease and then increase
  4. First increase and then decrease
Correct answer: (D)
First increase and then decrease

122. For a laminar flow of fluid in a circular tube, 'h1' is the convective heat transfer co-efficient at velocity 'V1'. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is

  1. 1.26 h1
  2. 0.794 h1
  3. 0.574 h1
  4. 1.741 h1
Correct answer: (B)

0.794 h1

123. For a liquid in laminar flow through a very long tube, when the exit fluid temperature approaches the wall temperature, the equation to be used is

  1. Nu = 0.023 Re0.8.4
  2. Nu = (π/2) Gz
  3. Nu = (2/π) Gz
  4. Nu = 2Gz0.5
Correct answer: (C)
Nu = (2/π) Gz

124. For a multipass shell and tube heat exchanger, the LMTD correction factor is always

  1. 1
  2. > 1
  3. < 1
  4. Between 1 & 2
Correct answer: (C)
< 1

125. For a perfectly transparent surface (like gases), the

  1. Absorptivity = 0
  2. Transmissivity = 1
  3. Reflectivity = 0
  4. All (A), (B) & (C)
Correct answer: (B)
Transmissivity = 1

126. For an ideal black body

  1. Absorptivity = 1
  2. Reflectivity = 1
  3. Emissivity = 0
  4. Transmissivity = 1
Correct answer: (A)
Absorptivity = 1

127. For concentrating an aqueous solution of a material like anhydrous Na2SO4, whose solubility decreases with rise in temperature, the most suitable evaporator is a _________ evaporator.

  1. High pressure
  2. Vacuum
  3. Backward feed
  4. None of these
Correct answer: (B)
Vacuum

128. For condensation of pure vapors, if the heat transfer co-efficients in filmwise and drop-wise condensation are respectively hf and hd, then

  1. hf = hd
  2. hf > hd
  3. hf < hd
  4. hf could be greater or smaller than hd
Correct answer: (C)

hf < hd

129. For evaporation of viscous solution in a multiple effect evaporator, the preferred feeding scheme is

  1. Forward
  2. Backward
  3. Parallel
  4. None of these
Correct answer: (B)
Backward

130. For flow over a flat plate, the ratio of thermal boundary layer thickness, 'xt' and hydrodynamic boundary layer thickness 'x' is equal to (where, NPr = Prandtl number)

  1. NPr
  2. NPr 1/3
  3. NPr -1
  4. NPr -1/3
Correct answer: (B)
NPr 1/3
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