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Heat Transfer - Engineering

Q1:

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

A 1.26 h1

B 0.794 h1

C 0.574 h1

D 1.741 h1

ANS:B - 0.794 h1

When the velocity of laminar flow in a circular tube is reduced by half, the convective heat transfer coefficient (h) changes according to the Dittus-Boelter equation for laminar flow: ℎ2ℎ1=(21)0.8h1​h2​​=(V1​V2​​)0.8 Where:

  • ℎ1h1​ is the convective heat transfer coefficient at velocity V1​.
  • ℎ2h2​ is the convective heat transfer coefficient at velocity V2​.
Given that the velocity is reduced by half (V2​=2V1​​), we can substitute this into the equation: ℎ2ℎ1=(1)0.8h1​h2​​=(V1​2V1​​​)0.8 ℎ2ℎ1=(12)0.8h1​h2​​=(21​)0.8 ℎ2ℎ1=(0.5)0.8h1​h2​​=(0.5)0.8 ℎ2ℎ1=0.794h1​h2​​=0.794 So, the new convective heat transfer coefficient (ℎ2h2​) is approximately 0.794×ℎ10.794×h1​. Therefore, the correct answer is: 0.794 ℎ10.794h1​