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
Answer: Option B
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For laminar flow, the empirical equation is
Nnu= 1.86[(NRe)(Npr)(D/L)]^1/3 * [u/uw]^0.14.
So the relationship between is
hd/k = (DV¶/u)^1/3
So, h = V^1/3
if V becomes half
then V2 = V1/2
h2/ h1 = (V2/V1)^1/3
h2 = h1(V1/2V1)^ 1/3= 0.793 h1
Nu=0.023Re^0.8*Pr^n
:.h is proportional to V^0.8
:. h2/h1=(V2/V1)^0.8=2^(-0.8) [V2=V1/2]
h2=0.574h1
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