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For a hydrodynamically and thermally fully developed laminar flow through a circular pipe of constant cross-section, the Nusselt number at constant wall heat flux (Nu_q) and that at constant wall temperature (Nu_T) are related as

1. Nu_q > Nu_T
2. Nu_q < Nu_T
3. Nu_q = Nu_T
4. Nu_q = (Nu_T)²

Correct answer: Nu_q > Nu_T

Solution

For fully developed laminar flow in a circular pipe, Nu_q=4.36 (constant heat flux) and Nu_T=3.66 (constant wall temperature). Therefore Nu_q > Nu_T; stored 'Nu_q = Nu_T' is wrong.

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