The main reason for increasing heat transfer coefficient in the entrance region?

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D. Andrew S. Rees

Could you describe precisely what configuration you are considering?

Mojtaba Bezaatpour

From the following equation, it is clear that the highest local heat transfer coefficient occurs at the entrance, where the temperature difference between the fluid and the surface is maximum.

h=q''/(Ts-Tb) ; Tb=balk temp Ts=surface temp

However, it decreases with the distance from entrance as the fluid temperature becomes closer to the surface temperature and temperature difference decreases.

Another justification can be boundary layer thickness. Boundary layers are thinner at the entrance and result in higher local heat transfer coefficient.

Best regards.