However, the fluid properties have to be evaluated at the mean bulk temperature of the fluid except μw which should be evaluated at the wall temperature.
The earlier relations were applicable for fully developed flow when entrance length was negligible. Nusselt recommended the following relation for the entrance region when the flow is not fully developed.
|
(4.7) |
where, L is the tube length and d is the tube diameter.
The fluid properties in eq. 4.7 should be evaluated at mean bulk temperature of the fluid.
Applicability conditions,
.
As different temperature terms will appear in the course therefore to understand these terms see the following details.
Bulk temperature/mixing cup temperature: Average temperature in a cross-section.
Average bulk temperature: Arithmetic average temperature of inlet and outlet bulk temperatures.
Wall temperature: Temperature of the wall.
Film temperature: Arithmetic average temperature of the wall and free stream temperature.
Free stream temperature: Temperature free from the effect of wall.
Log mean temperature difference: It will be discussed in due course of time