
230 Fuel Cells
For internal ows, thermal boundary layers develop from both top and bottom
surfaces and develop into two regions: thermal entry length and thermal fully devel-
oped regions similar to hydrodynamic internal ow as shown in Figure 6.8.
Thermal fully developed region: The region where the dimensionless temper-
ature prole remains invariable along the longitudinal length of the channel.
Thermal entry length (L
e,th
): The length required for the dimensionless tem-
perature prole to become fully developed.
Criterion for entry length: Criterion for hydrodynamic entry length for
laminar ow:
L
Re
D
e,th
≈ 006
(6.32)
For example, for a max