Criterion for Neglecting Axial Diffusion

Equation 8.26 includes two dimensionless parameters that govern the importance of molecular diffusion relative to convection. The parameters are t/R for diffusion in the radial direction and 3 111 for diffusion in the axial direction. They differ in magnitude by a factor of Expressed in terms of the dimensionless variables 

The corresponding criterion for ignoring the axial diffusion of heat is less certain. Replacing the molecular diffusivity in Equation 8.28 with the thermal diffusivity 

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