Prandtl Number

The Prandtl number is a parameter that relates the thicknesses of the velocity and thermal boundary layers and is given by:

where n is the kinematic viscosity, a is the thermal diffusivity, r is the fluid density, k is the fluid thermal conductivity, and c p is the fluid heat capacity at constant pressure.

The kinematic viscosity n of a fluid reveals information about the rate at which momentum may diffuse through the fluid because of molecular motion. Thermal diffusivity a reveals information about the diffusion of heat in the fluid. Thus the ratio of these two quantities expresses the relative magnitudes of diffusion of momentum and heat in the fluid.