Prandtl number, Pr, is a dimensionless parameter representing the ratio of diffusion of momentum to diffusion of heat in a fluid.
where ν is kinematic viscosity and κ thermal diffusivity.
Prandtl number is a characteristic of the fluid only. For air at room temperature Pr is 0.71 and most common gases have similar values. The Prandtl number of water at 17°C is 7.56. Liquids in general have high Prandtl numbers, with values as high as 105 for some oils.
Hewitt, G. F., Shires, G. L., and Bott, T. R. (1994) Process Heat Transfer, CRC Press, Boca Raton, FL.