Prandtl number is the ratio of momentum diffusivity (kinematic viscosity) and thermal diffusivity $Pr = \frac{\nu}{\alpha} = \frac{\mu/\rho}{k/(\rho c_p)} = \frac{c_p\mu}{k}$ Typical values for Prandtl number air - 0.71 hydrogen - 0.16-0.7 oxygen - 0.63 water - 7.56 engine oil - 100 to 40,000