Prandtl number, abbreviated as Pr, a dimensionless number, in fluid dynamics is used to calculate force by the ratio of momentum diffusivity (kinematic viscosity) and thermal diffusivities. This number helps characterize the relative thickness of the velocity boundary layer to the thermal boundary layer. Some common values of non-metallic thermal diffusivity values can be found here.
Prandtl number calculator
Prandtl number formula