The Prandtl Number (Pr) is the ratio of rate at which momentum (momentum diffusivity: kinematic viscosity) is transported to the rate at which thermal energy (thermal diffusivity) that is a dimensionless number, named by German physicist Ludwig Prandtl. The Prandtl Number is a characteristic of the fluid only. (Examples of Prandtl Number (Pr): Gases are 0.7 ~ 1.0; Water is ranges 1 ~ 10; Liquid Metals are 0.001 ~ 0.03; Oils are 50 – 2000)