ANS:B - Reyleigh

(NGr x NPr) is called the Graetz number. The Graetz number (Grz) is a dimensionless number used in fluid mechanics and heat transfer to characterize the ratio of thermal diffusion (Prandtl number) to momentum diffusion (Reynolds number) in a flow system with heat transfer. It is defined as the product of the Nusselt number (Nu), which describes the convection heat transfer, and the Prandtl number (Pr), which describes the ratio of momentum diffusivity to thermal diffusivity. Grz=NGr×NPr It is often used in the analysis of heat transfer in fluid flows, particularly in situations where both momentum and thermal diffusion are important, such as in forced convection flows over heated surfaces.