ANS:B - momentum diffusivity to thermal diffusivity.

The Prandtl number (Pr) is the ratio of momentum diffusivity to thermal diffusivity. Explanation:

• The Prandtl number (Pr) is a dimensionless quantity used in fluid mechanics and heat transfer to characterize the relative importance of momentum diffusivity (kinematic viscosity) to thermal diffusivity in a fluid.
• Mathematically, the Prandtl number (Pr) is defined as the ratio of the momentum diffusivity (kinematic viscosity, ν) to the thermal diffusivity (α) of the fluid. It is often expressed as: Pr=momentum diffusivitythermal diffusivity=Pr=thermal diffusivitymomentum diffusivity​=αν​
• Momentum diffusivity (kinematic viscosity) represents the ability of a fluid to transport momentum or velocity fluctuations through its mass.
• Thermal diffusivity represents the ability of a fluid to transport heat through its mass.
• The Prandtl number is significant in the analysis of forced convection heat transfer, particularly in determining the boundary layer characteristics and heat transfer coefficients in fluid flow over solid surfaces.
• In practical terms, the Prandtl number provides insights into the relative thicknesses of the momentum and thermal boundary layers near a solid surface, influencing the rate of convective heat transfer.
• Therefore, the correct statement is that the Prandtl number is the ratio of momentum diffusivity to thermal diffusivity.