ANS:A - Prandtl number

The Dietus-Boelter equation is commonly used to predict heat transfer coefficients in forced convection situations for fluids with moderate to high Prandtl numbers. The Prandtl number is a dimensionless parameter that represents the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity. Molten metals typically have very low Prandtl numbers. This means that their thermal diffusivity is much higher compared to their kinematic viscosity. The low Prandtl number of molten metals implies that they tend to conduct heat much more effectively than they transport momentum, which makes them behave differently in terms of heat transfer compared to fluids with higher Prandtl numbers. Therefore, the Dietus-Boelter equation cannot be used for molten metals mainly due to their very low Prandtl number.