ANS:B - is thinner than hydrodynamic boundary layer.

When the Prandtl number (PrPr) is greater than 1 (Pr>1Pr>1), it indicates that thermal diffusivity is lower compared to momentum diffusivity. In this case:

• The thermal boundary layer is thicker than the hydrodynamic boundary layer.

Here's why: The Prandtl number (PrPr) is defined as the ratio of momentum diffusivity (kinematic viscosity, ν) to thermal diffusivity (α). When Pr>1Pr>1, it means that thermal diffusivity is lower relative to momentum diffusivity. This implies that heat is transferred more slowly compared to momentum, resulting in a thicker thermal boundary layer compared to the hydrodynamic boundary layer. Therefore, when Pr>1Pr>1, the thermal boundary layer is thicker than the hydrodynamic boundary layer.