ANS:B -

The Nusselt number (Nu) is a dimensionless number used in fluid mechanics and heat transfer to characterize the heat transfer coefficient in a fluid flow system. It's named after Wilhelm Nusselt, a German engineer. The Nusselt number is defined differently depending on the specific heat transfer mode:

1. For forced convection: In forced convection heat transfer, where a fluid is forced to flow over a surface, the Nusselt number is defined as the ratio of the convective heat transfer coefficient (ℎh) to the thermal conductivity (k) of the fluid, multiplied by a characteristic length (L), typically the hydraulic diameter or the length of the flow path:

2. For natural convection: In natural convection heat transfer, where fluid motion is driven by buoyancy forces due to temperature differences, the definition of the Nusselt number depends on the specific geometry and flow conditions. It is often expressed in terms of empirical correlations obtained from experimental data.