ANS:C - Nu = Gz

When the exit fluid temperature approaches the wall temperature in laminar flow through a very long tube, the appropriate correlation to use is the Graetz number (𝐺𝑧Gz) correlation. The Graetz number (𝐺𝑧Gz) is defined as the product of the tube length (𝐿L) and the ratio of fluid velocity (𝑢u) to thermal diffusivity (𝛼α). It represents the relative importance of convection to conduction in the fluid flow. The equation for the Nusselt number (𝑁𝑢Nu) in terms of the Graetz number (𝐺𝑧Gz) is: 𝑁𝑢=2𝐺𝑧0.5Nu=2Gz0.5 So, the correct option is: 𝑁𝑢=2𝐺𝑧0.5Nu=2Gz0.5.