For a liquid in laminar flow through a very long tube, when the exit fluid temperature approaches the wall temperature, the equation to be used is

Nu $=0\mathrm{.}023$ Re$0\mathrm{.}8\mathrm{.}4$

Nu $=(/2)$ Gz

Nu $=(2/)$ Gz

Nu $=2$Gz$0\mathrm{.}5$