Graetz number (Gz) is a non dimensional number which is defined as heat transfer by conduction/ heat transfer by convection. This number in general is used in calculation of heat transfer coefficient by forced convection in heat exchanger using Inserts. It is given by the equation, Gz = Re Pr d/L, where Re is Reynolds number, Pr is Prandtl number, d is inside tube diameter and L is test section length.

Thanks Mr.@Pisipaty Srinivas Kishore about your answer .and we can say Gz=Nu^-1.