Abstract: A generalized factor D_n was introduced to the fluid to fluid modeling to take into account the effect of flow channel geometrical parameters on critical heat flux (CHF). Thirteen similarity dimensionless products were derived based on the dimensional analysis and similarity theory and compared with those derived by Ahmad. It is found that Ahmad’s results are the special situation of the paper’s when n is equal to 1, and a new equivalent characteristic size D_e was developed, which could reflect the effects of complex flow channels on the occurrence of CHF.