The prediction of the pressure drop in various two-phase flow systems has been the object of numerous studies by various investigators.

Most developments of the theory are seen to parallel the treatment for the single-phase pressure drop case with some factor, usually obtained empirically, being introduced to modify the expressions for two-phase applications. The available theory has been used, so far, in the design of various systems and, in some cases, for fluids other than those upon which the correlations have been based. Various parameters (both flow and geometry) interact in complex ways to affect the overall pressure drop observed in a two-phase system. Besides, for a system in which there is evaporation, the various flow regimes assumed by the two phases as they flow together in the channel further complicate the process of predicting the pressure drop. This is mainly because the assumptions made in arriving at the flow models in obtaining the pressure drop estimates are different for different flow regimes.

It is the purpose of this study to point out some of the limitations of existing models for predicting the pressure drop in an evaporative two-phase system and to offer suggestions that might prove useful in the improvement of the existing prediction schemes.