In gas–liquid two-phase flow, void fraction is the most unique parameter which influences all the transport processes. In the most general case, though the void fraction varies nonlinearly with the channel length, many practical simulations make use of the “system mean void fraction.” The present investigation makes a critical assessment of different system mean void fraction models for a wide range of slip velocity and density difference between the phases. To this end, different correlations for slip ratio have been considered and, for all the cases, closed form expression for the system mean void fraction has been presented. The local as well as the system mean void fractions have also been estimated numerically from a heat transfer based model. Predictions from the heat transfer based model and the slip ratio based model have been compared. As an application, the slip ratio based system mean void fraction is used in to build the moving boundary model for phase-change heat exchangers. The prediction of startup transients for both an evaporator and a condenser of an automotive air conditioning system (AACS) agrees well with the experimental results.