Oil-injected rotary positive displacement compressors (RPDC) like twin-screw machines are mainly used for the compression of refrigerants in HVAC (Heating, Ventilation and Air Conditioning) systems. The efficiency of such machines is largely determined by the inevitable two-phase surge and internal gap flows in these types of compressors. Therefore, it is quite remarkable that the calculation of these flows is still a challenge today. This is due to both the complexity of simulating the two-phase flow in the narrow gaps with moving boundaries as well as the lack of accurate models for the thermophysical properties of the highly asymmetric fluid mixtures of oil and refrigerant. Against this background, the proposed Research Unit provides a unique opportunity for coordinated, interdisciplinary efforts to address this knowledge gap, with seven project partners complementing each other’s capabilities. The three subprojects of group A deal with the development of new analysis and calculation methods for the two-phase gap flows while the four subprojects of group B cover the determination and modelling of the thermophysical properties of the oil-refrigerant mixtures. Within this consortium subproject A1 aims to develop validated models for the simplified calculation of two-phase gap flows with moving boundaries for mixtures of oil and refrigerant from the findings of the Research Unit in conjunction with its own experimental investigations. For this purpose, new calculation models for the two-phase inertial and wall friction forces are developed specifically for the typical gap geometries of RPDC. The enhanced models are validated against experimental data obtained from a generic test rig.