Oil-injected rotary type 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 gap flows in these types of compressors. Therefore, it is all the more 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. In this subproject, a highly efficient, high-resolution numerical method for the simulation of the gas-liquid multiphase flow in the screw compressor gap will be developed. This includes the complex interactions between the deformable gas bubbles and their interactions with the gap walls. The method will be validated with experimental results of the other subprojects of group A and applied to generic screw compressor configurations.