Oil-injected rotary type positive displacement compressors (RPDC) like twin-screw machines are mainly used for the compression of refrigerants in heating, ventilation and air conditioning (HVAC) systems. The efficiency of such machines is largely determined by the inevitable two-phase surge and gap flows in these types of compressors. Thus, it is 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 thermo-physical properties of the highly asymmetric fluid mixtures of oil and refrigerant. Against this back-ground, 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 modeling of the thermophysical properties of the oil-refrigerant mixtures. Within this consortium, subproject B3 aims at the further development and application of especially advanced experimental, but to some extent also of molecular dynamics simulation techniques for the accurate determination of viscosity, interfacial tension, and thermal conductivity of oil-refrigerant mixtures in and beyond the process-relevant range of composition, temperature, and pressure. With the data for systematically selected model and oil-refrigerant systems, structure-property relationships should be identified that support the development of property models.