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.
Within this consortium, subproject B4 aims to develop models for the viscosity in the Newtonian range and for the thermal conductivity for characterizing strongly asymmetric oil-refrigerant mixtures. For this purpose, existing literature models are first evaluated for their applicability and accuracy. With these results, two directions of development will be pursued. On the one hand, a physically based model, which so far can only describe simple mixtures with adequate accuracy, will be further developed for more complex mixtures. Due to its physically based character, it can then easily be transferred to different oil-refrigerant systems. The second branch of development intends to develop a purely empirical model. This will not be universally transferable to any mixture, but it will provide significantly more accurate results for selected systems.