Phase behavior of oil-refrigerant mixtures – Experimental investigation

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Oil-injected rotary type positive displacement compressors are broadly used for the compression of refrigerants in HVAC (Heating, Ventilation and Air Conditioning) systems. The compressor efficiency is largely determined by the two-phase surge and gap flows in the compressors’ mechanical components. Due to the complexity of the two-phase flow and the lack of accurate thermodynamic property models of the working fluids, the calculation of these flows is nowadays still challenging. To advance such calculations, thermodynamic property models must be improved, which requires accurate experimental property data. However, further development is currently limited by the significant lack of experimental thermodynamic data for the fluids in the compressors, which are highly asymmetric mixtures of oil and refrigerant. Against this background, the key objective of this subproject of the Research Unit FOR 5595 is to carry out comprehensive and accurate measurements of thermodynamic properties of oil-refrigerant mixtures, focusing on density and vapor-liquid phase behavior. These data will ultimately serve as basis for developing accurate thermodynamic property models of the oil-refrigerant mixtures. Certainly, the mixing behavior of the highly asymmetric mixtures poses challenges to the experimental setups originally developed for less asymmetric mixtures. Therefore, it is envisioned to improve and further develop existing experimental setups for the investigation of binary mixtures of increasing asymmetry with the final goal to study oil-refrigerant mixtures. Binary systems to be measured have been jointly selected within the Research Unit and are listed with increasing asymmetry as follows: (1) propane + n-pentane and CO2 + n-decane (2) propane + n-decane and CO2 + n-hexadecane and (3) if possible propane + squalane and CO2 + squalane. Raman spectroscopy will be used to measure gas (refrigerants) solubility in liquids (oil surrogates). For density measurements of pure oils and oil-refrigerant mixtures in the liquid phase, vibrating-tube densimetry will be utilized. To study the vapor-liquid phase behavior of the mixtures, microwave re-entrant cavity resonators will be employed, whereby a new microwave cavity design is to be developed to investigate the full vapor-liquid phase behavior including temperature, pressure, phase fraction as well as compositions and density of the coexisting phases. Moreover, inherent part of the experimental work is to advance the infrastructure for mixture preparation and sample handling at the measuring systems towards asymmetric mixtures to ensure that measurements will be conducted at known compositions.