Oil separator, Droplet, Visualization, Coalescence
Oil management is an important issue for compressors. Oil circulating in the vapor compression system is harmful for the heat exchanger performance and the compressor reliability. Therefore, oil separation is commonly needed in air conditioning or refrigeration systems to reduce the oil circulation ratio and return the oil back to the compressor. For compactness, oil separation structure integrated into the compressor is more and more popular than traditional external oil separator. However, a quantitative guideline for oil separation structure design is missing for irregular geometry and realistic flow condition at the compressor discharge. In this paper, coalescence, as one of major mechanisms of droplet separation, is presented as studied by flow visualization and analytical models. The oil mist flow through separator is visualized by high speed camera and analyzed quantitatively. Oil droplet size distribution is estimated by video processing. Important flow details are revealed, including oil droplet collision, oil droplet coalescence, oil film breakup and re-entrainment. Separation efficiency is estimated by the ratio of drained and incoming oil mass flow rate. Pressure drop is also measured to evaluate the cost brought by extra separation structures. An analytical model for coalescence type oil separator is developed, which considers the droplet re-entrainment. The model is based on the mass flow in multipath going through multilayer of coalescers. The results of the model is verified by experimental measurements of a full vapor compression cycle with R134a and PAG oil. The physics behind oil separator is illustrated by visualization and analyzed by models. The results and conclusion give useful guidelines for oil separator design and operation.