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Title:Flow and separation of the oil from compressor discharge to external separator
Author(s):Xu, Jiu
Director of Research:Hrnjak, Pega
Doctoral Committee Chair(s):Hrnjak, Pega
Doctoral Committee Member(s):Jacobi, Anthony; Miljkovic , Nenad; Zhang, Yuanhui
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):Oil
Compressor
Separation
Abstract:Oil is essential for most of the refrigerant compressors, but oil circulating in the refrigeration or air conditioning systems has a side effect on the heat transfer and total efficiency. Oil separator is an important component to reduce the amount of oil circulating in the system and to keep the oil in the compressor. Though commonly applied in the industry, oil separator still needs a systematic design guideline. The understanding of fundamental separation mechanisms is not sufficient for design optimization. This dissertation aims to elaborate on the characteristics of the oil mist generated by compressors, the coalescing and impinging mechanism of oil separation, and the design suggestions for oil separators. Characteristics of the oil mist at the compressor discharge are quantified by flow visualization techniques in this dissertation. Oil droplets are formed by oil film breakup during the periodic reed valve movement. The oil droplets are then entrained by the refrigerant vapor inside the discharge plenum and leave the compressor to form a mist-annular developing flow in the discharge pipe. The oil mist characteristics, including oil droplet size distribution and oil droplet velocity distribution, are affected by the compressor working condition, the internal geometry of the compressor, and the oil properties. The discrete phase model is used to simulate the trajectories of oil droplets of different diameters inside a scroll compressor plenum. The simulation requires the visualization results as the input, and its output is the oil droplet size distribution in the compressor discharge pipe. The simulation results are validated by the flow visualization results under the same conditions. The oil droplet trajectories show that the discharge plenum space has the potential to separate large oil droplets. The design of the oil separating structure has a wide variety of options, but three main mechanical separation mechanisms are most commonly applied in the industry: coalescence, impingement, and centrifugation. In this dissertation, experimental measurement and flow visualization are used to analyze the oil mist through coalescing and impinging oil separators under realistic compressor discharge conditions. The experimental data confirmed that separation efficiency decreases as the refrigerant superficial vapor velocity increases when the volume is constant. This trend can be attributed to higher oil mist load, smaller oil droplet size, and re-entrainment caused by higher overall flow rate. Also, the pressure drop introduced by the separating structure increases as the refrigerant vapor velocity increases. The effect of geometry on the oil separator performance is evaluated by testing different separating geometry under the same group of flow conditions. In general, denser separating structure provides better separation efficiency at the cost of more significant pressure drop. Detailed effect brought by the geometric feature of both coalescing and impinging separators is also discussed. A semi-empirical multilayer model is developed to predict the separation efficiency and pressure drop of the oil separators. The model simplifies the geometry of the separator to make it feasible for implementation. The framework of the model is built based on the mass balance between the capture, entrainment, and drainage of the oil. Both the separation model and the pressure drop model are validated by the experimental results. Design suggestions are given for oil separators by the experimental observation and the model results. The design of the oil separators should consider the trade-off between the separation efficiency, pressure drop, and occupying volume. With the knowledge of upstream oil mist characteristics, the separation efficiency and pressure drop can be estimated for design purpose by the extrapolation of the semi-empirical model. For design optimization, the effect brought by the oil separator should be evaluated on the system level.
Issue Date:2019-04-17
Type:Thesis
URI:http://hdl.handle.net/2142/105026
Rights Information:Copyright 2019 Jiu Xu
Date Available in IDEALS:2019-08-23
Date Deposited:2019-05


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