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4:00pm - 4:20pm ID: 1347 / C-12: 1 Paper for Compressor Engineering Conference
Transient Oil-refrigerant Mixture Flow Change after Compressor Shutdown at Suction and Discharge
Xin Wang1, Nenad Miljkovic1, Stefan Elbel2
1Air Conditioning and Refrigeration Center, Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801, USA; 2Technische Universität Berlin, Institut für Energietechnik, FG Wärmeübertragung und -wandlung, Marchstr. 18, 10587 Berlin, Germany
The compressor oil is retained at different locations of the vapor compression system during operation. After shutdown, the retained oil absorbs the vapor refrigerant and mixes with the liquid refrigerant with temperature and pressure changes. In this paper, the liquid mixture behaviors after shutdown at the compressor suction and discharge are observed by flow visualization. The liquid mixture property variations are estimated with existing models according to the temperature and pressure variations. The refrigerant concentration in the liquid phase increases rapidly at suction after shutdown. The viscosity of the liquid decreases quickly with temperature and refrigerant mass fraction change. Flow visualization shows that the mixture film breaks from the top of the tube wall and flows down. Transient vapor condensation and re-evaporation occur with a sudden pressure increase. The oil-refrigerant mixture liquid level increases and decreases within seconds. At the discharge, the refrigerant solubility fluctuates within a small range. Viscosity increases because of temperature decrease then decreases because of refrigerant mass fraction increase. After shutdown, the oil flow in contact with the inner wall of the discharge stops immediately while oil droplets in the vapor core flow slow down gradually. Three flow behaviors and property variations are compared after different operation conditions. The results show that larger flow property changes and more severe vapor condensation happen at the suction with the higher outdoor temperature. The vapor refrigerant, liquid refrigerant, and oil relationship is investigated and estimated after shutdown to better understand oil retention.
4:20pm - 4:40pm ID: 1379 / C-12: 2 Paper for Compressor Engineering Conference
Two-phase Simulations On Transient Flow In Scroll Compressor Using VOF Method
1Seoul national university, Korea, Republic of (South Korea); 2Energy Technology Lab., SAMSUNG ELECTRONICS Co., Korea, Republic of (South Korea)
In operating scroll compressors, lubrication oil is in general injected between the moving parts, to cool the working gas or to reduce the gaps between them. However, it is challenging to analyze the two-phase flow in a scroll compressor due to the complicated structure including narrow gaps. We conducted three-dimensional two-phase numerical simulations in the transient flow field in a scroll compressor using R32 as a working fluid and POE85 as a lubricating oil. Oil and refrigerant were modeled using the Volume of Fluid (VOF) method. The refrigerant property tables were generated from REFPROP and during calculation, each thermodynamic property of the refrigerant was interpolated using UDRGM (User-Defined Real Gas Model), thereby reducing the computational time. Bypass valves were included in the scroll compressor geometry and the mesh update during the valve motion occurred using the layering method. Additionally, the deformation of chamber part geometry was simulated using Structured Dynamic Mesh(SDM) method. By implementing these approaches, we could maintain high grid quality and a sufficient number of mesh including the flank gap region. In this paper, we investigated the influence of the oil on the compression performance and local flow field. In the presence of valve motions, a modest reduction in discharge temperature was observed, accompanied by an increase in mass flow rate.
