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Title:Experimental investigation of mist flow generated by reed valves in compressor discharge
Author(s):Zimmermann, Augusto Jose Pereira
Director of Research:Hrnjak, Predrag S.
Doctoral Committee Chair(s):Hrnjak, Predrag S.
Doctoral Committee Member(s):Jacobi, Anthony M.; Newell, Ty; Zhang, Yuanhui; Elbel, Stefan
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:This work presents an experimental investigation to characterize mist flow of oil and refrigerant at the discharge of air conditioning and refrigeration compressors. It was identified that in a vast majority of compressors the discharge port is the gateway for the oil to leave the compressor and get carried over to the other components of the system. Between the discharge port and the discharge plenum or volume in compressors there is usually a valve. The most common type of discharge valve in AC&R compressors is a reed type valve. The mist flow characteristics such as droplet size, drop velocity are determined by obtaining high speed videos of the flow in a compressor equipped with optical access and performing image processing in those videos to quantify the variables of interest. The evolution of this study starts from focusing on the discharge tube and makes its way inside the compressor discharge chamber by capturing the characteristics of the flow in each location. An attempt at enhancing droplet deposition at the tube walls by forcing the mist flow to go through a change in direction did not prove successful and the reasons were captured by pointing out increased instabilities on the oil film at the walls which created more droplet entrainment after the change in direction. Experiments with different reed valve assemblies used both in swash plate compressors (Automotive AC application) and hermetic reciprocating compressors (Household Refrigerators) were performed to give insights on the oil film breakup patterns that may be encountered in compressor real operation. The results show that the oil film is atomized into a mist when a pressure pulse is generated inside the discharge orifice and that sizes could be determined at those conditions. Some ligaments and a majority of droplets are present in the film breakup. For the case of the hermetic reciprocating compressor it was possible to observe that the oil film first breaks up into columns with equal spacing amongst themselves. As the valve continues to lift those structures then are atomized into droplets. Similar visualizations carried out inside a real compressor during start up conditions showed the same structures with a column spacing of 290µm on average. The opening and closing behavior of the reed valve was visualized and some dynamic parameters such as opening and closing velocities and displacement during the compression cycle were determined from the image analysis and agreed in general with what has been reported in the literature using alternative methods. Visualization experiments of the opening and closing process of the reed valve inside a scroll compressor in real operating conditions were performed and the following conclusions were drawn: - A cloud of droplets could be observed during the first moments of the opening cycle. - The cloud existence time and also its expanding velocity were determined from image analysis of the captured videos. - Droplet cloud velocity was very close to estimated gas velocity at the discharge based on the valve lift the compression process models. A fully transparent discharge plenum was constructed to enable characterization of the mist flow inside the compressor discharge space and at the valve vicinity, including determination of droplet sizes and velocities. The following conclusions can be drawn from this part of the work: - At the vicinity of the valve, the volume mean diameter decreases as the compressor volumetric flow increases with operating frequency. - Mean diameter ranges from 250µm down to 150µm from 30Hz to 60Hz operation. - Volume mean diameter showed to be 10 to 20% higher for POE120 oil vs PVE32 oil under same running conditions. - At the discharge plenum the volume mean diameter of the droplets found was between 40µm and 110µm. - Velocities at the discharge plenum location ranged from 2 to 10m/s with the mean velocity at around 5m/s. - There was no significant difference in droplet velocity between the two oil types evaluated. As final concluding remarks this work has provided information for flows inside a compressor discharge plenum, providing size and speed information as well as the origin of the droplets that make up the mist flow inside that space. This allows further work to be carried out in developing separation methods that can be integrated into the compressor design itself.
Issue Date:2016-11-21
Rights Information:Copyright 2016, Augusto Jose Pereira Zimmermann. All rights reserved.
Date Available in IDEALS:2017-03-01
Date Deposited:2016-12

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