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Flash gas bypass concept utilizing low pressure refrigerants

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Title: Flash gas bypass concept utilizing low pressure refrigerants
Author(s): Milosevic, Alen S.
Advisor(s): Hrnjak, Predrag S.
Department / Program: Mechanical Sci & Engineering
Discipline: Mechanical Engineering
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: M.S.
Genre: Thesis
Subject(s): Air-Conditioning Refrigeration Flash gas 1,1,1,2-Tetrafluoroethane (R134a)
Abstract: Flash gas bypass represents one of the prospective approaches to improve the efficiency of various small scale heat transfer systems that utilize refrigerant as a working fluid. It is characterized by the bypass of vapor created throughout the expansion process around the evaporator. As the advantages of flash gas removal in R744 systems are observed, it is of scientific interest to elaborate the same approach in R134a or similar low pressure refrigerant A/C systems. The main reason for this is the fact that lower pressure fluids are more affected by pressure drop, therefore flash gas removal which reduces low system side pressure drop will be more beneficial utilizing these fluids. This project developed a methodology for studying phase separation in compact vessels and analyzing their impact inR134a automotive A/C systems. The flash gas bypass concept would be attractive for commercial implementation if a compact separation vessel could efficiently separate refrigerant phases over wide ranges of flow conditions and the simple flow controlling device could be exploited in the flash gas bypass line. It would be convenient if the phase separation vessel could be implemented in the evaporator header. A methodic approach has been implemented analyzing T -junctions as a starting design, studying the mechanisms of phase separation and paving the way towards more advanced geometries. To our knowledge, the idea of implementing compact and efficient separation devices in the heat exchanger header is a unique approach that is not been studied elsewhere. . Two T-junctions with inlet tube diameters 8.7 mm and 12 mm, with body diameters 18.3 and 23.8 mm respectively, are examined and analyzed at a range of mass flow rates and vapor qualities typically found in automotive A/C systems. The quantification of both vapor and liquid phase separation efficiencies has been defined. The 18.3 mm body diameter T-junction could efficiently separate liquid up to 30 g/s and 20 % vapor quality, while the 23.8 mm diameter flash tank could perform efficient phase separation up to 45 g/s and 15 % vapor quality. Based on the observed phase separation phenomena, separation enhancers could increase the separation efficiency. The impact on the A/C system level has been analyzed using a T-junction with 23.8 mm body diameter. At matched capacities to the Baseline DX system, the Flash Gas Bypass system showed significant improvements in the system efficiency (COP), up to 55%. The reason for such high improvement lies in the fact that the evaporator was slightly undersized when compared to the rest of the A/C system components. A needle valve was utilized to control the flash gas bypass flow. The flash gas bypass valve opening had a significant effect on system performance. , The system had the highest improvement in COP with the flash gas bypass valve opened ½ turn (valve flow coefficient approximately 0.25), when compared to the Baseline DX configuration at matched capacity. This shows the prospect of implementing a simple regulating device such as an orifice tube.
Issue Date: 2011-01-21
URI: http://hdl.handle.net/2142/18510
Rights Information: Copyright 2010 Alen S. Milosevic
Date Available in IDEALS: 2011-01-21
2013-01-22
Date Deposited: 2010-12
 

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