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Title:Experimental and modeling study of lubricant effect on refrigerant distribution in microchannel evaporators
Author(s):Li, Huize
Advisor(s):Hrnjak, Predrag S.
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Microchannel heat exchanger
Refrigerant-oil mixture
Abstract:Microchannel heat exchanger has been used in industry for decades. But the problem of refrigerant maldistribution can significantly deteriorate the performance of heat exchanger (especially evaporator) and system. In real air conditioning systems especially automobile air conditioning systems where oil separator is less commonly used, the lubricant pumped out of the compressor is mixed with refrigerant and travels throughout the system. In this study, lubricant effect on refrigerant distribution in microchannel evaporators is investigated and the corresponding effects on evaporator performance and system performance are discussed. First, the effect of lubricant on distribution is studied by relating the flow regime in the horizontal inlet header and the corresponding infrared image of the evaporator. Visualization of the flow regime is performed by high-speed camera. R134a is used as the refrigerant with PAG 46, PAG 100 and PAG 150 lubricants, forming foam in all flow regimes. Quantitative information including foam location and foam layer thickness is obtained using a matlab-based video processing program. Oil circulation rate (OCR) effect and viscosity effect on flow regime are analyzed quantitatively. Second, lubricant effect on the performance of microchannel evaporators is investigated experimentally in a typical automobile air conditioning system. The increase of oil circulation rate and viscosity elevate the pressure drop of the evaporator. Increasing OCR has been found beneficial for refrigerant distribution, contributing to higher mass flow rates. But the addition of lubricant can significantly decrease the specific enthalpy difference of the working fluid mainly because of the non-evaporative nature of lubricant (in the temperature range air conditioning application) and the dissolution of refrigerant in lubricant. The overall OCR effect on capacity is the balance of these two opposing effects. Higher viscosity tends to slightly deteriorate the distribution and reduce capacity. Third, lubricant effect on system performance is examined. Increasing OCR reduces the compressor power. COP peaks are witnessed around 2-3% OCR under almost every test condition in this study. Higher viscosity results in higher compressor power and lower COP. In parallel with the experimental study, a simulation approach is carried out. A microchannel evaporator model developed by Tuo et al. (2012a) is enhanced by inclusion of the thermodynamic and transport properties of refrigerant-oil mixture and their impact on boiling heat transfer and pressure drop characteristics. Viscosity and OCR effect on refrigerant distribution are investigated using this model. By only considering lubricant effects among parallel microchannel tubes, the results show that 1) high viscosity is detrimental for refrigerant distribution; 2) as OCR increases, distribution becomes worse; but at very high OCR, distribution becomes better. Attempts have been made to take account of the lubricant effect in the inlet header by reversely calculating refrigerant distribution from infrared images. As a result, a distribution quantification method is developed which correlate liquid refrigerant distribution with measurable temperature parameters (evaporator air inlet temperature and evaporator wall temperature). This method can serve the same role as a quality distribution function in a microchannel heat exchanger model. By combining the lubricant effect in the inlet header taken into account by the newly proposed method and the parallel microchannel tubes model, a complete evaporator model is developed in which lubricant effect is fully considered. The new model incorporating lubricant effect provides better prediction of heat exchanger performance than the pure refrigerant models, especially at high OCR’s.
Issue Date:2014-01-16
Rights Information:Copyright 2013 Huize Li
Date Available in IDEALS:2014-01-16
Date Deposited:2013-12

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