Experimental and numerical investigation of the design and control of vapor-compression systems with integration of two-phase ejectors for performance enhancement through expansion work recovery
The use of ejectors to improve the efficiency and capacity of vapor-compression refrigeration cycles by means of expansion work recovery has received significant attention in the past decade. Research has focused primarily on the design and performance of the ejector and the effect the ejector has on cycle performance. However, recent research has shown that additional factors, such as cycle architecture, improvement in evaporator performance, and cycle control, can also have significant influence on ejector cycle performance. While these factors have been noted in several studies, they have yet to be thoroughly investigated. Thus, the objective of this research is to investigate how to properly integrate an ejector into a vapor-compression cycle (how to choose the proper cycle architecture and proper use of the ejector) and how to design and operate other system components in addition to the ejector, such as the evaporator and cycle controls, in order to gain the maximum benefit from the ejector in the given system. Two ejector cycles are investigated through the use of numerical modeling and experiments. The cycles of interest are the standard ejector cycle, which uses the ejector pressure increase to directly increase compressor suction pressure and reduce compressor power, and the ejector recirculation cycle, which uses the ejector to recirculate excess liquid through the evaporator and improve evaporator performance. The numerical results have shown that refrigerants and systems with inherently high throttling loss, such as transcritical CO2 (R744) systems, should use the ejector to directly supplement compressor power using the pressure increase provided by the ejector, while systems using lower-pressure refrigerants should use the ejector to improve the performance of the evaporator by means of liquid recirculation (overfeed). An experimental investigation of the two ejector cycles using R410A has been performed; two different microchannel evaporators with the same air-side geometry but different refrigerant-side cross-sectional area are used in the experimental investigation. The experimental results have shown that the more favorable ejector cycle depends on the design of the evaporator and on the operating conditions. The standard ejector cycle is more favorable at conditions of higher ambient temperature and with an evaporator with lower refrigerant-side cross-sectional area, achieving up to 9 % greater COP at matched capacity compared to an expansion valve cycle without an ejector. On the other hand, the ejector recirculation cycle is more favorable at lower ambient temperature and with an evaporator with greater refrigerant-side cross-sectional area (achieving up to 16 % greater COP at matched capacity). Further numerical investigation of the R410A system has provided additional insight into proper evaporator design and operation in ejector cycles. It has been found that the standard ejector cycle should operate with a low amount of evaporator overfeed to achieve greater ejector pressure increase and use the design of the evaporator to improve evaporator effectiveness. The ejector recirculation cycle, which cannot directly utilize the ejector pressure increase, should operate with higher overfeed and use the evaporator design to optimize mass flux by balancing pressure drop and heat transfer effectiveness. Finally, an experimental investigation of a transcritical CO2 standard ejector cycle has been performed in order to investigate ejector cycle control. The high-side pressure of the transcritical system has been controlled and optimized by changing the effective nozzle throat size of the ejector through use of an adjustable position needle. Compared to using an expansion valve upstream of a fixed geometry ejector to control high-side pressure, the adjustable ejector results in slightly higher expansion work recovery efficiency and slightly higher COP. A loss in COP of up to 4 % has been observed for not properly controlling high-side pressure, while a loss in COP of up to 11 % has been observed for not properly controlling evaporator flow rate, meaning that it is important to also control and optimize evaporator flow rate in addition to high-side pressure when using a transcritical CO2 standard ejector cycle. It has also been demonstrated that the high-side pressure optimization concept used to maximize COP under transcritical conditions can be extended and used to optimize the performance of the same system when it is operating under subcritical conditions as well.
Ejector, expansion work recovery, vapor-compression technology, evaporator design, carbon dioxide
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign