An experimental study of an ejector-boosted transcritical R744 refrigeration system including an exergy analysis

The field of refrigeration witness a massive transition in the supermarket with a strong focus reflected on energy consumption. The use of ejector allows for overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. In this study, a detailed experimental work and exergy analysis on the R744 transcritical ejector cooling system was investigated. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The impact of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature and receiver pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destruction were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provide a maximum pressure lift of 9.51 bar. Moreover, it was found out that the overall available work recovery potential increased by rising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the exergy consumed and destruction were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature. © 2021 Elsevier LtdacceptedVersio

Visual Investigation on Effect of Structural Parameters and Operation Condition of Two-phase Ejector

As an important component in transcritical CO2 refrigeration cycle, complex flow in ejector have not been clearly elucidated. In this paper, CO2 flow in two-phase rectangle ejector was investigated experimentally by visualization measurement. The phase transition and the relaxation phenomena in the ejector were observed. By analyze the picture and the data collected from this experiment, we study the relationship between efficiency of ejector and the phase transition position in the ejector. Firstly, the microstructure of the flow pattern in the ejector was captured by a high speed digital video camera with a microscope to analyze the mixing process in mixing chamber. It was found that there were two stages with different characteristics in mixing process ,which were named fluid mixing section and fluid equilibrium section. When fluid get through mixing channel, the ejector realize the majority functions of entrainment in the first stage, and the ejector also homogenize the velocity of primary fluid and secondary flow by the way of flow core expand to almost all the channel in the second stage. Secondly, based on the comparison of pictures collected from different ejectors under different operating conditions, we found that phase transition position and the form of phase transition was mainly depended on the entrance condition of motive nozzle. For an ejector that keeps the suction nozzle under the same operation condition, when the phase transition point trend to exit of motive nozzle, in mixing channel ,motive flow will occupy more space meanwhile the relaxation phenomena occurred in longer region. It was worth mentioning that the phase transition point will change with different operation condition. But there exist only one best position where the ejector contributes to best efficiency. So, it is of great significance to treat phase transition point as an important sign which was easy to be recognized. Visualization research of ejector will be an important reference for theoretical study of flow pattern in the ejector. It also can provide some date to validate the results from the numerical calculation. The visualization study of ejector will also be the basis of further learn of shock waves and delayed phase transition in the ejector

A Visualization Investigation on the Influence of the Operating Conditions on the Phase Change in the Primary Convergent-divergent Nozzle of a Transcritical CO2 Ejector

Complex flow processes exist in the primary convergent-divergent nozzle of a transcritical CO2 ejector because of the rapid expansion of the supercritical CO2 flow, which have a significant influence on the performance of a transcritical CO2 ejector expansion refrigeration system. A visualization experiment with the direct photography method was carried out to investigate the phase change phenomena in the primary convergent-divergent nozzle of a transcritical CO2 ejector. The visualization transcritical CO2 ejector was designed as a rectangular cross section to minimize the optical distortion. In order to better interpret the phase change phenomena of CO2 flow, four pressure measurement points were lumped in the convergent-divergent nozzle to get the pressure distribution along the convergent-divergent nozzle for various operating conditions. The results revealed that the phase change position in the convergent-divergent nozzle was closely related to the primary flow inlet conditions and the suction flow inlet pressure. .The results showed that the phase change could start after or before the nozzle throat, and the phase change position moved upstream by decreasing the primary flow inlet pressure and temperature simultaneously. As keeping the primary flow inlet pressure constant, the phase change position also moved upward by decreasing the suction flow inlet pressure. In addition, the measured pressure results indicated that the pressure differences in the convergent section of the primary convergent-divergent nozzle increased as the CO2 suction flow inlet pressure decreased because of more adequate expansion of the primary flow

A novel transcritical CO2 refrigeration cycle with two ejectors

In recent years, CO2 is being revisited as a fully environmentally friendly and safe refrigerant. However, basic CO2 transcritical refrigeration cycle suffers from large expansion loss due to high pressure difference between gas cooler and evaporator. Then, it is crucial to find effective and economic way to reduce the expansion loss. Here, a novel cycle with two ejectors is proposed for the first time. Compared with conventional ejector-expansion CO2 cycle with only one ejector, this novel cycle with two ejectors is able to recover more expansion loss, thus improving the system performance further. A computational model is designed to simulate the double ejector CO2 cycle. Simulation results show its high system COP. Effects of parameters, such as ejector nozzle efficiency, gas cooler pressure, entrainment ratios of the two ejectors, gas cooler outlet temperature, on the cycle performance are also analyzed by using the computational model. (C) 2012 Elsevier Ltd and IIR. All rights reserved.</p

Yearly energy analysis of CO2 refrigeration systems

La tesi prende in considerazione sei diverse tipologie di impianti frigoriferi ad anidride carbonica con lo scopo di studiarne le performance sotto il punto di vista di un analisi energetica annuale. Questo permette di capire quali tra i sistemi studiati è il migliore rispetto ad un ciclo a CO2 base e/o ad un ciclo che usa R404A. Lo studio è ststo svolto per due città italiane: Milano e Napoli. I modelli dei sistemi sono stati scritti usando EES (Engineer Equation Solver)ope

Experimental and Numerical Optimization of a Variable-Geometry Ejector in a Transcritical CO2 Refrigeration Cycle

Implementation of an ejector for expansion work recovery in transcritical carbon dioxide (CO2) cycles provides an opportunity to improve the efficiency of these environmentally-friendly refrigeration systems. However, literature outlining an approach to ejector design for a given application is lacking. This paper presents a tool to design a complete ejector applied in a vapor compression cycle. In this work, the developed design tool was validated using experimentally-derived polynomials for air-conditioning conditions. Then, constant values for nozzle and mixing section efficiencies were used as inputs into design tool to broaden the analysis outside of the application boundaries of the experimentally-derived polynomials to study a transcritical CO2 system with an ejector operating in the evaporating temperature and gas cooler pressure in the range of -15 °C to 20 °C and 80 bar to 110 bar, respectively. The design tool allows for the calculation of the motive and suction nozzle throat diameters, the mixing section diameter, and the diffuser outlet diameter, as well as the lengths of each section, to output a full internal geometry of the ejector based on performance requirements. Individual component sub-models are presented within the proposed model structure. The model which forms the basis of the design tool was experimentally validated with a mean absolute error (MAE) between 3% to 4%. Additionally, the sensitivity of the ejector geometry and performance to component efficiencies, operating conditions, and component versus system optimization was investigated. The optimization and parametric studies provided novel insights into the impact of desired efficiency and operating conditions on ejector geometry, thus allowing a designer to make decisions based on the tradeoff between ejector size and performance. For example, as the diffuser length increased by 5.1 mm to obtain an efficiency increase, to obtain a further efficiency increase of the same amount would require a 17.1 mm length increase in diffuser length. Potential model improvements and other future work are also discussed

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-boosted Transcritical CO2 Refrigeration System

Recently, the field of refrigeration and air conditioning has come under immense scrutiny as a result of their indirect contribution to global warming and climate change. This is due to the imminent danger posed on the globe by greenhouse gas emissions. This field is continually increasing, experiencing high-grade energy consumption, and calls for the developing of innovative alternative technologies for saving energy. The use of ejector allows overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. The thesis focused on two things: investigate detailed experimental work on the ejector supplied R744 transcritical cooling system and the impact of the ejector profile on the system performance. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The effect of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature, and liquid separator pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destructed related to the ejector profile were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provided a maximum pressure lift of 9.51 bar.Moreover, it was found out that the overall available work recovery potential increased by raising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the ejector exergy consumed and destructed were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature. The ejector-supported system was theoretically compared with the parallel compression concept as the baseline system and carried out at different pressure lifts and exit gas cooler properties. The result indicated a COP and exergy efficiency improvement up to 2.05% and 1.92% for the set conditions, respectively, while the COP could be improved to a maximum of 11.22% when the system cooling load is minimized.Additionally, the ejector played a vital role in the system input power. Up to 3.46% of the energy consumption was reduced at subcritical heat rejection conditions. Operating the system with an ejector at a lower cooling capacity allows further overall power consumption reduction to 18%. In addition, the exergy analysis revealed a prominent lack of total system exergy destruction by employing the ejector in parallel with the high-pressure valve, which recovered 21% of the expansion work and saved 46% of the HPV exergy losses. Furthermore, the result exhibited a maximum system exergy loss of 7.8% that could be saved at the set condition and a maximum of 13.2% total system exergy destruction rate recovered by the ejector depending on the cooling load.Recently, the field of refrigeration and air conditioning has come under immense scrutiny as a result of their indirect contribution to global warming and climate change. This is due to the imminent danger posed on the globe by greenhouse gas emissions. This field is continually increasing, experiencing high-grade energy consumption, and calls for the developing of innovative alternative technologies for saving energy. The use of ejector allows overcoming the significant exergy destruction lays on the expansion processes of the cooling systems and led to spark improvement in the system performance by recovering some of the expansion work. The thesis focused on two things: investigate detailed experimental work on the ejector supplied R744 transcritical cooling system and the impact of the ejector profile on the system performance. The experiment was implemented on the commercial ejector cartridge type (032F7045 CTM ELP60 by Danfoss). The effect of different operating conditions determined by exit gas cooler pressure and temperature, evaporation temperature, and liquid separator pressure was examined. The ejector performance of the pressure lift, mass entrainment ratio, work rate recovery and efficiency were evaluated. In addition, exergy efficiency and the variation of exergy produced, consumed, and destructed related to the ejector profile were assessed based on the transiting exergy. The result revealed better overall performance when the ejector operated at transcritical conditions. The ejector was able to recover up to 36.9% of the available work rate and provided a maximum pressure lift of 9.51 bar.Moreover, it was found out that the overall available work recovery potential increased by raising the gas cooler pressure. Out of the findings, the ejector could deliver maximum exergy efficiency of 23% when working at higher motive nozzle flow temperatures along with providing lower exergy destruction. The experiment results show that the amount of the ejector exergy consumed and destructed were gradually increased with higher gas cooler pressure and, in contrast, decreasing with higher motive nozzle flow temperature. The ejector-supported system was theoretically compared with the parallel compression concept as the baseline system and carried out at different pressure lifts and exit gas cooler properties. The result indicated a COP and exergy efficiency improvement up to 2.05% and 1.92% for the set conditions, respectively, while the COP could be improved to a maximum of 11.22% when the system cooling load is minimized.Additionally, the ejector played a vital role in the system input power. Up to 3.46% of the energy consumption was reduced at subcritical heat rejection conditions. Operating the system with an ejector at a lower cooling capacity allows further overall power consumption reduction to 18%. In addition, the exergy analysis revealed a prominent lack of total system exergy destruction by employing the ejector in parallel with the high-pressure valve, which recovered 21% of the expansion work and saved 46% of the HPV exergy losses. Furthermore, the result exhibited a maximum system exergy loss of 7.8% that could be saved at the set condition and a maximum of 13.2% total system exergy destruction rate recovered by the ejector depending on the cooling load