Estudo computacional de um ciclo de refrigeração transcrítico de CO2 com e sem ejetor

Abstract

Despite the large social and economic benefits of refrigeration, large-scale environmental impacts have prompted researchers to replace HCFC and CFC refrigerants with natural refrigerants due to their lower environmental impact and the need to comply with the Kigali Amendment. Among the alternatives to conventional refrigerants, CO2 stands out. As a refrigerant, it has attracted attention due to its thermophysical properties, with one of the most used cycles being the transcritical vapor compression cycle with CO2. Different studies have investigated the use of promising new equipment to increase the COP in a conventional transcritical CO2 cycle. Thus, new enhancement technologies have emerged, among which the ejector stands out. It is equipment that recovers part of the compressor's work to decrease the compression ratio, resulting in improved system performance. Therefore, the aim of this work is to computationally simulate the conventional transcritical CO2 cycle with an ejector and compare the results obtained with a conventional cycle without an ejector. For the simulation, models and considerations obtained from different studies in the literature are used. The different components of the system are thermodynamically analyzed through energy, mass, and momentum balances. The computationally obtained results show satisfactory agreement with the experimental results. For the ejector, two models were compared, concluding that the constant pressure mixing model showed the highest agreement with the experimental data. The compressor model used a fictitious wall to account for the gains and losses in heat transfer during the compression process. The heat exchanger model used small control volumes, which are considered independent heat exchangers. The expansion valve was modeled considering the expansion factors for two-phase fluids. Numerically, gains between 22 % and 25 % in COP were observed when implementing the ejector.