Search

With increasing awareness of the adverse effects of carbon emissions on the environment, researchers within the heating, ventilation, air conditioning, and refrigeration (HVAC&R) community have been pushing for lower global warming potential (GWP) and natural working fluids as well as systems that are more efficient than the higher-GWP systems they replace. One such working fluid is carbon dioxide (CO2). While CO2 has the advantages of being low-cost, non-flammable, and possessing a high volumetric heat capacity, it has a high critical pressure associated with a low critical temperature, thus often necessitating transcritical operation that requires significant compressor input power. As such, numerous cycle modifications have been proposed that enable the transcritical CO2 cycle to match, and in some cases surpass, the coefficient of performance (COP) of existing hydrofluorocarbon (HFC) cycles under the same operating conditions. This work provides an experimental comparison of four cycle architectures that utilize the same compressors and heat exchangers. This enables a meaningful comparison of these modifications, consisting of open economization with an evaporator bypass, as well as both electronic expansion valve (EXV) and ejector expansion strategies, along with a pump applied between the gas cooler outlet and the ejector motive nozzle inlet for control and increased recoverable pressure differential. Experimental parametric studies were conducted, and comparisons of architecture costs and benefits are presented. Design recommendations are provided along with future work.

Vapor compression cycles (VCCs) are a promising technology for refrigeration needs on future space craft due to their generally high cooling COP. However, due to microgravity there lies a risk of liquid flooding the compressor during start-up. Thus, to better prepare VCCs for microgravity applications, it is significant to understand the dependence of two-phase refrigerant on gravity during start-up. In this work, liquid flooding is evaluated at the start-up of a VCC and a possibility for passive compressor protection is considered. The experimental setup has two configurations. In the first, two-phase phenomena can be observed in a transparent tube and different tube insertions can be tested for their effectiveness as a liquid flooding obstruction. In the second configuration, liquid flooding from a commercial evaporator can be evaluated for different charge levels. The results show a clear effect of tube insertions on liquid flooding in a straight tube and find the felt tube insertion to be most effective at impeding flow. The evaporator test results also present a strong correlation of liquid flooding parameters with the charge level and show only a small dependence on the orientation of the evaporator.