CO2 has received extensive attentions in developing various energy conversion systems. In recent years, a CO2 ultra-low temperature cascade refrigeration system with dry ice sublimation has been introduced. The system is composed of two CO2 refrigeration compression cycles, namely high pressure cycle and low pressure cycle, to achieve a cryogenic refrigeration bellow the CO2 triple-point temperature of -56.6℃. In the expansion process of the liquid CO2 at the low pressure cycle, dry ice (solid-gas two-phase flow) is formed, and it has been know that the geometric configuration of the evaporator/sublimator is one of important factors in the CO2 ultra-low temperature cascade refrigeration system.
In this report, a tapered evaporator/sublimator is newly designed and installed into the existed CO2 ultra-low temperature cascade refrigeration system in order to investigate the heat transfer characteristics and its associated system performances. Based on the investigation, it is found that the CO2 ultra-low temperature cascade refrigeration system with the tapered evaporator/sublimator has the capability of achieving the ultra-low temperature of -66.3℃ continuously. Resultantly the achieved refrigeration temperature with the tapered evaporator/sublimator is 3.5℃ lower than that of the sudden expansion evaporator/sublimator which is installed in the previous system.