Experimental Observation of CO2 Dry-ice Behavior in an Evaporator/Sublimator

In the evaporator/sublimator process of CO2 (R744) ultra-low temperature cascade heat pump system, it is known, as a problem, that dry-ice blockage makes the system operation fail. The design of an expanding channel for the evaporation/sublimation process is one solution to solve the problem. In this study, a swirl promoter is newly designed and tested by means of experimental observation. In order to give better understanding to heat transfer process in the refrigeration system, particular attention is focused for CO2 dry-ice solid-gas two-phase flow to obtain the effectivity of heat transfer and to verify the flow-phenomena where a visualization test apparatus is placed horizontally. As results, by installing the swirl promoter in the inlet flow channel, it is found that the dry-ice particles uniformly dispersed in whole pipe cross-section with swirling flow, which induces an increase of heat absorption in CO2 solid-gas two-phase flow. In the case without a swirl promoter, a dry-ice ball shows lower heat transfer characteristic for higher inlet pressures, and resultantly leading to a possible down-stream blockage of the channel.