Characteristics of CO2 transcritical expansion process

Abstract

In order to improve the performance of the CO2 refrigeration cycle, the use of an expander as an expansion device has been examined. The CO2 expansion process in the expander is a transcritical expansion process in which the CO2 condition changes from supercritical to two-phase. In this study, the transcritical expansion process is investigated in detail using a single piston expander with glass windows. The expansion process is observed with a high-speed camera, and the heat transfer coefficient within the expansion chamber is examined based on the enthalpy change during the process. It is shown that the transcritical expansion process is accompanied by a non-equilibrium condition (delay of flash) when an initial temperature of the expansion process is low. When the expansion process passes across the saturation gas line or the saturation liquid line near the critical point, a blackout phenomenon occurs by generation of fine mist of the liquid phase. In addition, the heat transfer coefficient for the heat transfer from the expander wall to CO2 inside the expansion chamber is found to be about 2–7 kW/(m2K) (352–1233 Btu/(hft2°F)) in the two-phase region.