In order to improve the performance of CO2 refrigeration cycle, the use of expander as an expansion device has been examined. CO2 expansion process in the expander is a transcritical expansion process in which the condition of CO2 changes from supercritical to two phase. In this study, the transcritical expansion process is investigated in detail by using a single piston expander having glass windows. The expansion process is observed by a high speed camera and heat transfer coefficient in the expansion chamber is examined based on 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, a blackout phenomenon occurs by generation of fine mist of the liquid phase. In addition, the heat transfer coefficient in the expansion chamber is found to be about 2 - 7 kW/(m2K) in the two phase region.
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