Experimental study of the heat transfer problem in expansion devices in CO₂ refrigeration systems

Two expansion devices designed for CO₂ refrigeration systems namely, capillary tube and fixed-geometry ejector were experimentally tested in the SINTEF/NTNU laboratory in Trondheim, Norway to determine the influence of ambient conditions on their performance. The novelty of this research was to examine the influence of heat transfer between the expansion device wall and the ambient on the R744 ow inside. To achieve this goal, both aforementioned expansion devices were tested with thermally insulated and uninsulated walls. A wide range of operating conditions typical for transcritical CO₂ refrigeration systems were examined. The relatively high ambient temperatures, which occurred during experiments, simulated the realistic conditions in the machine-room of the refrigeration system. The capillary tube consisted of sections of the same length and number of bends. Temperature and differential pressure sensors were placed between those sections to investigate the characteristics of CO₂ within the capillary tube. To measure the inner wall temperature using thermocouples, a prototype R744 ejector with drilled channels was prepared. Ambient conditions did not significantly affect the operation of the capillary tube. Similar conclusions were made for the R744 ejector. However, a slight effect on the temperature of the ejector's wall was observed.