Characteristics of pressure recovery in a two-phase ejector applied to a carbon dioxide heat pump cycle.

The aim of this research is to elucidate the characteristics of pressure recovery of a two-phase ejector applied to a carbon dioxide heat pump cycle. The effects of the geometric parameters of the ejector on pressure recovery are investigated in this study. The diameter and length of the constant-area mixing tube are varied within ranges of 2.0-3.6 and 2-40 mm, respectively. The maximum pressure recovery is achieved at a distance of approximately 20 mm from the inlet of the mixing tube, irrespective of the diameter of the mixing tube, when the diameter of the nozzle throat is 0.82 mm. In addition, pressure recovery in the mixing section with a diameter of 3.6 mm is less in comparison to those with diameters of 2.0 and 2.5 mm; in the two latter cases, almost the same pressure recovery is observed. The clearance between the nozzle outlet and the inlet of the constant-area mixing tube is varied to compare the constant-area mixing ejector and the constant-pressure mixing ejector. Pressure recovery decreases when the clearance between the nozzle and the inlet of the mixing tube is large. Therefore, the constant-area mixing ejector is preferable for the carbon dioxide heat pump cycle. Finally, the performance of the ejector equipped with a diffuser is investigated; the measured ejector efficiency is 46%, which corresponds to a COP improvement of approximately 23-24%.