06 November 2008

CHARACTERISTICS OF PRESSURE RECOVERY IN TWO-PHASE EJECTOR APPLIED TO 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 mm
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 with 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 in order to compare the constant-area mixing ejector and the constantpressure
mixing ejector. Pressure recovery decreases when the clearance between the
nozzle and the inlet of the mixing tube is large. The constant-area mixing ejector is therefore
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 %.