Experimental investigation on the internal working process of a CO2 rotary vane expander

This paper presents the experimental investigation on the internal working process especially the internal leakage of a rotary vane expander prototype, which was developed to replace the throttling valve to improve the Coefficient of Performance (COP) of the transcritical CO2 refrigeration cycle. The pressure diagram as a function of the rotation angle (p–θ diagram) was recorded by two pressure sensors arranged within the expansion chamber, based on which the features of the working process were analyzed and effects of the some improvement measures on the internal leakage were discussed. Compared with the ideal p–θ diagram, the recorded diagrams presented more rapid decrease in the pressure during expansion process, which was attributed to serious leakage within the expander. Further analysis of the recorded p–θ diagrams showed that three adjacent vanes instead of two formed an integrated working chamber, which implied that the in-between vane did not contact the cylinder wall at all, and this guess was proved by the high speed video recording of the running rotor together with the vanes during operation of the expander in CO2 system. By arranging springs in the vane slots, tight contact between the vanes and the cylinder wall was ensured and hereby the working process was improved. The gap between the suction and discharge ports was another important leakage path and installation of a seal there could increase the pressure difference through the expander by 1.5–2.5 MPa by decreasing the leakage directly from the high-pressure suction chamber to the low-pressure discharge chamber. Performance test of the modified expander prototype with springs in the slots and seal at the seal arc showed that the volumetric efficiency increased from 17% to 30% and the isentropic efficiency from 9% to 23% at the speed of 800 rpm.