Simulating apex gap sizes in a small scale Wankel expander for air liquefaction

Abstract

The seals of a Wankel expander produce significant friction and wear, contributing its largest disadvantage. This friction necessitates lubricant to be mixed with the working fluid, increasing the complexity of the device. Removing the seals could therefore produce a significantly more desirable device, if the resulting leakage is acceptable. Currently there are no published studies assessing the performance of a Wankel expander without apex seals, therefore this paper will attempt to address this knowledge gap. A CFD (computational fluid dynamics) model of a Wankel expander without apex seals was developed to assess the performance and leakage amount. Using this model, a parametric study was performed using the apex gap sizes, the rotational speed and the inlet pressure as the variable parameters. Results showed that higher rotational speeds reduced leakage, but resulted in larger pressure drops over the ports, reducing the efficiency. A double-sided ports model helped to counter this, with speeds up to 6000RPM becoming viable. The friction from the apex seals was calculated in order to better compare the cases with and without apex seals. The best efficiency case with apex seals (at 1200RPM, 2 bar inlet pressure) gave 64% isentropic efficiency including apex seal frictional losses. The highest efficiency for the cases without apex seals was 55% (at 6000RPM, 2 bar inlet pressure), however due to the higher rotational speed, the power output was almost four times higher than the case with apex seals above (385 W compared to 100 W).