CFD analysis of the influence of variable wall thickness on the aerodynamic performance of small scale ORC scroll expanders

Abstract

This research paper presents a CFD analysis of small scale ORC scroll expanders using variable and constant wall thicknesses by providing back-to-back aerodynamic performance comparisons. The evaluation of the three-dimensional and transient CFD results shows that the shorter scroll profile length of the variable wall thickness design (VWD) generated lower average radial and axial gas forces. In addition, higher pressure gradients in between individual working chambers contributed to a higher peak of the tangential gas moment despite higher transient gas force and tangential gas moment variations. Moreover, the pressure trace analysis reveals that the expansion process was finished at a crank angle of 816° in VWD, compared to 996° in the constant wall thickness design (CWD). The studies of the static pressure distributions along the surface of the fixed scroll of the two geometries indicate that static pressure drops through local radial clearances were higher in VWD. However, a higher number of static pressure drops occurred in CWD. The expansion process of CWD was driven by lower pressure gradients resulting in a complete dissipation of the large-scale vortices in the expansion chambers of CWD at the crank angle of 672°, in contrast to 600° in the expansion chambers of VWD.