CFD modelling of small scale ORC scroll expanders using variable wall thicknesses

The built-in volume ratio of variable wall thickness scroll expanders can be increased without increasing the number of scroll turns and the expander size in contrast to constant wall thickness expanders. CFD models for these novel scroll-type designs are presented in this research paper. The validation, verification and the findings had proven consistency with the theory of small scale ORC scroll expanders. The performance analysis indicates that the optimum performance point was reached at a pressure ratio of 3.5. The decrease of radial clearance from 200 μm to 75 μm had a significant effect on the isentropic efficiency and the specific power output, with the isentropic efficiency significantly increasing from 31.9% up to 53.9%. Based on the second-law analysis, it is found that exergy of 336.5W (75 μm) and 864.2W (200 μm) were destroyed during the expansion processes. Furthermore, characteristic pressure imbalances were observed in the expansion chambers. The studies also reveal that the large-scale vortices, generated during the suction process, were completely dissipated in the expansion chambers at a crank angle of 600°. Analysis of the pressure-volume diagram shows that variable wall thickness scroll expanders with built-in volume ratios above 4.5 could fully expand the working fluid to the defined outlet pressure.