Selection of operating conditions and optimisation of design parameters for multi-vane expanders

Multi-vane expanders possess many advantages as prime movers for organic Rankine-cycle engines utilising low-grade heat as the energy inputs. The performances of these devices can be improved considerably by the proper selection of the operating conditions for existing units and by optimising their designs proposed for specified applications. Computer programs, composed for predicting the behaviours of multi-vane expanders, were employed to ascertain the most suitable operating conditions for two existing designs. A simple formula for selecting the optimal working pressure ratios of the expanders, using R-113 as the working fluid, is presented. Optimal values of the design parameters of the expanders were determined for selected operating conditions. The performances of the optimised designs of the considered expanders are relatively insensitive to changes in the operating conditions of the machines in the vicinity of their optimised values. This is a desirable feature of prime movers in Rankine engines utilising fluctuating low-grade energy sources, such as solar energy.