Pelton turbines are used for energy recovery in pressure-retarded osmosis (PRO) systems. The quality of jet sprays is a significant factor affecting the performance of these turbines. This study examined jet quality characteristics according to the nozzle geometry of Pelton turbines. Jet quality tests were conducted to analyze the degree of similarity between the actual jet flow and the ideal jet flow. Power loss, discharge coefficients, and performance characteristics of Pelton turbines were examined with respect to the nozzle shape through performance tests of the nozzles and turbines. Furthermore, jet flows were numerically analyzed. The test results and analyses confirm that for a given input power, the recirculation region increases and the jet quality decreases with increasing nozzle throat angle. On the basis of the results, the optimum range of nozzle throat angle for Pelton turbines in PRO systems is suggested.

In this study, we develop a theoretical formula for determining the optimum number of buckets required for achieving the maximum possible efficiency of a Pelton turbine. In addition, by deriving an improved formula for determining the speed ratio limit at which a missing jet begins, we obtain a valid condition for the selection of the number of buckets. To validate these formulas, we conduct a performance test on a pico-scale Pelton turbine by using runners with different numbers of buckets. The results of the performance test validate both the formula for bucket selection, by confirming that the calculated number of buckets maximizes the efficiency of the Pelton turbine, and the formula for the speed ratio limit. We conduct an additional performance test on a micro-scale Pelton turbine, the results of which also verify the formula for selecting the number of buckets. This study should help researchers determine the appropriate number of buckets when designing a highly efficient Pelton turbine.

The demands to apply transonic centrifugal compressor have increased in the advanced gas turbine engines. Various techniques are used to increase the aerodynamic performance of the centrifugal compressor. The effects of the inclined leading edges in diffuser vanes of a transonic centrifugal compressor on the flow-field unsteadiness and noise generation are investigated by solving the compressible, three-dimensional, transient Navier–Stokes equations. Diffuser vanes with various inclination angles of the leading edge from shroud-to-hub and hub-to-shroud are numerically modeled. The results show that the hub-to-shroud inclined leading edge improves the compressor performance (2.6%), and the proper inclination angle is effective to increase the stall margin (3.88%). In addition, in this study, the transient pressure variations and radiated noise prediction at the design operating point of the compressor are emphasized. The influences of the inclined leading edges on the pressure waves were captured in time/space domain with different convective velocities. The pressure fluctuation spectra are calculated to investigate the tonal blade passing frequency (BPF) noise, and it is shown that the applied inclination angles in the diffuser blades are effective, not only to improve the aerodynamic performance and stall margin, but also to reduce the BPF noise (7.6 dB sound pressure level reduction). Moreover, it is found that the diffuser vanes with inclination angles could suppress the separation regions and eddy structures inside the passages of the diffuser, which results in reduction of the overall sound pressure level and the broadband noise radiated from the compressor.