Youhwan Shin

Jet quality characteristics according to nozzle shape of energy-recovery Pelton turbines in pressure-retarded osmosis

AbstractPelton 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.

Theoretical method of selecting number of buckets for the design and verification of a Pelton turbine

ABSTRACT 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.

Effect of the Passage Area Ratio of an Impeller on the Performance of Two-Dimensional Centrifugal Compressors

This study is performed to understand the effect of the variation in the passage area of a two-dimensional impeller on its performance characteristics. We observe the results with changing the area ratio of inlet to outlet about . A comparison between the experimental and numerical results was performed for the same configuration in order to verify the reliability of the CFD code. Overall characteristics in the passages of impeller were analyzed in detail including streamline, Mach number, pressure and polytropic efficiency distribution. When the passage area ratio exceeds 2, the pressure ratio is high. An area ratio of 2.3 showed the highest efficiency. The results will be used as useful reference data to establish the design concept of two-dimensional impeller and to improve its performance.

Numerical Analysis Techniques and Flow Characteristics of Two-Stage Centrifugal Compressor for R134a Turbo-Chiller

In this study, flow structure in a two-stage centrifugal compressor for a turbo-chiller with the refrigerant, R134a, was numerically investigated at the design point of the compressor using a commercial code. Flow characteristics in the passages of impeller, diffuser and return channel were analyzed in detail including velocity vector, secondary flow, Mach number and pressure contours in blade spanwise and meridional plane for each stage. The estimation on the one-dimensional output from the preliminary design and three-dimensional shape of the impeller blade and the meridional shape of the return channel were performed through the flow analysis, while some numerical schemes and techniques including Multiple Frames of Reference technique, real gas property data and inlet boundary condition changes, which were used in CFD, were compared with their features. The results will be used as reference data for a new design of 3-D impeller shape to improve R134a compressor performance.