Analysis on gas dynamic effects and design of supersonic ORC stator nozzles for transcritical expansions

Abstract

Organic Rankine Cycles can reach high efficiency by using transcritical cycles, where the fluid expansion starts in the supercritical region and ends in the superheated vapor region. When molecularly complex fluids are used, their gas dynamic behavior differs significantly from that of expanding ideal gases, and their low speed of sound results in supersonic conditions even with moderate flow velocities. In this study, gas dynamic effects and supersonic nozzle designs for turbines are investigated. Isentropic expansions in conditions near the critical point are considered for different working fluids at varying nozzle inlet total conditions. The reduced total pressure is varied in the range 0.8 to 1.5 and the reduced total temperature in the range 0.95 to 1.20. The fluid and operating conditions have significant effect on the gas dynamic behavior and characteristics of the expansion, as well as on the resulting size of supersonic nozzles. The most significant non-ideal behavior was observed with siloxanes MDM and MM, and hydrocarbons toluene and cyclohexane. The variations in thermodynamic properties along the expansions were observed to be significantly large, especially if the expansions start from the supercritical region. Correspondingly, remarkable variations in the required passage areas and nozzle length were observed.