Experimental characterization of organic vapor flows in asymmetric curved supersonic nozzles

An experimental campaign investigated the supersonic expansion of linear siloxane MM (hexamethyldisiloxane, C6H18OSi2) in asymmetric curved planar nozzles. Two benchmark nozzle geometries were tested, both designed to discharge a parallel uniform flow of MM at Mach 1.6 under reservoir conditions characterized by a compressibility factor Zt=0.40. The nozzles feature gently-curved mean lines from inlet to outlet, discharging flow at geometric deviations of 6° and 17° from the horizontal. They differ in the shape of the channel near the minimum-area section (throat), with the first nozzle exhibiting upper and lower contours of opposite curvature, while the second nozzle has contours of concordant curvature. Shock-free expansions were observed under all tested conditions, ranging from highly nonideal (Zt=0.41) to ideal-like (Zt=0.95). Total pressure, temperature, static pressures from the inlet to the outlet, and schlieren images were recorded for each condition. A novel optical method, recently developed for asymmetric flows, enabled Mach number measurements without prior knowledge of the flow direction. The tested geometries mimic converging–diverging blade passages with a curved mean line. This study provides the first experimental validation of these asymmetric designs, previously introduced and verified only through CFD. The resulting dataset offers a valuable benchmark for validating numerical design and analysis tools in nonideal compressible flows, with direct relevance to the design of supersonic ORC turbine stages.