A novel blow-down wind tunnel is currently being commissioned at the Politecnico di Milano, Italy, to investigate real-gas behavior of organic fluids operating at subsonic-supersonic speed in the proximity of the liquid-vapor critical point and the saturation curve. The working fluid is expanded from a high-pressure reservoir, where it is kept at controlled super-heated or super-critical conditions, into a low-pressure reservoir, where the vapor is condensed and pumped back into the high-pressure reservoir. Expansion to supersonic speeds occurs through a converging-diverging Laval nozzle. Siloxane fluid MDM (octamethyltrisiloxane-C8H24O2Si3) is to be tested during the first experimental trials. A standard method of characteristics is used here to assess the influence of the molecular complexity of the working fluid on the design of the supersonic portion of the nozzle by considering different fluids at the same real-gas operating conditions, including linear and cyclic siloxanes, refrigerant R245fa, toluene, and ammonia. The thermodynamic properties of these fluids are described by state-of-the-art thermodynamic models. The nozzle length and exit area are found to increase with increasing molecular complexity due to the nonideal dependence of the speed of sound on density along isentropic expansion of organic fluids.

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