Influence of Molecular Complexity on Nozzle Design for an Organic Vapor Wind Tunnel

Abstract

A novel blowdown wind tunnel is currently being commissioned at the Politecnico di Milano, Italy, to investigate realgas behavior of organic fluids operating at subsonicsupersonic speed in the proximity of the liquidvapor critical point and the saturation curve. The working fluid is expanded from a highpressure reservoir, where it is kept at controlled superheated or supercritical conditions, into a lowpressure reservoir, where the vapor is condensed and pumped back into the highpressure reservoir. Expansion to supersonic speeds occurs through a convergingdiverging Laval nozzle. Siloxane fluid MDM (octamethyltrisiloxaneC8H24O2Si3) 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 realgas operating conditions, including linear and cyclic siloxanes, refrigerant R245fa, toluene, and ammonia. The thermodynamic properties of these fluids are described by stateoftheart 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.