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Session: Parallel Session: Prototypes and Experiments
Session starts: Friday 23 September, 09:00
Presentation starts: 09:20
Room: Auditorium

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Alberto Guardone (Dipartimento di Ingegneria Aerospaziale, Politecnico di Milano, Italy)
Andrea Spinelli (Dipartimento di Energia, Politecnico di Milano, Italy)
Vincent Vandecauter (Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands)
Vincenzo Dossena (Dipartimento di Energia, Politecnico di Milano, Italy)

Abstract:
A novel test rig for investigating the real-gas behavior of organic fluids operating at subsonic-supersonic speed in the proximity of the liquid-vapor critical point and the saturation curve has been constructed at the Politecnico di Milano, Italy. This is a blow-down facility in which an organic vapor is expanded from a high-pressure reservoir 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. A standard design technique based on the Method Of Characteristics (MOC) is used to design of the supersonic portion of the nozzle [1]. The transonic potential equation is solved by means of the approximate solution procedure of Sauer [2], which is applicable to real-gas flow without significant modifications, to compute the transonic flow at the nozzle throat. The transonic flow solution provides the initial data curve for the MOC. The expansion through the divergent section to the desired exit pressure is achieved via an initial circular profile followed by the so-called turning region, in which the nozzle upper wall geometry is determined by imposing the conservation of the mass flow at each section. The resulting flow at the nozzle exit is with uniform Mach number and parallel to the x axis. Further details on the design procedure, including comparisons with available ideal gas solutions, are given in [3]. Differently from the well-known ideal-gas results, the shape of the supersonic expander - the divergent section of the nozzle - depends on the reservoir or total flow conditions and therefore diverse designs are obtained for a given exit Mach number depending on the relative location of the initial state in the volume-pressure thermodynamic plane with respect to the liquid-vapor saturation curve. For flow states close to the liquid-vapor saturation curve and critical point, the nozzle length and height are larger than the corresponding ideal gas designs. Four different operating conditions for siloxane fluid MDM (Octamethyl-trisiloxane) and refrigerant R245fa are considered and the resulting nozzle designs are thoroughly discussed. The effect of the molecular complexity of the fluid on the final design is throughly investigated for the cyclic siloxanes D4 (Octamethyl-cyclotetrasiloxane), D5 (Decamethyl-cyclopentasiloxane) and D6 (Dodecamethyl-cyclohexasiloxane) and for the linear ones MM (Hexamethyl-disiloxane), MDM, MD2M (Decamethyl-tetrasiloxane), MD3M (Dodecamethyl-pentasiloxane).