Numerical simulations of non-ideal supersonic nozzle expansions

Non-Ideal Compressible-Fluid Dynamics (NICFD) is the branch of fluid mechanics devoted to the study of compressible flows whose behaviour deviates from the one predicted by the ideal-gas model. This behaviour is typical of molecularly complex compounds in the vapour phase, namely when the operating thermodynamics conditions are close to the liquid-vapour saturation curve and critical point. NICFD flows are characterized by peculiar features which are not physically admissible under the ideal flow assumption, like the non-monotone variation of Mach number along isentropic expansions in converging-diverging nozzles. In the present thesis, numerical simulations are performed using the open-source NICFD solver SU2 to compute the flow field of siloxane MM (hexamethyldisiloxane, C6H18OSi2) expanding isentropically into a converging-diverging nozzle, and then the numerical solutions will be compared to experimental data, that were obtained in the frame of past MSc and PhD theses at the Test Rig for Organic VApors (TROVA) the blow-down wind tunnel of the Laboratory of Compressible-fluid dynamics for Renewable Energy Applications (CREA Lab) of Politecnico di Milano. Two experimental campaigns are considered to set the test conditions of the numerical computation. The first one was composed of four tests with different total conditions. These conditions are used to perform CFD simulations to study the behaviour of the fluid in order to understand the effect of the boundary layer on the flow field. For this purpose, three different simulations are performed for each condition: two- dimensional computations based on Euler and RANS equations and three-dimensional ones based only on RANS equations. Also, the four simulated conditions have different total compressibility factor (ZT ), this allows verifying the non-ideal dependence of isentropic expansions on total conditions. The second experimental campaign was composed of twenty-six tests with different stagnation conditions covering a significant part of the vapour phase of fluid siloxane MM. Numerical simulations performed on these several conditions and grouped in terms of their ZT allow proving numerically the suitability of ZT as a similarity parameter for non-ideal flows. The studied supersonic expansions are relevant for practical applications since the simulations are representative of the typical operating conditions of Organic Rankine Cycle (ORC) power systems, more precisely of supersonic flows within the turbine vanes of the first stator. Also, they represent a contribution to the experimental research aimed at providing assessment to NICFD theory and simulation tools.