NIMOC: A design and analysis tool for supersonic nozzles under non-ideal compressible flow conditions

Abstract

A computational tool for non-ideal supersonic nozzle flows is developed to perform direct design of nozzle geometries and flow analysis of shock-free off-design conditions. So-called non-ideal flows are characterized by the departure of the fluid thermodynamics from the ideal-gas law, which is the case of supersonic expansions close to the critical point, either in the superheated vapour or in the supercritical region. Deviations from the ideal-gas model of remarkable significance are observed in connection with, but not limited to, fluids made of complex molecules. NIMOC (Non-Ideal Method Of Characteristics) implements a formulation of the Method Of Characteristics (MOC) valid for non-ideal flows: the classical formulation of the MOC is complemented with state-of-the-art nonlinear multiparameter Equations of State (EoS) implemented in external thermodynamic libraries. NIMOC implements a two-dimensional and axisymmetric formulation of the MOC, which is applied to the solution of the isentropic expansion through nozzles of different geometrical configurations: straight-axis symmetric and axisymmetric conventional wind-tunnel nozzles, straight-axis symmetric and axisymmetric minimum-length nozzles, and asymmetric nozzles with curved meanline. Results are presented, which demonstrate nozzle design and flow analysis in the presence of paradigmatic non-ideal flow phenomena. Verification of MOC computations is performed by means of inviscid Computational Fluid Dynamics (CFD) simulations and experimental data. Results are presented for selected geometrical configurations.