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Abstract Consistent simulations of compressor speedlines from choke to surge are challenging for CFD codes. Changing the outlet boundary condition along the speedline is often necessary to obtain physically plausible and numerically robust results. The root of the problem is the compressor characteristic: Close to choke, a mass flow rate as outlet boundary condition can produce ambiguous values and close to surge, a pressure boundary condition can yield multi-valued results. The paper discusses an outlet boundary condition based on a reduced outlet mass flow rate to overcome this problem. The boundary condition behaves like a pressure boundary condition close to choking and like a boundary condition for the mass flow rate close to surge, allowing for consistent simulations of whole speedlines. The paper explains the mechanics and advantages of this boundary condition in detail. The authors applied outlet boundary conditions for static pressure, mass flow rate, and reduced mass flow rate to the High-Efficiency Centrifugal Compressor (HECC) experimentally investigated at the NASA Glenn Research Centre. The HECC has an impeller with 15 blade-splitter pairs, a diffusor with 20 blade-splitter pairs and 60 exit guide vanes. It is representative of the final stage of a compressor for rotorcraft applications. The HECC is a challenging test case for every CFD method because of its small diameter, the total pressure ratio of 4.7, the high work factor of 0.81, and the complex shock structures in the flow close to the choke condition. In the paper’s results section, the authors discuss the pros and cons of static pressure, mass flow rate and reduced mass flow rate as boundary conditions regarding numerical accuracy and robustness. The simulations are conducted on a sequence of three homogeneously refined meshes with an effective refinement factor of 1.6 between the meshes. For three-dimensional problems, an effective refinement factor of 1.6 increases the number of cells by four. The solutions on the three meshes serve to quantify the numerical accuracy using Richardson extrapolation. The mathematical model for the simulations consists of the steady-state averaged conservation equations for mass, momentum, and total enthalpy, complemented by two-equation eddy-viscosity turbulence models, namely the SST and BSL two-equation eddy-viscosity models and an eddy-diffusivity model for the turbulent heat fluxes. The rotor-stator interaction is modelled with a steady-state mixing plane algorithm using single-blade passages.
Hansen et al. (Mon,) studied this question.