Analysis of Radial Compressor Performance at Low Pressure Ratios and Compressor Choke

Strong transient engine load steps can result in low pressure ratio (⁠$ΠC$⁠) compressor operation for single stage turbocharged (TC) systems. For conventional full load TC engine matching using one-dimensional (1D)-engine process simulation, these operating points are of limited relevance and are consequently less studied. However, for the layout of sequential turbocharging systems, low pressure ratio compressor operation has to be thoroughly understood. Therefore, in this paper, three-dimensional (3D)-computational fluid dynamics (CFD) simulations will be presented, which analyze the stationary compressor behavior at low pressure ratios. Operating points at $ΠC<1$ are investigated by reducing the compressor outlet pressure. The simulation results are validated against measurement data acquired at a stationary hot gas test bench. The compressor performance is quantified by a corrected compressor torque. Opposed to the well-known operation at $ΠC>1$⁠, the compressor generates power close to zero speed for $ΠC<1$ (turbine operation). At higher mass flowrates and $ΠC<1$⁠, the compressor consumes power. Pressure build-up in the wheel is overcompensated by losses in the diffusor and the volute resulting in a net pressure drop across the stage. The 3D-CFD simulations also allow a speed-dependent evaluation of the choking cross section inside the compressor. At low circumferential speeds, compressor choke occurs in the volute or at the wheel outlet. At higher speeds, choking is observed at the wheel inlet. This behavior must be accounted for compressor map extrapolation methods for 1D-engine process simulations in order to correctly predict the choking mass flowrate.