Some Modelling Development for Unsteady Flows in Axial Compressors

Abstract: In compressors, there exist various complex unsteady flow structures with 5 to 6 orders of magnitudes variations in length scale. Turbulence, rotor-stator interaction and rotating stall are typical flows for "small scale", "medium scale" and "large scale" flow phenomena, respectively. RANS method is still in high demand for engineering applications. And modelling errors are the main sources for the compressor CFD. In this presentation, emphasis is placed on some modelling development for the above three typical unsteady flows that are compatible with the industrial design environment. The developed models could offer the potential of providing fast and credible results. For turbulence modelling, a new method for modifying turbulence models using helicity to consider the energy backscatter in vortical flows is proposed. The SA model and the SST model are modified. By comparing the numerical results with experiments, it can be concluded that the modification for SA and SST model with helicity can greatly improves the predictive accuracy for simulating the corner separation flow. For unsteady rotor-stator interaction, an exponential decay deterministic correlations (DC) model using meridional distance between blade rows as characteristic length is proposed to close the average-passage equation system proposed by Adamczyk. Based on the unsteady simulation, the proposed model is validated by comparing DC distributions and mean flow fields. The comparison indicates that the proposed model can take into account the major part of rotor-stator interaction and provide significant improvements for predicting spanwise distributions of flow properties in axial compressors, compared with the steady mixing plane method. For rotating stall, a computational model for axial compressor stall inception and its nonlinear evolution using unsteady RANS method as an initial boundary value problem. The initial disturbance with the most unstable mode is obtained by a stall inception eigenvalue approach. Several unsteady inlet boundary conditions with different frequencies are used as initial perturbations for full-annulus unsteady simulations of a transonic compressor rotor flow. Based on comparison and analysis for different simulation examples, the present computational model provides a fast and reliable route to implement nonlinear simulation of compressor rotating stall.