Numerical Research Of Unshrouded Impeller At Small Flow Rate Condition

The centrifugal compressor is widely used in the gas compression process.Many kinds of method have been invented for stall margin improvement.For instance,holed casing treatment has been used in a certain centrifugal compressor,and it has been proved to be effective for stall margin improvement through experiment and numerical simulation.Steady and unsteady numerical method have been used to seek the position where the unstable flow will occur and to know how shall aerodynamic parameter changes during this process,hoping for providing basis for holed casing treatment method.For validation of the numerical method,mesh generation method,computational residual and advection scheme were discussed.Method reliability was made by comparing simulation results with experimental data.Thereafter,the match between the centrifugal impeller and the diffuser of different width were analyzed,to provide an effective reference for the appropriate design of centrifugal compressor.After that,the centrifugal impeller passage model,which is uniform to the real condition,with the whole passage grid,was numerically simulated with steady simulation method at design and small flow rate conditions.The overall performance of full passage was similar to that of single blade passage model.The flow field at near blade tip region of these two models show remarkable difference.The impeller internal tip clearance leakage flow,reflux in the tip clearance region,and the streamline near the blade tip of the full passage model has been analyzed,non-periodic characteristics of the flow field were found at low flow rate conditions.At 0.4~0.6 normalized blade length,non-periodic characteristics became more obvious as the decrease of the mass flow rate,showing high probability that this region may be the first place for the initiation of flow instability.Then,the influence of the circumferential total pressure distortion at inlet on the performance and internal flow field was discussed with the steady state simulation using the full passage model.It was found that with the decreasing of the flow rate,the impact of inlet distortion on the impeller internal flow field was more noticeable.The single blade passage model was used to conduct the transient flow simulation.Analysis and discussion of the convergence criterion of unsteady calculation was made.Considering the limitation of periodic boundary condition of the single passage,unsteady simulation of full passage model was conducted.Unsteady feature of velocity and pressure at blade tip region was analyzed.FFT was adopted for frequency domain analysis and the spectrum feature of the flow fields was obtained.Nonperiodic flow feature of the steady and time-averaged unsteady simulation results was similar,proving superiority of the full passage model.