Analyzing The Mechanism And Signal Of Rotating Stall On Eckardt Centrifugal Impeller

The flow stability in turbomachinery,which is the core component of power equipment in aeronautic and petrochemical industry,severely restricts its aerodynamic performance and operating range,and it influences the security and efficiency of power equipment.As an important phenomenon of flow instability,rotating stall may result in global periodic flow oscillations(surge),leading to power decrease and even mechanical damage.Due to the high rotating speed,complex geometry and sharp unsteady flow of centrifugal impeller,the stall mechanism and stall signal recognition are difficult to be detected.The further research on stall characteristics helps to better understand stall mechanism,then new methods for stall mode recognizing and monitoring can be developed,which provide the theory basis for impeller optimization.To clarify the stall mechanism,the unsteady flow characteristics were analyzed based on the detailed numerical results.Since the circumferential periodicity of flow is destroyed under the stall condition,single passage simulation cannot truly reflect the stall process.In order to capture accurate stall flow structure,the full-annulus unsteady simulation was carried out on Eckardt centrifugal impeller.As a result,the detailed flow characteristics under different operating conditions are obtained.Under design condition,the periodic variation features in passages and the formation mechanism of low-velocity secondary vortex at downstream suction side near shroud are clarified.Under stall condition,4 symmetrical stall cells are identified at blade leading edge.Similar to the spike-type stall in axial impeller,leading edge spillage also occurs in centrifugal impeller.Additionally,due to the strong meridional and circumferential twist of centrifugal impeller,the trailing edge backflow shows new characteristics: the confluence of the tip leakage flow and secondary flow leads to multiple smaller-scale larger-area secondary vortices,unlike axial impeller,the trailing edge backflow cannot directly impinge on the pressure surface.The stall mode was recognized and monitored by spectral analysis method.The static pressure on different streamwise and spanwise was analyzed by spatial Fourier transform.Under design condition,only 20 order mode influenced by blade pass frequency was found.Under stall condition,4 order mode shows the maximum amplitude.By analyzing the phase variation of the 4 order mode,the circumferential velocity of the stall cells moving towards downstream is determined,to be about 0.63-0.73 times of the rotor speed.Spatial Fourier analysis can accurately identify spatial non-uniformity and the circumferential moving rules of the stall cells,thus it can be used for stall signal monitoring and stall warning in real-time.