Research On The Mechanism Of Rotating Stall In Wide Vaneless Diffusers Of Centrifugal Compressors

Micro gas turbine is widely used in small distributed power generation and cogeneration,in which centrifugal compressor is one of the important equipment of micro gas turbine.Centrifugal compressor operating in small flow conditions will produce unsteady flow phenomena such as rotating stall and surge.Diffuser is an important component of the centrifugal compressor,the wide vaneless diffusers are widely used because of the advantage for the wide operation range with higher efficiency.The research of stall induced mechanism in centrifugal compressor with wide vaneless diffuser has important significance for optimization design of vaneless diffusers and active control of the rotating stall.In this paper,two centrifugal compressors with wide vaneless diffusers are disigned as the research subjects to analysis the flow distributions in vaneless diffusers under different flow coefficients.The results show that the field distributions in vanless diffusers of two centrifugal compressor under different flow coefficients have certain regularity.Namely,the vaneless diffuser reflux zone first appears at the diffuser plate side,with the decrease of the flow coefficient the backflow zone disappears in the plate side and appears on the cover side.The flow coefficient is further reduced,two walls of the diffuser have the reflux zone,cover side flow appears in the diffuser entrance while plate side is extended to the diffuser outlet.Due to the separation of the vortices at the boundary layer and the presence of the return flow at the outlet,the core flow in the vaneless diffuser is distorted and the rotating stall is induced.From the simulation results,it is found that there is no necessary connection between the backflow region and the rotation stall in the vaneless diffuser.The presence of a recirculation zone does not necessarily lead to a rotational stall.The inlet flow angle of the vaneless diffuser decreases with the decrease of the flow coefficient,and the wider the vaneless diffuser,the more sensitive the inlet flow angle to the change of the flow coefficient.The unsteady state simulation of the centrifugal compressor under stall flow coefficient was carried out.The number of stall cells and rotation speeds in the vaneless diffuser under stall conditions were calculated and analyzed.The unsteady evolution of the flow field was analyzed.Centrifugal compressor with vaneless diffuser width ratio of 0.141,The inlet critical flow angle of vaneless diffuser is 12.3°,the speed of stall in the diffuser is 0.42 wr,and the number of stall cell is one,When the flow coefficient decreases further,the number of stall cells increases from one to four.Centrifugal compressor model with vaneless diffuser width ratio of 0.182.The diffuser inlet critical flow angle is 9.98°,the speed of stall in the diffuser is 0.38 wr,and the stall cell is three.The increase of the centrifugal impeller speed makes the stall critical angle smaller,while the increase of the vaneless diffuser width leads to the decrease of the non-dimensional rotational speed of the stall cell.The formation of the rotating stall of the vaneless diffuser is as follows: the return flow first appears at the outlet boundary of the diffuser,and as the impeller rotates,the return flow gradually expands to the interior of the diffuser,forming the same number of flow channels as the time,The recirculation zone gradually grows and merges with each other to form a stable low-velocity fluid mass.The resulting low velocity fluid mass rotates at about 40% of the impeller speed and the low velocity fluid mass also rotates in a direction opposite to the impeller rotation.