Study On Two Stall Modes In Centrifugal Compressor Wide Vaneless Diffuser

As a kind of turbomachinery,centrifugal compressor is widely used in various sectors of national economy.However,stall and surge are the main problems that limit the stable operation range of centrifugal compressor.As an important part of the centrifugal compressor to improve the pressure,the wide vaneless diffuser has a higher efficiency in a larger range.It is of great significance for the design of wide vaneless diffuser and the active control of rotating stall to study the difference of stall mode and mechanism of wide vaneless diffuser with different radius ratio.In this paper,a wide vaneless diffuser with different radius ratio is taken as the research object.The numerical simulation of the instability process of the wide vaneless diffuser is carried out,and the differences of the stall cell numbers,the circumferential propagation speed,the stall frequency and the stall induced process are analyzed.The results show that: there are different stall modes in the wide vaneless diffuser with different radius ratio.For a wide vaneless diffuser with a radius ratio of 1.8,the stall cell number is four at the initial flow rate,the circumferential propagation speed is small,the number of stall cells does not change,and the circumferential propagation speed gradually increases from low speed to high speed as the decrease of flow rate,and finally stabilizes at about 31% of the impeller speed.The frequency of stall cells also decreases with the flow rate.For a wide vaneless diffuser with a radius ratio of 1.5,the number of stall clusters at the initial flow rate is 2,which has a large circumferential propagation speed.With the reduction of flow conditions,the number of stall clusters does not change,and the circumferential propagation speed of stall clusters is stable at about 38% of the impeller speed,the frequency of vaneless diffuser stall clusters also almost does not change with the reduction of flow rate.Secondly,the rotating stall of wide vaneless diffuser with different radius ratio is caused by the development of outlet reflux,however,the evolutionary process are different.When the radius ratio is 1.8,part of the outlet reflux enters the inlet of the vaneless diffuser after a long process of movement,the inlet reflux of the diffuser.As the outlet reflux increases gradually,the streamline shorter.When the reflux area develops to 50% of the blade height,the outlet reflux and the inlet reflux begin to converge.When the "S" type indirect reflux structure formed in the diffuser,stable stall cells formed.When the radius ratio is 1.5,part of the reflux flow back to the inlet of the diffuser after a short movement process,which causes the inlet reflux.When the reflux area develops to 50% of the blade height,the outlet return area and the inlet return area begin to converge,leading to the appearance of stall precursor.Finally,when a "1" type direct reflux structure is formed from the outlet to the inlet,stable stall cells are formed in the diffuser.Finally,reducing the diffuser radius ratio can increase the stability of vaneless diffuser.When the radius ratio of the diffuser is 1.5,the initial flow rate of rotating stall is lower than that of the diffuser when the radius ratio is 1.8.