Study On The Influence Of Radius Ratio In Centrifugal Compressor Wide Vaneless Diffuser On Stall Mode

Centrifugal compressors have a wide range of applications in distributed power generation and cogeneration,Because of their high gas volume,high pressure ratio,simple structure and easy maintenance,they have an extremely important position in the national economic system.The rotating stall of the vaneless diffuser is the most common type of rotary stall in centrifugal compressors,and its working range is larger than that of the blade type,and the frequency of use is higher.Therefore,it is important to study the rotation failure mechanism and starting point of the vaneless diffuser.The flow in the vaneless diffuser is not only affected by the upstream impeller,but also related to the size structure of the diffuser itself.Therefore,for Vaneless Diffusers with different radius ratios,their stall mechanisms and stall modes are also different.In this paper,the numerical simulation of two kinds of centrifugal compressors with different radius ratios of the vaneless diffuser is carried out.The influence of the radius ratio of the diffuser on the induction mechanism of the diffuser stall is mainly studied.The main conclusions are as follows:By changing the outlet flow of the diffuser for steady-state simulation,it is found that the flow field changes at the entrance of the two diffusers are consistent: under the large flow coefficient,the recirculation zone is formed on the disk side of the diffuser and the core flow is close to the cover side,with the flow With the reduction,the recirculation zone disappears on the disk side and appears on the cover side,and the core flow moves from the cover side to the disk side.In the circumferential direction of the diffuser with a radius ratio of 1.5,the flow is uneven,the high-speed zone and the low-speed zone are alternately distributed,and the jet wake structure affects the diffuser outlet;for the diffuser outlet with a radius ratio of 1.8,the flow is uniform on circumferential direction.The jet wake structure is difficult to influence the exit.It is found that the instability of the boundary layer is the key for inducing the rotational stall of the short vaneless diffuser.The formation process of the stall under its influence is as follows: Since the radius of the vaneless diffuser is relatively small,the jet-wake flow at the impeller outlet has a greater impact on the vaneless diffuser flow,resulting in the non-uniform circumferential flow at the diffuser outlet,and the alternate distribution of high-speed and low-speed regions.When the flow rate is decreases,a backflow zone first appears in the low-speed zone of the vaneless diffuser.Then,three recirculation zones appeared to evolve with the stall,eventually forming three stall cells.Through the study of a wide diffuser with a radius ratio of 1.8,it is found that the instability of the core flow is critical to the stall.Before the stall occurs,there is a backflow region at the hub side and the boundary layer on the shroud side was stable of vaneless diffuser,the outlet of the diffuser flows uniform.With the impeller rotating,the backflow zone is simultaneously generated on the hub side and the shroud side,resulting in instability of core flow and the outlet of the diffuser is nonuniform.Backflow occurs at the outlet of the diffuser hub side and the number of baceflow zones are the same as the number of compressor passages.Over time,the reflow intensity of the diffuser outlet is strengthened,and gradually merges to form four stable stall cells eventually.By changing the stall flow rate of the diffuser with a radius ratio of 1.8,it is found that there are two different stall phenomena.Under the large stall flow rate,the number of stall cell is four,and the rotation speed of stall cell is lower at 30.9% impeller speed.When the stall flow rate decreases to 0.4kg/s,the number of stall cell is reduced to three,but the rotation speed of stall cell is increased to 38.5% impeller speed.