Induced Mechanism And Energy Loss Of Rotating Stall In The Centrifugal Compressor With Vaneless Diffuser

Centrifugal compressor is one of the most important devices in micro gas turbine.The existence of unsteady flow is an important source which will influence the efficient and safe operation of centrifugal compressor.According to the location of stall,the instability of centrifugal compressor mainly includes impeller stall and diffuser stall.And the pressure fluctuation caused by diffuser stall is larger than that of impeller stall,which may bring more damage.Vaneless diffuser is widely used because of its simple structure,wide stable operation range and low production cost.However,the study of three-dimensional complex unsteady flow in vaneless diffuser,especially the instability mechanism of flow field under different width ratio and inflow conditions,and the correlation between energy loss and stall are not clear.So,this thesis focus on the investigation of induced mechanism of rotating stall in vaneless diffuser of centrifugal compressor.This work can not only deepen and improve people's understanding of the instability mechanism of centrifugal compressor,but also help to form advanced diffuser design and control technology,which preserved important practical value for the safe and stable operation of centrifugal compressor.In this thesis,the centrifugal compressor with vaneless diffuser was taken as the research object.Through theoretical modeling,experimental testing and numerical simulation,the mechanism of rotating stall induced by vaneless diffuser and the mechanism of energy loss were studied.Based on the eigenvalue method,the inviscid three-dimensional stability model of vaneless diffuser was constructed,and the steady and unsteady flow field numerical simulation of centrifugal compressor was carried out.Based on numerical simulation and stability model analysis,the stall mechanisms of narrow vaneless diffuser and wide vaneless diffuser under asymmetric inflow were revealed respectively.Based on the grid mapping technique,the stability model of vaneless diffuser with non-parallel wall was constructed.The stall inducing mechanism of vaneless diffuser with non-parallel wall was studied by using the dynamic mode decomposition method.Based on the field synergy analysis,the correlation between energy loss and stall mechanism of vaneless diffuser was established.The main results of this paper are as follows:(1)Based on the eigenvalue method,the inviscid three-dimensional stability model of vaneless diffuser was constructed,and the unsteady numerical simulation of the stall process of vaneless diffuser in centrifugal compressor was carried out.The accuracy of the stability model and numerical calculation was verified by experiments.The results show that the inlet jet wake structure of vaneless diffuser is significantly weakened,and the recirculation region is significantly larger than that of the design condition.The flow separation near the hub side is the main reason for the stall of vaneless diffuser.(2)The stall inducing mechanism of wide and narrow vaneless diffusers in a centrifugal compressor with axial asymmetric inflow was revealed.For the isolated narrow vaneless diffuser with a given asymmetric inflow condition,the upstream velocity distribution have a significant effect on the distribution of instability disturbance near the inlet.And there is a strong outlet reflux disturbance in the narrow vaneless diffuser.For an isolated wide vaneless diffuser with a given asymmetric inflow condition,the flow separation near the inlet wall and outlet wall result in the generation and amplification of the velocity disturbance.(3)The influence of nonparallel wall on the instability of vaneless diffuser in centrifugal compressor was clarified.Different shapes of shroud side not only have a significant impact on the aerodynamic performance of vaneless diffuser,but also change the key spatial position and stall mechanism induced by stall.For a parallel wall diffuser,the separation of the flow near the outlet of the diffuser from the flow near the wall is the main factor causing the stall.For the inlet broken line vaneless diffuser,the main cause of stall is the near wall flow separation on the hub side.The local backflow at the inlet is the main factor leading to stall.(4)Based on the field synergy theory,the relationship between the energy loss and stall induced mechanism of vaneless diffuser was revealed.The one-dimensional energy loss prediction model of centrifugal compressor was built.By constructing the weight coefficient to measure the contribution of various losses to the total pressure ratio of compressor,it is found that the influence of friction loss and secondary loss gradually decreased from design condition to near stall condition,while the influence of diffusion loss gradually increased.Although the secondary flow and the diffusion process can lead to the separation of the boundary layer in the diffuser,the momentum loss caused by the strong diffusion effect have a great influence on the generation of the hydrodynamic instability in the vaneless diffuser with large width ratio and small inflow angle.