采用不同黏性处理方法的宽无叶扩压器不稳定流动研究

STUDY ON INSTABILITY OF THE WIDE VANELESS DIFFUSER WITH DIFFERENT TREATMENTS OF VISCOSITY

径向无叶扩压器的全局稳定性可能受到核心主流失稳,出口回流与壁面边界层分离等因素影响,对于宽无叶扩压器,无黏核心主流与壁面边界层流动对不稳定扰动诱发的作用机理是当前研究的重点.本文首先通过数值计算获得了大宽度比孤立无叶扩压器平均流动,然后基于小扰动理论和周向均质假设,分别对欧拉方程与Navier-Stokes方程进行线性化,建立了基于无黏核心流动的稳定性分析方法,以及基于涡黏性与分子黏性的混合稳定性分析方法;通过与实验结果的对比,验证了混合稳定性分析方法预测所得流动失稳频率和全局直接模态的准确性;最后基于伴随方法获得了特征值的结构敏感性,揭示了不同黏性处理条件下宽无叶扩压器内全局不失稳扰动的源发区域.在只考虑核心主流的无黏条件下,宽无叶扩压器内流动不稳定扰动来源于流场中部,为二维的离心失稳;在同时考虑核心主流与边界层的作用时,宽无叶扩压器不稳定扰动不仅来源于扩压器流场中部的核心主流,壁面回流对于不稳定扰动的产生了重要影响.

The global stability of vaneless diffuser in the centrifugal compressor is often influenced by the main flow stability,backflow at exit and the boundary layer separation.For the vaneless diffuser with large axial width ratio,the effect of the main flow and boundary layer on the instability perturbations is the main subject.In this paper,the mean flow of the wide vaneless diffuser is firstly obtained with numerical simulations.The Eulers’and Navier-Stokes equations are linearized respectively based on the small perturbations assumption.Then the inviscid stability approach considering the inviscid main flow and the mixed stability approach considering the effect of both eddy viscosity and molecular viscosity are established.The prediction results are validated against the experimental results.At last,the structural sensitivity based on the adjoint method is adopted.And the wave-maker region is revealed under different treatments of viscosity.The instability perturbations are located at the middle of the flow field when inviscid main flow is only considered.And a centrifugal instability maybe the main cause of the vaneless diffuser stall.When the inviscid main flow and boundary layer are both considered,the wave-maker region not only lies at the main flow near the middle of the vaneless diffuser,but also lies at the reverse flow region of the boundary layer.