【作者】 沈枫；

【导师】 杜朝辉；

【作者基本信息】 上海交通大学 ， 动力机械及工程， 2011， 博士

【摘要】 无叶扩压器失速是离心压缩机系统内常见的非稳定流动现象之一,它限制了扩压器的稳定工作范围,也限制了离心压缩机整机的稳定工作范围,并影响整机的运行可靠性。认识无叶扩压器内的非定常流动特性,掌握预测无叶扩压器失速的方法,对于扩展其运行范围,改善其工作效率意义重大。而深入研究和理解无叶扩压器内的流动现象,对于无叶扩压器的优化以及与压缩机系统中其他部件的匹配也具有指导意义。本文建立了三种离心压缩机系统内无叶扩压器失速的三维模型,研究了无叶扩压器内的非稳定流动及其失速特性。本文首先通过求解线性化的三维不可压缩欧拉方程,建立了研究无叶扩压器失速问题的三维不可压缩理论模型。研究了无叶扩压器入口非扰动流径向速度在轴向上的分布对扩压器扰动流场以及对失速特性的影响。此外还研究了周向波数,叶轮叶片后弯以及无叶扩压器宽度对扩压器稳定性的影响。模型的计算结果显示,无叶扩压器入口流动的轴向分布会激发内流场轴向速度扰动。同时,对三种不同的入口流动轴向分布类型的研究发现,符合Fj0rtoft提出的非稳定流动必要条件的分布类型临界流量系数最大,即最不稳定。对周向波数的研究发现,最不稳定波数与实验中常见的失速团个数一致,通常在2~4。对叶片后弯的研究证实了后弯对于提高无叶扩压器稳定性的作用,且无叶扩压器越长,扩稳效果越明显。在叶片后弯且入口流动轴向分布不均一情况下发现的多重共振现象则为实验中得到无叶扩压器内可能相继出现两种不同转速失速团的现象提供了一种理论解释。此外,研究还发现增加无叶扩压器的宽度,在保持入口流动轴向分布不变的情况下可以增加长扩压器的稳定性。在三维不可压缩模型的基础上,本文进一步通过求解线性化的三维可压缩欧拉方程,建立了研究无叶扩压器失速问题的三维可压缩理论模型。研究了不同入口马赫数下,无叶扩压器入口非扰动流径向速度在轴向上的分布,周向波数,叶轮叶片后弯角以及无叶扩压器宽度对于扩压器失速的影响,得到了相应的临界流量系数和失速团转速。模型的计算结果显示,无叶扩压器内的扰动流参数幅值随入口马赫数的升高明显增大。对于较短的无叶扩压器,入口流动的轴向分布对其稳定性的影响较为明显;而对于较长的无叶扩压器,入口马赫数对其稳定性的影响较大。周向波数对无叶扩压器稳定性的影响效果随马赫数的变化较小,叶片后弯的相对扩稳效果随马赫数的变化亦不明显,而增加无叶扩压器的宽度但保持入口流动轴向分布不变的所带来的扩稳效果,随着入口马赫数的上升而增强。在较高的入口马赫数下,临界流量系数很接近但相应失速团转速相差明显的多重共振现象,不仅出现在较短的无叶扩压器内,也会出现在中等长度的无叶扩压器内。在平行壁面无叶扩压器失速模型的基础上,本文通过求解非正交坐标系下的线性化三维可压缩欧拉方程,建立了一种可用于研究非平行壁面无叶扩压器失速问题的理论模型,研究了不同轮盖型线对扩压器稳定性的影响,以及非平行壁面无叶扩压器内,周向波数,叶轮叶片后弯角,以及无叶扩压器入口段收缩对于扩压器失速的影响,得到了相应的临界流量系数和失速团转速。模型的计算结果验证了收缩型轮盖对无叶扩压器的扩稳效果,且扩稳效果随着收缩率以及入口马赫数的上升而增强。在非平行壁面无叶扩压器内,最不稳定波数有所减小,但仍然大于1。叶片后弯对非平行壁面无叶扩压器同样具有扩稳效果,但多重共振在较短的非平行壁面无叶扩压器内被抑制。研究还发现,在无叶扩压器入口宽度与出口宽度保持不变的前提下,无叶扩压器入口段收缩越大,稳定性越高。最后,将本论文所开发的无叶扩压器三维不可压缩模型,三维可压缩模型以及用于非平行壁面无叶扩压器的可压缩模型得到的失速预测结果,与文献中以及课题组的低速离心压缩机系统的实验结果,Honeywell高速离心压缩机系统的CFD结果,以及文献中非平行壁面无叶扩压器实验结果进行了比较。结果表明,对于宽度较大的无叶扩压器,忽略边界层的核心流模型可以得到较好的预测结果,且三维模型的预测结果比二维模型更准确。更多还原

【Abstract】 Vaneless diffuser stall is one of the most commonly seen unsteady flow phenomena in centrifugal compression system, which limits the steady operating range of the compression system as well as of the diffuser itself, and impacts operation reliability. In order to extend steady operation range and to improve the performance of the vaneless diffusers as well as the whole compression system, it is important to investigate the unsteady flow and to predict the stall limit of vaneless diffusers. Also, a further study and better understanding of the flow field in vaneless diffusers will have instructional value in optimizing diffuser geometries and matching of diffusers with other parts of the compression system.In this thesis, three three-dimensional vaneless diffuser stall models in centrifugal compression system were developed. Unsteady flow and stall characteristics in vaneless diffusers were studied. A three-dimensional vaneless diffuser stall model for incompressible flow was first proposed by solving linearized three-dimensional Euler equations for incompressible flow, and the effects of axial distribution of inlet radial velocity of undisturbed flow on disturbed flow and on stall characteristics were studied. Also studied were the effects of wave number, impeller backswept angle and diffuser width on diffuser stability. The results showed that axial velocity component of the disturbed flow would be provoked by inlet distortion of undisturbed radial velocity. And study of three different types of inlet distribution showed that the one which meet the conditions of general unsteady flow proposed by Fj0rtoft was least stable. The study of wave number showed that the least stable wave number consisted with the stall cell number found in most experimental measurements, which is 2 to 4. Impeller backswept angle stabilized vaneless diffusers and the stabilizing effect was more obvious for long diffusers. The multiple resonances found in diffusers with impeller backswept angle and inlet distortion provided a theoretical explanation of the experimental findings that a low-speed rotating stall cell could happen just after a high-speed rotating stall cell in vaneless diffusers. If the undisturbed inlet radial velocity distribution slop was kept, stability would be improved for long diffusers when the diffuser width increases.A three-dimensional vaneless diffuser stall model for compressible flow was then established based on the previous model for incompressible flow by solving linearized three-dimensional Euler equations for compressible flow. The effects of axial distribution of inlet radial velocity of undisturbed flow , wave number, impeller backswept angle and diffuser width on stall characteristics under different inlet Mach number were studied, and critical inlet mass flow rate and rotating speed of stall cells were predicted. The results showed that the amplitudes of disturbed flow parameters increased with inlet Mach number. For short diffusers, axial distribution of inlet radial velocity had more obvious effects on diffuser stability, while for long diffusers inlet Mach number had more obvious effects on diffuser stability. The effects of wave number on diffuser stability changed little with inlet Mach number, so did the relative stability extension brought by impeller backswept angle. However, the stabilizing effects of increasing diffuser width but keeping distribution slop increased with inlet Mach number. Under higher inlet Mach number, multiple resonances of which critical inlet mass flow rates were close to each other but rotating speed of stall cells were of difference were found not only in short diffusers but also in diffusers of medium length.Based on the developed stall models for vaneless diffusers with parallel walls, a three-dimensional stall model for vaneless diffusers with unparallel walls was then established via solving linearized three-dimensional Euler equations for compressible flow in non-orthogonal coordinates. The effects of different types of shroud wall on diffuser stability were studied. Also studied were the effects of wave number, impeller backswept angle and contraction of diffuser inlet part on diffuser stability in unparallel wall diffusers, and the critical inlet mass flow rate and rotating speed of stall cells were predicted. The results showed the stabilizing effects of shroud walls with contractions, and the effects increased with contraction ratio and inlet Mach number. For unparallel wall diffusers, the least stable wave number was smaller than that for parallel wall diffusers, but still higher than one. Impeller backswept angle showed stabilizing effect in unparallel wall diffusers. However, multiple resonances found in parallel wall diffusers were restrained in unparallel wall diffusers with short length. The study also found that contraction of diffuser inlet part had stabilizing effect on diffusers and the effect increased with contraction ratio even the inlet and outlet width of these diffusers were not changed.At last, the predicted stall characteristics from the developed three-dimensional vaneless diffuser stall model for incompressible flow and for compressible flow, and from the three-dimensional model for diffusers with unparallel walls were compared with the results from open literatures and the test rig established in our lab of low speed centrifugal compression system, the results from Honeywell CFD simulation of high speed centrifugal compression system, and the results from open literature of experimental measurements in an unparallel wall diffuser respectively. The comparison showed that for wide diffusers the predictions from the developed models based on core flow theory were satisfying and three-dimensional models gave better predictions than two-dimensional ones.更多还原