Investigation into the Interaction of Centrifugal Compressor Impeller and Vaneless Diffuser

Centrifugal compressors with parallel-wall and contracting wall vaneless diffuser are designed by using centrifugal compressor computer-aided integrated design system. The internal flow fields of the compressor are calculated by solving three-dimensional Navier-Stokes equation. Four aspects are investigated and calculation results show that the total efficiencies and total pressure ratios of the compressor with contracting wall vandess diffuser is higher than that of the compressor with parallel-wall. The jet and wake don＇t mix rapidly inside vandess diffuser. The outlet blade lean angle doesn＇t affect the compressor performance. The greater the mass flow rate through impeller, the more uneven the velocity distribution at impeller outlet is.

Experimental and Computational Analysis of the Unstable Flow Structure in a Centrifugal Compressor with a Vaneless Diffuse

The unstable flow phenomena in compressors, such as stall and surge, are closely related to the e ciency and the operating region. It is indispensable to capture the unstable flow structure in compressors and understand the mechanism of flow instability at low flow rates. Cooperated with the manufacturer, an industrial centrifugal compressor with a vaneless di user is tested by its performance test rig and our multi-phase dynamic measurement system. Many dynamic pressure transducers are circumferentially mounted on the casing surface at seven radial locations, spanning the impeller region and the di user inlet region. The pressure fields from the design condition to surge are measured in details. Based on the multi-phase dynamic signals, the original location of stall occurring can be determined. Meanwhile, the information of the unstable flow structure is obtained, such as the circumferential mode and the propagating speed of stall cells. To get more details of the vortex structure, an unsteady simulation of this tested compressor is carried out. The computational result is well matched with the experimental result and further illustrates how the unstable flow structure in the impeller region gradually a ects the stability of the total machine at low flow rates. The dynamic mode decomposition(DMD) method is applied to get the specific flow pattern corresponding to the stall frequency. Both experimental and computational analysis show that the flow structure at a particular radial location in the impeller region has a great impact on the stall and surge. Some di erences between the computational and experimental result are also discussed. Through these two main analytical methods, an insight into the unstable flow structure in an industrial compressor is gained. The result also plays a crucial role in the guidance of the compressor stabilization techniques.

Stall Evolution Mechanism of a Centrifugal Compressor with a Wide-Long Vaneless Diffuser

The rotating stall mechanism is of high importance for the stability of centrifugal compressors and thermal power cycles.The majority of research concerning this topic has concentrated on the initial stall phase.However,the evolution of stall cells in wide-long diffusers has not been comprehensively studied.In this paper,the causes of rotating stall in the wide-long diffuser and the three-dimensional evolution mechanism of stall cells during the stall process were thoroughly analyzed.During the stall induction phase,an annulus vortex structure was found in the reverse-flow zone near the hub side of the diffuser outlet,which was the initial form of stall cells.The whole evolution process of stall cells was divided into three phases as the flow rate decreased.During the initial stall phase,the dynamic equilibrium was built under effects of the impeller wake and the adverse pressure gradient.As a result,the number of stall cells was kept at seven and the size of stall cells remained constant.During the transition phase,the flow in the diffuser became unstable.Stall cells extended to the impeller outlet,and the effect of the wake flow was strengthened significantly.Stall cells started integrating and separating regularly.As a result,the number and propagation speed of stall cells varied periodically at a constant mass flow rate.During the deep stall phase,the size of stall cells remained unchanged,and the number of stall cells kept at one.This study has important practical guidance and engineering value for the high-efficiency design and safe operation of centrifugal compressors.

小尺寸无静叶对转风扇/压气机气动设计与变工况性能分析研究

无静叶对转风扇/压气机能有效减小发动机陀螺力矩,在轴向长度方面具有优势。然而,无静叶往往导致下游叶片攻角变化较大,变工况性能难以保证,且这一特点随叶尖切线速度的增大而恶化。本文利用小尺寸涡扇发动机风扇叶片根部周向速度较低的特点,尝试对其进行风扇与压气机无静叶对转方案设计。通过速度三角形分析了无静叶对转风扇/压气机的性能特点,开发了无静叶对转风扇/压气机一维设计程序进行参数方案的筛选优化,设计得到小尺寸无静叶对转风扇/压气机构型方案,设计点效率为85.1%,压比为3.16。结果表明,对转压气机的下排叶片相对进口气流角的变化范围较常规转叶+静叶相对进口气流角范围更小,而对转下排叶片的进口速度却将增大1.5~3倍,因此,需要合理选择对转风扇/压气机的轮毂比及转速比,才能保证对转风扇/压气机变转速工况下的设计点效率及失速裕度。减小对转风扇/压气机转速比有利于提高设计点效率和裕度,但本文受发动机总体指标限制,对转速比选择进行了折中。

A Review of Rotating Stall in Vaneless Diffuser of Centrifugal Compressor

At present, off-design centrifugal compressor suffers from great instabilities, such as rotating stall and surge, which lead to work failure, strong pressure fluctuations, and violent vibrations of the entire system. One cause of these instabilities is the rotating stall occurring in vaneless diffuser. In this review, recent researches on rotating stall of vaneless diffuser in centrifugal compressor are reviewed, and its inductive mechanism and characteristic description are emphatically introduced. In vaneless diffuser, the rotating stall starts up and fully develops under non-design conditions with the characteristic of increasing pressure fluctuation amplitude. The negative effects of rotating stall include reverse flow and vortex, both of which have significant effects on the instability of centrifugal compressors. Some improvement methods of rotating stall are also introduced, and a lot of examples are given. Finally, some suggestions and discussions are put forward for the future work.

Numerical Investigation of Rotating Stall in Centrifugal Compressor with Vaned and Vaneless Diffuser

This study presents a numerical simulation of the stall and surge in a centrifugal compressor and presents a description of the stall development in two different cases.The first case is for a compressor with vaneless diffuser and the second is for a compressor with vaned diffuser of the vane island shape.The main aim of this study is to compare the flow characteristics and behavior for the two compressors near the surge operating condition and provide further understanding of the diffuser role when back flow occurs at surge.Results showed that for a location near the diffuser entrance,the amplitude of the static pressure fluctuations for the vaneless diffuser case is higher than that for the vaned diffuser case near surge condition.These pressure fluctuations in the case of the vaneless diffuser appear with a gradual decrease of the mean pressure value as a part of the surge cycle.While for the case of the vaned diffuser,the pressure drop during surge occurs faster than the case of the vaneless diffuser.Also,results indicated that during surge in the case of vaneless diffuser,there is a region with low velocity and back flow that appears as a layer connecting all impeller passages near shroud surface and this layer develops in size with time.On the other hand,for the case of vaned diffuser during surge,the low velocity regions appear in random locations in some passages and these regions expand with time towards the shroud surface.Results showed that during stall,the impeller passages are exposed to identical impact from stall cells in the case of vaneless diffuser while the stall effect varies from passage to another in the case of the vaned diffuser.

Study of Rotating Stall in a Centrifugal Compressor with Wide Vaneless Diffuser

To study the stall mechanism in a wide vaneless diffuser,a three-dimensional centrifugal compressor model was established.The numerical simulation method was used to study the fluid characteristics in a centrifugal compressor with a wide vaneless diffuser under stall conditions.The simulated results showed that the wide vaneless diffuser stall was related to the instability of the core flow.Due to the influence of the reflow,the core flows moved between the hub and shroud side and the directions were not consistent.This was called core flow distortion,which triggered the rotating stall.The reflow first appeared at the outlet boundary of the diffuser,and the reflux gradually expanded to the inside of the diffuser as the impeller rotated.Eventually,the reflux zones grew and merged,forming a stable low-speed fluid regiment,which produced a stall cell.The stall cells rotated around themselves with an opposite direction with impeller.

Unsteady Pressure Field in a Vaneless Diffuser of a Centrifugal Compressor: An Experimental and Computational Analysis

Unsteady static pressure was measured and time-accurate numerical simulations were conducted in order to analyze unsteady behaviour of the centrifugal compressor diffuser. The unsteady static pressure was measured at the diffuser inlet and outlet at three different circumferential positions. Operation points near the surge, the design operation point and operation point near the choke were used in measurements. The time-accurate simulations were conducted at the design operation point and operation point near the choke. The whole compressor was modelled and the low Reynolds number k-εturbulence model was utilized in numerical simulations. The gathered numerical and experimental unsteady pressures were compared with each other.

Rotating instability in the vaneless diffuser of a radial flow pump

The paper refers to the behavior of a vaneless diffuser of a radial flow pump in partial flow operating conditions. Some experimental data have been obtained using 2D/2C PIV and unsteady pressure measurements within the diffuser, in various operating conditions. The experimental results at the lower flow rate are compared with two-dimensional numerical calculations.

Computational and Experimental Study of Pinch on the Performance of a Vaneless Diffuser in a Centrifugal Compressor

This study focuses on the vaneless diffuser of a centrifugal compressor. The examined stage consists of an unshrouded impeller, a parallel wall vaneless diffuser and a volute. The walls of the diffuser were movable allowing different pinch configurations to be investigated. The baseline geometry had no pinch i.e. the height of the diffuser was equal to the height of the impeller flow channel plus the axial running clearance. The work consists of both numerical and experimental parts. Quasi-steady, turbulent, fully 3D numerical simulations were conducted. The inlet cone, rotor and diffuser were modelled. Six different configurations were studied. The height of the pinch was altered and the pinch made to different walls was tested. Two of the numerically studied cases were also experimentally investigated. The overall performance of the compressor, the circumferential static and total pressure and the spanwise total pressure distribution before and after the diffuser were measured. The numerical and experimental studies showed that the pinch improved the efficiency of the compressor.

Receptivity and structural sensitivity study of the wide vaneless diffuser flow with adjoint method

The stability of the flow in the vaneless diffuser of a centrifugal compressor is studied with the linear theory. The characteristics of direct and adjoint perturbation modes are investigated,and the receptivity of the instability mode to momentum forcing or mass injection is identified based on the adjoint modes. Analysis shows that the perturbation with the largest amplitude is located at the outlet of the vaneless diffuser, while the highest-receptivity region is located in the middle of the vaneless diffuser along the radial direction. The large difference between the direct and adjoint modes indicates that the instability mechanism cannot be identified from a study of either eigenmode separately. Therefore, the structural sensitivity analysis is adopted to study the feedback of the instability mode. The structural sensitivity of the eigenvalue which is proportional to the perturbation pressure and velocity is used to explain the mechanism of flow control for the vaneless diffuser.

Behavior of Vaneless Diffuser Stall in a Centrifugal Compressor

The rotating stall in a centrifugal compressor with a vaneless diffuser was investigated both experimentally and numerically with focus on the effect of the internal flow field within the impeller on the diffuser stall.Through numerical analysis,the boundary layer separation at the impeller outlet was found to play an important role in the expansion and rotation processes of the diffuser stall.In particular,the expanded boundary layer separation near the hub side at the outlet of the main blade(M.B.)suction surface passage was considered to be the main cause of the expansion and rotation processes.A longitudinal vortex existed at the throat of the M.B.passage,and the mass flow rate in the M.B.passage was significantly reduced by the blockage effect.In addition,the longitudinal vortex induced the rolling up flow near the hub side at the impeller exit.Thus,the boundary layer separation expanded.

宁海抽水蓄能电站机组相位共振风险评估

通过研究诱发相位共振的相关因素,消除抽水蓄能电站机组相位共振,提高抽蓄机组的运行稳定性,利用归纳分析法,从基础模型选择及模型试验对比结果、无叶区宽度比较、不同转轮叶片数与活动导叶数配比的水压脉动频率及优缺点、相位共振风险因子分析及业绩比较等几方面评估了宁海抽水蓄能电站机组由于动静干涉引起的相位共振风险。研究结果表明:宁海抽水蓄能电站基础模型选择合适,优化后模型各项稳定性指标优于基础模型;无叶区D_(0)/D_(1)=1.23属于国内抽蓄电站较高水平;10+16的转轮叶片数与活动导叶数配比合理,水压脉动频率较低;较低波速下相位共振风险因子高于国内工程经验值,但低于国外工程统计值。研究成果说明宁海抽水蓄能电站机组水力开发选型合理先进,发生相位共振风险较低,评估过程可为后续新建抽水蓄能电站评估提供一定参考和借鉴。

水泵水轮机水泵工况顶盖下腔特殊压力脉动模型试验研究

随着抽水蓄能电站建设的加速推进,高水头、大容量的抽水蓄能机组逐渐成为发展主流。部分高水头大容量抽水蓄能机组在水泵工况运行时,监测到顶盖下腔出现特殊频率的压力脉动现象,引起了业内的广泛关注。为了深入揭示该现象的产生机理,通过设置干扰物的方法,改变流道中的流态,开展顶盖下腔特殊频率模型试验研究。试验结果表明:通过对无叶区流态的干扰,顶盖下腔与无叶区压力脉动发生变化,特殊频率幅值与区域被减弱,水泵工况顶盖下腔出现的特殊频率压力脉动现象主要受无叶区流态影响,而受顶盖下腔体流场影响较小。

桅杆长径比对有限长锥台形捕能柱涡激横向摆动特性的影响

无叶片风力机是利用捕能柱的风致横向摆动收集风能并将其转换成电能的装置。通过引入桅杆结构并将其与有限长锥台型捕能柱构成一个捕能系统,在建立有限长锥台型捕能系统简化模型的基础上,推导了与捕能系统几何形状相关的无量纲结构系数计算式,探讨了桅杆长径比对有限长锥台型捕能柱涡激摆动特性及捕能效率的影响。所得结果表明,桅杆结构的引入能有效地提升有限长捕能柱的涡激横向摆幅峰值及捕能效率,且其均随桅杆长径比的增大呈现先增大后减小的趋势。与无限长捕能柱结构相比,在桅杆长径比为6.67时有限长捕能柱的能量收集效率提升了44.8%;增大桅杆的长径比会使锁频区间增宽,但捕能系统的固有频率会减小;对于给定的捕能系统存在一个与其相匹配的最佳桅杆长径比。所得结论对于无叶片风力机的设计与实际工程应用具有重要的指导作用。

高水头抽水蓄能机组减振降噪评估及工程实践

振动和噪声超标问题在高水头抽水蓄能机组较为常见,其产生原因与无叶区压力脉动相关。因此,抽水蓄能电站减振降噪的有效措施之一是降低无叶区压力脉动。本文开展真机尺度三维非定常可压缩数值计算,预测转轮修型对无叶区压力脉动的降低程度,进而评估转轮修型的减振降噪效果。计算结果显示,修型方案能够有效降低无叶区压力脉动混频幅以及18倍转频幅值,对改善机组振动和噪声问题非常有效。现场实测数据显示,改造后无叶区压力脉动降低程度与数值预测结果非常接近,机组顶盖振动和水车室噪声也得到了明显的改善。

抽水蓄能机组甩负荷时无叶区压力幅值特性

甩负荷是抽水蓄能机组的正常设计工况,此时无叶区经历过流系统最为严重的压力脉动,对机组运行稳定产生深远影响。长期以来,对无叶区压力及其脉动研究主要集中于数值模拟、模型观测,对真机试验研究少。以某抽水蓄能电站6台机组单机甩额定负荷时无叶区压力信号为分析对象,基于Savitzky-Golay滤波器对压力信号进行处理,分离出了趋势压力与脉动压力,阐释了趋势压力与脉动压力的主要影响因素,并指明了亟须解决的问题。研究表明:甩负荷过程中无叶区趋势压力主要受转速影响,与转速的相关系数在0.99左右;脉动压力峰峰值受动静干涉、旋转失速等影响,是额定负荷稳定运行时的7~22倍,显著高于数值模拟结果。

离心压缩机扩稳改进的数值分析

对已有的离心压缩机模型进行数值模拟得出其压比、效率随流量变化的特性曲线。在保证压缩机整机尺寸不变的情况下,对压缩机叶片数、叶片角分布方式、机匣线曲率、无叶扩压器收缩段进行调节改进,最终实现对压缩机稳定运行范围的扩充。通过对比改进前后的数值仿真特性曲线,结果显示:叶片进出口叶片角较小,以及采用前加载叶片角分布时有利于压缩机小流量稳定范围;叶轮机匣线曲率较大时有利于压缩机小流量稳定范围扩充,较小时有利于设计点性能;无叶扩压器收缩段位置对压缩机稳定范围影响较大。研究表明,压缩机的扩稳改进是各项参数共同调节的结果,掌握各项参数的调整规律,可以快速实现压缩机稳定范围的扩充调整。

降低变速水泵水轮机无叶区压力脉动的活动导叶优化研究

通过数值模拟,研究了某变速水泵水轮机活动导叶型线优化后水轮机工况内部流场、无叶区压力脉动和效率的变化情况,分析了活动导叶优化对水泵工况水力性能的影响。结果表明:活动导叶优化后,水轮机工况内部流态改善,无叶区压力脉动幅值减小,效率提高,且部分负荷时的效果更为明显,在最小水头40%负荷,活动导叶优化后无叶区压力脉动幅值减小了1.2%,效率提高了1.64%;活动导叶优化对水泵工况的水力性能无不利影响,且最低扬程下的流道效率提高了0.1%,无叶区压力脉动幅值减小了0.2%,说明活动导叶优化是提高变速水泵水轮机稳定性能和能量性能的有效手段。

VGT涡轮喷嘴出口气流速度分布的试验研究

使用 IFA30 0型热膜风速仪 ,通过选择合适的低通滤波器、采样频率和采样时间 ,用 X形探针测量了无叶喷嘴出口速度沿蜗壳周向和轴向的分布 ,并用概率统计方法对测量结果进行了处理。测试方法可行、方便、快速、自动化程度高。测量结果表明 :沿蜗壳周向 ,其径向速度分布较均匀 ,切向速度分布变化较大 ;沿蜗壳轴向 。