离心压气机宽无叶扩压器旋转失速的数值研究

无叶扩压器作为离心压气机的重要部件,研究其内部旋转失速的诱发过程及机理对离心压气机无叶扩压器旋转失速的主动控制具有重要意义。利用ANSYS平台对离心压气机建模并对其旋转失速过程进行非稳态数值模拟。结果表明:离心压气机无叶扩压器旋转失速是随着流量的逐渐减小建立起来的,失速工况点位于性能曲线的负斜率区;无叶扩压器内部,低速区靠近扩压器盖侧,高速区靠近扩压器盘侧;宽无叶扩压器的旋转失速与扩压器出口回流区有关,出口处回流区逐渐向扩压器内部发展,导致核心流发生畸变,最终导致旋转失速的发生。     

带无叶扩压器离心压气机的稳定性分析

结合实验数据,采用数值模拟方法对某带无叶扩压器离心压气机的性能与稳定性进行了详细分析,比较了设计点与近失速点下无叶扩压器内部的流动特征．结果表明：在发生喘振瞬间,扩压器进口最先出现了压力崩溃;扩压器进口流动特别是轮盖侧流动是导致压气机失稳的重要因素,对扩压器进口逆叶轮旋转方向的轮盖侧啧蒸汽能够提高扩压器的稳定性．     

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.     

离心压缩机无叶扩压器的改进新方法

研究了某高压比离心压缩机无叶扩压器的性能，提出在无叶扩压器轮盘侧加装一定高度机翼型导叶的改进方法。结果表明，这种扩压器可以改善其内部流动，提高自身的压力恢复系数和效率，从而改善离心压缩机的整级性能。     

叶片安装角对离心压气机扩压器气动性能及失速机理影响研究

为了研究离心压气机扩压器异常叶片对于失速现象的诱发效果，以带有叶扩压器的高速离心压气机为研究对象，通过整体或局部改变扩压器叶片安装角，开展非定常数值模拟研究并与实验结果对比验证，研究叶片安装角改变对离心压气机性能、动态特性以及失速机理的影响规律。研究表明：整体负方向旋转叶片安装角会促使扩压器更加不稳定，旋转角度从-5°到5°，最高效率点对应的质量流量逐渐增大。其中安装角偏转+5°扩压器叶片前缘靠近轮缘壁面发生流动分离，诱使无叶区间产生回流；而偏转-5°扩压器轮毂附近的流动分离主要发生在尾缘，造成扩压器叶片吸力面附近产生大范围回流。单个叶片安装角发生较大偏转(大于10°)时，扩压器比叶轮更早进入失速状态，且失速的机制可能会随着安装角偏移的增大发生改变。     

基于动态模式分解的离心压缩机动态流场分析

动静流道相互作用引起的非定常流动是离心式压缩机内流的主要特征,借助模态分解方法可以有效提取出该流场中相干结构,进而可以说明该动静干涉流场的主要特征。本文将动态模式分解(Dynamic Mode Decomposition,DMD)方法应用于带无叶扩压器通道的离心式压缩机流场分析中,通过对动静干涉速度场和压力场的模态提取和分析,直观揭示出动静流道中的流体运动特征。在设计流量工况下,叶轮出口叶顶间隙的流体流动和动静流道干涉效应影响了叶轮出口的速度参数和压力参数的分布。在叶轮和扩压器流道中分别出现了频率1 715和725Hz的湍流相干结构,两频率和叶轮通过频率有关。动静干涉流道中压力沿叶高方向动态规律相似,而速度场沿叶高方向波动规律存在一定的差异。该特征可以为压缩机流场测试提供参考。     

Unsteady behavior of leading-edge vortex and diffuser stall in a centrifugal compressor with vaned diffuser

The characteristics of a rotating stall of an impeller and diffuser and the evolution of a vortex generated at the diffuser leading-edge (i.e., the leading-edge vortex (LEV)) in a centrifugal compressor were investigated by experiments and numerical analysis. The results of the experiments revealed that both the impeller and diffuser rotating stalls occurred at 55 and 25 Hz during off-design flow operation. For both, stall cells existed only on the shroud side of the flow passages, which is very close to the source location of the LEV. According to the CFD results, the LEV is made up of multiple vortices. The LEV is a combination of a separated vortex near the leading- edge and a longitudinal vortex generated by the extended tip-leakage flow from the impeller. Therefore, the LEV is generated by the accumulation of vorticity caused by the velocity gradient of the impeller discharge flow. In partial-flow operation, the spanwise extent and the position of the LEV origin are temporarily transmuted. The LEV develops with a drop in the velocity in the diffuser passage and forms a significant blockage within the diffuser passage. Therefore, the LEV may be regarded as being one of the causes of a diffuser stall in a centrifugal compressor.     

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 un-shrouded impeller,a parallel wall vaneless diffuser and a volute.The walls of the diffuser were movable allowingdifferent pinch configurations to be investigated.The baseline geometry had no pinch i.e.the height of the dif-fuser was equal to the height of the impeller flow channel plus the axial running clearance.The work consists ofboth 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 thepinch was altered and the pinch made to different walls was tested.Two of the numerically studied cases werealso experimentally investigated.The overall performance of the compressor,the circumferential static and totalpressure and the spanwise total pressure distribution before and after the diffuser were measured.The numericaland experimental studies showed that the pinch improved the efficiency of the compressor.     

An experimental description of the flow in a centrifugal compressor from alternate stall to surge

The present paper gives the experimental results obtained in a centrifugal compressor stage designed and built by SAFRAN Helicopter Engines.The compressor is composed of inlet guide vanes,a backswept splittered unshrouded impeller,a splittered vaned radial diffuser and axial outlet guide vanes.Previous numerical simulations revealed a particular S-shape pressure rise characteristic at partial rotation speed and predicted an alternate flow pattern in the vaned radial diffuser at low mass flow rate.This alternate flow pattern involves two adjacent vane passages.One passage exhibits very low momentum and a low pressure recovery,whereas the adjacent passage has very high momentum in the passage inlet and diffuses efficiently.Experimental measurements confirm the S-shape of the pressure rise characteristic even if the stability limit experimentally occurs at higher mass flow than numerically predicted.At low mass flow the alternate stall pattern is confirmed thanks to the data obtained by high-frequency pressure sensors.As the compressor is throttled the path to instability has been registered and a first scenario of the surge inception is given.The compressor first experiences a steady alternate stall in the diffuser.As the mass flow decreases,the alternate stall amplifies and triggers the mild surge in the vaned diffuser.An unsteady behavior results from the interaction of the alternate stall and the mild surge.Finally,when the pressure gradient becomes too strong,the alternate stall blows away and the compressor enters into deep surge.     

Numerical Simulation of Rotating Stall in a Centrifugal Compressor with Vaned Diffuser

This paper reports a numerical study on the process from normal operating conditions to rotating stall in a cen-trifugal compressor with vaned diffuser.The purpose is to better understand the flow characteristics near stallpoint under the interactions between centrifugal impeller and vaned diffuser.Numerical results show that undercertain conditions just preceding stall point the tip leakage vortex begins to fluctuate at roughly half of the bladepassing frequency.This phenomenon is similar to rotating instability in axial compressors.With the flow rate re-duced further the impeller stalls and five stall cells propagating at a frequency of 85 percent of impeller rotationspeed are found.     

Experimental Investigations of Micro Air Injection to Control Rotating Stall

Steady discrete micro air injection at the tip region in front of the first compressor rotor has been proved to be aneffective method to delay the inception of rotating stall in a low speed axial compressor.Considering the practicalapplication a new type of micro injector was designed and described in this paper,which was imbedded in thecasing and could be moved along the chord.In order to verify its feasibility to other cases,such as high subsonicaxial compressor or centrifugal compressor,some other cases have been studied.Experimental results of the samelow speed axial compressor showed that the new injector could possess many other advantages besides success-fully stabilizing the compressor.Experiments performed on a high subsonic axial compressor confirmed the ef-fectiveness of micro air injection when the relative velocity at the blade tip is high subsonic.Meanwhile in orderto explore its feasibility in centrifugal compressor,a similar micro injector was designed and tested on a lowspeed centrifugal compressor with vaned diffuser.The injected mass flow was a bit larger than that used in axialcompressors and the results showed micro injection could also delay the onset of rotating stall in the centrifugalcompressor.     

Numerical Simulation of Active Suppression of Rotating Stallin Axial Compression Systems

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Computational and experimental study of an industrial centrifugal compressor volute

IntrotctionThe overall performance of a centrifugal compressoris a combination of the dueller, diffuser, and voluteperformances. Each component has to be designedcarefully in order tO' minimize the losses and maximizethe efficiency. In this paper, a centrifugal compressorvolute (or spiral) is stUdied numerically, and the CFDresults are compared with measured data.A volute is used to collect the flow from a vaned orvaneless diffuser and to gUide it Ollt of the compressor.The volute geometry…     

Unstable Head-Flow Characteristic Generation Mechanism of a Low Specific Speed Mixed Flow Pump

This paper treats the flow instabilities in a mixed flow pump with a vaned diffuser. Test pump has a positive slope of a head-flow performance curve at 65% flow rate of BEP (Best Efficiency Point) because of a rotating stall. Dynamic Particle Image Velocimetry (PIV) and pressure fluctuation measurements are used for investigating the propagation mechanism of a rotating stall. It was found that unstable performance was caused by periodical large scale abrupt backflow generated from the vaned diffuser to the outlet of impeller. Further, the relation between the static pressure at the inlet of diffuser vane and the internal flow condition was clarified. From these experimental results, in order to improve the positive slope of a head-flow performance curve, to suppress the growth of strong vortex toward the inlet of diffuser vane was proved to be a key point.     

基于本征正交分解的无叶扩压器流动稳定性Galerkin降阶模型

将分析湍流拟序结构中的本征正交分解技术运用到离心压气机无叶扩压器的流动稳定性分析中,并将其与Galerkin投影法相结合建立了无叶扩压器内部流场的降阶模型。使用本征正交分解技术提取无叶扩压器在某来流入口角下非定常流场的主要流动模态,将流动控制方程通过Galerkin方法投影至由有限阶本征正交模态所组成的空间上,得到无叶扩压器流动稳定性降阶模型。计算结果表明:该模型可用于判断不同来流入口角下的扩压器流动稳定性,并将降阶模型的计算结果与流场数值模拟结果和线性稳定性预测结果进行对比,验证了此降阶方法的合理性。     

Optimal ratio of bend channel in centrifugal compressor

A numerical investigation on the flow in a bend channel by coupling the impeller with the vaneless diffuser in a centrifugal compressor with different r/b ratios (bend radius r to bend channel width b) is presented. The jet-wake effect of the impeller outlet is considered and flow pattern in the bend channel and the performance of the centrifugal compressor stage are investigated. The results indicate that there is an optimal r/b ratio for increasing the stage efficiency to the highest for a specific compressor stage. The change in r/b ratio significantly affects the flow angle of the bend channel outlet. The prime reason for the total pressure loss in the bend channel is the wall friction in the bend channel.     

Numerical analysis of rotating stall characteristics in vaneless diffuser with large width-radius ratio

A two-dimensional model, where the influence of wall boundary layers is neglected and inlet jet-wake velocity patterns are prescribed, was applied to simulate one vaneless diffuser with a large width-radius ratio. The impact of diffuser length, impeller blade number, etc. on the rotating stall was analyzed. Computational results show that a different mechanism does exist for diffusers with large width-radius ratios. Comparison with related conclusions and references is supportive of the model.     

基于小波变换的离心风机弱失速特征分析

结合实验室4—73Nd8D风机进行了旋转失速试验，将谐波小波变换引入旋转失速的研究，通过对弱失速阶段机壳压力信号的时频分析，确定了弱失速阶段的失速能量间歇现象、频率特征以及在弱失速阶段对调节作用的敏感性，从而说明离心风机的弱失速阶段是一个应该引起运行、维修人员足够重视的重要区域，这也为离心风机旋转失速的深入开展研究莫定了基础。     

Unsteady behavior and control of vortices in centrifugal compressor

Two examples of the use of vortex control to reduce noise and enhance the stable operating range of a centrifugal compressor are presented in this paper.In the case of high-flow operation of a centrifugal compressor with a vaned diffuser,a discrete frequency noise induced by interaction between the impeller-discharge flow and the diffuser vane,which appears most notably in the power spectra of the radiated noise,can be reduced using a tapered diffuser vane（TDV） without affecting the performance of the compressor.Twin longitudinal vortices produced by leakage flow passing through the tapered portion of the diffuser vane induce secondary flow in the direction of the blade surface and prevent flow separation from the leading edge of the diffuser.The use of a TDV can effectively reduce both the discrete frequency noise generated by the interaction between the impeller-discharge flow and the diffuser surface and the broadband turbulent noise component.In the case of low-flow operation,a leading-edge vortex（LEV） that forms on the shroud side of the suction surface near the leading edge of the diffuser increases significantly in size and blocks flow in the diffuser passage.The formation of an LEV may adversely affect the performance of the compressor and may cause the diffuser to stall.Using a one-side tapered diffuser vane to suppress the evolution of an LEV,the stable operating range of the compressor can be increased by more than 12 percent,and the pressure-rise characteristics of the compressor can be improved.The results of a supplementary examination of the structure and unsteady behavior of LEVs,conducted by means of detailed numerical simulations,are also presented.     

A New Simplified Model of Post Stall Transients inAxial Compression Systems

INTRODUCTIONItiswellknown,thattheusefuloperatingrangeofturbomachinerycompressionsystemsislimitedbytheonsetofoneofthetwokindsoffluiddynamicinstabilities:rotatingstallandsurgetandbothofthemwilldegradecompressionsystemperformanceanddurability[1].Thesetw...