A study of the unsteady tip flow field of a transonic compressor

An experimental investigation on the unsteady tip flow field of a transonic compressor rotor has been performed.The casing-mounted high frequency response pressure transducers were arranged along both the blade chord and the blade pitch.The chord-wise ones were used to indicate both the ensemble averaged and time varying flow structure of the tip region of the rotor at different operating points under 95% design speed and 60% design speed.The pitch-wise circumferential transducers were mainly used to analyze the unsteadiness frequency of the tip leakage flow in the rotor frame at the near stall condition.The contours of casing wall pressure show that there were two clear low pressure regions in blade passages,one along the chord direction,caused by the leakage flow and the other along the tangential direction,maybe caused by the forward swept leading edge.Both low pressure regions were originated from the leading edge and formed a scissor-like flow pattern.At 95% design speed condition,the shock wave interacted with the low pressure region and made the flow field unsteady.With the mass flow reduced,the two low pressure regions gradually contracted to the leading edge and then a spike disturbance emerged.     

Stall Behaviour in a Mixed-flow Compressor with Axial Slot Casing Treatment

Casing treatment is an effective technique in extending stall margin of axial and centrifugal compressor.However,its impacts on the stall behaviour of mixed-flow compressor are still not completely understood until now.To conquer this issue,unsteady full-annulus simulations were conducted to investigate the stall mechanism of a mixed-flow compressor with and without axial slot casing treatment(ASCT).The circumferential propagating speed of spike inception resolved by the numerical approach is 87.1%of the shaft speed,which is identical to the test data.The numerical results confirmed that the mixed-flow compressor fell into rotating stall via spike-type with and without ASCT.The flow structure of the spike inception was investigated at 50%design rotational speed.Instantaneous static pressure traces extracted upstream of the leading edge had shown a classic spiky wave.Furthermore,it was found that with and without ASCT,the mixed-flow compressor stalled through spike with the characteristic of tip leakage spillage at leading edge and tip leakage backflow from trailing edge,which is different from a fraction of the centrifugal compressor.The resultant phenomenon provides conoborating evidence for that unlike in axial-flow compressor,the addition of ASCT does not change the stall characteristics of the mixed-flow compressor.The flow structure that induced spike inception with ASCT is similar to the case with smooth casing.In the throttling process,tip leakage flow vortex had been involved in the formation of tornado vortices,with one end at the suction side,and the other end at the casing-side.The low-pressure region relevant to the downward spike is caused by leading-edge separation vortex or tornado vortex.The high-pressure region relevant to the upward spike is induced by blockage from the passage vortex.These results not only can provide guidance for the design of casing treatment in mixed-flow compressor,but also can pave the way for the stall waring in the highly-loaded compressors of next-generation aeroengines.     

间隙变化对轴流压气机转子近失速工况叶顶流场的影响研究

为研究间隙变化对轴流压气机转子近失速工况下叶顶流场结构的影响,以轴流压气机转子Rotor37为研究对象,对其叶顶流场进行定常和非定常的数值模拟。计算结果表明：随着叶顶间隙的减小,压气机的总压比和等熵效率均有所提高,稳定运行范围扩大;2倍设计间隙下,叶尖泄漏涡经激波作用后发生膨胀破碎,堵塞来流通道,诱发压气机堵塞失速;0.5倍设计间隙下,吸力面流动分离加剧,发生回流,部分回流与来流在压力面前缘上游发生干涉,进口堵塞加剧,致使部分来流从前缘溢出,导致压气机叶尖失速;不同间隙下压气机失速过程的主导因素不同,大间隙下失速由叶尖泄漏涡破碎的非定常波动引起,小间隙下失速主要由流动分离引发的周期性前缘溢流所主导。     

Circumferential propagation of tip leakage flow unsteadiness for a low-speed axial compressor

Full-annulus three-dimensional unsteady numerical simulations were conducted for a low-speed isolated axial compressor rotor, intending to identify the behavior of self-induced unsteady tip leakage flow within multi-blade passages. There is a critical mass flow rate near stall point, below it, the self-induced unsteadiness of tip leakage flow can propagate circumferentially and thus initiates two circumferential waves. Otherwise, the self-induced unsteady tip leakage flow oscillates synchronously in each single blade passage. The major findings are: 1) while the self-induced unsteadiness of tip leakage flow is a single-passage phenomenon, there exist phase shifts among blade passages in multi-passage environments then evolving into the first short length wave propagating at about two times of rotor rotation speed after the transient period ends; and 2) the time traces of the pseudo sensors located on the rotor blade tips reveal another much longer length-scale wave modulated with the first wave due to phase shift propagating at about half of rotor rotation speed. Features of the short and long length-scale circumferential waves are similar to those of rotating instability and modal wave, respectively.     

System identification modeling and unstable behavior of the dynamics of flows within the tip region of an axial compressor blade passage

In recent years, the correlation coefficient of pressure data from the same blade passage in an axial compressor unit has been used to characterize the state of flow in the blade passage. In addition, the correlation coefficient has been successfully used as an indicator for active control action using air injection. In this work, the correlation coefficient approach is extended to incorporate system identification algorithms in order to extract a mathematical model of the dynamics of the flows within a blade passage. The dynamics analyzed in this research focus on the flow streams and pressure along the rotor blades as well as on the unsteady tip leakage flow from the rotor tip gaps. The system identification results are used to construct a root locus plot for different flow coefficients, starting far away from stall to near stall conditions. As the compressor moves closer to stall, the poles of the identified models move towards the imaginary axis of the complex plane, indicating an impending instability. System frequency data is captured using the proposed correlation based system identification approach. Additionally, an oscillatory tip leakage flow is observed at a flow coefficient away from stall and how this oscillation changes as the compressor approaches stall is an interesting result of this research. Comparative research is analyzed to determine why the oscillatory flow behavior occurs at a specific sensor location within the tip region of the rotor blade.     

Numerical study on the influence mechanism of inlet distortion on the stall margin in a transonic axial rotor

A numerical study is conducted to investigate the influence of inlet flow condition on tip leakage flow (TLF) and stall margin in a transonic axial rotor.A commercial software package FLUENT,is used in the simulation.The rotor investigated in this paper is ND_TAC rotor,which is the rotor of one-stage transonic compressor in the University of Notre Dame.Three varied inlet flow conditions are simulated.The inlet boundary condition with hub distortion provides higher axial velocity for the incoming flow near tip region than that for the clean inflow,while the incoming main flow possesses lower axial velocity near the tip region at tip distortion inlet boundary condition.Among the total pressure ratio curves for the three inlet flow conditions,it is found that the hub dis-torted inlet boundary condition improves the stall margin,while the tip distorted inlet boundary condition dete-riorates compressor stability.The axial location of interface between tip leakage flow (TLF) and incoming main flow (MF) in the tip gap and the axial momentum ratio of TLF to MF are further examined.It is demonstrated that the axial momentum balance is the mechanism for interface movement.The hub distorted inflow could de-crease the axial momentum ratio,suppress the movement of the interface between TLF and MF towards blade leading edge plane and thus enhance compressor stability.     

Unsteady tip clearance flow in an isolated axial compressor rotor

Introduction Background It is well known that the rotor tip clearance flow has profound effects on the performance and stability of axial compressor (Wisler[1], Howard[2]). Numerous studies on the tip clearance flow were carried out in the past fifty years. Rain[3] proposed a model to predict the loss due to tip leakage flow assuming that the kinetic energy of the leakage flow velocity component normal to the mean chamber line would be dissipated. Lakshminarayana[4] developed a model to pre…     

Numerical analysis of the tip leakage flow field in a transonic axial compressor with circumferential casing treatment

This paper reports on numerical investigations aimed at understanding the influence of
circumferential casing grooves on the tip leakage flow and its resulting vortical structures.The results
and conclusions are based on steady state 3D numerical simulations of the well-known transonic axial
compressor NASA Rotor 37 near stall operating conditions.The calculations carried out on the casing
treatment configuration reveal an important modification of the vortex topology at the rotor tip
clearance.Circumferential grooves limit the expansion of the tip leakage vortex in the direction
perpendicular to the blade chord,but generate a set of secondary tip leakage vortices due to the
interaction with the leakage mass flow.Finally,a deeper investigation of the tip leakage flow is
proposed.     

零间隙压气机失稳机理的全通道数值模拟研究

为了探究零间隙压气机流动失稳机理,采用全通道非定常数值模拟方法研究了一台零间隙斜流压气 机转子的失稳机理,数值模拟过程中在转子出口施加了随时间动态变化的背压模拟压气机转子节流,非定常 数值计算结果表明零间隙斜流压气机转子仍然表现为典型突尖流动失稳特征。通过详细地分析斜流压气机 转子节流过程中不同阀系数对应的压气机内部流场结构,结果表明:尽管零间隙斜流压气机无叶顶泄漏特 征,但随着对压气机节流,转子叶片尾缘率先出现流动分离,进一步节流,尾缘流动分离表现为一方面在周向 范围加剧,另一方面分离点逐渐向上游移动,造成通道严重堵塞,最终引发相邻叶片通道尾缘回流和叶片前 缘流动溢出进而诱发叶片通道内部出现径向涡结构,从而形成压气机突尖失速先兆。     

Effects of Width Variation of Pressure-Side Winglet on Tip Flow Structure in a Transonic Rotor

Tip leakage flow has become one of the major triggers for rotating stall in tip region of high loading transonic compressor rotors.Comparing with active flow control method,it’s wise to use blade tip modification to enlarge the stable operating range of rotor.Therefore,three pressure-side winglets with the maximum width of 2.0,2.5 and 3.0 times of the baseline rotor,are designed and surrounded the blade tip of NASA rotor 37,and the three new rotors are named as RPW1,RPW2,and RPW3 respectively.The numerical results show that the width of pressure-side winglet has significant influence on the stall margin and the minimum throttling massflow of rotor,while it produces less effect on the choking massflow and the peak efficiency of new rotors.As the width of the pressure-side winglet increases from new rotor RPW1 to RPW3,the strength of leakage massflow has been attenuated dramatically and a reduction of 20%in leakage massflow rate has appeared in the new rotor RPW3.By contrast,the extended blade tip caused by winglet has not introduced much more aerodynamic losses in tip region of rotor,and the new rotors with different width of pressure-side winglet have the similar peak efficiency to the baseline.The new shape of the leakage channel over blade tip which replaces of the static pressure difference near blade tip has dominated the behavior of the leakage flow in tip gap.As both the new aerodynamic boundary and throat in tip gap have reshaped by the low-velocity flow near the solid wall of extended blade tip,the discharging velocity and massflow rate of leakage flow have been suppressed obviously in new rotors.In addition,the increasing inlet axial velocity at the entrance of new rotor has increased slightly as well,which is attributed to the less blockage in the tip region of new rotor.In consideration of the increased inlet axial velocity and the weakened leakage flow,the new rotor presents an appropriately linear increase of the stall margin when the width of pressure-side winglet increases,and has a nearly 15%increase in new rotor RPW3.     

转速对跨声速轴流压气机的性能影响分析

为了研究跨声速压气机内部流动失稳对压气机性能的影响,对跨声速轴流压气机NASA转子37进行三维定常数值模拟,研究不同设计转速下跨声速轴流压气机稳定运行及内部流动失稳现象。研究发现：转速不变时近堵塞点的等熵效率高于近失速点;随着转速降低,压气机稳定运行范围变宽、效率增大及流动损失变小;压气机叶栅通道出现堵塞情况的叶高截面范围随着转速的降低而逐渐增大,这导致压气机叶片在近失速点处的流动失稳情况变严重。     

Unsteady tip clearance flow pattern in an isolated axial compressor rotor with micro tip injection

A numerical study of the effect of discrete micro tip injection on unsteady tip clearance flow pattern in an isolatedaxial compressor rotor is presented,intending to better understand the flow mechanism behind stall control meas-ures that act on tip clearance flow.Under the influence of injection the unsteadiness of self-induced tip clearanceflow could be weakened.Also the radial migration of tip clearance vortex is confined to a smaller radial extentnear the rotor tip and the trajectory of tip clearance flow is pushed more downstream,So the injection is benefi-cial to improve compressor stability and increase static pressure rise near rotor tip region.The results of injectionwith different injected mass flow rates show that for the special type of injector adopted in the paper the effect ofinjection on tip clearance flow may be different according to the relative strength between these two streams offlow.For a fixed injected mass flow rate,reducing the injector area to increase injection velocity can improve theeffect of injection on tip clearance flow and thus the compressor stability.A comparison of calculations betweensingle blade passage and multiple blade passages validates the utility of single passage computations to investi-gate the tip clearance flow for the case without injection and its interaction with injected flow for the case with tipinjection.     

Low speed axial compressor stall margin improvement by unsteady plasma actuation

This research investigates the use of single dielectric barrier discharge(SDBD) actuators for energizing the tip leakage flow to suppress rotating stall inception and extend the stable operating range of a low speed axial compressor with a single rotor.The jet induced by the plasma actuator adds momentum to the flow in the tip region and has a significant impact on the tip-gap flow.Experiments are carried out on a low speed axial compressor with a single rotor.The static pressure is measured at both the rotor inlet and outlet.The flow coefficient and pressure rise coefficient are calculated.Then the characteristic line is acquired to show the overall performance of the compressor.With unsteady plasma actuation of 18kV and 60W the compressor stability range improvement is realized at rotor speed of 1500 r/min – 2400 r/min.     

Transition from Unsteady Flow Inception to Rotating Stall and Surge in a Transonic Compressor

Since the transition from rotating stall to surge in a transonic compressor at high speed is very quick,quite often there is no time to take measures to prevent the surge.Therefore,it is desired to find any rotating stall precursors,of which the occurrence can offer sufficient time for stall or surge prevention.In this study,a series of unsteady flow analyses were performed on a transonic compressor under operating conditions before rotating stall with unsteady results scrutinized to find rotating stall precursors.Particular attention is paid to the spatial modes and time modes of static pressure near the casing and around the blade leading and trailing edges.The results show that the characteristics of the precursor in both spatial and time domains can be used as rotating stall warnings.     

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.     

Evolution of unsteady flow near rotor tip during stall inception

Previously the features of circumferential propagation of self-induced tip leakage flow unsteadiness for a low speed isolated axial compressor rotor in the authors’ laboratory were discovered and investigated via numerical simulation,which only occurs below a critical stable flow point that is close to but not yet at the stall limit.Further in this paper,the detailed investigation on evolution of tip leakage flow during the throttling process into spike rotating stall was conducted by adopting the valve-throttling model.During this process,the development of the circumferential propagation of tip leakage flow unsteadiness was especially focused on.According to the unsteady characteristics of pressure signals,the evolvement of compressor flow field can be classified into four stages.As compressor throttled,the oscillation frequency of self-induced unsteady tip leakage flow decreased gradually,and thus resulted in the decrease of its circumferential propagation speed.The circumferential propagation of self-induced tip leakage flow unsteadiness is closely related with rotating instability.When the forward spillage of tip leakage flow at the leading edge occurred,the spike type rotating stall was initiated.Its flow struc-tures were given in the paper.     

Effects of probe support on the flow field of a low-speed axial compressor

This paper presents an investigation on the effect of probe support on the flow field of an axial compressor.The experiment is carried out in a large-scale low-speed research compressor.A cylindrical probe support intruding to 50% blade span was installed at 50% chord upstream from the rotor leading edge.The region from 5° to 32° off the probe support in the direction of rotation at the rotor outlet was measured with a 5-hole probe and a high-response total pressure probe.The experiment is performed at both near-design and near-stall points.The measuring results of 5-hole probe and high-response total pressure probe indicate that the probe blockage effect is different at different blade spans.The wake of the probe support weakens the leakage vortex intensity at the tip region,leading to greater total pressure rise.At near-design condition,the presence of probe support has a negative effect on the region from 75% to 92% span,while improves the flow field below 75% span.At near stall condition,the probe support has a negative effect on the region from 70% to 90% span,and almost has no influence on the flow field below 70% span.     

小流量工况汽轮机末级流动不稳定数值研究

针对汽轮机低压末级在小负荷工况下出现的流动不稳定现象,进行了非定常数值模拟研究和分析。对末三级叶片耦合排气缸进行建模,其中末级采用整圈形式,对17%设计质量流量工况进行非定常计算,小流量工况下汽轮机末级表现出类似于压气机旋转失速的现象,对流场监控数据进行周向模态分解及相关分析,确定了扰动的数目为30个,其周向传播速度约为转子转速的56%。最后,结合内部流动特征对非稳定现象的形成机理进行了探讨,小流量下由于径向流动阻塞了通道,并在叶顶间隙射流的作用下形成了通道内的周期性高压区,而前缘溢流和叶顶间隙射流耦合作用促成了叶顶进口附近周期性低压区的形成。     

Investigation on multiple cylindrical holes casing treatment for transonic axial compressor stability enhancement

Casing treatment is one possible way of regaining axial compressor operating range. However, most of casing treatments extend the operating range with the cost of efficiency penalty. A new form of multiple cylindrical holes casing treatment (MHCT) with pre-swirl blowing for the NASA Rotor-37 has been designed based on profound understanding of the stall inception. Unsteady numerical simulations have been performed for Rotor-37 with and without MHCT. Parametric studies of the total extraction holes area and their axial locations show that the compressor performance deteriorates as the area ratio increases but the stall margin is extended and there is an optimum extraction holes axial location for stall margin extending. The better configuration of MHCT could extend the stall margin by 6.2% with only 0.23% peak efficiency reduction. Detailed analysis of the physical mechanism behind the stall margin improvement shows that the casing treatment could eliminate the passage blockage by suppressing breakup of tip leakage vortex and decrease the blade load in tip region, which both contribute to improve stall margin of transonic axial compressors.     

对跨音速转子附面层发展控制研究

采用数值方法模拟了低雷诺数条件下NASA Rotor 37跨音速压气机转子内部流场。结果表明,附面层径向涡是该压气机转子流动失稳的一个很重要的原因。通过在该压气机转子叶片吸力面叶顶附近处进行抽吸,发现可以很好地抑制附面层径向涡的发展,压气机转子的稳定工作范围明显扩大。此外,还比较了不同的抽吸量对压气机性能的影响。