Numerical simulation on the effect of tip clearance size on unsteadiness in tip clearance flow

Unsteadiness of tip clearance flow with three different tip clearance sizes is numerically investigated in this paper. NASA Rotor 67 is chosen as the computational model. It is found that among all the simulated cases, the un- steadiness exists when the size of the tip clearance is equal to or larger than design tip clearance size. The relative total pressure coefficient contours indicate that region of influence by tip leakage flow augments with the increase of tip clearance size at a fixed mass flow rate. Root Mean Square contours of static pressure distribution in the rotor tip region are provided to illustrate that for design tip clearance （1.1% tip chord） the strongest fluctuating region is located on pressure side of blade near leading edge, while for the larger tip clearance （2.2% tip chord）, it is in the region of the interaction between the shock wave and the tip leakage flow.     

The Self-induced unsteadiness of tip leakage flow in an axial low-speed compressor with single circumferential casing groove

Numerical investigation on the self-induced unsteadiness of tip leakage flow （TLF） for an axial low-speed com- pressor with smooth wall and six single grooved casings are presented. A ten-passage numerical scheme is used to solve the unsteady Reynolds averaged Navier-Stokes （URANS） equations. It is found that the single grooves at various axial locations could have a large impact on the self-induced unsteadiness and the stall margin improve- ment （SMI） of compressor. The trend of SMI with groove center location demonstrates that the groove located near the mid of blade tip chord generates the best SMI. The worst groove is located about 20% Cax after the blade leading edge. The root-mean-squre of static pressure （RMSP） contours at 99.5% span and fast Fourier transform for the static pressure traces recorded in the tip clearance region for each casing are analyzed. The results demon- strate that the single groove location not only affects the oscillating strength but also the frequency of the un- steady tip leakage flow. At the near-stall point of smooth casing, the self-induced unsteadiness of TLF is enhanced most by the best grooved casing for SMI. While, the self-induced unsteadiness disappears when the worst groove for SMI is added. The characteristic frequency of TLF is about 0.55 blade passing frequency （BPF） with smooth casing. The frequency components become complicated as the single groove moves from the leading edge to the trailing edge of the blade.     

汽轮机叶顶间隙内非定常流动的数值分析

为了分析叶顶间隙泄漏涡的影响范围、运行轨迹和强度的变化规律,以某汽轮机高压级为研究对象,采用SSTκ-ω湍流模型,应用PISO算法对叶项间隙内的非定常流动进行了数值模拟．结果表明：叶顶间隙泄漏流是有规律的周期性的非定常流动,泄漏涡的影响范围、运行轨迹和强度随时间和叶顶间隙的变化而变化;泄漏流对主流的影响呈现出从弱到强、再从强到弱的周期性变化规律;叶顶间隙泄漏涡在丁／4时刻的强度和影响范围均达到最大,在T／2时刻,静叶脱落涡和动叶吸力面前部的泄漏涡混合形成新的涡系,而动叶吸力面后部的泄漏涡却与其边界层的脱涡混合,离开吸力面．     

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.     

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…     

Effect of winglet geometry arrangement and incidence on tip clearance control in a compressor cascade

An experimental study is conducted to investigate the influences of blade tip winglet on the flow field of a compressor cascade. The tests are performed in a low speed linear cascade with stationary endwall, with three blade tip configurations, including the baseline tip, the suction-side winglet tip and the pressure-side winglet tip. The flowfield downstream of the cascade is measured using five-hole probe, from which the three-dimensional velocity field, vorticity field and pressure field are obtained. Static pressure measurements are made on the endwall above the blade row using pressure taps embedded in the plywood endwall. All measurements are made at both design and off-design conditions for tip clearance level of about 2 percent of the blade chord. The results revealed the incidence variation significantly affects the secondary flow and the associated loss field downstream of the cascade, where the tip leakage vortex and passage vortex exist as the major contributors on the field. The winglet geometry arrangements can change the trajectory of the tip leakage vortex. The suction-side winglet tip blade provides a lower overall total pressure loss coefficient when compared to the baseline tip blade and pressure-side winglet tip blade at all incidence angles.     

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.     

Experimental and numerical investigation on compressor cascade flows with tip clearance at a low Reynolds number condition

High flow rate aeroengines typically employ axial flow compressors,where aerodynamic loss is predominantly due to secondary flow features such as tip leakage and corner vortices.In very high altitude missions,turbomachinery operates at low density ambient atmosphere,and the recent trend toward more compact engine core inevitably leads to the reduction of blade size,which in turn increases the relative height of the blade tip clearance.Low Reynolds number flowfield as a result of these two factors amplifies the relative importance of secondary flow effects.This paper focuses on the behavior of tip leakage flow,investigating by use of both experimental and numerical approaches.In order to understand the complex secondary flow behavior,cascade tests are usually conducted using intrusive probes to determine the loss.However relatively few experimental studies are published on tip leakage flows which take into account the interaction between a rotating blade row and its casing wall.Hence a new linear cascade facility has been designed with a moving belt casing in order to reproduce more realistic flowfield as encountered by a rotating compressor row.Numerical simulations were also performed to aid in the understanding of the complex flow features.The experimental results indicate a significant difference in the flowfield when the moving belt casing is present.The numerical simulations reveal that the leakage vortex is pulled by the shearing motion of the endwall toward the pressure side of the adjacent blade.The results highlight the importance of casing wall relative motion in analyzing leakage flow effects.     

Experimental study of effects of one kind of tangentially non-uniform tip clearance on the flow field at an exit of a compressor cascade passage

This paper presents an experimental investigation of effects of one kind of tangentially non-uniform tip clearance on the flow field at an exit of a compressor cascade passage.The tests were performed in a low-speed large-scale cascade with the uniform tip clearance and the non-uniform clearance.The three-dimensional flow field was measured at the exit at three incidence angles of 0°,5°,and 8° using a mini five-hole pressure probe.The measurement results show that the non-uniform tip clearance can moderate the leakage flow and blow down more low-energy fluids at the tip corner and decrease the accumulation of low-energy fluids which cause the flow blockage in the blade passage.In the meantime,the non-uniform clearance can weaken the tangential migration of the low-energy fluids in the endwall boundary layer and reduce the secondary loss and the flow blockage in the tip region.     

基于SPOD方法的压气机转子叶顶区域非定常流动分析

为了研究叶顶区域非定常流动特性,对跨声速轴流压气机转子NASA Rotor37在多工况下进行了三维非定常数值模拟,采用谱本征正交分解（Spectral Proper Orthogonal Decomposition,SPOD）方法从叶顶区域流场中提取出时空耦合的单频相干结构进行分析。研究结果表明：相比于常规分析方法,SPOD方法能够高效地从非定常流场中识别出流动特征,有助于揭示叶顶区域流动规律;在“小流量”工况下叶顶区域流动呈现出强的非定常性,且随着质量流率的减小叶顶区域非定常流动增强、波动范围增加、波动频率呈现出“阶跃式”下降;造成叶顶区域流场非定常周期性波动的主要原因是叶顶间隙泄漏涡破碎区的扰动以及叶顶间隙泄漏涡破碎后与主流相互作用所形成的叶尖二次涡的波动。     

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.     

凹槽式叶顶流动换热的数值研究

针对叶尖间隙高度对凹槽式叶顶流动与换热的影响展开数值研究,评估4种湍流模型在叶顶换热方面的预测能力.结果表明：凹槽肩壁顶部、凹槽腔底部近前缘区域和叶顶尾缘为高换热区,凹槽腔底的中部和尾部区域为低换热区;不同湍流模型对叶尖间隙泄漏量预测差别很小,但泄漏流流动状态差异很大,这是造成不同湍流模型对叶顶换热预测存在重大差别的原因;在研究的间隙范围内,叶尖间隙泄漏量和叶顶换热强度随间隙高度的增大而增加;在所选的4种湍流模型中,k-ω模型是叶顶换热数值模拟较好的湍流模型选择.     

A similitude method and the corresponding blade design of a low-speed large-scale axial compressor rotor

A similitude method to model the tip clearance flow in a high-speed compressor with a low-speed model is presented in this paper. The first step of this method is the derivation of similarity criteria for tip clearance flow, on the basis of an inviscid model of tip clearance flow. The aerodynamic parameters needed for the model design are then obtained from a numerical simulation of the target high-speed compressor rotor. According to the aerodynamic and geometric parameters of the target compressor rotor, a large-scale low-speed rotor blade is designed with an inverse blade design program. In order to validate the similitude method, the features of tip clearance flow in the low-speed model compressor are compared with the ones in the high-speed compressor at both design and small flow rate points. It is found that not only the trajectory of the tip leakage vortex but also the interface between the tip leakage flow and the incoming main flow in the high-speed compressor match well with that of its low speed model. These results validate the effectiveness of the similitude method for the tip clearance flow proposed in this paper.     

汽轮机动叶顶部间隙泄漏流动特性的数值模拟

以一个小展弦比轴流透平级为研究对象,采用数值方法对不同动叶顶部间隙情况下的间隙泄漏流动进行了分析,研究了间隙流和间隙涡的形成、发展及其对透平级性能的影响.以三维流线和极限流线为手段,分析了6种间隙尺寸下动叶顶部的泄漏流和泄漏涡造成的损失及其与主流掺混的过程.结果表明:动叶顶部间隙两侧压力面和吸力面之间的压力差使汽流从压力面被吸入间隙,跨过叶顶,进入相邻叶栅通道的吸力面,导致泄漏流动;与无间隙的情况相比,叶顶间隙的存在使上端壁处的流场发生明显变化,引起损失迅速增长;随着间隙的增大,泄漏涡的产生位置提前,强度增大,从而导致更大的流动损失.     

Numerical investigation of the unsteady tip leakage flow and rotating stall inception in a transonic compressor

<正>It is well known that tip leakage flow has a strong effect on the compressor performance and stability. This paper reports on a numerical investigation of detailed flow structures in an isolated transonic compressor rotor-NASA Rotor 37 at near stall and stalled conditions aimed at improving understanding of changes in 3D tip leakage flow structures with rotating stall inception.Steady and unsteady 3D Navier-Stokes analyses were conducted to investigate flow structures in the same rotor.For steady analysis,the predicted results agree well with the experimental data for the estimation of compressor rotor global performance.For unsteady flow analysis, the unsteady flow nature caused by the breakdown of the tip leakage vortex in blade tip region in the transonic compressor rotor at near stall condition has been captured with a single blade passage.On the other hand, the time-accurate unsteady computations of multi-blade passage at near stall condition indicate that the unsteady breakdown of the tip leakage vortex triggered the short length-scale-spike type rotating stall inception at blade tip region.It was the forward spillage of the tip leakage flow at blade leading edge resulting in the spike stall inception. As the mass flow ratio is decreased,the rotating stall cell was further developed in the blade passage.     

气冷涡轮转子变叶尖间距性能试验及数值研究

为量化评估工程应用的气冷低压涡轮带冠转子叶片的叶尖间距大小对涡轮气动性能的影响,综合现有涡轮部件试验能力,以单级轴流低压涡轮性能试验件为基础,通过控制圆度的机加方式磨削转子外环内壁以实现叶尖间距的变化,采用控制冷气流量比的方法,开展5次不同叶尖间距大小的涡轮级性能试验,得到多工况下涡轮效率、换算流量和换算功率等特性参数。采用加载冷气及考虑转子叶冠结构的数值模型进行三维仿真计算,并与试验结果对比分析。研究表明：叶尖间距由0.6 mm增加至3.2 mm,低压涡轮流通能力增大1%,叶冠泄漏量增多3.4%,但做功能力下降2.3%。涡轮效率变化与叶尖间距大小近似呈线性关系,叶尖间距每增加1 mm,效率约降低0.7%,同时,叶尖间距的增加导致了叶冠腔的旋涡结构、气流掺混及主流入侵强度逐渐增大,引起动叶总压损失的增大,叶尖间距增加至3.2 mm导致叶间位置总压损失由0.88增至2.3。     

Experimental Investigation of Aerodynamic Performance due to Blade Tip Clearance in a Gas Turbine Rotor Cascade

This study examines how the complex flow structure within a gas turbine rotor affects aerodynamic loss. An unshrouded linear turbine cascade was built, and velocity and pressure fields were measured using a 5-hole probe. In order to elucidate the effect of tip clearance, the overall aerodynamic loss was evaluated by varying the tip clearance and examining the total pressure field for each case. The tip clearance was varied from 0% to 4.2% of blade span and the chord length based Reynolds number was fixed at 2×10⁵. For the case without tip clearance, a wake downstream of the blade trailing edge is observed, along with hub and tip passage vortices. These flow structures result in profile loss at the center of the blade span, and passage vortex related losses towards the hub and tip. As the tip clearance increases, a tip leakage vortex is formed, and it becomes stronger and eventually alters the tip passage vortex. Because of the interference of the secondary tip leakage flow with the main flow, the streamwise velocity decreases while the total pressure loss increases significantly by tenfold in the last 30% blade span region towards the tip for the 4.2% tip clearance case. It was additionally observed that the overall aerodynamic loss increases linearly with tip clearance.     

汽轮机叶顶汽封间隙内的流动损失分析

为了揭示叶顶汽封结构变化对泄漏损失的影响,提高汽轮机运行效率,数值研究了平齿汽封、高低齿汽封和侧齿汽封3种不同叶顶汽封结构下汽轮机高压转子间隙泄漏的流动形态、间隙涡系的形成机理和发展规律,研究表明:在叶顶汽封腔室复杂的周向螺旋状的涡动中,泄漏流体的周向速度是影响漩涡耗散的一个重要因素;高低齿及侧齿的汽封结构可以增强漩涡之间的相互作用,降低泄漏流体的周向速度,使漩涡在腔室内的耗散更加充分;由于掺混损失降低,高低齿及侧齿汽封的泄漏总损失较平齿汽封相比分别下降7.1%和9.8%。     

叶顶开槽——小翼结构对提升轴流风机性能影响的数值研究

为了有效抑制叶顶泄漏流的发展,降低叶顶泄漏损失,针对两级动叶可调轴流风机提出在吸力面构造叶顶小翼并开设斜槽的新型叶顶改型方案。采用Fluent数值模拟了5种叶顶改型方案对风机性能和流场特征的影响,分析了不同方案下流场、叶顶静压、叶顶泄漏量和动叶区做功能力的变化。结果表明：吸力面小翼可有效降低叶顶损失,小翼上开设顺流向斜槽可进一步提高风机性能,逆流向斜槽会使性能略有降低;顺流向单斜槽为最佳改型方案,在设计流量下全压和效率分别提升166 Pa和0.942%;叶顶间隙处产生额外的涡流,叶顶泄漏流得到抑制,动叶区做功能力得以提升。