Measurement and analysis of tip clearance unsteady flow spectrum in axial-flow fan rotor

The dynamic pressure measurement device and test technology are described in this study. The tip clearance unsteady flow development from the inlet to the outlet of an axial-flow rotor was revealed by analyzing pressure frequency spectrum acquired from measuring the unsteady pressure field of the tip endwall. The experiment provides test basis for thoroughly understanding the tip clearance unsteady flow and building interaction models of tip clearance flow and main flow.     

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 3-D unsteady flow in a vaneless counter-rotating turbine

To reveal the unsteady flow characteristics of a vaneless counter-rotating turbine (VCRT), a three-dimensional, viscous, unsteady computational fluid dynamics (CFD) analysis was performed. The results show that unsteady simulation is superior to steady simulation because more flow characteristics can be obtained. The unsteady effects in upstream airfoil rows are weaker than those in downstream airfoil rows in the VCRT. The static pressure distribution along the span in the pressure surface of a high pressure turbine stator is more uniform than that in the suction surface. The static pressure distributions along the span in the pressure surfaces and the suction surfaces of a high pressure turbine rotor and a low pressure turbine rotor are all uneven. The numerical results also indicate that the load of a high pressure turbine rotor will increase with the increase of the span. The deviation is very big between the direction of air flow at the outlet of a high pressure turbine rotor and the axial direction. A similar result can also be obtained in the outlet of a low pressure turbine rotor. This means that the specific work of a high pressure turbine rotor and a low pressure turbine rotor is big enough to reach the design objectives. Translated from Journal of Engineering Thermophysics, 2006, 27(1): 35–38 [译自: 工程热物理学报]     

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。     

Flow fields with tip leakage vortex in a small axial cooling fan

In order to improve the fan characteristics, especially efficiency and noise level of a small axial cooling fan with a large tip clearance, the internal flow measurements with tip leakage vortex were carried out at fan rotor outlet us- ing an I-type hot-wire probe. The probe was set toward two directions, parallel and normal to the meridional plane of test fan, and the two directional velocity components were measured. From the result of fan test it was found that the test fan didn＇t have the unstable characteristic with a positive gradient on its pressure - flow-rate curve. From the results of velocity measurement it was observed that the tip leakage vortex exited at maximum efficiency flow-rate and large flow-rate conditions. However, at small flow-rate conditions the tip leakage vortex was not observed and it was found that the flow field were enlarged toward radial outwards     

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。     

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.     

1.5级变几何涡轮非定常流动特性研究

可转导叶由于端部间隙和转轴的存在,会产生复杂的二次流动。本文对LISA涡轮进行变几何改型,采用几何约化法对该1.5级变几何涡轮进行数值模拟,详细探究了可转导叶间隙高度对可转导叶（S1）涡系的流动细节和载荷的影响,并深入研究其非定常流动对下游叶排的干涉及二次流输运过程的影响。计算结果表明：泄漏涡（LV）、角涡（CV）和通道涡（PV）共同组成了可转导叶的涡系;可转导叶端部间隙高度影响流动损失和级效率大小,设计间隙下该变几何涡轮S1时均总压损失系数Y为10.32%,涡轮时均总总效率ηtt为82.26%;可转导叶的尾缘泄漏涡使第1级动叶（R1）流动产生强非定常性;可转导叶的尾缘泄漏涡和R1泄漏涡、壁面涡是造成第2级静叶（S2）流动非定常性的主要因素。     

Numerical analysis of aerodynamic noise radiated from cross flow fan

The flow field in a cross flow fan was simulated by solving the 2-D unsteady Reynolds-averaged Navier-Stokes equations. The calculated pressure fluctuations of the blades, the vortex wall, and the rear wall were then used as noise sources to calculate the sound field. The Ffowcs Williams-Hawkings (FW-H) equation was employed to predict the noise field caused by these sources. The predictions show that the rear wall and the vortex wall sources contribute significantly to the total noise and that both the predicted aerodynamic performance and noise agree well with the experimental results. __________ Translated from J Tsinghua Univ (Sci & Tech), 2007, 47(2): 236–239 [译自: 清华大学学报(自然科学版)]     

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．     

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.     

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。     

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.     

Effects of a kind of non-smooth blade on the unsteady flow field at the exit of an axial fan

An experimental investigation of effects of a kind of streamwise-grooved blade on the unsteady flow field at an exit of an axial-flow fan was performed. The flow field at 25% chord downstream from the trailing edge at hub was measured using a fast-response five-hole pressure probe at different mass-flow conditions. The unsteady flow of the grooved blades was compared with that of the smooth blades. The measurement results indicate that： （1） the grooved blades restrain the velocity fluctuation and the pressure fluctuation by modulating the blade boundary layers, which contributes to the flow loss reduction in the hub region and in the rotor wake region at the design condition; （2） the stream-wise grooves play an important role in restraining the radial migration in the blade boundary layer and abating the tip flow mixing, which contributes to the flow loss reduction in the tip region at the design condition; （3） at the near stall condition, the grooved surface can not reduce the flow loss, even increase the loss nearby when the separation happens in the blade boundary layer.     

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.     

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 3D model of viscous turbulent flow in one stage gas turbine and its experimental validation

The paper describes 3D numerical Reynolds Averaged Navier-Stokes (RANS) model and approximate sector approach for viscous turbulent flow through flow path of one stage axial supercharge gas turbine of marine diesel engine. Computational data are tested by comparison with experimental data. The back step flow path opening and tip clearance jet are taken into account. This approach could be applied for variety of turbine theory and design tasks: for offer optimal design in order to minimize kinetic energy stage losses; for solution of partial supply problem; for analysis of flow pattern in near extraction stages; for estimation of rotational frequency variable forces on blades; for sector vane adjustment (with thin leading edges mainly), for direct flow modeling in the turbine etc. The development of this work could be seen in the direction of unsteady stage model application.     

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.     

Experimental and computational analysis on guide vane losses of impulse turbine for wave energy conversion

This paper deals with the detailed flow analysis of impulse turbine with experimental and computed results for wave energy power conversion. Initially, several turbulence models have been used in two-dimensional (2-D) computational fluid dynamic (CFD) analysis to find a suitable model for this kind of slow speed unconventional turbine. Experiments have been conducted to validate the CFD results and also to analyze the aerodynamics at various stations of the turbine. The three-dimensional (3-D) CFD model with tip clearance has been generated to predict the internal flow and to understand the effect of tip clearance leakage flow on behavior of the turbine in design and off-design conditions. As a result, it is found from the 2-D results that the comparison between computed and experimental data is good, qualitatively and the turbulence model, standard k–ε can predict the experimental values reasonably well, especially the efficiency of the turbine. Experimental results reveal that the downstream guide vanes are more responsible for low efficiency of the turbine and it is measured that 21% average pressure is lost due to downstream guide vanes. It is proved from the 3-D CFD model with tip clearance that it can predict the experimental values quantitatively and qualitatively. Furthermore, it is estimated from the computed results that the efficiency of the turbine has been reduced about 4%, due to tip clearance leakage flow at higher flow coefficients.     

Study of two-phase flow regime identification in horizontal tube bundles under vertical upward cross-flow condition using wavelet transform

A wavelet-transform based approach for flow regime identification in horizontal tube bundles under vertical upward cross-flow condition was presented. Tests on two-phase flow pattern of R134a were conducted under low mass velocity and flow boiling conditions over ranges of mass flux 4–25 kg/m²·s, vapor quality 0.02–0.90. Time series of differential pressure fluctuations were measured and analyzed with discrete wavelet transform. Different time-scale characteristics in bubbly flow, churn flow and annular flow were analyzed. The wavelet energy distributions over scales were found to be appropriate for flow regime identification. Based on the wavelet energy distribution over characteristic scales, a criterion of flow regime identification was proposed. The comparison with experiment results show that it is feasible to use the discrete wavelet transform as the tool of flow regime identification in horizontal tube bundles under vertical upward cross-flow condition. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 337–341, 346 [译自: 上海交通大学学报]