高压涡轮动叶前缘新型几何特征对流场结构影响研究

本文提出一种前缘前伸内凹且根部叶片略厚的新型动叶片设计方法,旨在通过这种新型叶片控制前缘马蹄涡的生成和发展,通过根部叶片较厚到中间区域叶片厚度略低的三维设计和积叠方法,实现动叶片的弯叶片效果。论文以某高压涡轮叶片为研究对象,从能量损失系数、型面压力分布、旋涡沿流向变化对比分析了新型叶片与原型叶片对流场结构的影响。研究表明叶型前缘新的几何特征使马蹄涡得到控制,流场内流动得到改善。     

凹凸前缘压气机叶栅旋涡结构探索

为改善压气机叶栅内的分离流动、优化气动性能,以仿生凹凸前缘叶栅为研究对象,基于数值方法分析吸力面特殊流动形成的原因,研究零攻角工况下凹凸前缘叶栅的流动特性,并基于涡系变化和附面层结构的分析,总结了凹凸前缘叶栅的流动控制机理。研究结果表明:由于前缘压力梯度作用使凹凸前缘叶栅形成了特殊的流向涡对,在下游向两侧发展形成特殊的三维分离结构,挤压局部流管收缩,提高了流动附着性并重组附面层结构,降低了角区分离范围且避免了大尺度集中脱落涡的形成,改善了下游流动。探索了凹凸前缘叶栅的典型旋涡模型,并基于对流动控制机理的理解,给出若干优化方案,得到叶栅气动性能提升,其中WFB-2-9叶栅相比原始叶栅总压损失系数降低了10.47%。     

Influence of air-jet vortex generator diameter on separation region

Control of shock wave and boundary layer interaction continues to attract a lot of attention. In recent decades several methods of interaction control have been investigated. The research has mostly concerned solid (vane type) vortex generators and transpiration methods of suction and blowing. This investigation concerns interaction control using air-jets to generate streamwise vortices. The effectiveness of air-jet vortex generators in controlling separation has been proved in a previous research. The present paper focuses on the influence of the vortex generator diameter on the separation region. It presents the results of experimental investigations and provides new guidelines for the design of air-jet vortex generators to obtain more effective separation control.     

中等折转角涡轮直叶栅端壁流场结构和涡系效应的分析

应用商用软件Fine/Turbo对一中等折转角涡轮直叶栅的流场进行了数值模拟,清晰地捕捉到了叶栅端壁处叶片前缘的马蹄涡及其吸、压力面分支,对马蹄涡的吸、压力面分支的相互作用过程和通道涡的发生发展过程给出了较为清晰的描述,获得了较为详细的叶栅端壁处二次流流动结构。通过对叶栅流道中各个近似垂直于流动方向的截面上的总压分布图的分析,揭示了在上述各个截面上端壁区域流动损失产生的机理。     

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.     

Effects of Damköhler number on vortex–flame interaction

Oscillating heat release associated with large-scale vortex structures in a dump combustor was investigated experimentally. The objectives were to understand vortex–heat release interactions that may lead to combustion instability and to characterize spatial patterns in fluctuating heat release for future control purposes. The inlet jet, which consisted of a turbulent premixed ethylene–air flow, was acoustically forced to control shedding of periodic vortices. The reacting flowfield was characterized using a phase-locked schlieren technique, while the corresponding heat release pattern was investigated using CH^* chemiluminescence. Phase-resolved data taken at various flow conditions were compared to identify important physical parameters. In the context of vortex-driven combustion instability, the results showed that the Damköhler number was a critical parameter for vortex–heat release interaction. When the Damköhler number was relatively high, the leading edge of the vortices coincided with the high point in the local heat release cycle and the trailing edge with the low point. However, a clear shift in the heat release pattern was observed at Damköhler numbers lower than four. Under these conditions, the high point in local heat release was observed either in the vortex core or at the trailing edge of the vortex. Accordingly, the effect of the Damköhler number must be taken into consideration when designing an active control strategy based on secondary fuel injection.     

Investigation of flow behind vortex generators by stereo particle image velocimetry on a thick airfoil near stall

Stereoscopic Particle Image Velocimetry measurements investigating the effect of vortex generators (VGs) on the flow near stall were carried out in a purpose‐built wind tunnel for airfoil investigations on a DU 91‐W2‐250 profile. Measurements were conducted at Re = 0.9?10⁶, corresponding to free stream velocity U_∞ = 15 m s^?1. The objective was to investigate the flow structures induced by the vortex generators and study their separation controlling behavior on the airfoil. The uncontrolled flow (no VGs) displayed unsteady behavior with separation as observed from large streamwise velocity variations. The corresponding controlled flow (with VGs) showed the same unsteadiness, where the appearance of the vortex structures alternated with a much less separated or even attached boundary layer as also seen in the measured airfoil data: C_L = 1.56, C_D = 0.116 with VGs and C_L = 1.16, C_D = 0.135 without. On average, the controlled flow left an attached flow as opposed to the uncontrolled one. Mixing close to the wall, transferring high momentum fluid into the near wall region, was observed, and the hypothesis of variations in the streamwise velocity component in the boundary layer was supported by a Snapshot Proper Orthogonal Decomposition analysis. This analysis also revealed some of the dynamics of the induced vortices. Copyright © 2012 John Wiley & Sons, Ltd.     

An integral boundary layer engineering model for vortex generators implemented in XFOIL

To assess and optimize vortex generators (VGs) for flow separation control, the effect of these devices should be modelled in a cost and time efficient way. Therefore, it is of interest to extend integral boundary layer models to analyse the effect of VGs on airfoil performance. In this work, the turbulent boundary layer formulation is modified using a source term approach. An additional term is added to the shear‐lag equation, to account for the increased dissipation due to streamwise vortex action in the boundary layer, forcing transition at the VG leading edge where applicable. The source term is calibrated and a semi‐empirical relation is set up and implemented in XFOIL . The modified code is capable of addressing the effect of the VG height, length, inflow angle, and chordwise position on the airfoil's aerodynamic properties. The predicted polars for airfoils with VGs show a good agreement with reference data, and the code robustness is demonstrated by assessing different airfoil families at a wide range of Reynolds numbers.     

Flow visualization of annular and delta winlet vortex generators in fin-and-tube heat exchanger application

This study presents flow visualization and frictional results of enlarged fin-and-tube heat exchangers with and without the presence of vortex generators. Two types of vortex generators and a plain fin geometry were examined in this study. For plain fin geometry at Re=500, the horseshoe vortex generated by the tube row is not so pronounced, and the horseshoe vortex separates into two streams as it flows across the second row and consequently loses its vortical strength. This phenomenon may supports the “maximum phenomenon” in low Reynolds number region reported by previous studies. With the presence of annular vortex generator, the presence of a pair of longitudinal vortices formed behind the tube is seen. The strength of the counter-rotating vortices increases with the annular height and the strength of the longitudinal vortices is so strong that may swirl with the horseshoe vortices and other flow stream. For the same winlet height, the delta winlet shows more intensely vortical motion and flow unsteadiness than those of annular winlet. This eventually leads to a better mixing phenomenon. However, it is interesting to know that the corresponding pressure drops of the delta winlet are lower than those of annular winlet. Compared to the plain fin geometry, the penalty of additional pressure drops of the proposed vortex generators is relatively insensitive to change of Reynolds number.     

Visualization and Experiment of Tip Vortex Phenomenon in Cooling Fan using Digital Particle Image Velocimetry

The Digital Particle Image Velocimetry (DPIV) is an efficient method for measuring the internal flow field of a low-speed cooling fan. This paper studied the velocity field by means of PIV technology for a leading edge swept axial-flow fan without casing, and the tip vortex phenomenon was observed. Time-averaged velocity measurements were taken near the pressure surface, the suction surface and the tip of blade, etc. Moreover, the flow characteristics were visualized using numerical techniques. Experimental results showed that this tip vortex existed at the leading edge of the blade. The generating, developing and dissipating evolvement process of the tip vortex from the blade leading edge to downstream were discussed in detail. In addition, by comparing DPIV results and numerical results, a good agreement between them indicated a possibility to predict flow field using CFD tools. The experimental data provided in this paper are reliable for improving the aerodynamic characteristics of the open axial fan.     

不同相对厚度前缘缝翼对S809翼型气动性能的影响

文章通过数值模拟方法研究了不同相对厚度的前缘缝翼对S809翼型气动性能的影响,并揭示了前缘缝翼相对厚度对流动控制产生影响的机理。研究结果表明:在大攻角下,空气流经过前缘缝翼会在其尾部产生涡旋,尾缘涡旋的形成有助于抑制S809翼型流动分离,进而改善翼型绕流场;不同相对厚度的前缘缝翼产生尾缘涡旋不同的流动轨迹,对翼型的流动控制作用效果不同;相同条件下,前缘安装最大相对厚度为35%的前缘缝翼能够将S809翼型最大升力系数提升至1.25,失速攻角推迟至17.21°;安装最大相对厚度为14%的前缘缝翼,能够使S809翼型最大升力系数提升至1.53,并使翼型在攻角为20.16°时仍未发生失速。     

Flow-field analysis of anti-kidney vortex film cooling

Film cooling is an important measure to enable an increase of the inlet temperature of a gas turbine and, thereby, to improve its overall efficiency. The coolant is ejected through spanwise rows of holes in the blades or endwalls to build up a film shielding the material. The holes often are inclined in the downstream direction and give rise to a kidney vortex. This is a counter-rotating vortex pair, with an upward flow direction between the two vortices, which tends to lift off the surface and to locally feed hot air towards the blade outside the pair. Reversing the rotational sense of the vortices reverses these two drawbacks into advantages. In the considered case, an anti-kidney vortex is generated using two subsequent rows of holes both inclined downstream and yawed spanwise with alternating angles. In a previous study, we performed large-eddy simulations (which focused on the fully turbulent boundary layer) of this anti-kidney vortex film-cooling and compared them to a corresponding physical experiment. The present work analyzes the simulated flow field in detail, beginning in the plenum (inside the blade or endwall) through the holes up to the mixture with the hot boundary layer. To identify the vortical structures found in the mean flow and in the instantaneous flow, we mostly use the λ 2 criterion and the line integral convolution (LIC) technique indicating sectional streamlines. The flow regions (coolant plenum, holes, and boundary layer) are studied subsequently and linked to each other. To track the anti-kidney vortex throughout the boundary layer, we propose two criteria which are based on vorticity and on LIC results. This enables us to associate the jet vortices with the cooling effectiveness at the wall, which is the key feature of film cooling.     

环形扩压叶栅弯叶片对流场性能的影响

对比研究了直叶片叶栅与弯叶片叶栅吸力面角区和下端壁流场显示的不同表现,发现弯叶片对角区分离流结构影响较大,它对减小端区马蹄涡尺度和减弱横向二次流作用明显。将不同叶栅中三维流向涡（通道涡和集中脱落涡）沿流向截面内的位置与强度作为研究对象,细致地分析了在采用弯叶片前后涡位置和强度的变化,分析表明两种涡的位置受弯叶片影响较大;通道涡沿流向的强度变化受弯叶片影响较为明显,而集中脱落涡强度受弯叶片影响却很小。来流马赫数、叶型折转角和稠度在一定范围内对弯叶片作用有规律性影响：当马赫数为0．7时,最佳弯角弯叶片降低损失7％．而马赫数为0．2时,最佳弯角弯叶片降低损失仅4％。     

An improved prediction model of vortex shedding noise from blades of fans

The main source of the noise of an axial flow fan is the fluctuating pressure field on blade surfaces caused by the shedding of vortices at the trailing edge of blades. An analytical model to predict the vortex shedding noise generated at the trailing edge of blades of axial flow fans was proposed by Lee in 1993. In this model, for mathematical convenience, an idealized vortex street is considered. However, the agreement between the analytical results and the experimental data needs to be improved because of the simplification about the Karman vortex street in the wake of blade. In the present study, a modified model is proposed based on the prediction model by Lee. The boundary layer theory is used to analyze and calculate the boundary layer development on both the pressure and the suction sides of blades. Considering the effect of boundary layer separation on the location of noise source, the predicted overall sound pressure level compares favorably with the experimental data of an axial fan. In the calculation of A-weighted sound pressure level (L_A), considering the effect of static pressure on radiate energy, the predicted broadband noise with the modified model compares favorably with the experimental data of a multiblade centrifugal fan.     

Flow visualization of wave-type vortex generators having inline fin-tube arrangement

This study presents visual observation of enlarged fin-and-tube heat exchangers with and without the presence of vortex generators. Three samples of fin-and-tube heat exchanger having inline arrangements are examined, including one plain fin and two wave-type vortex generators. For plain fin geometry at Re=500, the horseshoe vortex generated by the tube row is not so pronounced, and a very large secondary flow circulation is seen between the first and second row. This flow re-circulation phenomenon is almost disappeared with the presence of proposed vortex generators. The presence of vortex generators significantly increase the vortrical motions of the horseshoe vortices hitting on the tubes. A much better mixing characteristics is seen by introducing the vortex generators. The frictional penalty of the proposed vortex generators are about 25-55% higher than that of the plain fin geometry. The penalty of pressure drops of the proposed vortex generators relative to plain fin geometry is relatively insensitive to change of Reynolds number.     

Effects of incidence angle on endwall convective transport within a high-turning turbine rotor passage

Effects of incidence angle on the endwall convective transport within a high-turning turbine rotor passage have been investigated. Surface flow visualizations and heat/mass transfer measurements at off-design conditions are carried out at a fixed inlet Reynolds number of 2.78 × 10⁵ for the incidence angles of −10°, −5°, 0, 5°, and 10°. The result shows that the incidence angle has considerable influences on the endwall local transport phenomena and on the behaviors of various endwall vortices. In the negative incidence case, convective transport is less influenced by the leading edge horseshoe vortex and by the suction-side corner vortex along their loci but is increased along the pressure-side corner vortex. In the case of positive incidence, however, convective transport is augmented remarkably along the leading edge horseshoe vortex, and is much influenced by the suction-side corner vortex. Moreover, heat/mass transfer is enhanced significantly along the pressure-side leading edge corner vortex. Local endwall convective transport in the area other than the endwall vortex sites is influenced significantly by the cascade inlet-to-exit velocity ratio which depends strongly on the incidence angle.     

Contribution of vortex structures and flow separation to local and overall pressure and heat transfer characteristics in an ultralightweight lattice material

Ultralightweight lattice-frame materials (LFMs) with open, periodic microstructures are attractive multifunctional systems that can perform structural, thermal, actuation, power storage and other functions [A.G. Evans, J.W. Hutchinson, M.F. Ashby, Multifunctionality of cellular metal systems, Prog. Mater. Sci. 43 (1999) 171–221]. This paper presents experimental and numerical studies of local fluid flow behaviour and its contribution to local and overall pressure and heat transfer characteristics of such a lattice material with tetrahedral unit cells. A single layer of the LFM with porosity of 0.938 is sandwiched between impermeable endwalls that receive uniform heat flux and the heat transfer is subjected to forced air convection.Experimental measurements with particle image velocity (PIV) and thermochromic liquid crystal (TLC), backed by computational fluid mechanics (CFD) simulations, revealed two dominant local flow features in the LFM. Distinctive vortex structures near the vertices where the LFM meets the endwalls and flow separation on the surface of LFM struts were observed. The vortex structures formed around the vertices include horseshoe vortices and arch-shaped vortices. The horseshoe vortex increases local heat transfer on the endwall region up to 180% more than that in regions where the least influence of the horseshoe vortex is present. The arch-shaped vortex behind the vertices creates regions of flow recirculation and reattachment, leading to relatively high heat transfer.The location of flow separation along the struts varies with the spanwise position due to the presence of vertices (or endwalls). The regions on the strut surface before flow separation contribute approximately 40% of the total heat transfer in the LFM. The delay of the flow separation leads to an increase in the overall heat transfer.Comparisons with foams and other heat dissipation media such as packed beds, louvered fins and microtruss materials suggest that the LFMs compete favourably with the best available heat dissipation media.     

叶片锯齿前缘控制流动分离的数值研究

针对锯齿前缘结构调控叶片近壁面流场特性,以NACA0018叶片为对象,采用大涡模拟方法研究不同锯齿前缘结构对叶片近壁面流场的影响机制。获得了来流速度为30 m·s^-1、雷诺数为513 440、0°攻角下叶片近壁面流场分布特性。分析了锯齿前缘和叶片前缘和尾缘处压力脉动及分离涡的影响。数值结果表明：对正弦波齿而言,随着振幅的增大,在波谷处的小涡开始向前缘移动,整体上小尺度涡增多,前缘近壁面压力脉动增大,尾缘近壁面压力脉动减小;对叠加波形齿而言,尾迹涡进一步破碎,厚度变薄,叶片表面出现破碎的小尺度涡,在尾缘处叶片压力脉动幅值下降最为明显,且未出现明显的窄带尖峰。     

Experimental Investigations of the Three—Dimensional Flow in a Large Deflection Turbine Cascade with Tip Clearance

INTRoDUCTI0NThetipleakaeflowisnowrecognizedasanimpor-tantsourceoflossesinbothcompressorsandturbines,asasourceofcoolingprobleminturbinesandasourceofinstabilityincomPressorsandfans.Manyturbo-maChinimPellersarenotshroudedandtheleakaeflowthroughthetipgaPofthebladeisanunavoidablefaCtorwhichdeterioratestheperformance.Den-tonandCumpsty[1]melltionedabouttwodistinctandequallyimportantaspects.tothetipclearanceflows.First,thereisareducti0ninthebladeforce,there-fore,theworkdone.Thisoccursbecausethe…     

叶根变径倒角的设计与流动分析

为了研究倒角结构对流动的影响,采用数值方法对带有不同类型倒角的叶栅模型进行模拟,综合评估了各种类型倒角对于二次流结构的影响。从中优选获得较为优秀的"变径倒角结构",并详细分析了该结构对于二次流结构的影响。结果表明:选用合适的设计参数构建变径倒角能够减小前缘位置马蹄涡强度,使马蹄涡与通道涡发展受到抑制,使出口的总压损失系数下降5. 35%;变径倒角控制点位置及前缘倒角半径的选择对于流动结果有较大影响。