Tip Clearance Flows in Turbine Cascades

This article describes the effects of some factors on the tip clearance flow in axial linear turbine cascades. The measurements of the total pressure loss coefficient are made at the cascade outlets by using a five-hole probe at exit Mach numbers of 0.10, 0.14 and 0.19. At each exit Mach number, experiments are performed at the tip clearance heights of 1.0%, 1.5%, 2.0%, 2.5% and 3.0% of the blade height. The effects of the non-uniform tip clearance height of each blade in the pitchwise direction are also studied. The results show that at a given tip clearance height, generally, total pressure loss rises with exit Mach numbers proportionally. At a fixed exit Mach number, the total pressure loss augments nearly proportionally as the tip clearance height increases. The increased tip clearance heights in the tip regions of two adjacent blades are to be blame for the larger clearance loss of the center blade. Compared to the effects of the tip clearance height, the effects of the exit Mach number and the pitchwise variation of the tip clearance height on the cascade total pressure loss are so less significant to be omitted.

EFFECT OF TIP CLEARANCE ON PERFORMANCE AND FLOW FIELD OF SMALL SIZED HIGH SPEED CENTRIFUGAL FAN

A centrifugal fan with the high speed and compact dimensions is studied numerically and experimentally. The centrifugal fan consists of a shrouded impeller rotating at 34 000 r/min with a small tip clearance 0.7 mm to the fixed outer casing. Computational models with/without the tip clearance are built and the κ-ω shear stress transport （SST） turbulence model and the unstructured mesh are applied to the numerical simulation for unsteady solutions. The overall performance is measured on a standard experimental bench and the major flow feature of each component inside the centrifugal fan is numerically investigated. In the presence of the tip clearance due to the difference of static pressure between leading and trailing edges of the clearance, i. e. , leading and trailing edges of the impeller, a strong return flow exists inside the clearance passage and re-circulates the main stream inside the impeller passage, and produces the strong flow interaction, thus changing the flow field and influencing the overall performance.

COMPARISON OF TWO METHODS TO INCREASE TIP CLEARANCE AND ITS EFFECT ON PERFORMANCE OF TURBOCHARGER CENTRIFUGAL COMPRESSOR STAGE

Tip clearance between the blade tip and casing of a centrifugal compressor can be varied through two methods： by changing the blade height （MI） or by changing the casing diameter （M2）. Numerical simulations are carried out to compare these two methods and their effect on the stage and impeller performance. The impeller and diffuser are connected through rotor stator boundary using frozen rotor approach. Overall stage performance and the flow configuration have been investigated for nine tip clearance levels from no gap to 1 mm. Impeller and diffuser performances are also presented separately. It has been found that the overall and impeller performance are comparatively better for MI below tip clearance of 0.5 mm whereas M2 is found advantageous above 0.5 mm of tip clearance. Both MI and M2 show performance degradation with the increase in tip clearance. Two models have been proposed for the stage total pressure ratio and efficiency, which are found to be in agreement with experimental results. The impeller efficiency and the pressure ratio are found to be maximum at tip clearance of 0.1 mm for both the cases however minimum diffuser effectiveness is also observed at the same clearance level. Diffuser effectiveness is found to be maximum at zero gap for both cases. As it is practically impossible to have zero gap for unshrouded impellers so it is concluded that the optimum thickness is 0.5 mm onwards for MI and 0.5 mm for M2 in terms of diffuser effectiveness. Mass averaged flow parameters, entropy, blade loading diagram and relative pressure fields are presented, showing the loss production within the impeller passage with tip clearance.

Topology and Vortex Structures of Turbine Cascade with Different Tip Clearances

This paper uses the topology theory to analyze the surface flowspectrums of straight, positively curved and negatively curvedcascades with relative tip clearances of 0.023 and 0.036, findsapparent differences of topology and vortex structures in the bladetip and the suction side wall corner of single type of cascade withthis two clearances, and studies the mechanism of the differenceformation as well as their effects o the energy loss.

Influence of Tip Clearance on the Flow Field and Aerodynamic Performance of the Centrifugal Impeller

This paper studies numerically the influence of the tip clearance on the three dimensional viscous flowfield and performance of the NASA Low Speed Centrifugal Compressor (LSCC) impeller with a vaneless diffuser A three dimensional viscous code developed by the authors is applied with several acceleration methods: local time step, multigrid and residual smoothing The computations were performed under several operating conditions with four different tip clearance sizes(0 0%,50%,100% and 200% design t...

Influence of Tip Clearance on Unsteady Flow in Automobile Engine Pump

The automobile engine pump is an important part of the automobile cooling system,and has a direct influence on the engine performance.Based on the SST k-ωturbulence model,unsteady numerical simulation for an automobile engine pump with different tip clearances was carried out by Fluent.To study the flow field characteristics and pressure fluctuation,the characteristics of secondary flow distribution in volute are also analyzed.The result shows that the pressure fluctuation characteristics of the flow field show obvious periodic variation at different levels of tip clearances.The peak value of pressure fluctuation at each monitoring point is dependent on the blade frequency.At the same time,with the increase of the tip clearance,the pressure fluctuation in the blade and volute is gradually increased,while the pressure fluctuation at the tip is reduced clearance.The pressure gradient in the pump also varies periodically with the rotation of the impeller.With the increase of the tip clearance,the pressure of the impeller,volute and tip clearance is gradually decreased.There are secondary flow vortexes inside the impeller,volute outlet and volute section.With the increase of tip clearance,the vortex intensity in the impeller channel is weakened,and the vortex strength at the volute outlet is intensified.On the cross section of the volute,the morphology of most vortexes has insignificant changes,but the vortex intensity decreased.

TOPOLOGY AND VORTEX STRUCTURES OF A CURVING TURBINE CASCADE WITH TIP CLEARANCE (Ⅰ)- EXPERIMENTAL MODEL AND TOPOLOGICAL FLOW PATTERNS ON BOTH ENDWALLS AND BLADE SURFACES

By means of ink trace visualization of the flows in conventional straight, positively curved and negatively curved cascades with tip clearance, and measurement of the aerodynamic parameters in the transverse section, and by appling topology theory, the structures on both endwalls and blade surfaces were analyzed. Compared with conventional straight cascade, blade positive curving eliminates the separation line of the upper passage vortex and leads the secondary vortex to change from close separation to open separation, while blade negative curving effects merely the positions of singular points and the intensities and scales of vortex.

TOPOLOGY AND VORTEX STRUCTURES OF A CURVING TURBINE CASCADE WITH TIP CLEARANCE (Ⅱ)- TOPOLOGICAL FLOW PATTERN AND VORTEX STRUCTURE IN THE TRANSVERSE SECTION OF A BLADE CASCADE

By means of ink trace visualization of the flows in conventional straight, positively curved and negatively curved cascades with tip clearance, and measurement of the aerodynamic parameters in transverse section, and by appling topology theory, the topological structures and vortex structure in the transverse section of a blade cascade were analyzed. Compared with conventional straight cascade, blade positive curving eliminates the separation line of the upper passage vortex, and leads the secondary vortex to change from close separation to open separation, while blade negative curving effects merely the positions of singular points and the intensities and scales of vortex.

The Tip Leakage Flow Structure of an Axial Fan with Tip Clearance

Experiment and numerical simulation technique are used to investigate the tip leakage flow in an axial fan with tip clearance at the design condition. The flow field in the tip region of fan is measured using a PDA (Particle Dynamics Analysis) system. The flow is surveyed across the whole passage at fifteen axial locations (from the 100% axial chord in front of the leading edge to the 100% axial chord behind the trailing edge), mainly focusing on the outer 90% blade span. Both experiment measurement and numerical simulation indicates the leakage flow originated from the tip clearance along the chord rolls up into three dimensional spiral structure to form leakage flow vortex. The interaction of leakage flow and main flow will produce the low velocity zone, and block the flow. The leakage flow almost occupies the most part of flow passage behind the trailing edge.

Three-dimensional atomic force microscopy based on tailored cantilever probe with flared tip

In order to meet the requirements of nondestructive testing of true 3D topography of micro-nano structures,a novel three-dimensional atomic force microscope(3D-AFM)based on flared tip is developed.A high-precision scanning platform is designed to achieve fast servo through moving probe and sample simultaneously,and several combined nanopositioning stages are used to guarantee linearity and orthogonality of displacement.To eliminate the signal deviation caused by AFM-head movement,a traceable optical lever system is designed for cantilever deformation detection.In addition,a method of tailoring the cantilever of commercial probe with flared tip is proposed to reduce the lateral force applied on the tip in measurement.The tailored probe is mounted on the 3D-AFM,and 3D imaging experiments are conducted on different samples by use of adaptive-angle scanning strategy.The results show the roob-mean-square value of the vertical displacement noise(RMS)of the prototype is less than 0.1 nm and the high/width measurement repeatability(peak-to-peak)is less than 2.5 nm.

流线隧道式涡轮增压器转子动力学特性分析

为研究流线隧道式涡轮对涡轮增压器转子动力学特性的影响,将某型号车用涡轮增压器叶片式涡轮替换为流线隧道式涡轮,建立了密封流体激振、叶尖间隙激振和隧道涡轮轮缘间隙激振耦合条件下的转子系统模型,计算分析了涡轮材料和轮缘间隙对转子临界转速、稳态响应以及瞬态响应的影响。结果表明,K418高温合金隧道涡轮转子一阶临界转速大幅降低,稳态响应稳定性较好,但瞬态响应振动剧烈、失稳,表明隧道涡轮采用K418高温合金需要调整结构设计;SiC陶瓷隧道涡轮转子一阶临界转速下降、稳态响应振幅增加,但对转子稳定性影响不大,二阶临界转速有微小增加,稳态响应振幅下降,瞬态响应稳定性较佳。

叶顶间隙对多级轴流压气机性能退化的影响

以某F级燃机压气机为研究对象,考虑燃机实际运行过程中腐蚀和磨损作用导致的压气机叶顶间隙变化,通过三维数值模拟方法开展了不同腐蚀/磨损状态下多级轴流压气机的性能退化机理研究,分析了叶顶间隙变化对多级轴流压气机性能的影响规律。结果表明:叶顶间隙增大会导致压气机特性曲线整体向质量流量减小的方向偏移,使得压气机的质量流量、效率及压比均产生一定程度的衰退;随着叶顶间隙的增大,压气机峰值效率逐渐降低,喘振裕度明显降低;叶顶间隙增大会导致压气机通道内的流动分离加剧,动叶叶顶间隙变化对压气机流动分离的影响更严重;静叶吸力面尾缘附近的流动分离导致形成低马赫数区,同时逆流的低速涡团导致叶根附近的损失显著增大,造成压气机性能衰退。

叶顶间隙对低比转数小流量泵性能的影响

为建立叶顶间隙对低比转速小流量泵性能的影响关系,采用数值计算的方法,以叶顶间隙宽s和叶轮出口宽b的比值(s/b)表征间隙大小,对不同间隙下泵的性能展开对比分析。分析发现,随着间隙的增加,泵的扬程和效率均逐渐降低,最大和最小间隙下的扬程和效率差分别达15.4%,5.5%。当间隙较小即s/b<0.2时,扬程曲线在小流量工况会出现不稳定的“驼峰”现象,在s/b>0.3后,设计工况附近效率的降幅显著增大。从流动特性来看,随着叶顶间隙的增大,间隙泄漏流对叶轮内主流区影响加剧,会诱导严重的二次流和旋涡的产生,同时间隙内不稳定流动也会产生能量损失,造成扬程和效率下降。为低比转数小流量泵的设计与优化提供借鉴意义。

叶顶间隙对螺旋混流式喷水推进泵内流及推进特性的影响

喷水推进泵是舰船泵喷推进系统的核心部件,喷水推进泵在工作过程中,叶顶间隙处所产生的泄漏流会严重影响混流泵的工作性能。为了探讨叶轮叶顶间隙对喷水推进泵推进特性的影响,本文定义了叶顶间隙系数τ,设计了六种不同叶顶间隙系数模型并使用Ansys进行仿真分析,最终得到叶顶间隙对于喷水推进泵内流以及推进特性的影响规律。结果表明:随着叶顶间隙系数的增大,叶顶间隙处泄漏流逐渐增加,泄漏涡影响范围变大,轴向推力随着叶顶间隙系数的增大呈负指数下降,叶顶间隙处泄漏量随指数上升,喷水推进泵内流损失增加,推进效率持续减小,并且在大流量工况下叶顶间隙对喷水推进泵推进特性影响最大,在设计的六套模型中,叶顶间隙系数为0.08,叶顶间隙为0.16mm时效率最高。通过本文的研究可以发现,叶顶间隙对于喷水推进泵内流以及推进特性影响较大,在设计时应尽量减小叶顶间隙。

压气机内部旋转不稳定的研究综述

旋转不稳定是压气机工作在高负荷近失速工况时的一种常见现象。研究旋转不稳定在降低压气机工作噪声、减小流致振动以及保障航空发动机稳定工作等方面具有重要意义。首先对旋转不稳定现象进行了回顾,详细讨论了旋转不稳定的特征。其次,重点调研了旋转不稳定的起源和机理,将旋转不稳定产生的原因归纳为叶尖泄漏流、涡脱落以及流动剪切等类别。此外,回顾了模拟旋转不稳定的数值方法,讨论了多种流动控制手段对旋转不稳定的作用效果。最后,对旋转不稳定的研究现状进行了总结,对未来的研究趋势进行了展望。

叶顶间隙对蜗壳式混流泵流动特性影响研究

采用计算流体力学(CFD)数值模拟技术,研究了蜗壳式混流泵在不同叶顶间隙下的水力性能及内部流动特性。通过对流场、涡流和压力分布的分析,揭示了叶顶间隙变化对泵装置扬程、效率及运行稳定性的影响。结果表明,随着间隙尺寸的增大,混流泵的整体效率逐渐下降,且对大流量工况的影响尤为显著。同时,叶顶间隙的增大会导致叶轮区域泄漏涡的增强,影响泵的运行稳定性。

Experimental Investigations of the Three-DimensionalFlow in a Large Deflection Turbine Cascade with Tip Clearance

Results obtained from an experbontal study of the threedimensional flow survey within and exit of a large defiection linear turbine cascade are presented for a tip clearance levels of 0.08, 1.5, 3.0 percent of chord and compared with the help of boundary layer probes and that within and exit of a blade passage was done with a miniaturised five hole probe. End wall and blade tip surface static pressures were also obtained, in addition to flow visualisation studies. A strong horse-shoe vortex forms in front of the leading edge for zero clearance whereas this vortex does not appear for 3 percent clearance indicating that for large clearance the pressure forces have dominating infiuence than the viscous forces. In addition to normally known clearance vortex, a small tip separation vortex was noticed on the blade tip surface inside the tip gap. Due to the area contraction caused by the tip separation vortex, the fluid movign towards the tip gap from the pressure side is accelerated. Downstream of the vortex, the endwall pressure increases due to flow mixing. Both vortices increase in size and strength along the chord. The miring is incomplete in the aft portion of the blade. The tip gap velocity profiles exhibit wak like characteristics especially at axial positions where the mixing is incomplete. The passage vortex in the present investigations did not diminish with increase in clearance. The discharge coefhcient and the total pressure loss coefficient within the tip gap show similar tendency with lower values near the leading and trailing edge regions.

凹槽叶顶的气膜冷却特性

涡轮工作时,叶顶间隙内高速高温的间隙泄露流会直接冲刷动叶顶部,产生局部高温区域,因此控制间隙泄露流至关重要。基于压敏漆涂料(PSP)实验,研究了密度比、吹风比和间隙高度对凹槽叶顶气膜冷却特性的影响。研究结果表明:密度比增大,凹槽叶顶气膜冷却效果逐渐提高,尤其是大间隙下冷效提高较为明显,叶顶面平均冷效增加了35%;间隙高度和吹风比对叶顶表面冷效的作用不是单一的,小间隙条件下,叶顶表面气膜冷效随着吹风比增大而增大,大间隙条件下,吹风比的增加使得叶顶表面气膜冷效先减小后增加,同时小吹风比时,间隙高度增加提高了叶顶尾缘区域和孔周围的气膜冷效,叶顶面平均冷效提高了6.85%。

半开式离心叶轮叶顶泄漏诱发流道流动堵塞特性研究

为探究半开式离心叶轮叶顶泄漏量诱发的流道中流动堵塞作用,采用大涡模拟对流场进行精确计算,基于流动堵塞预估模型对叶顶间隙0.5、1.0和1.5 mm的3种半开式离心叶轮在不同流量工况下的流动堵塞特性进行研究,发现在叶轮流道内存在流动堵塞峰值,且在3种叶顶间隙下均在截面2处,在经过截面2之后流动堵塞发生大幅度的减小,并在后面4个截面发生小幅度的波动;通过对不同叶顶间隙、不同流量工况下在叶轮尾缘处相对流动堵塞作用的分析,发现尾缘处的流动堵塞在不同叶顶间隙下,随流量的变化具有相同的趋势,相对流动堵塞均在1.0Q_(d)时达到最小值,在0.6Q_(d)时出现峰值;然而流动堵塞在相同流量下与叶顶间隙的关系不是单一的线性关系,且存在一个最优叶顶间隙在设计工况点达到最小的流动堵塞情况;同时通过对泄漏涡发展情况的观察,发现泄漏涡的破碎和位移会增大叶片尾缘处的流动堵塞;通过对叶片静压差的分析,发现运行流量对叶片载荷的影响不大,然而叶顶间隙的改变会大大影响叶片载荷,同时对叶片尾缘处的流动堵塞造成影响。

复杂背景和光照下双旋翼直升机桨尖间距实时测量方法

针对现有双旋翼直升机桨尖间距测量方法不适应室外复杂环境的问题,提出一种基于深度网络的桨尖间距实时测量方法。首先,采集桨尖图像制作数据集,搭建并训练YOLOv3-tiny网络;其次,利用训练好的网络定位桨尖区域;再次,采用OTSU分割区域图像并提取桨尖轮廓,定位上下桨尖气动中心,计算桨尖间距;最后,通过在模拟和真实环境分别开展测量实验,模拟实验结果表明本文方法精度高,相机离桨尖20 m时桨尖间距测量的最大误差为1.99 mm;真实实验结果表明本文方法适应能力强和速度快,可以适应不同复杂背景和光照下的桨尖间距测量,帧率达50 fps,已用于现场实验。