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.     

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…     

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.     

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.     

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.     

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.     

Numerical Investigation of Influence of Tip Leakage Flow on Secondary Flow in Transonic Centrifugal Compressor at Design Condition

In a centrifugal compressor, the leakage flow through the tip clearance generates the tip leakage vortex by the interaction with the main flow, and consequently makes the flow in the impeller passage more complex by the interaction with the passage vortex. In addition, the tip leakage vortex interacts with the shock wave on the suction surface near the blade tip in the transonic centrifugal compressor impeller. Therefore, the detailed examination for the influence of the tip leakage vortex becomes seriously important to improve the aerodynamic performance especially for the transonic centrifugal compressor. In this study, the flows in the transonic centrifugal compressor with and without the tip clearance at the design condition were analyzed numerically by using the commercial CFD code. The computed results revealed that the tip leakage vortex induced by the high loading at the blade tip around the leading edge affected the loss generation by the reduction or the suppression of the shock wave on the suction surface of the blade.     

Numerical simulation of tip clearance flow passive control in axial turbine

This paper focuses on the effects of five different passive turbine tip clearance flow control methods on the tip clearance flow physics, which consists of a partial suction side squealer tip, a double squealer tip, a pressure side tip shelf with inclined squealer tip on a double squealer tip, a tip platform extension edge in pressure side and in suction side respectively. A pressure-correction based, 3D Reynolds-averaged Navier-Stokes equations CFD code with Reynolds Stress Model was adopted. The variable specific heat was considered. The detailed tip clearance flow field with different squealer rims was described with the streamline and the velocity vector. Accordingly, the mechanisms of five passive controls were elucidated; the effects of the passive controls on turbine efficiency and tip clearance flow field were illuminated. The results showed that the secondary flow loss near the outer casing including the tip leakage losses and the passage vortex losses could be reduced in all the five passive control methods. The turbine efficiency could be increased via the rational passive turbine tip clearance flow control. The Improved PS Squealer had the best effect on turbine efficiency, and the efficiency increased by 0.215%.     

Study on the performance deterioration of mixed flow impeller due to change in tip clearance

Performance of mixed flow compressor with un-shrouded impeller strongly depends upon unsteady, asymmetrical flow fields in the axial directions. The flow through the mixed flow impeller is complex due to three-dimensional nature of geometry. In mixed flow impeller, there are clearances between the rotating impeller blades and the casing as the high pressure ratio compressors are usually open shrouded impellers. As a result, certain amount of reduction in the performance is unavoidable due to clearance flows. In the present investigations, numerical analysis is performed using a commercial code to investigate tip clearance effects on through flow. The perform- ance of mixed flow impeller with four different clearances between impeller and stationary shroud are evaluated and compared with experimental results. The impeller performance map was obtained for different operating speeds and mass flow rates with different tip clearances. The result shows that the tip leakage flow strongly inter- acts with mainstream and contributes to total pressure loss and performance reduction. The pressure and per- formance decrement are approximately linearly proportional to the gap between impeller and stationary shroud. The analysis showed scope for improvement in design of the compressor for better performance in terms of effi- ciency and operating range.     

Effect of Nozzle Vane and Rotor Clearance on Performance of Variable Nozzle Turbine

In a variable nozzle turbine(VNT), the nozzle vane and rotor clearance can generate complicated tip leakage flow, which produces a large flow loss and results in a noticeable reduction in the VNT performance. Therefore, it is necessary to study the influence of the nozzle vane and rotor clearance on the VNT performance to reveal the underlying mechanisms. In this study, the variation of VNT performance resulting from the nozzle vane and rotor clearance was studied numerically using the commercia...     

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…     

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.     

Factors affecting small axial cooling fan performance

Many factors such as outer diameter,hub ratio,blade numbers,shape and stagger angle affect the performance of small cooling fans.A small cooling fan was simulated using CFD software for three blade stagger angles (30.5°,37.5°,44.5°)and obtained the internal flow field and the static characteristics.Research indicated that the stagger angle has an obvious effect on the static characteristics of a fan.For flow rates below 0.0104 m3/s,total pressure is the greatest when the stagger angle is 37.5°;flow rates higher than 0.0104 m3/s,the total pressure is greatest when the stagger angle is 44.5° For the same flow rates,the velocity at inlet of pressure surface increases with increasing stagger angle,but the change of velocity on the suction surface is very small.For one model,vortices and the speed of revolution surfaces decrease with tip clearance increasing.But for other three models,increasing the stagger angle,the vortex intensity and speed of revolution surfaces at same height tip clearance increases,simultaneously,the position of vortex offset from the top of the rotor blade to the suction surface.     

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。     

机匣喷气量对涡轮间隙流动控制的影响

采用数值模拟方法研究机匣喷气量大小对涡轮间隙流动控制的影响。结果显示,在10％轴向弦长位位置喷气时,增大喷气量,喷气在间隙内轴向上影响范围增大,对间隙流阻塞作用增加,间隙涡出现位置推迟。同时减小了间隙涡、上通道涡区熵增,尤其是上通道涡区损失大幅减小,并减弱机匣喷气引起的气流偏转不足／过偏现象。叶顶压力面附近由间隙流动引起的低压区减小,并向叶片尾缘移动。但由于喷气量增大使得动叶输出功率下降,使得涡轮效率降低。     

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．     

Effects of suction side squealer tip on the performance of a low-speed axial compressor

This paper presents the investigation of the effects of suction side squealer tip on the performance of an axial compressor.The experiment is carried out in a single-stage large-scale low-speed compressor.The investigated tip geometries include flat tip as the baseline and suction side squealer tip.The tip clearance of the baseline is 0.5% of the blade span.The static pressure rise characteristic curves of both the rotor and the stage are measured.The flow field at the exit of the rotor is measured by a 5-hole probe under design and off-design conditions.The static pressure on the endwall of the rotor passage is also obtained.The results show that the pressure rise characteristic curves obtained by measuring the pressure on the end wall are almost unchanged by using the suction side squealer tip.The measuring results of the 5-hole probe show the static pressure and the total pressure in tip region is slightly greater than that of the flat tip at the design condition at the exit of the rotor.It also leads to greater av-eraged static pressure rise and total pressure.At the near stall condition,the averaged static pressure and total pressure is lower than the baseline which is related to the redistribution of the blade load caused by the suction side squealer tip.     

L型叶尖小翼对风力机性能影响的研究

采用标准的k-ε湍流模型对添加L型叶尖小翼叶片与原叶片在不同风速条件下进行三维流场的数值研究。通过分析叶尖区域流场和压力分布得到:对比原叶片,L型小翼对通过叶尖的气流具有导流作用,使通过叶尖的气流变得平缓流畅,同时小翼能有效改善叶尖吸力面的气流分离,使得气流分离位置远离叶片前缘,减小压差阻力。L型叶尖小翼加大叶尖部位吸力面与压力面的压差,增大风轮转矩,使风力机出力增加。添加L型小翼后,风力机推力系数最大增幅为0.81%,风力机功率最大增幅为4.2%。