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.     

Numerical investigation of tip clearance effects in an axial transonic compressor

Numerical investigations of the Darmstadt transonic single stage compressor (DTC), in the Rotor1-Stator1 configuration, aimed at advancing the understanding of the effect of different rotor tip gaps and transition modelling on the blade surfaces are presented. Steady three dimensional Reynolds Averaged Navier Stokes (RANS) simulations were performed to obtain the flow fields for the different configurations at different operating conditions using the RANS-Solver TRACE. The stage geometry and the multi-block structured grid were generated by G3DMESH and a grid sensitivity analysis was conducted. For the clearance gap region, a fully gridded special H-grid was chosen. Comparisons were made between the flow characteristic at design speed, representative for a transonic flow regime, and at 65% speed, representative for a subsonic flow regime. The computations were used to analyse the flow phenomena through the tip clearance region for the different configurations and their impact on the performance of the compressor stage.     

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. __________ Translated from Compressor, Blower & Fan Technology, 2007, (5): 12–15 [译自: 风机技术]     

Experimental and numerical investigation of the unsteady tip leakage flow in axial compressor cascade

For convenience of both measurement and adjusting the clearance size and incidence, the current research is mainly conducted by experiments on an axial compressor linear cascade. The characteristics and the condition under which the unsteadiness of tip leakage flow would occur were investigated by dynamic measuring in different clearances, inlet velocities and incidences. From the experiment it is found that increasing tip clearance size or reducing rotor tip incidence can affect the strength of the tip clearance flow. Then the experimental results also indicate the tip leakage shows instability in certain conditions, and the frequency of unsteadiness is great influenced by inflow angle. The condition of occurrence of tip leakage flow unsteadiness is when the leakage flow is strong enough to reach the pressure side of the adjacent blade. The main cause of tip leakage flow unsteadiness is the tip blade loading.     

Relationship between the flow blockage of tip leakage vortex and its evolutionary procedures inside the rotor passage of a subsonic axial compressor

The near casing flow fields inside the rotor passage of a 1.5 stage axial compressor with different blade-loading levels and tip gap sizes were measured by using stereoscopic particle image velocimetry （SPIV）. Based on a carefully defined blockage extracting method, the variations of blockage parameter inside the blade passage were analyzed. It was found that the variation of blockage parameter appeared as a non-monotonic behavior inside the blade passage in most cases. This non-monotonic behavior became much more remarkable as the blade loading increases or mass flow rate decreases.The variations of the blockage parameter inside the blade passage had close relation to the evolutionary procedures of the tip leakage vortex （TLV）. The destabilization of the TLV caused a rapid increasing of the blockage parameter. After the TLV lost the features of a concentrated streamwise vortex, the blockage parameter usually got a peak value. And then, because of the intense turbulent mixing between the TLV low momentum flow and its surrounding flows, the flow deficit inside the TLV recovered.     

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.     

涡轮叶顶间隙内部流动的数值研究

动叶叶顶间隙流动是引起涡轮中流动损失的主要原因之一,但由于间隙尺寸较小,且流动方向与叶片转动方向相同,因此很难通过试验测量间隙内部详细的流动及压力分布。通过对"LISA"1.5级轴流涡轮设计工况进行计算,并与试验测量结果进行对比,进而研究间隙内部压力场、流场的分布情况,同时分析间隙高度变化对间隙内部流动的影响。     

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。     

Impact of Wake on Downstream Adjacent Rotor in Low-speed Axial Compressor

This paper investigates the effect of upstream wake on unsteady separated flow field of downstream adjacent vanes by solving 2D unsteady Reynolds-averaged Navier-Stokes equations discretized by a high-order scheme. The results indicated that the maximum relative reduction of loss coefficient was 27.2% when the relative passing frequency (the ratio of wake passing frequency to the characteristic frequency of trailing-vortex shedding of downstream adjacent rotor) is less than 2.5. The amplitude of wake defect exists a threshold value and the aerodynamic performance is enhanced monotonically with the amplitude of wake defect basically. The effective range of incidence must be greater than 8°, which is near the stall boundary.     

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.     

Stall characteristics and tip clearance effects in forward swept axial compressor rotors

Tilting the blade sections to the flow direction (blade sweep) would increase the operating range of an axial compressor due to modifications in the pressure and velocity fields on the suction surface. On the other hand, blade tip gap, though finite, has great influence on the performance of a turbomachine. The present paper investigates the combined effect of these two factors on various flow characteristics in a low speed axial flow compressor. For this present study, nine computational domains were modeled; three rotor sweep configurations (0°, 20° and 30°) and for three different clearance levels for each rotor. Commercial CFD solver ANSYS CFX 11.0 is used for the simulations. Results indicated that tip chordline sweep is found to improve the stall margin of the compressor by modifying the suction surface boundary layer migration phenomenon. Diffusion Factor (DF) contours showed the severity of stalling with unswept rotor. For the swept rotors, the zones of high probable stall are less severe and they become less in size with increasing sweep. Increment in the tip gap is found to gradually affect the performance of unswept rotor, while the effect is very high for the two swept rotors for the earlier increments. As a minimum clearance is unavoidable, swept rotors suffer relatively higher deviation from the idealistic behavior than the unswept rotor due to tip clearance.     

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.     

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.     

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.     

Unsteady behavior of surge and rotating stall in an axial flow compressor

Unsteady behaviors as well as unsteady cascade flow fields of a single-stage axial flow compressor were experimentally investigated by detail measurements of unsteady performance characteristics and casing wall pressure and internal flow velocity fluctuations. The main feature of the test compressor is a capacity tank connected directly to the compressor outlet in series through slits and a concentric duplex pipe, and also jet nozzles in order to inject compressed air toward the rotor tip region. Research attention is focused on the post-stall characteristics of surge and rotating stall which occur simultaneously. When the compressor was connected to the capacity tank, surge was generated with rotating stall in accordance with the capacitance increment of whole compressor system. The surge behavior changed irregularly with throttling valve installed behind the compressor, and several types of surge cycles were observed. In addition, the surge cycle changed by jet injection to the rotor tip region. The results suggested that the blockages of the cascade flow which were generated by a stall cell play an important role in deciding the surge behaviors.     

Unsteady flow field under surge and rotating stall in a three-stage axial flow compressor

The unsteady flow structure between rotor blade-to-blade passages in a three-stage axial flow compressor is experimentally investigated by detailed measurements of unsteady performance characteristics,casing wall pressure fluctuations and their wavelet analyses.The main feature of the test compressor is a capacity tank facility connected in series to the compressor outlet in order to supply compression and/or expansion waves from downstream of the compressor.Research attention is focused on the post-stall characteristics of the surge and rotating stall which occur simultaneously.The influence of the compressor operating point on the unsteady performance curve shows that the surge cycle changes irregularly depending on the steady-state resistance characteristics,and the results of the wavelet analyses of the wall pressure fluctuations suggest that the surge cycle may selectively be determined by the rotating stall cell structure within the rotor cascade.     

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.     

Flow Visualization and Topological Analysis of a Turbine Rotor Cascade with Tip Clearance

INTRoDUCTIoNRecentlysignificantattentionhasbeengiventothefinestructurewithintheflowfieldofturbomachinery.Ingeneral,thestudyofthisproblemcontainstwoas-pects.Thefirstistovisualizethesolidsurfaceflowandtopologicallyanalyzethesingularpointstoinves-tigatetheirnumbers,typesandlocationsandtheskinfrictionlinepatternsintheflowfie1dnearesttothewall.Thisaspectaimsatobtainingacertainamountofwallflowinformation.Thesecondaspect,ontheotherhand,carriesoutdetailedflowfieldmeasure-mentsandvisualizationstoac…     

Evolution of unsteady flow near rotor tip during stall inception

Previously the features of circumferential propagation of self-induced tip leakage flow unsteadiness for a low speed isolated axial compressor rotor in the authors’ laboratory were discovered and investigated via numerical simulation,which only occurs below a critical stable flow point that is close to but not yet at the stall limit.Further in this paper,the detailed investigation on evolution of tip leakage flow during the throttling process into spike rotating stall was conducted by adopting the valve-throttling model.During this process,the development of the circumferential propagation of tip leakage flow unsteadiness was especially focused on.According to the unsteady characteristics of pressure signals,the evolvement of compressor flow field can be classified into four stages.As compressor throttled,the oscillation frequency of self-induced unsteady tip leakage flow decreased gradually,and thus resulted in the decrease of its circumferential propagation speed.The circumferential propagation of self-induced tip leakage flow unsteadiness is closely related with rotating instability.When the forward spillage of tip leakage flow at the leading edge occurred,the spike type rotating stall was initiated.Its flow struc-tures were given in the paper.     

Numerical analysis of the tip leakage flow field in a transonic axial compressor with circumferential casing treatment

This paper reports on numerical investigations aimed at understanding the influence of
circumferential casing grooves on the tip leakage flow and its resulting vortical structures.The results
and conclusions are based on steady state 3D numerical simulations of the well-known transonic axial
compressor NASA Rotor 37 near stall operating conditions.The calculations carried out on the casing
treatment configuration reveal an important modification of the vortex topology at the rotor tip
clearance.Circumferential grooves limit the expansion of the tip leakage vortex in the direction
perpendicular to the blade chord,but generate a set of secondary tip leakage vortices due to the
interaction with the leakage mass flow.Finally,a deeper investigation of the tip leakage flow is
proposed.