基于进气射流的压气机失速主动控制系统仿真

针对一轴流压气机，构造了利用进气射流对旋转失速进行主动控制的控制系统，发展了以Ｍｏｏｒｅ－Ｇｒｅｉｔｚｅｒ模型为基础的系统数学模型并进行了数值仿真研究。结果表明，该控制系统可有效地抑制压气机中旋转失速的发生。

离心压气机近失速工况叶尖流场特征及泄漏涡面涡模型

为研究航空微型涡轮发动机离心压气机的失速机理,采用数值模拟的方法,对压气机在设计点和近失速点的流场特征进行了研究,并对叶尖泄漏涡进行了建模。结果表明:在近失速工况下,更多的来自轮毂处的低能流和二次流会汇入叶轮通道近壁面的低速区,使得叶尖的堵塞加重,泄漏涡轨迹的偏转及其卷吸作用可能是造成这种现象的原因。通过在压气机机匣外侧引入虚拟镜像涡的方法,应用机翼涡对的不稳定性理论和面涡模型对泄漏涡建立了模型。应用该模型对本文研究的离心压气机以及三个国内外具有典型代表性的压气机进行了预测,表明模型得到的频率同计算和实验结果具有较好的吻合性,对德国宇航院压气机的预测频率与其实际频率的误差仅为2.8%。总体而言,当间隙高度较大时,模型的预测更为准确。

缝式机匣处理对对转压气机最先失速级的影响

为探究缝式机匣处理对对转压气机最先失速级的影响规律,以双级对转压气机为研究对象,采用数值模拟的方法,开展了缝式机匣处理对对转压气机最先失速级的影响研究。研究表明:缝式机匣处理下该对转压气机的失速初始扰动类型仍为突尖型失速,机匣处理前移压气机的最先失速级由转子R2转换为转子R1,而机匣处理后移未改变该压气机的失速级。机匣处理前移抑制了转子R2前缘溢流的发生,降低了叶片通道内的非定常脉动强度,而转子R1在近失速工况下叶片前缘溢流加剧,主流和泄漏流的交界面被推出叶片通道,同时叶片通道内的非定常脉动强度增大,最终使得转子R1首先进入失速状态;机匣处理向转子R2下游移动,难以抑制前缘溢流的发生,虽然此时转子R1也出现了前缘溢流现象,但转子R2前缘溢流更剧烈,主流和泄漏流交界面的位置更远离叶片前缘,更容易使压气机发生失速。

低速轴流压气机转子尾迹三维动态测量

对一台低速轴流压气机的转子尾迹进行了三维动态测量,采用锁相平均、方均根和功率谱密度等分析方法对低速轴流压气机转子在节流过程中尾迹流场的稳态和动态特征进行了研究.结果显示:在绝对坐标系下,转子尾迹有高总压和高方均根特征,且在不同叶高的强度不同;转子尾迹与主流的掺混作用导致平均总压升系数的快速衰减;功率谱密度分析表明,在接近失速点时,转子尾迹90%叶高以上出现0.5倍叶片通过频率的特征频带且其功率随着流量减小而增大.

EFFECT OF AXIAL SPACING ON COMPRESSOR STABILITY

Experimental studies are carried out at a low speed axial compressor with five different rotor/stator gaps. Analysis of the effect of axial spacing of two successive blade rows on the measured mean flow coefficient at stall inception and on the flow range of compressor under multi-cell rotating stall operating conditions proves that the stator can suppress the flow disturbance in the compressor and strengthen the stability of the compressor. Experimental data show that the stall flow coefficient decreases by reducing the axial spacing of successive blade rows. Moreover, by reducing the axial spacing, the stall pattern transition pace from multi-cell stall to single-cell stall can be shifted. And the compressor directly slips into single-cell stall at 21.0% CR axial spacing. By analyzing the pressure fluctuation closed to the surge line, it can be known that there exists an eigenfrequency where the amplitude of the oscillating pressure suddenly and dramatically increases as the compressor runs close to the surge line and this pressure disturbance is relevant to the compressor instability.

Wet Compression Performance of a Transonic Compressor Rotor at its Near Stall Point

In order to study the effects of wet compression on a transonic compressor,a full 3-D steady numerical simulation was carried out under varying conditions.Different injected water flow rates and droplet diameters were considered.The effect of wet compression on the shock,separated flow,pressure ratio,and efficiency was investigated.Additionally,the effect of wet compression on the tip clearance when the compressor runs in the near-stall and stall situations was emphasized.Analysis of the results shows that the range of stable operation is extended,and that the pressure ratio and inlet air flow rate are also increased at the near-stall point.In addition,it seems that there is an optimum size of the droplet diameter.

预旋对周向槽处理机匣扩稳能力的影响

以Rotor 67转子为研究对象,通过数值模拟的手段分析了周向槽(CG)提高转子失速裕度(SM)的机理,并在此基础上研究了周向槽处理机匣与转子前预旋的匹配问题。结果表明:在Rotor 67转子中,叶尖泄漏流与激波相互干渉产生的低速区是转子失速的重要原因,而周向槽内气流离开时形成的与泄漏涡(LV)方向相反的涡会抑制泄漏涡的周向发展,是周向槽的扩稳机理之一。通过分析周向槽处理机匣在不同预旋条件下扩稳效果的变化,显示在不同流量点、不同预旋条件下,随着转子叶尖负荷位置的变化,起主要扩稳作用的处理槽不同,并在此原则下,提出了一种周向槽设计的思路。

Prediction of stall inception in multi-stage compressors based on an eigenvalue approach

A stability model for multi-stage compressor is developed on the basis of the eigenvalue approach.This model assumes that the unsteady flow field can be decomposed into pressure,vortex and entropy waves.Besides,a linear cascade of blades is modeled by three-dimensional semi-actuator disk theory and the characteristics of steady flow field are also considered in the present model.The connection between the analytical solution for stator,rotor and gap can be established by applying mode matching approach,the relevant stability equation can be expressed in the form of matrix,while the compressor system stability can be judged by the imaginary part of the matrix eigenvalue.The capacity of the stall inception model to predict the stall inception point of multi-stage compressor is assessed against the experimental data of National Aeronautics and Space Administration(NASA) two stage fan.The theoretical results show that this model can predict the stall onset points of a two-stage fan at different operating speeds with a reasonable accuracy.

An Experimental Description of the Flow in a Centrifugal Compressor from Alternate Stall to Surge

The present paper gives the experimental results obtained in a centrifugal compressor stage designed and built by SAFRAN Helicopter Engines. The compressor is composed of inlet guide vanes, a backswept splittered un- shrouded impeller, a splittered vaned radial diffuser and axial outlet guide vanes. Previous numerical simulations revealed a particular S-shape pressure rise characteristic at partial rotation speed and predicted an alternate flow pattern in the vaned radial diffuser at low mass flow rate. This alternate flow pattern involves two adjacent vane passages. One passage exhibits very low momentum and a low pressure recovery, whereas the adjacent passage has very high momentum in the passage inlet and diffuses efficiently. Experimental measurements confirm the S-shape of the pressure rise characteristic even if the stability limit experimentally occurs at higher mass flow than numerically predicted. At low mass flow the alternate stall pattern is confirmed thanks to the data obtained by high-frequency pressure sensors. As the compressor is throttled the path to instability has been registered and a f＇wst scenario of the surge inception is given. The compressor first experiences a steady alternate stall in the dif- fuser. As the mass flow decreases, the alternate stall amplifies and triggers the mild surge in the vaned diffuser. An unsteady behavior results from the interaction of the alternate stall and the mild surge. Finally, when the pres- sure gradient becomes too strong, the alternate stall blows away and the compressor enters into deep surge.

Investigations on Low Speed Axial Compressor with Forward and Backward Sweep

In the recent past, experimental studies have shown some advantages of blade lean and sweep in axial compressors. As most of the experimental results are combined with other features, it is difficult to determine the effect of individual parameters on the performance of the compressor. The present numerical studies are aimed at understanding the performance and three-dimensional flow pattern at the exit of swept and unswept rotors. Three rotors, namely; unswept, 200 forward swept and 200 backward swept rotors are analysed with a specific intention of understanding the pattern of the blade boundary layer flow. The analysis was done using a fully three-dimensional viscous CFD code CFX-5. Results indicated reduction in pressure rise with sweep. Backward sweep is detrimental as far as the performance near endwalls is considered. On the other hand total pressure loss in the wake in mid span region is less with backward sweep, which favours its application here. However, backward sweep adversely affects the stall margin. The ability of the forward sweep to deflect the streamlines towards hub gets diminished at low flow rates. Forward sweep changes the streamline pattern in such a way that the suction surface streamlines are deflected towards the hub and the pressure surface streamlines are deflected towards the easing. An opposite behaviour is observed in backward swept rotors.

Experimental Investigations of Micro Air Injection to Control Rotating Stall

Steady discrete micro air injection at the tip region in front of the first compressor rotor has been proved to be an effective method to delay the inception of rotating stall in a low speed axial compressor. Considering the practical application a new type of micro injector was designed and described in this paper, which was imbedded in the casing and could be moved along the chord. In order to verify its feasibility to other cases, such as high subsonic axial compressor or centrifugal compressor, some other cases have been studied. Experimental results of the same low speed axial compressor showed that the new injector could possess many other advantages besides successfully stabilizing the compressor. Experiments performed on a high subsonic axial compressor confirmed the effectiveness of micro air injection when the relative velocity at the blade tip is high subsonic. Meanwhile in order to explore its feasibility in centrifugal compressor, a similar micro injector was designed and tested on a low speed centrifugal compressor with vaned diffuser. The injected mass flow was a bit larger than that used in axial compressors and the results showed micro injection could also delay the onset of rotating stall in the centrifugal compressor.

Numerical Simulation of Rotating Stall in a Centrifugal Compressor with Vaned Diffuser

This paper reports a numerical study on the process from normal operating conditions to rotating stall in a centrifugal compressor with vaned diffuser. The purpose is to better understand the flow characteristics near stall point under the interactions between centrifugal impeller and vaned diffuser. Numerical results show that under certain conditions just preceding stall point the tip leakage vortex begins to fluctuate at roughly half of the blade passing frequency. This phenomenon is similar to rotating instability in axial compressors. With the flow rate reduced further the impeller stalls and five stall cells propagating at a frequency of 85 percent of impeller rotation speed are found.

Effects of Probe Support on the Stall Characteristics of a Low-Speed Axial Compressor

In order to investigate the effects of probe support on the stall characteristics of micro compressors, an experiment was carried out on a large-scale low-speed research compressor according to the principle of geometric similarity. A cylindrical probe support intruding to 50% blade span was mounted at 50% chord upstream from the rotor blade leading edge. The static pressure rise characteristic of the compressor is measured, with and without the probe support respectively. The dynamic compressor behavior from pre-stall to full stall was also measured. The results indicate that the stability margin of the compressor is lowered after installing the probe support. The stall inception is aroused by modal wave disturbances. The disturbances developed into two stall cells smoothly before installing the probe support, while the disturbances first developed into a single stall cell then splitting into two stall cells after installing the probe support. The presence of probe support lowers the initial intensity of the rotating stall of the compressor, while it doesn't alter the intensity of the rotation stall after the compressor enters into full stall.

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 experi- mentally 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 di- rectly 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 re- suits suggested that the blockages of the cascade flow which were generated by a stall cell play an important role in deciding the surge behaviors.

Numerical Investigation of the Unsteady Tip Leakage Flow and Rotating Stall Inception in a Transonic Compressor

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.

Numerical investigation of the casing treatment mechanism with a single circumferential groove

The paper investigates the effect of a single circumferential groove casing treatment(CGCT) on a transonic compressor rotor numerically.In particular,the effect of the groove at different axial locations on the flow field is studied in detail and stall margin improvement is also discussed.The present results show that the groove close to the leading edge plays a crucial role in stabilizing the near stall flow structures and,hence,improves the stall margin.The groove at the mid-chord-section of the blade can help exchange and transfer momentums between different directions,and suppress the flow unsteadiness,leading to increased efficiency in rotor performance and extended operation range.The groove located near the blade trailing edge has limited effects on stall margin improvement and may cause additional penalty in efficiency.Through comparison with the recent work on CGCT,some common flow physics can be observed.

Low Speed Axial Compressor Stall Margin Improvement by Unsteady Plasma Actuation

This research investigates the use of single dielectric barrier discharge(SDBD) actuators for energizing the tip leakage flow to suppress rotating stall inception and extend the stable operating range of a low speed axial compressor with a single rotor.The jet induced by the plasma actuator adds momentum to the flow in the tip region and has a significant impact on the tip-gap flow.Experiments are carried out on a low speed axial compressor with a single rotor.The static pressure is measured at both the rotor inlet and outlet.The flow coefficient and pressure rise coefficient are calculated.Then the characteristic line is acquired to show the overall performance of the compressor.With unsteady plasma actuation of 18kV and 60W the compressor stability range improvement is realized at rotor speed of 1500 r/min – 2400 r/min.

Coexisting Phenomena of Surge and Rotating Stall in an Axial Flow Compressor

The unsteady inner flow structure of a single-stage axial flow compressor under the coexisting conditions of surge and rotating stall was experimentally investigated via detailed measurements of the unsteady characteristics and the internal flow velocity fluctuations. The main relevant feature of the tested compressor is a shock tube with a capacity tank connected in series to the compressor outlet through slits and a concentric duplex pipe: surge and rotating stall can both be generated by connecting the shock tube. Research attention is focused on the unsteady behavior of a rotating stall during the surge cycle. The size of the rotating stall cell during the recovery process of an irregular surge cycle was experimentally determined by the circumferential flow velocity fluctuations ahead of the rotor blade. The results suggested that the size of the rotating stall cell at the switching point of the performance curve between large and small cycles is considered to be the key parameter in determining the following surge cycle. In addition, the surge cycle is largely influenced by the unsteady behavior of the rotating stall cell.

Unsteady flow structures in the tip region for a centrifugal compressor impeller before rotating stall

To get an insight into the occurrence and the mechanism of flow unsteadiness in the tip region of centrifugal compressor impellers, the flow in Krain’s impeller is investigated by using both steady and unsteady RANS solver techniques. It is found that the flow unsteadiness on the pressure side is much stronger than that on the suction side. The periodical frequency of the unsteady flow is around half of the blade passing frequency. The originating mechanism of the flow unsteadiness is illustrated with the time-dependent tip leakage flow and blade loading at the tip region. Due to the blockage caused by the joint effects of broken-downed tip leakage vortex, separated fluids and tip leakage flow at downstream, a low pressure region is formed on the pressure side, consequently the blade loadings is altered. In turn, the changed blade loadings will alter the intensity of tip leakage flow. Such alternative behavior finally results in the periodic process. By comparing the calculated flow field in the cases of single-passage and four-passage models, it is confirmed that the investigated flow unsteadiness is confined in each single passage, as no phase differences are found in the model of four passages. This is different from the situation in axial compressor when the rotating instability is encountered. The flow unsteadiness only occurs at the working conditions with small mass flow rates, and the oscillation intensity will be enhanced with the decrease of mass flow rate. When the mass flow rate is too small, the flow unsteadiness in a single passage may trigger rotating stall, as the disturbance propagates in the circumferential direction.