Numerical study on unsteadiness of tip clearance flow and performance prediction of axial compressor

Numerical calculations were performed to investigate unsteady features of tip clearance leakage flow in an axial compressor. The first stage rotor of a low speed axial compressor with a large tip clearance was examined. It was confirmed that the numerically calculated performance data were in good agreement with the experimentally measured performance data. Using frequency analysis, the flow characteristic near the casing induced by tip clearance leakage flow was found to be not associated with the rotating speed of the rotor. This characteristic is called rotating instability or self-induced unsteadiness. We found that the circumferential length scale of the rotating instability of the compressor was longer than a pitch of a blade passage; therefore, a multi-blade passage was adopted to study the flow structure more precisely. The flow characteristic was described by the frequency, the circumferential length, and the phase velocity, and was changed by operating points toward stall. The behavior of the flow was characterized by circumferentially traveling waves. Hence, the mechanism governing the development of the unsteady feature was further examined in terms of the rotating wave pattern of the pressure distribution. Furthermore, the unsteady feature of the tip clearance leakage flow affected the prediction of compressor performance by altering blockage, flow turning, and loss near the casing.     

多级离心压缩机级间弯道与回流器内流动的数值研究

对某型多级离心压缩机的气动性能进行了CFD数值研究,计算方法基于Jameson格式,湍流模型选择BaldwinLomax模型.对弯道与回流器等多级压缩机级间的静止部件内的气动特点进行研究,旨在揭示气流经弯道与回流器后气动参数的变化特征.结果表明,不同位置的弯道对流动的影响是截然不同的.由于这些静止部件对流体的扰动,使次级动叶的进口来流工况点偏离设计值,易于造成对整机气动性能的下降.因此多级压缩机的动叶设计应对来流的不均匀性加以充分考虑.     

Discussion on back-to-back two-stage centrifugal compressor compact design techniques

Design a small flow back-to-back two-stage centrifugal compressor in the aviation turbocharger, the compressor is compact structure, small axial length, light weighted. Stationary parts have a great influence on their overall performance decline. Therefore, the stationary part of the back-to-back two-stage centrifugal compressor should pay full attention to the diffuser, bend, return vane and volute design. Volute also impact downstream return vane, making the flow in circumferential direction is not uniformed, and several blade angle of attack is drastically changed in downstream of the volute with the airflow can not be rotated to required angle. Loading of high-pressure rotor blades change due to non-uniformed of flow in circumferential direction, which makes individual blade load distribution changed, and affected blade passage load decreased to reduce the capability of work, the tip low speed range increases.     

EXPERIMENTAL INVESTIGATION OF ROTATING STALL FOR A LOW-SPEED CENTRIFUGAL COMPRESSOR

Unsteady flows and rotating stall of a low-speed centrifugal compressor are investigated by measuring vaneless diffuser wall static pressure fluctuation and internal flow fields at different small flow fluxes. During the experiment, firstly the real time static pressure fluctuations on the vaneless diffuser shroud at different circumferential and radial position were acquired by high-frequency dynamic pressure transducers. Discrete Fourier transformation analysis and cross-correlation analysis were applied to the experimental results to ascertain the rotating stall beginning operation conditions and stall cells numbers and rotating speed. Secondly, the vaneless diffuser inlet flow angle distribution along diffuser width direction was acquired by single hotwire, which was compared with SENOO＇s analysis results. At last, the internal flow fields of the centrifugal compressor were investigated with a particle image velocimetry （PIV） system at different small flow fluxes. The flow field development of vaneless diffuser and blade flow passage are given at rotating stall conditions. The experiments enrich the understanding of rotating stall flow phenomenon of the low-speed centrifugal compressor and provide full experiment data for designing high performance centrifugal compressor.     

A novel clocking effect between inlet bend and volute in an automotive turbocharging system

Numerical methods were carried out on a turbocharger compressor with inlet bent pipe to research a novel clocking effect between the inlet bend and the volute. It was found that the clocking effect with 3.4 percent of variations (1.9 percentage points) in compressor efficiency positively exists at the research point near chock. The reason for the changed efficiency loss is that the inlet bend induces a serious distortion of approaching flow to impeller inlet on one hand, and the downstream volute causes a circumferentially non-uniform distribution of pressure in vaneless diffuser. By adjusting the clocking positions between inlet bend and volute, not only is the unsteadiness of the flow rate through single impeller channel changed, but the uniformity of the incoming flow at the vaneless diffuser inlet is modulated as well. It is the dominant reason for the novel clocking effect that the bad uniformity causes more flow loss in both diffuser and volute.

     

Inlet Recirculation Influence to the Flow Structure of Centrifugal Impeller

Inlet recirculation is proved as an effective way for centrifugal compressor surge margin extension,and is successively used in some engineering applications.Unfortunately its working mechanism is still not being well understood,which leads to redesigning of inlet recirculation mostly by experience.Also,most study about inlet recirculation is steady to date.It is necessary to study surge margin extension mechanism about inlet recirculation.To expose the mechanism in detail,steady and unsteady numerical simulations were performed on a centrifugal compressor with and without inlet recirculation.The results showed that,with inlet recirculation,the inlet axial velocity is augmented,relative Mach number around blade tip leading edge area is significantly reduced and so is the flow angle.As the flow angle decreased,the incidence angle reduced which greatly improves the flow field inside the impeller.Moreover,inlet recirculation changes the blade loading around blade tip and restrains the flow separation on the blade suction side at the leading edge area.The unsteady results of static pressure around blade surface,entropy at inlet crossflow section and vorticity distributions at near tip span surface indicated that,at near stall condition,strong fluctuation exists in the vicinity of tip area due to the interaction between tip leakage flow and core flow.By inlet recirculation these strong flow fluctuations are eliminated so the flow stability is greatly enhanced.All these improvements mentioned above are the reason for inlet recirculation delays compressor stall.This research reveals the surge margin extension reason of inlet recirculation from an unsteady flow viewpoint and provides important reference for inlet recirculation structure design.     

叶片型线对离心风机性能影响的研究

首先对某采用双圆弧叶片的高效离心风机进行实验和数值对比,并在叶轮前盘形状、轮盖和叶片进出口安装角都相同的情况下,分析了采用双圆弧叶片和等减速叶型的模型风机的性能变化。结果表明,使用双圆弧长短叶片风机模型的稳定工况范围更宽、全压更高,等减速模型则在设计流量和小流量下效率更高、流动损失更小。     

进口探针支杆诱发压气机转子叶片振动的流固耦合研究

针对某压气机试验中发现进口探针支杆的引入导致下游转子叶片发生振动失效的问题开展数值计算研究,采用MPCCI把Numeca和ABAQUS联合起来构建成流固耦合计算平台,数值分析了试验故障工况条件下有/无探针支杆对压气机转子叶片振动特性的影响。结果表明:无探针支杆时转子叶片振动收敛,有探针支杆时不同转子叶片的振动特征不同,至少存在一个转子叶片的振动位移及应变发散,对应于叶片振动失效。     

离心压缩机动静干涉信号特征识别方法

离心式压缩机由于内部静、动叶片排的相对运动,气流在压缩机内部流动时会产生动静干涉,从而会产生不定常气流激振力和高振幅压力脉动。这种非定常动静干涉对压缩机的稳定正常运行和实际工作效率有着重要的影响。为了有效研究动静干涉的信号特征,首先,对某模型级压缩机试验台离心叶轮进行运行模态仿真计算从而得到叶轮干涉图;其次,确定进口导叶(inlet guide vanes,简称IGV)和出口扩压器(outlet guide vanes,简称OGV)对叶轮存在干涉的转速;最后,在确定的试验转速工况下进行应变测试和压力脉动测试。通过分析对比,验证了仿真转速工况下干涉特征频率的存在,也证实了应变信号更适用于识别干涉信号特征。通过总结干涉较严重的转速工况干涉特征频率与叶轮固有频率的关系,为离心压缩机进一步改进设计奠定了基础。     

LP compressor blade vibration characteristics at starting conditions of a 100 MW heavy-duty gas turbine

In this paper are presented the blade vibration characteristics at the starting conditions of the low pressure multistage axial compressor of heavy-duty 100 MW gas turbine. Vibration data have been collected through strain gauges during aerodynamic tests of the model compressor. The influences of operating modes at the starting conditions are investigated upon the compressor blade vibrations. The exciting mechanisms and features of blade vibrations are investigated at the surge, rotating stall, and buffeting flutter. The influences of operating modes upon blade dynamic stresses are investigated for the first and second stages. It is shown that a high dynamic stress peak of 120 MPa can occur in the first stage blades due to resonances with stall cell excitations or with inlet strut wake excitations at the stalled conditions.     

The characteristics investigation under the unsteady cavitation condition in a centrifugal pump

Numerical simulation and experimental method are combined to investigate the pump inlet and outlet pressure fluctuations, the vibration characteristics and the internal flow instabilities under the unsteady cavitation condition in a centrifugal pump. It is found that the unsteady cavitation starts to generate as the NPSHa is lower than 5.93 m. Apparent asymmetric and uneven cavity volume distribution on each blade and in the impeller can be observed as the NPSHa decreases from 4.39 m to 1.44 m, which includes the cavitation develops from cavitation surge, rotating cavitation to asymmetric cavitation. The flow vortexes in each blade channel are produced in the cavity trailing edges by the shedding and collapse of cavitation, which interfere with each other and aggravate the flow instabilities. The dominant frequencies of the pump inlet and outlet pressure fluctuations are the shaft frequency and blade passing frequency under the unsteady cavitation conditions, respectively. Broadband pulses are obtained from both the pump inlet and outlet pressure pulsations, which results from the random shedding and collapse of unsteady cavitation bubbles. Obvious corresponding relationship between the root mean squares of the vibration measured in different positions and the suction performance curve is found under both the non-cavitation and unsteady cavitation conditions.

     

二氧化碳汽车空调器变工况性能分析

建立了超临界二氧化碳汽车空调器循环计算模型,结合美国Ｉｌｌｉｎｏｉｓ大学制冷空调中心（ＡＣＲＣ）二氧化碳汽车空调样机实验结果,对两种压缩机转速和两种气体冷却器空气进口温度的不同组合条件下的工况,进行了循环计算,并对计算结果作了分析。计算结果表明,压缩机转速越高,或者气体冷却器空气进口温度越高,二氧化碳汽车空调的工部越恶劣,ＣＯＰ值越低。     

Computation of turbulent flows and radiated sound from axial compressor cascade

The losses at off-design points from a compressor cascade occur due to the deviation from a design incidence angle at the inlet of the cascade. The self-noise from the blade cascade at off-design points comes from a separated boundary layer and vortex sheddings. If the incidence angle to the cascade increases, stalling in blades may occur and the noise level increases significantly. This study applied Large-Eddy Simulations (LES) using deductive and deductive dynamic SGS models to low Mach-number, turbulent flow with each incidence angle to the cascade ranging from −40° to +20°, and compared numerical predictions with measured data. It was observed that the oscillating separation bubbles attached to the suction surface do not modify wake flows dynamically for cases of negative incidence angles. However, an incidence angle greater than 8° caused a separated vortex near the leading edge to be shed downstream and created stalling. The computed performance parameters such as drag coefficient and total pressure loss coefficient showed good agreement with experimental results. Noise from the cascade of the compressor is summarized as sound generated by a structure interacting with unsteady, turbulent flows. The hybrid method using acoustic analogy was observed to closely predict the measured overall sound powers and directivity patterns at design and off-design points of blade cascade.     

Numerical investigation of two-phase flow characteristics in multiphase pump with split vane impellers

Multiphase pump is a cost-effective option for subsea oil and gas field development. The ability to handle different inlet gas volume fractions (GVFs) especially high inlet GVF is critical to the development of pump performance. In this study, the two-phase flow characteristics in normal impeller and split vane impeller at different inlet GVFs were investigated by steady numerical simulations. The gas distribution on blade-to-blade plane and meridional flow channel at different inlet GVFs were analyzed and compared. Gas accumulation area and movement characteristics of the gas-liquid flow in impeller flow passage were also pointed out by unsteady simulations. Experimental results of the pump differential pressure were compared with the numerical simulation results, to validate the accuracy of numerical simulation method. The flow characteristics in pump with modified impeller and its performance at different inlet GVFs were both compared with that of the normal impeller. The steady simulation results of normal impeller in different inlet GVFs show that gas concentrating area in the flow passage increases as inlet GVF grows. The unsteady simulation results indicate that gas pocket firstly occurs on the pressure side of impeller, then moves to the suction side in the middle area of blade and finally transfers to outlet of impeller and disappears. The errors between numerical simulation results and experiment data are below 10 %, which validated the feasibility of the numerical simulation method. Simulation results on the split vane impeller demonstrate that the gas accumulation area in flow passage of the modified impeller is dramatically decreased compared to that of the normal impeller. The performance of the modified impeller is generally better than the normal impeller especially in high inlet GVF conditions.

     

水轮机模式液力透平叶片进口安放角影响分析

针对转轮叶片进口安放角 β1对超低比转速水轮机模式液力透平性能影响规律认识不足的问题,在超低比转速范围内,考虑液力透平水力性能,取不同 β1建立超低比转速混流式转轮三维模型,配合过流部件构建超低比转速水轮机模式二级液力透平模型,采用ANSYS FLUENT仿真软件对液力透平内部流动特性和能量特性进行计算,对比数值模拟结...     

打磨修理对跨声速压气机性能的影响研究

为了控制航空发动机压气机叶片损伤继续发展,通常采用孔探打磨进行修理。为研究其带来的影响,选取跨声速压气机Rotor37为研究对象,构建了不同叶高的叶片前缘打磨修理模型,采用数值模拟方法,研究了打磨修理对跨声速压气机气动性能的影响,结果表明:叶片前缘打磨修理使压气机整体性能下降,压比减小,效率降低,使堵塞点流量减小,而对失速流量点基本没有影响;不同叶高的打磨修理中心对压气机气动性能影响趋势一致影响幅度相当,但打磨叶高的不同造成压气机内部流场变化;叶片前缘打磨后,打磨处激波前马赫数增加,激波强度增大,影响效果随着叶高的增加而增加;压气机进口因为叶片打磨而造成相对动能在叶片打磨处附近降低。     

Automotive wheel with cooling fan for brake system and in-wheel motor

For suppressing the automotive troubles like an overheating of the drum/disc brake units or the in-wheel motor devices, authors propose the new automotive wheel with the blade shape spokes, which can work as the cooling fan, by use of the blade design procedure of the turbomachinery. In this paper, the prototype automotive wheels with the centrifugal or axial type blades were prepared to confirm the flow generation by proposed wheels experimentally and numerically, as the preliminary step. The proposed wheel brings the increase of the air flow through the wheel compared with the traditional wheel, which has the rectangle shape spokes. At a same time, the required power for the rotation wheel rotation with cooling fan also increase about 2–4% of the traditional one. Furthermore, the swirling inlet flow, which generates by the rotating cylindrical rim, may greatly influences the discharge performance of the wheel with the axial blades.     

不同流量系数下叶顶间隙对离心压缩机模型级性能的影响

通过CFD技术,分析了不同流量系数下叶顶间隙对离心压缩机模型级性能的影响。结果表明,对于中、小流量系数模型级,叶顶间隙越大,其效率曲线下降越大,而对于大流量系数模型级,有一定的叶顶间隙可改善叶轮内流场,反而有利于离心压缩机效率性能。无论何种流量系数模型级,存在一定的叶顶间隙则可提升能头曲线。     

基于结构参数相关性的液力缓速器优化设计

以液力缓速器为研究对象, 通过计算流体动力学(CFD)数值模拟方法, 借助滑动网格技术对缓速器内部非稳态不可压缩流动进行数值计算。分析了不同叶片前倾角下缓速器内部流场的特性和制动转矩。在最优前倾角的基础上, 基于各结构参数之间的相关性, 分别研究了不同流道腔型、叶片数对液力缓速器性能的影响。当叶片倾角为40°, 截面形状为扁圆形, 转子外环叶片数为40, 内环叶片数为20, 定子叶片数为43时, 模型制动转矩最大, 制动效果最好。     

静叶尾迹对压气机动叶非定常气动载荷的影响

以某型航空发动机压气机转子系统为研究对象,建立了单级叶盘的三维结构及流场模型。通过对滑移网格(sliding mesh,简称SM)方法与运动坐标系(moving reference frame,简称MRF)方法在计算耗时和收敛性方面的比较,证明了运动坐标系方法的准确性和高效性。考虑前一级静叶尾迹的影响,求解压气机内部在不同时刻的流动特性,得到静叶尾迹对动叶流场的非定常干扰情况。经过对压气机叶顶和轮毂、动叶压力面和吸力面非定常气动载荷的分析发现:在动叶流场的前缘形成了较主流区压力和速度较低的不均匀流场,且在动叶前缘叶顶位置受到的气动载荷最为显著;动叶压力面和吸力面气动载荷的分布规律相反,从叶顶至轮毂、前缘至尾缘,压力面非定常气动载荷的大小和波动幅度逐步递减,而吸力面却与其相反,呈现出逐步递增的趋势。该研究为某型航空发动机压气机叶盘转子系统的动力学设计提供了理论依据。