Aerodynamic Performance and Noise Characteristics of a Centrifugal Compressor with Modified Vaned Diffusers

Improvement of aerodynamic performance and reduction of interaction tone noise of a centrifugal compressorwith vaned diffusers are discussed by experiments and visualization techniques using a colored oil-film method.The focus of the research is concentrated on the leading edge shape of diffuser vanes that are deeply related to thegeneration mechanism of the interaction tone noise.The compressor-radiated noise can be reduced by more thanten decibels by using modified diffuser vanes which have 3-D tapered shapes on both pressure and suction sur-faces of the leading edge.Furthermore,by adopting the proposed modified diffuser vanes,the secondary flowwhich is considered to be an obstruction of diffuser pressure recovery can be suppressed,and also the pressuredecrease observed in the throat part of the diffuser flow passage is reducible.Thus,the proposed diffuser vanesshow a favorable result for both noise and the aerodynamic performance of the centrifugal compressor,and offera few basic guidelines for the diffuser vane design.     

Noise reduction and surge margin improvement using tapered diffuser vane in a centrifugal compressor

Improvement of surge margin and reduction of interaction tone noise radiated from a centrifugal compressor with a vaned diffuser is discussed from experiments and CFD analyses. Two types of one-side tapered diffuser vanes, which are shroud- and hub-side tapered vanes, were used in the experiments. The height of the leading edge of the tapered vanes varied from 100% to 10%. Both types of tapered vanes are effective for the reduction of the inter- action tone noise radiated from the compressor, because of the smaller interaction area between impel- ler-discharge flow and diffuser vane surface. Especially, the hub-side tapered diffuser vane suppresses the evolution of the leading-edge vortex, which causes reverse-flow in the diffuser passage. Furthermore, the hub-side tapered diffuser vane, which has an optimized diffuser leading edge shape, can improve the compressor pressure-rise characteristics at low-flow operation and can enlarge the surge margin.     

Effects of Diffuser Vane Geometry on Interaction Noise Generated from a Centrifugal Compressor

The characteristics of interaction tone noise radiated from a centrifugal compressor with a vaned diffuser are discussed by experiments, including visualization techniques using the oil-film method. Research attention is paid to the leading edge geometries of the diffuser vanes that are deeply related to the generation mechanism of the interaction tone noise. The compressor-radiated noise can be reduced by several decibels by setting some clearances in both the hub and shroud surfaces of the diffuser wall along with some decline in the pressure-rise coefficient. Since the decline turned out to be caused by the flow impingement and also by the secondary flow within the diffuser passages, several new types of diffuser vane geometries which do not detract from both the performance and noise level are developed and utilized for the experiments. The presented diffuser vane geometries will offer a few basic guidelines for the diffuser vane design.     

Unsteady behavior and control of vortices in centrifugal compressor

Two examples of the use of vortex control to reduce noise and enhance the stable operating range of a centrifugal compressor are presented in this paper.In the case of high-flow operation of a centrifugal compressor with a vaned diffuser,a discrete frequency noise induced by interaction between the impeller-discharge flow and the diffuser vane,which appears most notably in the power spectra of the radiated noise,can be reduced using a tapered diffuser vane（TDV） without affecting the performance of the compressor.Twin longitudinal vortices produced by leakage flow passing through the tapered portion of the diffuser vane induce secondary flow in the direction of the blade surface and prevent flow separation from the leading edge of the diffuser.The use of a TDV can effectively reduce both the discrete frequency noise generated by the interaction between the impeller-discharge flow and the diffuser surface and the broadband turbulent noise component.In the case of low-flow operation,a leading-edge vortex（LEV） that forms on the shroud side of the suction surface near the leading edge of the diffuser increases significantly in size and blocks flow in the diffuser passage.The formation of an LEV may adversely affect the performance of the compressor and may cause the diffuser to stall.Using a one-side tapered diffuser vane to suppress the evolution of an LEV,the stable operating range of the compressor can be increased by more than 12 percent,and the pressure-rise characteristics of the compressor can be improved.The results of a supplementary examination of the structure and unsteady behavior of LEVs,conducted by means of detailed numerical simulations,are also presented.     

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 unshrouded 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 first scenario of the surge inception is given.The compressor first experiences a steady alternate stall in the diffuser.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 pressure gradient becomes too strong,the alternate stall blows away and the compressor enters into deep surge.     

小型离心压气机径向扩压器复杂涡系结构分析

对某小型离心压气机进行详细数值模拟,构建径向扩压器复杂涡系结构模型,重点分析设计、堵塞、失速工况下径向扩压器内部复杂涡系结构。研究表明：径向扩压器内部涡系结构主要包括前缘涡、两个通道涡（压力面侧通道涡与吸力面侧通道涡）以及喉部涡;主叶片吸力面的前缘涡是机匣侧低能流体在展向与流向压力梯度作用下形成的,喉部涡是吸力面侧通道涡沿分流叶片前缘的回流与前缘涡构成的;喉部涡在喉部的堆积是导致径向扩压器失速的原因,径向扩压器喉部的激波则是堵塞的原因;随流量的减小,前缘涡的涡核越向相邻主叶片压力面迁移。     

串列叶栅前后排叶片周向相对位置对离心压气机性能的影响

串列叶栅前后排叶片相对位置对串列扩压器的性能有重要影响.根据离心叶轮出口气流参数设计了一离心式串列叶栅扩压器,并利用数值模拟方法在前、后排叶栅周向相对位置分别为10%、20%、30%、40%、50%、60%、70%、80%和90%时对离心压气机级进行了计算和分析,研究周向相对位置变化对离心压气机性能的影响以及作用机理.数值模拟结果表明:随着前后排叶栅周向相对位置变化,后排叶栅前缘滞止高压区相对前排叶栅的位置发生了变化,影响了前排叶栅压力面的压力分布,从而改变了前排叶栅压力分布及大小;当前后排叶栅周向相对位置为30%时,扩压器性能达到最佳,使压气机总压比和等熵效率最大,稳定工作范围增大;前后排叶栅所形成的渐缩通道可抑制后排叶栅吸力面边界层的分离.     

Transient numerical analysis of rotor–stator interaction in a Francis turbine

Pressure fluctuation due to rotor–stator interaction and occurrence of vortex rope in draft tube at partial load operation are obvious phenomena in Francis type reaction hydro turbines. These hydrodynamic effects are important issues and should be addressed during the design of hydraulic machines. A 3-dimensional transient state turbulent flow simulation in the entire flow passage of a 70 kW-Francis turbine having specific speed of 203.1 is conducted to investigate the rotor–stator interaction by adopting kω based SST turbulence model. The commercial 3D Navier–Stokes CFD solver Ansys-CFX is utilized to study the flow through this vertical shaft Francis turbine in its stationary and transient passages, at 100% optimum load and 72% of part load. The investigated turbine consists of a spiral casing with 16 guide vanes, 8 stay vanes, a runner with 13 blades and a draft tube. With a time step of 2° of a rotational period of the runner for 10 full rotations, the time dependent pressure and torque variations are monitored at the selected locations during the unsteady state calculation. A periodical behavior is observed for the pressure distribution in guide vanes, runner blades and torque in the runner blades. The pressure distribution curve in runner blades reveals the two dominating frequencies – the lower peaks due to runner speed and the upper peaks corresponding to the number of guide vanes interacting with the flow. The flow acceleration toward inside of the runner is depicted by the expanding wakes behind the stay vanes. Vortex rope is observed in draft tube, downstream the runner, at part-load operation.     

基于流固耦合的斜流泵空间导叶多工况强度分析

空间导叶作为斜流泵的主要过流部件,对改善叶轮出口流场具有重要的意义,探究其内部流动以及与固体之间的相互作用尤为必要。通过对0.6Q_d、0.8Q_d、1.0Q_d、1.2Q_d和1.4Q_d5个不同流量工况点进行CFD仿真及流固耦合计算分析,结果显示空间导叶承受的总压随着流量的增加逐渐减小,空间导叶与上下盖板连接位置承受压力较大;随着流量的增加,空间导叶最大应力值和变形量均减小,其所承受应力主要集中于上盖板进口边以及导叶与上下盖板连接处,在上盖板两导叶叶片中间位置变形较为明显。研究结果可为斜流泵空间导叶结构设计和强度优化提供参考。     

Numerical simulation of air flow through turbocharger compressors with dual volute design

In this paper, turbocharger centrifugal compressors with dual volute design were investigated by using Computational Fluid Dynamics (CFD) method. The numerical simulation focused on the air flow from compressor impeller inlet to volute exit, and the overall performance level and range are predicted. The numerical investigation revealed that the dual volute design could separate the compressor into two operating regions: “high efficiency” and “low efficiency” regions with different air flow characteristics, and treating these two regions separately with dual diffuser design showed extended stable operating range and improved efficiency by comparing with conventional single volute design. The “dual sequential volute” concept also showed the potential to further extend the stable operating range by closing one of the volutes at low air flow rates. Furthermore, by comparing with other alternate designs such as variable diffuser vanes and variable inlet guide vanes, the operation of the dual sequential volute also features relatively simple control and calibration.     

Computed effect of guide vane shape on performance of impulse turbine for wave energy conversion

This paper deals with the computational fluid dynamics (CFD) analysis on effect of guide vane shape on performance of impulse turbine for wave energy conversion. Initially, experiments have been conducted on the impulse turbine to validate the present CFD method and to analyse the aerodynamics in rotor and guide vanes, which demonstrates the necessity to improve the guide vanes shape. The results showed that the downstream guide vanes make considerable total pressure drop leads low performance of the turbine and hence three‐dimensional (3‐D) inlet and downstream guide vanes have been designed based on well‐known vortex theory to improve the efficiency of the turbine. In order to prove the improvement in efficiency due to 3‐D guide vanes, CFD analysis has been made on impulse turbine with 2‐D and 3‐D guide vanes for various flow coefficients. As a result, it is seen that the present CFD model can predict the experimental values with reasonable accuracy. Also, it is showed from the numerical results that the efficiency of the turbine can be improved by average of 4.5 percentage points by incorporating 3‐D guide vanes instead of 2‐D guide vanes. The physical reason for improvement in efficiency of the turbine due to 3‐D guide vanes has been explained with the CFD flow insight pictures. As the turbine operates in fluctuating flow conditions, the performance of the turbine with 2‐D and 3‐D guide vanes have been calculated numerically using quasi‐steady analysis. Furthermore, the performance of the turbine has been predicted for one year based on Irish wave climate to show the feasibility of using 3‐D guide vanes in actual sea wave conditions. Copyright © 2005 John Wiley & Sons, Ltd.     

Performance evaluation of a downwind diffuser on vertical axis wind turbine

Research has proven that the performance of a horizontal axis wind turbine (HAWTs) can be increased significantly by the application of a diffuser. It serves as a power augmented feature to draw higher wind flow toward the HAWT. However, research on integrating a diffuser onto vertical axis wind turbines (VAWTs) is scant, where most of the available power augmentation devices used for VAWTs are the convergent duct, deflector plate, shroud, and guide vanes which are placed in a proper configuration at the upwind. In this paper, laboratory tests and computational simulations have been carried out to study the impacts of a downwind diffuser on the performance of a VAWT. The diffuser is designed with the absence of a concentrator or flange and is placed downwind of the VAWT. Parametric computational fluid dynamics (CFD) studies were carried out for the downwind diffuser length and semi-opening angle. A five-bladed H-rotor was selected as the testing wind turbine, whereas the diffuser used was made up of flat plates. Both simulations and experiment results are consistent. From the experiments, it was found that a downwind diffuser increases the VAWT performance remarkably. The diffuser-augmented VAWT produced an increment in the maximum coefficient of power of 31.42% at the TSR 0.65 to 0.75. Moreover, the diffuser induced a better self-start ability on the VAWT. The simulation showed that the flow field at the diffuser promotes a flow expansion which created a lower-pressure region at downstream that accelerates the wind toward the VAWT, hence increasing the turbine performance significantly.     

Influence of diffuser on aerodynamic noise of a forward curved fan

In order to clarify the influence of a diffuser on the characteristics of a forward curved fan,the influence of the bare ratio and the outlet angle on the characteristics of the fan were measured through an experiment performed with an actual fan as well as through a numerical simulation,respectively.The mechanism of the discrete frequency noise generated by the separated flow of the diffuser was analyzed.The optimized bare ratio was approximately 17%.The flow separated inside of the diffuser generated discrete frequency noise owing to the interaction between the reversed flow from the diffuser and the impeller rotating at the blade passing frequency.The diffuser outlet angle influenced the pressure ratio more than that by the bare ratio.Furthermore,it was confirmed that restraining the separation in the diffuser effectively decreases the fan noise.     

Unsteady behavior of leading-edge vortex and diffuser stall in a centrifugal compressor with vaned diffuser

The characteristics of a rotating stall of an impeller and diffuser and the evolution of a vortex generated at the diffuser leading-edge (i.e., the leading-edge vortex (LEV)) in a centrifugal compressor were investigated by experiments and numerical analysis. The results of the experiments revealed that both the impeller and diffuser rotating stalls occurred at 55 and 25 Hz during off-design flow operation. For both, stall cells existed only on the shroud side of the flow passages, which is very close to the source location of the LEV. According to the CFD results, the LEV is made up of multiple vortices. The LEV is a combination of a separated vortex near the leading- edge and a longitudinal vortex generated by the extended tip-leakage flow from the impeller. Therefore, the LEV is generated by the accumulation of vorticity caused by the velocity gradient of the impeller discharge flow. In partial-flow operation, the spanwise extent and the position of the LEV origin are temporarily transmuted. The LEV develops with a drop in the velocity in the diffuser passage and forms a significant blockage within the diffuser passage. Therefore, the LEV may be regarded as being one of the causes of a diffuser stall in a centrifugal compressor.     

不同结构消声器内部流场的比较

通过对两种不同结构消声器内部流场的模拟,研究了气流对消声器的影响。以两种典型消声器Ⅰ和消声器Ⅱ为例,对内部流场进行对比分析。消声器结构不同,内部产生的涡流也不同,气流经过消声器Ⅰ的内插管后,进入第二腔产生两处大的涡流,而消声器Ⅱ有4处大的涡流,强度也比消声器Ⅰ的大。模拟计算出的入口绝对压力可以作为设计优化消声器结构流动阻力大小的依据。消声器尾管采用穿孔管,可以对消声器再生噪声在出口处进行衰减,但同时也增加了流动阻力。     

Control of secondary flow in a low solidity circular cascade diffuser

According to the previous experimental works on the low solidity circular cascade diffuser (LSD), a pressure recovery of a centrifugal blower was improved by the LSD significantly in a wide range of flow rate, and the pres-sure recovery was improved further by the LSD with a tandem cascade in comparison with the LSD with a sin-gle-row cascade. In the present study, the flow behavior in the LSD with the tandem cascade has been analyzed numerically by using the commercial CFD code of ANSYS-CFX12. It was shown clearly that the higher pressure recovery was achieved by applying the LSD with the tandem cascade, and the high pressure recovery is based on the high pressure rise in the vaneless space upstream of the LSD and the high blade loading of the front blade of the LSD. The high pressure recovery in the LSD could be achieved by controlling the flow separation on the suc-tion surface of the front blade and also on that of the rear blade due to formation of the favorable secondary flow and due to increase in mass flow passing through the slit section between the front and rear blades.     

Effect of Placing a Diffuser around a Wind Turbine

An actuator disc CFD model of the flow through a wind turbine in a diffuser is developed and validated. Further, it is shown theoretically from a 1D analysis that the Betz limit can be exceeded by a factor proportional to the relative increase in mass flow through the rotor induced by the diffuser. The theoretical 1D result is verified by the CFD model. Copyright © 2000 John Wiley & Sons, Ltd.     

超低比转速离心风机无叶扩压器改进设计研究

为了提高超低比转速离心通风机的气动性能,通过理论分析提出了降低超低比转速离心风机内部损失的扩压器设计方法,采用数值模拟的方法得到不同直径旋转扩压器和静止扩压器的性能特点,从风机静压、效率、扩压器静压恢复系数和滞止压力损失系数等方面对其分析比较。研究得到：在设计流量下,旋转扩压器直径增大8.7%后,风机静压提升6.8%,效率提高3%,扩压器静压恢复系数提高62.9%,扩压器内滞止压力损失减少71.9%,表明在尺寸允许范围内适当增大扩压器直径可获得更高的静压升,同时降低损失、提高效率;另外,静止扩压器壁面与通流之间的剪切力更大,损失更多,设计流量下静止扩压器比等直径旋转扩压器滞止压力损失增加75.2%,表明旋转扩压器比静止扩压器有更好的性能。     

压气机喘振的空气动力学分析和防范措施

本文从空气动力学的角度分析了压气机的气流在扩压器内的流动状态;指出压气机发生喘振的机理是由于种种原因使进入压气机的空气流量减少所致。实践表明,改变扩压器的叶片进口角,使其通流面积的直径适当减小,是防止压气机喘振的一个特例。     

Lessons from Rotor 37

INTanDUCTIONhi1992theturbomachinerycommittee0ftheIoter-nationalGasTurbineInstitute(IGTI)decidedtosetuPatestcaseforCFDcalculations.Aftersomedebateitwasdecidedthatthetestcasesh0uldbecalculated'blind',i.e.thattheexperimentalresultssh0uldn0tbemadeawilableulltilafterthesolutionshadbeensubndtted.NASA(LewisResearchCenter)offeredasuitabletestca-seintheformofahighlyloadedtransoniccompressorrot0rwhichwasthenbeingtestedwitheXtensiveuseoflaseranemometrytomeasuretheiliternalflow.Themeasurementsw…