Design and optimization of a single stage centrifugal compressor for a solar dish-Brayton system

According to the requirements of a solar dish-Brayton system,a centrifugal compressor stage with a minimum total pressure ratio of 5,an adiabatic efficiency above 75% and a surge margin more than 12% needs to be designed.A single stage,which consists of impeller,radial vaned diffuser,90° crossover and two rows of axial stators,was chosen to satisfy this system.To achieve the stage performance,an impeller with a 6:1 total pressure ratio and an adiabatic efficiency of 90% was designed and its preliminary geometry came from an in-house one-dimensional program.Radial vaned diffuser was applied downstream of the impeller.Two rows of axial stators after 90° crossover were added to guide the flow into axial direction.Since jet-wake flow,shockwave and boundary layer separation coexisted in the impeller-diffuser region,optimization on the radius ratio of radial diffuser vane inlet to impeller exit,diffuser vane inlet blade angle and number of diffuser vanes was carried out at design point.Finally,an optimized centrifugal compressor stage fulfilled the high expectations and presented proper performance.Numerical simulation showed that at design point the stage adiabatic efficiency was 79.93% and the total pressure ratio was 5.6.The surge margin was 15%.The performance map including 80%,90% and 100% design speed was also presented.     

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。     

带无叶扩压器离心压气机的稳定性分析

结合实验数据,采用数值模拟方法对某带无叶扩压器离心压气机的性能与稳定性进行了详细分析,比较了设计点与近失速点下无叶扩压器内部的流动特征．结果表明：在发生喘振瞬间,扩压器进口最先出现了压力崩溃;扩压器进口流动特别是轮盖侧流动是导致压气机失稳的重要因素,对扩压器进口逆叶轮旋转方向的轮盖侧啧蒸汽能够提高扩压器的稳定性．     

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.     

Detailed analysis of the flow in the inducer of a transonic centrifugal compressor

Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. A detailed analysis of the flow in the inducer (i.e. the entry zone of the impeller between the main blade leading edge and the splitter blade leading edge) is proposed from choke to surge. Steady and unsteady simulations were performed using the code elsA, which uses a multi-domain approach on structured meshes and solves the compressible RANS equations, associated with a two-equation turbulence model k-l in the rotating frame of reference. The 1MW LMFA-ECL test rig was used for carrying out the tests in the compressor stage. Unsteady pressure measurements up to 150 kHz and Laser Doppler Anemometry measurements were performed in the inducer. A good agreement is obtained between the experimental and numerical data even if an over dissipation is noticed in the numerical results. The change in flow pattern from choke to surge is mainly due to a change in the tip leakage flow trajectory which straightens, leading to a flow blockage of an individual passage near shroud. A spectral analysis shows that only the blade passing frequency and its harmonics compose the various spectra obtained from choke to surge.     

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.     

叶片安装角对离心压气机扩压器气动性能及失速机理影响研究

为了研究离心压气机扩压器异常叶片对于失速现象的诱发效果，以带有叶扩压器的高速离心压气机为研究对象，通过整体或局部改变扩压器叶片安装角，开展非定常数值模拟研究并与实验结果对比验证，研究叶片安装角改变对离心压气机性能、动态特性以及失速机理的影响规律。研究表明：整体负方向旋转叶片安装角会促使扩压器更加不稳定，旋转角度从-5°到5°，最高效率点对应的质量流量逐渐增大。其中安装角偏转+5°扩压器叶片前缘靠近轮缘壁面发生流动分离，诱使无叶区间产生回流；而偏转-5°扩压器轮毂附近的流动分离主要发生在尾缘，造成扩压器叶片吸力面附近产生大范围回流。单个叶片安装角发生较大偏转(大于10°)时，扩压器比叶轮更早进入失速状态，且失速的机制可能会随着安装角偏移的增大发生改变。     

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。     

Unstable Head-Flow Characteristic Generation Mechanism of a Low Specific Speed Mixed Flow Pump

This paper treats the flow instabilities in a mixed flow pump with a vaned diffuser. Test pump has a positive slope of a head-flow performance curve at 65% flow rate of BEP (Best Efficiency Point) because of a rotating stall. Dynamic Particle Image Velocimetry (PIV) and pressure fluctuation measurements are used for investigating the propagation mechanism of a rotating stall. It was found that unstable performance was caused by periodical large scale abrupt backflow generated from the vaned diffuser to the outlet of impeller. Further, the relation between the static pressure at the inlet of diffuser vane and the internal flow condition was clarified. From these experimental results, in order to improve the positive slope of a head-flow performance curve, to suppress the growth of strong vortex toward the inlet of diffuser vane was proved to be a key point.     

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.     

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 cen-trifugal compressor with vaned diffuser.The purpose is to better understand the flow characteristics near stallpoint under the interactions between centrifugal impeller and vaned diffuser.Numerical results show that undercertain conditions just preceding stall point the tip leakage vortex begins to fluctuate at roughly half of the bladepassing frequency.This phenomenon is similar to rotating instability in axial compressors.With the flow rate re-duced further the impeller stalls and five stall cells propagating at a frequency of 85 percent of impeller rotationspeed are found.     

Effect of high negative incidence on the performance of a centrifugal compressor stage with conventional vaned diffusers

Three vaned diffusers, designed to have high negative incidence (-8°) at the design operating point, are studied experimentally. The overall performance (efficiency and pressure ratio) are measured at three rotational speeds, and flow angles before and after the diffuser are measured at the design rotational speed and with three mass flow rates. The results are compared to corresponding results of the original vaneless diffuser design. Attention is paid to the performance at lower mass flows than the design mass flow. The results show that it is possible to improve the performance at mass flows lower than the design mass flow with a vaned diffuser designed with high negative incidence. However, with the vaned diffusers, the compressor still stalls at higher mass flow rates than with the vaneless one. The flow angle distributions after the diffuser are more uniform with the vaned diffusers.     

Effects of Tapered Diffuser Vane on the Flow Field and Noise of a Centrifugal Compressor

The effects of a three-dimensional tapered diffuser vane on the flow field and noise radiated from a centrifugalcompressor are investigated by both CFD analyses and experiments.Tapered diffuser vanes are very useful notonly for the reduction of the interaction tone noise but also for the improvement of the pressure recovery charac-teristics within the diffuser passage.By using tapered diffuser vanes,the interaction area between the impel-ler-discharge flow and diffuser vanes becomes small,and then the noise level of the discrete tone can be reducedremarkably as a result.Furthermore,by utilizing the visualization technique of vortical structures based on theCFD results,the scale of vortex shedding leaving from the leading edge of the diffuser vanes is found to be con-tracted and a tendency for the turbulence level to decrease is observed.This may be the cause of the attenuation ofbroadband noise components.The secondary flow,which is considered to be an obstruction of diffuser pressurerecovery,can also be suppressed by the tapered diffuser vanes,and the pressure decrease observed in the throatpart of the diffuser passage is further reducible.     

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.     

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 aneffective method to delay the inception of rotating stall in a low speed axial compressor.Considering the practicalapplication a new type of micro injector was designed and described in this paper,which was imbedded in thecasing and could be moved along the chord.In order to verify its feasibility to other cases,such as high subsonicaxial compressor or centrifugal compressor,some other cases have been studied.Experimental results of the samelow speed axial compressor showed that the new injector could possess many other advantages besides success-fully stabilizing the compressor.Experiments performed on a high subsonic axial compressor confirmed the ef-fectiveness of micro air injection when the relative velocity at the blade tip is high subsonic.Meanwhile in orderto explore its feasibility in centrifugal compressor,a similar micro injector was designed and tested on a lowspeed centrifugal compressor with vaned diffuser.The injected mass flow was a bit larger than that used in axialcompressors and the results showed micro injection could also delay the onset of rotating stall in the centrifugalcompressor.     

带可调进口导叶的多级轴流压气机非设计点性能计算

多级轴流压气机为保证非设计转速下的性能,通常采用进口导叶(IGV)以及静叶可调的扩稳方法。为获得可靠的进口导叶性能以用于压气机初步设计和扩稳方案的初步筛选,基于Banjac和Petrovic等提出的IGV损失和落后角模型与平均中径计算相结合,开发了带有IGV的多级轴流压气机性能分析工具。选用E3压气机作为算例进行计算分析,结果表明,所选用的IGV模型在较大开度内具有较好的精度和有效性。研究结果可以为多级轴流压气机初步设计和扩稳方案确定提供参考。     

Improvement in solar air collector performance with vaned diffuser

Modifications made in a solar air collector inlet duct to achieve uniform velocity of air in the absorber duct are described. Measurements of temperature and pressure at various points in the duct gave information on the distribution of air in the absorber duct. A thermal performance test conducted on the collector with a vaned diffuser showed some significant improvement compared with a diffuser without vanes.     

Numerical investigation of effect of inlet swirl and total-pressure distortion on performance and stability of an axial transonic compressor

This paper represents numerical simulation of flow inside an axial transonic compressor subject to inlet flow distortion, to evaluate its effect on compressor performance and stability. Two types of inlet distortion, namely inlet swirl and total pressure distortion are investigated. To study the effect of combined distortion patterns, different combinations of inlet swirl and total pressure distortion are also studied. Results for cases with total pressure distortion indicate that hub radial distortion improves stability range of the compressor while tip radial distortion deteriorates it. An explanation for this observation is presented based on redistribution of flow parameters caused by distortion and the way it interacts with stall inception mechanisms in a transonic axial compressor. Results also show that while co-swirl patterns slightly improve stability range of the compressor, counter-swirl patterns diminish it. Study of combined distortion cases reveals that superimposition of effects of each individual pattern could predict the effect of a combined pattern on compressor’s performance within an accuracy of 1%. However, it is unable to predict the associated effect on compressor’s stability.     

Transition from Unsteady Flow Inception to Rotating Stall and Surge in a Transonic Compressor

Since the transition from rotating stall to surge in a transonic compressor at high speed is very quick,quite often there is no time to take measures to prevent the surge.Therefore,it is desired to find any rotating stall precursors,of which the occurrence can offer sufficient time for stall or surge prevention.In this study,a series of unsteady flow analyses were performed on a transonic compressor under operating conditions before rotating stall with unsteady results scrutinized to find rotating stall precursors.Particular attention is paid to the spatial modes and time modes of static pressure near the casing and around the blade leading and trailing edges.The results show that the characteristics of the precursor in both spatial and time domains can be used as rotating stall warnings.