Investigation into the Interaction of Centrifugal Compressor Impeller and Vaneless Diffuser

Centrifugal compressors with parallel-wall and contracting wall vaneless diffuser are designed by using centrifugal compressor computer-aided integrated design system. The internal flow fields of the compressor are calculated by solving three-dimensional Navier-Stokes equation. Four aspects are investigated and calculation results show that the total efficiencies and total pressure ratios of the compressor with contracting wall vandess diffuser is higher than that of the compressor with parallel-wall. The jet and wake don＇t mix rapidly inside vandess diffuser. The outlet blade lean angle doesn＇t affect the compressor performance. The greater the mass flow rate through impeller, the more uneven the velocity distribution at impeller outlet is.

Experimental and Computational Analysis of the Unstable Flow Structure in a Centrifugal Compressor with a Vaneless Diffuse

The unstable flow phenomena in compressors, such as stall and surge, are closely related to the e ciency and the operating region. It is indispensable to capture the unstable flow structure in compressors and understand the mechanism of flow instability at low flow rates. Cooperated with the manufacturer, an industrial centrifugal compressor with a vaneless di user is tested by its performance test rig and our multi-phase dynamic measurement system. Many dynamic pressure transducers are circumferentially mounted on the casing surface at seven radial locations, spanning the impeller region and the di user inlet region. The pressure fields from the design condition to surge are measured in details. Based on the multi-phase dynamic signals, the original location of stall occurring can be determined. Meanwhile, the information of the unstable flow structure is obtained, such as the circumferential mode and the propagating speed of stall cells. To get more details of the vortex structure, an unsteady simulation of this tested compressor is carried out. The computational result is well matched with the experimental result and further illustrates how the unstable flow structure in the impeller region gradually a ects the stability of the total machine at low flow rates. The dynamic mode decomposition(DMD) method is applied to get the specific flow pattern corresponding to the stall frequency. Both experimental and computational analysis show that the flow structure at a particular radial location in the impeller region has a great impact on the stall and surge. Some di erences between the computational and experimental result are also discussed. Through these two main analytical methods, an insight into the unstable flow structure in an industrial compressor is gained. The result also plays a crucial role in the guidance of the compressor stabilization techniques.

Stall Evolution Mechanism of a Centrifugal Compressor with a Wide-Long Vaneless Diffuser

The rotating stall mechanism is of high importance for the stability of centrifugal compressors and thermal power cycles.The majority of research concerning this topic has concentrated on the initial stall phase.However,the evolution of stall cells in wide-long diffusers has not been comprehensively studied.In this paper,the causes of rotating stall in the wide-long diffuser and the three-dimensional evolution mechanism of stall cells during the stall process were thoroughly analyzed.During the stall induction phase,an annulus vortex structure was found in the reverse-flow zone near the hub side of the diffuser outlet,which was the initial form of stall cells.The whole evolution process of stall cells was divided into three phases as the flow rate decreased.During the initial stall phase,the dynamic equilibrium was built under effects of the impeller wake and the adverse pressure gradient.As a result,the number of stall cells was kept at seven and the size of stall cells remained constant.During the transition phase,the flow in the diffuser became unstable.Stall cells extended to the impeller outlet,and the effect of the wake flow was strengthened significantly.Stall cells started integrating and separating regularly.As a result,the number and propagation speed of stall cells varied periodically at a constant mass flow rate.During the deep stall phase,the size of stall cells remained unchanged,and the number of stall cells kept at one.This study has important practical guidance and engineering value for the high-efficiency design and safe operation of centrifugal compressors.

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.

引气结构对引气压气机性能影响的研究

基于压气机扩压器引入废气再循环(diffuser exhaust gas recirculation,DEGR)方案,通过计算流体力学(computational fluid dynamics,CFD)数值模拟的方法研究引气压气机性能的影响。研究了无引气时引气结构对压气机性能的影响,分析了有无涡旋进气段时引气压气机的性能。结果表明:与压气机原机相比,在无引气时,引气结构使引气压气机峰值效率降低近2%;有无涡旋进气结构仿真结果表明采用涡旋进气能够有效降低引气结构出口与叶轮出口气体的切向速度差,降低引气进入扩压器造成的流动损失和混合损失,进而提高引气压气机绝热效率。

基于可调叶高扩压器的离心压气机构型流动机理研究

离心叶轮/扩压器匹配关系恶化直接限制了离心压气机的稳定工作范围.以Radiver离心压气机为研究对象,借助经过校核的数值模拟方法开展了基于可调叶高扩压器的离心压气机叶轮/扩压器匹配关系及流动机理研究.结果表明:通过建立能够指导扩压器叶高调节方向和量值的“压气机工作流量-扩压器最佳叶高”正相关对应关系,即可在压气机近失稳点等小流量点主动降低扩压器叶高以改善压气机稳定工作裕度.在设计转速下,引入可调叶高扩压器能够使离心压气机的稳定裕度提升18.8%、流量范围拓宽385%且峰值效率提高0.45%;随着压气机工作点向小流量工况偏移,降低扩压器叶高可以抑制扩压器来流正攻角,优化重构其通道内部涡系结构,减小扩压器吸力面和压力面/轮毂角区的低速区,进而改善叶轮/扩压器匹配关系,抑制扩压器内部流动失稳发生,提升离心压气机稳定裕度和效率指标.

Mechanism of stall and surge in a centrifugal compressor with a variable vaned diffuser

To expand the stable operating range of compressors, understanding the mechanism of flow instability at low flow rates is necessary. In this paper, the mechanism of stall and surge in a centrifugal compressor with a variable vaned diffuser is experimentally investigated, where the diffuser blade setting angle can be adjusted. Many dynamic pressure transducers are mounted on the casing surface of the compressor. From the design condition to surge, dynamic pressure data is recorded throughout the gradual process. According to the signal developing status, the typical modes of compressor instability are defined in detail, such as stall, mild surge, and deep surge. A relatively high-frequency stall wave originates in the impeller and propagates to the diffuser, and finally stimulates a deep surge in the compressor. The compressor behavior during surge differs at different diffuser vane angles. When the diffuser vane angle is adjusted, both the unstable form and the core factor affecting the overall machine stability change. A specific indicator is proposed to measure the instability of each component in a compressor, which can be used to determine the best region for stability extension technologies, such as a holed casing treatment, in different compressor applications.

A Review of Rotating Stall in Vaneless Diffuser of Centrifugal Compressor

At present, off-design centrifugal compressor suffers from great instabilities, such as rotating stall and surge, which lead to work failure, strong pressure fluctuations, and violent vibrations of the entire system. One cause of these instabilities is the rotating stall occurring in vaneless diffuser. In this review, recent researches on rotating stall of vaneless diffuser in centrifugal compressor are reviewed, and its inductive mechanism and characteristic description are emphatically introduced. In vaneless diffuser, the rotating stall starts up and fully develops under non-design conditions with the characteristic of increasing pressure fluctuation amplitude. The negative effects of rotating stall include reverse flow and vortex, both of which have significant effects on the instability of centrifugal compressors. Some improvement methods of rotating stall are also introduced, and a lot of examples are given. Finally, some suggestions and discussions are put forward for the future work.

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 compressor with vaned diffusers are discussed by experiments and visualization techniques using a colored off-film method. The focus of the research is concentrated on the leading edge shape of diffuser vanes that are deeply related to the generation mechanism of the interaction tone noise. The compressor-radiated noise can be reduced by more than ten 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 flow which is considered to be an obstruction of diffuser pressure recovery can be suppressed, and also the pressure decrease observed in the throat part of the diffuser flow passage is reducible. Thus, the proposed diffuser vanes show a favorable result for both noise and the aerodynamic performance of the centrifugal compressor, and offer a few basic guidelines for the diffuser vane design.

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.

Numerical Investigation of Rotating Stall in Centrifugal Compressor with Vaned and Vaneless Diffuser

This study presents a numerical simulation of the stall and surge in a centrifugal compressor and presents a description of the stall development in two different cases.The first case is for a compressor with vaneless diffuser and the second is for a compressor with vaned diffuser of the vane island shape.The main aim of this study is to compare the flow characteristics and behavior for the two compressors near the surge operating condition and provide further understanding of the diffuser role when back flow occurs at surge.Results showed that for a location near the diffuser entrance,the amplitude of the static pressure fluctuations for the vaneless diffuser case is higher than that for the vaned diffuser case near surge condition.These pressure fluctuations in the case of the vaneless diffuser appear with a gradual decrease of the mean pressure value as a part of the surge cycle.While for the case of the vaned diffuser,the pressure drop during surge occurs faster than the case of the vaneless diffuser.Also,results indicated that during surge in the case of vaneless diffuser,there is a region with low velocity and back flow that appears as a layer connecting all impeller passages near shroud surface and this layer develops in size with time.On the other hand,for the case of vaned diffuser during surge,the low velocity regions appear in random locations in some passages and these regions expand with time towards the shroud surface.Results showed that during stall,the impeller passages are exposed to identical impact from stall cells in the case of vaneless diffuser while the stall effect varies from passage to another in the case of the vaned diffuser.

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 centrifugal compressor are investigated by both CFD analyses and experiments. Tapered diffuser vanes are very useful not only for the reduction of the interaction tone noise but also for the improvement of the pressure recovery characteristics within the diffuser passage. By using tapered diffuser vanes, the interaction area between the impeller-discharge flow and diffuser vanes becomes small, and then the noise level of the discrete tone can be reduced remarkably as a result. Furthermore, by utilizing the visualization technique of vortical structures based on the CFD results, the scale of vortex shedding leaving from the leading edge of the diffuser vanes is found to be contracted and a tendency for the turbulence level to decrease is observed. This may be the cause of the attenuation of broadband noise components. The secondary flow, which is considered to be an obstruction of diffuser pressure recovery, can also be suppressed by the tapered diffuser vanes, and the pressure decrease observed in the throat part of the diffuser passage is further reducible.

Design of a Centrifugal Compressor with Low Solidity Vaned Diffuser(LSVD) for Large-Scale Compressed Air Energy Storage(CAES)

Compressed Air Energy Storage(CAES) has tremendous promotional value in the intermittent renewable energy supply systems. CAES has special requirements for compressor(e.g. heavy load, high pressure ratio, wide range). With advantages of higher efficiency and wider operation range, IGC(Integrally Geared Compressors) is selected to fulfill the special requirements of the large-scale CAES. To get a better aerodynamic performance, in this paper, based on the analysis of internal flow of centrifugal compressor, a multi-objective one-dimensional optimization design program was put forward combined with modified Two-Zone model and a low solidity vaned diffuser(LSVD) design method. Then, a centrifugal compressor aerodynamic component optimization design system was established with the three-dimensional blade optimization design method based on neural network and genetic optimization algorithm. Then a validation was done by redesigning the Krain-Impeller to get better performance. Finally, the aerodynamic design of the first stage of IGC was completed. The CFD calculation results indicated that the total-to-total pressure ratio of the first stage was 2.51 and the polytropic efficiency was 91.0% at the design point. What’s more, an operation margin and surge margin of the compressor was about 26.5% and 16.4% respectively.

Influence of volute distortion on the performance of turbocharger centrifugal compressor with vane diffuser

As the geometry of the volute of turbocharger compressor is non-axisymmetric,it causes a distortion at the outlet of the diffuser and influences the upstream components.A distortion model in which a pressure distortion was applied as outlet boundary condition was established to simulate the distortion induced by the volute.It turned out to be sufficient to impose a circumferentially asymmetric pressure distribution at the outlet of the diffuser to replace the volute.Based on the distortion model which was verified,the influence of the amplitude of the distortion on the performance of centrifugal compressor was studied in detail.The results show that the distortion severely harms aerodynamic stability of the investigated compressor.The larger the amplitude of the distortion,the worse the performance of the compressor.The distortion induced by asymmetric volute propagates to upstream components and causes local flow separation at part of diffuser and impeller,and then causes the compressor surge.When the amplitude of the volute distortion is 10%,the stable flow range of the centrifugal compressor decreases to near zero.To authors’knowledge,the relationship between the compressor performance and distortion amplitude is first obtained quantitatively,which provides evidence to improve the performance of turbocharger compressor by decreasing the distortion induced by asymmetric volute.

Unsteady Pressure Field in a Vaneless Diffuser of a Centrifugal Compressor: An Experimental and Computational Analysis

Unsteady static pressure was measured and time-accurate numerical simulations were conducted in order to analyze unsteady behaviour of the centrifugal compressor diffuser. The unsteady static pressure was measured at the diffuser inlet and outlet at three different circumferential positions. Operation points near the surge, the design operation point and operation point near the choke were used in measurements. The time-accurate simulations were conducted at the design operation point and operation point near the choke. The whole compressor was modelled and the low Reynolds number k-εturbulence model was utilized in numerical simulations. The gathered numerical and experimental unsteady pressures were compared with each other.

Temporal Behaviour of a Corner Separation in a Radial Vaned Diffuser of a Centrifugal Compressor Operating near Surge

The temporal behaviour of a flow separation in the hub-suction side comer of a transonic diffuser is studied thanks to unsteady numerical simulations based on the phase-lagged approach. The validity of the numerical re- sults is confn＇med by comparison with experimental unsteady pressure measurements. An analysis of the instan- taneous skin-friction pattern and particles trajectories is presented. It highlights the topology of the separation and its temporal behaviour. The major result is that, despite of a highly time-dependent core flow, the separation is found to be a ＂fixed unsteady separation＂ characterized by a fixed location of the main saddle of the separation but an extent of the stall region modulated by the pressure waves induced by the impeller-diffuser interaction.

Study of Rotating Stall in a Centrifugal Compressor with Wide Vaneless Diffuser

To study the stall mechanism in a wide vaneless diffuser,a three-dimensional centrifugal compressor model was established.The numerical simulation method was used to study the fluid characteristics in a centrifugal compressor with a wide vaneless diffuser under stall conditions.The simulated results showed that the wide vaneless diffuser stall was related to the instability of the core flow.Due to the influence of the reflow,the core flows moved between the hub and shroud side and the directions were not consistent.This was called core flow distortion,which triggered the rotating stall.The reflow first appeared at the outlet boundary of the diffuser,and the reflux gradually expanded to the inside of the diffuser as the impeller rotated.Eventually,the reflux zones grew and merged,forming a stable low-speed fluid regiment,which produced a stall cell.The stall cells rotated around themselves with an opposite direction with impeller.

Effect of Diffuser Vane Shape on the Performance of a Centrifugal Compressor Stage

The present paper reports the results of experimental investigations on the effect of diffuser vane shape on the performance of a centrifugal compressor stage. These studies were conducted on the chosen stage having a back- ward curved impeller of 500 turn tip diameter and 24.5 mm width and its design flow coefficient is Фd=0.0535. Three different low solidity diffuser vane shapes namely uncarnbered aerofoil, constant thickness flat plate and circular arc cambered constant thickness plate were chosen as the variants for diffuser vane shape and all the three shapes have the same thickness to chord ratio （t/c=0.1）. Flow coefficient, polytropic efficiency, total head coeffi- cient, power coefficient and static pressure recovery coefficient were chosen as the parameters for evaluating the effect of diffuser vane shape on the stage performance. The results show that there is reasonable improvement in stage efficiency and total head coefficient with the use of the chosen diffuser vane shapes as compared to conven- tional vaneless diffuser. It is also noticed that the aero foil shaped LSD has shown better performance when com- pared to flat plate and circular arc profiles. The aerofoil vane shape of the diffuser blade is seen to be tolerant over a considerable range of incidence.

The Influence of Wedge Diffuser Blade Number and Divergence Angle on the Performance of a High Pressure Ratio Centrifugal Compressor

Wedge diffuser is widely used in centrifugal compressors due to its high performance and compact size. This paper is aimed to research the influence of wedge diffuser blade number and divergence angle on centrifugal compressor performance. The impact of wedge diffuser blade number on compressor stage performance is investigated, and then the wedge diffusers with different divergence angle are studied by varying diffuser wedge angle and blade number simultaneously. It is found that wedge diffuser with 27 blades could have about 0.8% higher adiabatic efficiency and 0.14 higher total pressure ratio than the wedge diffuser with 19 blades and the best compressor performance is achieved when diffuser divergence angle is 8.3°.These results could give some advices on centrifugal compressor 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 interaction tone noise radiated from the compressor,because of the smaller interaction area between impeller-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.