Flow Behavior of a Centrifugal Compressor With Vaneless Diffuser at Design and Off Design Conditions

This paper concerns with the numerical and experimental aspects of both steady and unsteady flow behavior in a centrifugal compressor with vaneless diffuser and downstream collector. Specifically, the appearance of flow instabilities i.e., rotating stall and surge is investigated in great detail. As the first step, the static performance of both stage and component was analyzed and possible root cause of system surge was put forward based on the classic stability theory. Then the unsteady pressure data was utilized to find rotating stall and surge in frequency domain which could be classified as mild surge and deep surge. With the circumferentially installed transducers at impeller inlet, backward travelling waves during stall ramp could be observed. The modes of stall waves could be clearly identified which is caused by impeller leading edge flow recirculation at Mu = 0.96. However, for the unstable flow at Mu = 1.08, the system instability seems to be caused by reversal flow in vaneless diffuser where the pressure oscillation was strongest. Thus steady numerical simulation were performed and validated with the experimental performance data. With the help of numerical analysis, the conjectures are proved.

Download Full-text

Effects of Non-Axisymmetric Volute on Rotating Stall in the Vaneless Diffuser of a Centrifugal Compressor

Volume 2E: Turbomachinery ◽

10.1115/gt2020-14729 ◽

2020 ◽

Author(s):

Zitian Niu ◽

Zhenzhong Sun ◽

Baotong Wang ◽

Xinqian Zheng

Keyword(s):

Numerical Simulation ◽

Flow Field ◽

Centrifugal Compressor ◽

Stability Boundary ◽

Rotating Stall ◽

Evolution Process ◽

Vaneless Diffuser ◽

Unstable Flow ◽

Centrifugal Compressors ◽

Specific Location

Abstract Rotating stall is an important unstable flow phenomenon that leads to performance degradation and limits the stability boundary in centrifugal compressors. The volute is one of the sources to induce the non-axisymmetric flow in a centrifugal compressor, which has an important effect on the performance of compressors. However, the influence of volute on rotating stall is not clear. Therefore, the effects of volute on rotating stall by experimental and numerical simulation have been explored in this paper. It’s shown that one rotating stall cell generates in a specific location and disappears in another specific location of the vaneless diffuser as a result of the distorted flow field caused by the volute. Also, the cells cannot stably rotate in a whole circle. The frequency related to rotating stall captured in the experiment is 43.9% of the impeller passing frequency (IPF), while it is 44.7% of IPF captured by three-dimensional unsteady numerical simulation, which proves the accuracy of the numerical method in this study. The numerical simulation further reveals that the stall cell initialized in a specific location can be split into several cells during the evolution process. The reason for this is that the blockage in the vaneless diffuser induced by rotating stall is weakened by the mainstream from the impeller exit to make one initialized cell disperse into several ones. The volute has an important influence on the generation and evolution process of the rotating stall cells of compressors. By optimizing volute geometry to reduce the distortion of the flow field, it is expected that rotating stall can be weakened or suppressed, which is helpful to widen the operating range of centrifugal compressors.

Download Full-text

Investigation of the instability mechanisms in a turbocharger centrifugal compressor with a vaneless diffuser by means of unsteady simulations

Proceedings of the Institution of Mechanical Engineers Part D Journal of Automobile Engineering ◽

10.1177/0954407016677435 ◽

2016 ◽

Vol 231 (11) ◽

pp. 1558-1567 ◽

Cited By ~ 6

Author(s):

Xinqian Zheng ◽

Anxiong Liu ◽

Zhenzhong Sun

Keyword(s):

Flow Field ◽

Centrifugal Compressor ◽

High Speed ◽

Simulation Method ◽

Great Accuracy ◽

Rotating Stall ◽

Tip Clearance ◽

Vaneless Diffuser ◽

Unstable Flow ◽

Low Velocity

The stable-flow range of a compressor is predominantly limited by surge and stall. In this paper, an unsteady simulation method was employed to investigate the instability mechanisms of a high-speed turbocharger centrifugal compressor with a vaneless diffuser. In comparison with the variation in the pressure obtained by dynamic experiments on the same compressor, unsteady simulations show a great accuracy in representing the stall behaviour. The predicted frequency of the rotating stall is 22.5% of the rotor frequency, which agrees with to the value for the high-frequency short-term rotating stall obtained experimentally. By investigating the instability of the flow field, it is found that the unstable flow of the turbocharger compressor at high rotational speeds is caused by the tip clearance leakage flow and the ‘backflow vortices’ originating from the interaction of the incoming flow and the backflow in the tip region of the passages. The asymmetric volute helps to induce the occurrence of stall in certain impeller passages because it generates an asymmetric flow field. The high-pressure low-velocity area from the 180° circumferential position to the 270° circumferential position is dominant and strengthens the backflow at the trailing edge of the impeller, finally triggering the stall.

Download Full-text

Experimental Investigation of Rotating Instabilities in a Two-Stage Centrifugal Compressor With Vaneless Diffuser

10.1115/imece2001/fed-24938 ◽

2001 ◽

Author(s):

Vladimir V. Golubev

Keyword(s):

Experimental Investigation ◽

Centrifugal Compressor ◽

Pressure Fluctuations ◽

Rotating Stall ◽

Sensitive Area ◽

Flow Conditions ◽

Vaneless Diffuser ◽

Unstable Flow ◽

Two Stage ◽

Multi Stage

Abstract This paper examines unsteady response of an experimental two-stage centrifugal compressor with vaneless diffusers. A thorough investigation of pressure fluctuations along the compressor channel is carried out in order to examine the onset of unstable flow conditions. Rotating stall structures are localized, with special attention paid to identifying the most sensitive area of the multi-stage compressor channel which may serve as a precursor to unstable compressor operation. It is shown that both stages may develop rotating stall structures simultaneously, however the 2nd stage tends to destabilize first and reveals higher magnitudes of the unsteady response.

Download Full-text

Numerical Simulation of Surge in Turbocharger Centrifugal Compressor: Influences of Downstream Plenum

Volume 7: Turbomachinery, Parts A, B, and C ◽

10.1115/gt2011-45163 ◽

2011 ◽

Cited By ~ 1

Author(s):

Strong Guo ◽

Hua Chen ◽

Xiaocheng Zhu ◽

Zhaohui Du

Keyword(s):

Numerical Simulation ◽

Flow Field ◽

Mass Flow ◽

Centrifugal Compressor ◽

Stokes Equations ◽

Numerical Technique ◽

Pressure Ratio ◽

Rotating Stall ◽

Vaneless Diffuser ◽

Unstable Flow

Surge is an important instability seriously affecting compression systems. This paper presents a numerical simulation of surge flow phenomenon inside a turbocharger centrifugal compressor with a vaneless diffuser. The compressor was discharged into a plenum and the effect of the plenum on surge behavior of the compressor system was investigated. The entire geometry of the compressor, including the impeller, vaneless diffuser, volute housing and downstream plenum, were included in the simulation. Three-dimensional Reynolds averaged compressible Navier–Stokes equations were solved with the k–ε turbulence model using commercial software CFX and two different sizes of plenum were studied. A new plenum model is proposed which allows temporal variation of temperature inside the plenum. The numerical technique employed to set up CFD (computational fluid dynamics) with such an unstable flow system are described. The results show that when the plenum volume was nearly doubled, the dominating frequency of the system suddenly dropped from 72Hz to 23Hz. During the surge cycle, the compressor characteristic (pressure ratio ν mass flow curve) showed distinct differences. With the smaller plenum, the characteristic showed random traces with little global backflow at the compressor inlet, while with the larger plenum, clear surge cycles are displayed with strong global backflow at the inlet. The flow fields of the two systems are presented as functions of time and show distinct differences. In the case of the smaller plenum, the circumferential flow field inside the impeller is non uniform, showing influences of rotating stall, while in the case of the large plenum, the circumferential uniformity returns and the flow field behaves quasi-steadily during the surge cycle. With the larger plenum, the volute flow synchronises with the inlet mass flow oscillation in time and a completed vortex break down occurs at every volute cross section, but with the smaller plenum the synchronisation disappears and vortex break down only occurs partially at the centers of some volute cross sections.

Download Full-text

Flow responses alteration by geometrical effects of tubercles on plates under the maximal angle of attack

Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science ◽

10.1177/0954406220975434 ◽

2020 ◽

pp. 095440622097543

Author(s):

KS Mu ◽

ABH Kueh ◽

PN Shek ◽

MR Mohd Haniffah ◽

BC Tan

Keyword(s):

Flow Direction ◽

Flow Behavior ◽

Angle Of Attack ◽

Leading Edge ◽

Control Case ◽

Reattachment Length ◽

Flow Recirculation ◽

Pressure Profiles ◽

Maximal Angle ◽

Geometrical Effects

Plates with leading-edge tubercles experience beneficially more gradual aerodynamics stalling when entering the post-stall regime. Little is known, however, about the corresponding aquatic flow responses when these tubercles-furnished plates are subjected to the maximal angle of attack, with the flow direction perpendicular to their planar area. Hence, this study presents numerically, by means of the flow behavior solver ANSYS, the flow responses alteration in terms of the geometrical effects of tubercles on plates through changes in amplitudes (5 mm, 10 mm, 15 mm) and wavelengths (50 mm, 100 mm, 150 mm) under the maximal angle of attack in comparison to a control case, i.e., without tubercles. Additional to the commonly examined flow velocity and pressure, characteristics such as wake (area, reattachment length, flow recirculation intensity) and newly defined downstream vortical parameters (area, perimeter, and Feret diameters) for the vortex region have been proposed and assessed. It is found that the drag increases with the tubercle wavelength but corresponds inversely with the tubercle amplitude. By correlating with the best beneficial velocity and pressure profiles, it has been characterized that the optimally performing plate is the one that generates the greatest flow recirculation intensity, wake area, and reattachment length, corresponding to the capability to produce also the highest vortical area, perimeter, and major Feret diameter. Compared to the control case, all plates with tubercles alter beneficially these flow behaviors. In conclusion, plates with tubercles contribute favorably to the flow behaviors under the maximal angle of attack compared to the control case while the newly proposed downstream parameters could serve capably as alternatives in corroborating the flow physics description in future studies.

Download Full-text

Tip Leakage Flow Instability in a Centrifugal Compressor

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4049821 ◽

2021 ◽

Vol 143 (4) ◽

Author(s):

Teng Cao ◽

Tadashi Kanzaka ◽

Liping Xu ◽

Tobias Brandvik

Keyword(s):

Shear Layer ◽

Centrifugal Compressor ◽

Large Scale ◽

Flow Instability ◽

Leading Edge ◽

Main Stream ◽

Leakage Flow ◽

Tip Leakage Flow ◽

Unstable Flow ◽

Tip Leakage

Abstract In this paper, an unsteady tip leakage flow phenomenon is identified and investigated in a centrifugal compressor with a vaneless diffuser at near-stall conditions. This phenomenon is associated with the inception of a rotating instability in the compressor. The study is based on numerical simulations that are supported by experimental measurements. The study confirms that the unstable flow is governed by a Kelvin–Helmholtz type instability of the shear layer formed between the main-stream flow and the tip leakage flow. The shear layer instability induces large-scale vortex roll-up and forms vortex tubes, which propagate circumferentially, resulting in measured pressure fluctuations with short wavelength and high amplitude which rotate at about half of the blade speed. The 3D vortex tube is also found to interact with the main blade leading edge, causing the reduction of the blade loading identified in the experiment. The paper also reveals that the downstream volute imposes a once-per-rev circumferential nonuniform back pressure at the impeller exit, inducing circumferential loading variation at the impeller inducer, and causing circumferential variation in the unsteady tip leakage flow.

Download Full-text

Stall Inception in a High Speed Centrifugal Compressor During Speed Transients

Volume 2C: Turbomachinery ◽

10.1115/gt2017-64499 ◽

2017 ◽

Author(s):

Fangyuan Lou ◽

John C. Fabian ◽

Nicole L. Key

Keyword(s):

Heat Transfer ◽

Centrifugal Compressor ◽

High Speed ◽

Flow Instability ◽

Leading Edge ◽

Fast Response ◽

Rotating Stall ◽

Transient Performance ◽

Stall Inception ◽

Pressure Transducers

The inception and evolution of rotating stall in a high-speed centrifugal compressor are characterized during speed transients. Experiments were performed in the Single Stage Centrifugal Compressor (SSCC) facility at Purdue University and include speed transients from sub-idle to full speed at different throttle settings while collecting transient performance data. Results show a substantial difference in the compressor transient performance for accelerations versus decelerations. This difference is associated with the heat transfer between the flow and the hardware. The heat transfer from the hardware to the flow during the decelerations locates the compressor operating condition closer to the surge line and results in a significant reduction in surge margin during decelerations. Additionally, data were acquired from fast-response pressure transducers along the impeller shroud, in the vaneless space, and along the diffuser passages. Two different patterns of flow instabilities, including mild surge and short-length-scale rotating stall, are observed during the decelerations. The instability starts with a small pressure perturbation at the impeller leading edge and quickly develops into a single-lobe rotating stall burst. The stall cell propagates in the direction opposite of impeller rotation at approximately one third of the rotor speed. The rotating stall bursts are observed in both the impeller and diffuser, with the largest magnitudes near the diffuser throat. Furthermore, the flow instability develops into a continuous high frequency stall and remains in the fully developed stall condition.

Download Full-text

Rotating Stall in Centrifugal Compressor Vaneless Diffuser: Experimental Analysis of Geometrical Parameters Influence on Phenomenon Evolution

International Journal of Rotating Machinery ◽

10.1155/s1023621x04000430 ◽

2004 ◽

Vol 10 (6) ◽

pp. 433-442 ◽

Cited By ~ 22

Author(s):

Giovanni Ferrara ◽

Lorenzo Ferrari ◽

Leonardo Baldassarre

Keyword(s):

Centrifugal Compressor ◽

Dynamic Pressure ◽

Pressure Sensors ◽

Experimental Tests ◽

Fast Response ◽

Rotating Stall ◽

Geometrical Parameters ◽

Vaneless Diffuser ◽

Response Dynamic ◽

Frequency Domains

The rotating stall is a key problem for achieving a good working range of a centrifugal compressor and a detailed understanding of the phenomenon is very important to anticipate and avoid it. Many experimental tests have been planned by the authors to investigate the influence on stall behavior of different geometrical configurations. A stage with a backward channel upstream, a 2-D impeller with a vaneless diffuser and a constant cross-section volute downstream, constitute the basic configuration. Several diffuser types with different widths, pinch shapes, and diffusion ratios were tested. The stage was instrumented with many fast response dynamic pressure sensors so as to characterize inception and evolution of the rotating stall. This kind of analysis was carried out both in time and in frequency domains. The methodology used and the results on phenomenon evolution will be presented and discussed in this article.

Download Full-text