Compressible Modal Instability Onset in an Aerodynamically Mistuned Transonic Fan

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Christoph Brandstetter ◽  
Benoit Paoletti ◽  
Xavier Ottavy
Keyword(s):  
Pressure Fluctuations ◽  
Low Frequency ◽  
Stability Margin ◽  
Acoustic Resonance ◽  
Feedback Mechanism ◽  
Rotating Stall ◽  
Rotor Blades ◽  
The Stability ◽  
Transonic Fan ◽  
Aeromechanical Stability

This paper describes observed modal oscillations arising from a feedback mechanism between an acoustic resonance in the exit flow channel and aerodynamic and aeroelastic disturbances in a transonic fan stage. During tests, the fan suffered from rotating stall and surge which were preceded by low frequency pressure fluctuations. Through a range of aerodynamic and aeromechanical instrumentations, it was possible to determine a clear chain of cause and effect, whereby geometrical asymmetries trigger local instabilities and modal oscillations through an interaction with the system acoustics. To the authors knowledge, this is the first time that modal oscillations occurring before stall are attributed to multiphysical interactions, showing that acoustic characteristics of the system can influence the aerodynamic as well as the aeromechanical stability of fans. This bears implications for the stability assessment of fans and compressors because first, the stability margin may be affected by standing waves generated in bypass ducts or combustion chambers, and second, geometrical variations of the rotor blades which are believed to be beneficial for aeromechanical stability may lead to complex coupling phenomena.

scholarly journals A Novel Piecewise Frequency Control Strategy Based on Fractional-Order Filter for Coordinating Vibration Isolation and Positioning of Supporting System

Sensors ◽  
2020 ◽  
Vol 20 (18) ◽  
pp. 5307
Author(s):  
Yeying Tao ◽  
Wei Jiang ◽  
Bin Han ◽  
Xiaoqing Li ◽  
Ying Luo ◽  
...  
Keyword(s):  
Fractional Order ◽  
Control Strategy ◽  
Vibration Isolation ◽  
Frequency Control ◽  
Low Frequency ◽  
Stability Margin ◽  
System Stability ◽  
Supporting System ◽  
Frequency Position ◽  
The Stability

A piecewise frequency control (PFC) strategy is proposed in this paper for coordinating vibration isolation and positioning of supporting systems under complex disturbance conditions, such as direct and external disturbances. This control strategy is applied in an active-passive parallel supporting system, where relative positioning feedback for positioning and absolute velocity feedback for active vibration isolation. The analysis of vibration and deformation transmissibility shows that vibration control increases low-frequency position error while positioning control amplifies high-frequency vibration amplitude. To overcome this contradiction across the whole control bandwidth, a pair of Fractional-Order Filters (FOFs) is adopted in the PFC system, which increases the flexibility in the PFC design by introducing fraction orders. The system stability analysis indicates that the FOFs can provide a better stability margin than the Integral-Order Filters (IOFs), so the control gains are increased to get a better performance on the AVI and positioning. The PFC based on FOFs can suppress the peak amplitude at the natural frequency which cannot be avoided when using the IOFs. The constrained nonlinear multivariable function is formed by the required performance and the stability of the system, then the controller parameters are optimized effectively. Lastly, the effectiveness of the proposed method is verified by experiments.

Detection of Precursor Waves Announcing Stall in Two 3-Stage Axial Compressors

1998 ◽  
Author(s):  
F. Grauer ◽  
W. Volgmann ◽  
H. Stoff ◽  
T. Breuer
Keyword(s):  
Pressure Fluctuations ◽  
Safety Margin ◽  
Stability Limit ◽  
Rotating Stall ◽  
Time Dependent ◽  
Axial Compressors ◽  
Performance Map ◽  
Fourier Transform Methods ◽  
The Stability ◽  
Stall Control

Rotating stall and surge limit the operating range of a compressor towards low throughflow and high pressure in the performance map. Usually a safety margin must be observed to prevent the compressor from entering unintentionally aerodynamic instabilities. As the range of highest performance and efficiency lies in the vicinity of the stability limit, efforts concentrate on recognizing imminent onset of unstable operation prior to its occurrence. The present investigation centers on means of detecting information about onsetting instability from signals of pressure fluctuations in two transonic medium-pressure axial compressors of 3 stages. Fourier-transform-methods as well as artificial neural networks are applied for the data reduction of the time-dependent pressure signals. The methods of investigation presented here detected stall precursors announcing the onset of instability. Some of them seem appropriate to be used in connection with active stall control.

Inducer Stall in a Centrifugal Compressor With Inlet Distortion

1987 ◽  
Vol 109 (1) ◽  
pp. 27-35 ◽  
Author(s):  
I. Ariga ◽  
S. Masuda ◽  
A. Ookita
Keyword(s):  
Flow Rate ◽  
Centrifugal Compressor ◽  
Stability Margin ◽  
Rotating Stall ◽  
Characteristic Parameters ◽  
Inlet Distortion ◽  
Rate Region ◽  
Compressor Performance ◽  
The Stability ◽  
Lower Flow Rate

The effects of inlet distortion on the inducer stall in a centrifugal compressor are investigated. Cases of both radial and circumferential distortion are investigated. It is shown that the rotating stall onset is amplified by radial distortions, and restrained by circumferential distortions. These results are compared with calculations based on the small disturbance theory. The authors find that the stall onset is governed by the characteristic parameters related to the lower flow rate region for radial distortions, but affected by those of the higher flow rate region for circumferential distortion. It is shown that the process of stall is different for each distortion pattern. Existence of inlet distortion reduces compressor performance characteristics and strongly influences the stability margin.

Proposed Forcing Mechanisms and Non-Linear Effects on Subsynchronous Vibrations in High Performance Turbomachinery

1997 ◽  
Author(s):  
M. F. White ◽  
S. H. Chan
Keyword(s):  
High Speed ◽  
High Performance ◽  
Fluid Dynamic ◽  
Low Frequency ◽  
Rotating Stall ◽  
Rotor Blades ◽  
Stable Form ◽  
Flow Distortion ◽  
Non Linear ◽  
Forcing Mechanisms

This paper suggests that the subsynchronous “instability” observed in many high speed, high performance turbomachines while operating in the supercritical speed range may in some cases be a stable form of lightly damped vibrations. They could be excited by low frequency process forces due to unsteady flow conditions. The non-linearity in the mass, stiffness or damping of the system may have provided a coupling or frequency transformation between the excitation forces and the subsynchronous vibrations. Depending on the kind of non-linear characteristics the critical speeds as defined for a linear system may become regions of “instability”. The degree of non-linearity of the bearing-seal-rotor system has an influence on the sensitivity of the machine to subsynchronous vibrations. Some forcing mechanisms are presented, including non-identical rotor blades, inlet flow distortion and rotating stall. The effect on response of mode shape, internal shaft rotatory damping and frequency dependence of bearing damping at subsynchronous frequencies are discussed. It is recommended that the unsteady fluid dynamic forces, together with the effects of non-linear dynamic characteristics be further investigated to provide more experimental evidence for this hypothesis.

Stability Analysis of a Complex Geared Rotor-Bearing System in a Turbine Reduction Gearbox

2013 ◽  
Author(s):  
Yuanhong Guan ◽  
Edward W. Sieveking ◽  
Varad Sampathkumar
Keyword(s):  
Stability Analysis ◽  
High Speed ◽  
Low Frequency ◽  
Stability Margin ◽  
Journal Bearings ◽  
Root Cause ◽  
Rotor Bearing ◽  
Noise Vibration ◽  
The Stability ◽  
Load Conditions

It is well known that the rotor system will meet several critical speeds or unstable regions as its rotation speed increases, especially when the rotor system is supported by journal bearings, since there exists a strong fluid-structure coupling which is rather prone to stability issues. Stability analysis of rotor-bearing systems (such as turbine-compressor) has been extensively studied in the literatures over the past 50 years. However, few studies have been performed on geared rotor-bearing systems, especially for complex multi-stage gear train systems. In this paper, the abnormal noise/vibration problem on a high speed 2 stage epicyclic reduction gearbox of a turbine-generator system is studied. This gearbox showed abnormal low frequency vibrations at low speed cranking and high speed partial load conditions. Further detailed probe testing showed that the gear bodies which were supported by 6 journal bearings had quite large sub-synchronized vibrations and shaft whirls were developed when the abnormal noise was present. In order to better understand the root cause and to fully eliminate such low frequency noise/vibration, a detailed finite element model for the whole turbine-gearbox-generator was developed under different speed / load conditions. The linearized journal bearing stiffness and damping matrix were calculated using a separate tool and then plugged into the above FE model. The gears are modeled as rigid bodies and connected by gear mesh stiffness. Gyroscopic force terms have also been included in the model. The stability of the whole system was evaluated by a complex eigenvalue analysis and the stability margin evaluated by the corresponding damping factor (or log decrement). The model predicts a range of instability regions and has good correlation with testing data. The root cause of this abnormal noise/vibration is due to the strong torsional-lateral coupling of gear systems, and further coupling with the fluid dynamics of the journal bearings under certain speed/load conditions. Some sensitivity studies are also performed in order to increase the stability margin and eliminate the sub-synchronized vibrations.

Self-Excited Blade Vibration Experimentally Investigated in Transonic Compressors: Acoustic Resonance

2015 ◽  
Author(s):  
F. Holzinger ◽  
F. Wartzek ◽  
M. Nestle ◽  
H.-P. Schiffer ◽  
S. Leichtfuß
Keyword(s):  
Rotor Blade ◽  
Guide Vane ◽  
Separated Flow ◽  
Acoustic Resonance ◽  
Rotating Stall ◽  
Rotor Blades ◽  
Inlet Guide Vane ◽  
Blade Vibration ◽  
Excited Vibration ◽  
Experimental Findings

This paper investigates the acoustically induced rotor blade vibration that occurred in a state-of-the-art 1.5-stage transonic research compressor. The compressor was designed with the unconventional goal to encounter self-excited blade vibration within its regular operating domain. Despite the design target to have the rotor blades reach negative aerodamping in the near stall region for high speeds and open inlet guide vane, no vibration occurred in that area prior to the onset of rotating stall. Self-excited vibrations were finally initiated when the compressor was operated at part speed with fully open inlet guide vane along nominal and low operating line. The mechanism of the fluid-structure-interaction behind the self-excited vibration is identified by means of unsteady compressor instrumentation data. Experimental findings point towards an acoustic resonance originating from separated flow in the variable inlet guide vanes. A detailed investigation based on highly resolved wall pressure data confirms this conclusion. The paper documents the spread in aerodynamic damping calculated by various partners with their respective aeroelastic tools for a single geometry and speed line. This significant spread proves the need for calibration of aeroelastic tools to reliably predict blade vibration. The paper contains a concise categorization of flow induced blade vibration and defines criteria to quickly distinguish the different types of blade vibration. It further gives a detailed description of a novel test compressor and thoroughly investigates the encountered rotor blade vibration.

Unstable Flow Characteristics in S-Shaped Region of Pump-Turbine Runners With Large Blade Lean

2017 ◽  
Author(s):  
Zhe Ma ◽  
Baoshan Zhu ◽  
Cong Rao ◽  
Lei Tan
Keyword(s):  
Guide Vane ◽  
Pressure Fluctuations ◽  
Vortex Formation ◽  
Flow Characteristics ◽  
Low Frequency ◽  
Leading Edge ◽  
Rotating Stall ◽  
Scale Model ◽  
Unstable Flow ◽  
Discharge Point

As the reversible pump-turbines operate in the S-shaped region, instability problems including backflow, vortex formation and rotating stall may appear. Previous researches studied instabilities at different guide vane opening (GVO) on their inception and evolution but few studies explored the effect of the blade lean at the leading edge. In present work, two runners tested by experiments, the runner A with a negative and the runner B with a positive blade lean at leading edge, were studied in CFD mode with a reduced scale model. Six operating points, namely, best efficiency point (OP#1), two points in the normal operating region (OP#2, OP#3), two points near runaway line (OP#4, OP#5) and a low discharge point in turbine brake (OP#6) were calculated for both runners. As the discharge reduces, the flow in the runners loses its symmetry and the efficiency becomes lower and lower. The flow of OP#1, OP#2 and OP#3 is healthy but slight separations locate near the inlet of the passages. At OP#4, obvious vortexes occupy the passages and the visible vortexes prevent the flow from entering the channels. The blockage generates strong backflow near the inlet of the runner. Moreover, the main backflow area locates near the hub for runner A while for runner B it is near the shroud. Unsteady vortex formation and rotating stall respectively exist at the near runaway points (OP#4 and OP#5) and low discharge point (OP#6). At these three points, the pressure fluctuations in the vaneless gap between the runner and guide vanes are very high and the amplitude shows a small difference between the two runners. Dramatic distinction appears on the frequency of the fluctuation. For both of the two runners, a peak corresponding to 70% fn, where fn is the runner rotating frequency, rises in the spectra of OP#4 and OP#5. This peak appears at all the monitors in the vaneless space at the same time standing for the unsteady vortex formation, which does not rotate with the blades. In addition, at OP#6, 40% and 50% fn are detected as the dominant frequencies for runner A and runner B respectively. In addition, the propagation of such two low frequency signal along the annulus in the vaneless space proves the existence of the rotating stalls.

Experimental and Numerical Investigations on the Origins of Rotating Stall in a Propeller Turbine Runner Operating in No-Load Conditions

2018 ◽  
Vol 140 (11) ◽  
Author(s):  
Sébastien Houde ◽  
Guy Dumas ◽  
Claire Deschênes
Keyword(s):  
Pressure Fluctuations ◽  
Low Frequency ◽  
High Amplitude ◽  
Rotating Stall ◽  
Spinning Reserve ◽  
Electrical Grids ◽  
Large Pressure ◽  
Turbine Runner ◽  
Synchronous Rotation ◽  
Hydraulic Turbines

Hydraulic turbines are more frequently used for power regulation and thus spend more time providing spinning reserve for electrical grids. Spinning reserve requires the turbine to operate at its synchronous rotation speed, ready to be linked to the grid in what is termed the speed-no-load (SNL) condition. The turbine's runner flow in SNL is characterized by low discharge and high swirl leading to low-frequency high amplitude pressure fluctuations potentially leading to blade damage and more maintenance downtime. For low-head hydraulic turbines operating at SNL, the large pressure fluctuations in the runner are sometimes attributed to rotating stall. Using embedded pressure transducer measurements, mounted on runner blades of a model propeller turbine, and numerical flow simulations, this paper provides an insight into the inception mechanism associated with rotating stall in SNL conditions. The results offer evidence that the rotating stall is in fact associated with an unstable vorticity distribution not associated with the runner blades themselves.

Self-Excited Blade Vibration Experimentally Investigated in Transonic Compressors: Acoustic Resonance

2015 ◽  
Vol 138 (4) ◽  
Author(s):  
F. Holzinger ◽  
F. Wartzek ◽  
H.-P. Schiffer ◽  
S. Leichtfuss ◽  
M. Nestle
Keyword(s):  
Rotor Blade ◽  
Guide Vane ◽  
Separated Flow ◽  
Acoustic Resonance ◽  
Rotating Stall ◽  
Rotor Blades ◽  
Inlet Guide Vane ◽  
Blade Vibration ◽  
Excited Vibration ◽  
Experimental Findings

This paper investigates the acoustically induced rotor blade vibration that occurred in a state-of-the-art 1.5-stage transonic research compressor. The compressor was designed with the unconventional goal to encounter self-excited blade vibration within its regular operating domain. Despite the design target to have the rotor blades reach negative aerodamping in the near stall region for high speeds and open inlet guide vane, no vibration occurred in that area prior to the onset of rotating stall. Self-excited vibrations were finally initiated when the compressor was operated at part speed with fully open inlet guide vane along nominal and low operating line. The mechanism of the fluid–structure interaction behind the self-excited vibration is identified by means of unsteady compressor instrumentation data. Experimental findings point toward an acoustic resonance originating from separated flow in the variable inlet guide vanes (VIGV). A detailed investigation based on highly resolved wall-pressure data confirms this conclusion. This paper documents the spread in aerodynamic damping calculated by various partners with their respective aeroelastic tools for a single geometry and speed line. This significant spread proves the need for calibration of aeroelastic tools to reliably predict blade vibration. This paper contains a concise categorization of flow-induced blade vibration and defines criteria to quickly distinguish the different types of blade vibration. It further gives a detailed description of a novel test compressor and thoroughly investigates the encountered rotor blade vibration.

scholarly journals Effect of Stall Cells on Pressure Fluctuations Characteristics in a Centrifugal Pump

Symmetry ◽  
2019 ◽  
Vol 11 (9) ◽  
pp. 1116 ◽  
Author(s):  
Peijian Zhou ◽  
Jiacheng Dai ◽  
Chaoshou Yan ◽  
Shuihua Zheng ◽  
Changliang Ye ◽  
...  
Keyword(s):  
Centrifugal Pump ◽  
Flow Rate ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Rotating Stall ◽  
Design Flow ◽  
Eddy Simulation ◽  
Leading Role ◽  
Stall Cells ◽  
Design Flow Rate

Rotating stall is an unsteady flow phenomenon, which always leads to instability and efficiency degradation. In order to reveal pressure fluctuations in the impeller of centrifugal pump induced by stall cells, the flow structures in a volute-type centrifugal pump were calculated using Large Eddy Simulation (LES) method. The predicted results of the numerical model were compared with experimental flow-head curve. The simulation results were in good agreement with the experimental results. The stall phenomenon occurred when the flow rate dropped to 70% of design flow rate. Three stall cells located at the entrance of passages could be observed, which remained stationary relative to the rotating impeller. With the decrease of flow rate, the area occupied by stall cells gradually increased. The peak value of pressure fluctuation at 25% of design flow rate is obviously larger than that at 50% of design flow rate. For the unstalled or stalled passage, the impeller-volute interaction played a leading role in the pressure fluctuations of the impeller. For the stalled passage, the amplitude of the low frequency induced by stall cell is relatively insignificant.

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