Simulation of Radial Compressor Aeroacoustics Using CFD

This paper focuses on the application of CFD to the prediction of radial compressor aeroacoustics. It concentrates mainly on automotive turbocharger operations in the low mass-flow range where blade leading-edge and tip separation reduce the compressor performance and induce transient flow behaviour. Whereas the blade-passing is tonal and at high frequency, usually beyond the human hearing range, transience in the flow are turbulence-dominated, broad-band in nature, and in magnitude a significant source of aeroacoustics which appears well within the range of peak human hearing (1–5kHz). Other noise sources occur due to distortions in the flow upstream of the compressor face, and rotating stall. The simulation methodology enumerated here pays attention to all the above flow-induced aeroacoustics. Due consideration is given to turbulence modelling, to ensure that both the narrow-band and broad-band sources are directly resolved in the CFD. Appropriate discretisation practices are adopted, so as to capture both turbulent-convection and sound-propagation mechanisms. Pressure-wave non-reflective boundary conditions are applied to the computational boundaries to remove any artificial resonances in the domain. STAR-CCM+, the commercial CFD code used here, was previously benchmarked against experimental data for the same compressor under ideal installation conditions, then the compressor performance assessed under real installation conditions [1]. The main foci of the studies reported here are to exploit possible improvements in modelling of the device performance and efficiency curves using more detailed wall modelling, comparing low-y+ versus high-y+ wall resolution, and to explore the viability for transient CFD calculations to capture the noise sources in the compressor at the challenging low mass flow end of the performance characteristic.

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Delay of Unsteady Flow in a Radial Diffuser

Volume 2: Fora ◽

10.1115/fedsm2008-55342 ◽

2008 ◽

Author(s):

Saad A. Ahmed

Keyword(s):

Mass Flow ◽

Flow Rate ◽

Flow Instability ◽

Rotating Stall ◽

Operating Conditions ◽

Flow Coefficient ◽

Width Ratio ◽

Flow Rates ◽

Lower Flow ◽

Low Mass

The operation of centrifugal compressor systems is limited at low-mass flow rates by fluid flow instabilities leading to rotating stall or surge. These instabilities limit the flow range in which the compressor can operate. They also lower the performance and efficiency of the compressor. Experiments were conducted to investigate a model of radial vaneless diffuser at stall as well as stall-free operating conditions. The speed of the impeller was kept constant at 2000 RPM, while the mass flow rate was reduced gradually to scan the steady and unsteady operating conditions of the compressor. The flow rate through the compressor was gradually decreased until flow instability is initiated at the diffuser. The flow rate was further reduced to study the characteristics of rotating stall. These measurements were reported for diffuser diameter ratios, Do/Di, of 2.0 with diffuser width ratio, b/Di, of 0.055. At lower flow rates than the critical, the rotating stall pattern with one stall cell was dominant over the pattern with two cells. In addition, the instability in the diffuser was successfully delayed to a lower flow coefficient when rough surfaces were attached to one or both sides of the diffuser with the lowest values achieved by attaching the rough surface to the shroud. Results show that the roughness has no significant effect on stall cell characteristics.

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Validation of Numerical Simulation for Rotating Stall in a Transonic Fan

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

10.1115/gt2011-46109 ◽

2011 ◽

Author(s):

Minsuk Choi ◽

Nigel H. S. Smith ◽

Mehdi Vahdati

Keyword(s):

Mass Flow ◽

Transient Flow ◽

Rotating Stall ◽

Implicit Time ◽

Stall Inception ◽

Rotating Speed ◽

Design Speed ◽

Fan Speed ◽

Loading Parameter ◽

Transonic Fan

This paper addresses a comparison of numerical stall simulations with experimental data at 60% (subsonic) and 95% (supersonic) of the design speed in a modern transonic fan rig. The unsteady static pressures were obtained with high frequency Kulite transducers mounted on the casing upstream and downstream of the fan. The casing pressure variation was clearly visible in the measurements when a stall cell passed below the transducers. Numerical stall simulations were conducted using an implicit, time-accurate 3D compressible RANS solver. The comparisons between the experiment and simulation mainly cover performance curves and time-domain pressure traces of Kulites during rotating stall. At two different fan speeds, the stall characteristics such as the number and rotating speed of the stall cells were well-matched to the experimental values. The mass flow rate and the loading parameter under the fully-developed rotating stall also showed good agreement with the experiment. In both numerical and experimental results, a large stall cell was eventually formed after stall inception regardless of the fan speed. Based on the validation, the detailed flow has been evaluated to understand rotating stall in a transonic fan. In addition, it was found that the mass flow measurement using casing static pressure might be wrong during transient flow if the Kulites were mounted too close to the fan blade.

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Control of Unstable Flow Using Rough Surfaces

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.431.155 ◽

2013 ◽

Vol 431 ◽

pp. 155-160

Author(s):

Saad Ahmed

Keyword(s):

Mass Flow ◽

Rough Surfaces ◽

Flow Characteristics ◽

Rotating Stall ◽

Operating Conditions ◽

Flow Coefficient ◽

Width Ratio ◽

Unstable Flow ◽

Flow Rates ◽

Low Mass

The function of centrifugal blowers/compressors is limited at low-mass flow rates by fluid flow instabilities leading to rotating stall. These instabilities limit the flow range in which they can operate. An experimental investigation was conducted to investigate a model of radial vaneless diffuser at stall as well as stall-free operating conditions. The speed of the blower was kept constant at 2000 RPM, while the mass flow rate was reduced gradually to investigate the steady and unsteady flow characteristics of the diffuser. These measurements were reported for diffuser diameter ratios, Do / Di, of 1.5, 1.75 and 2.0 with diffuser width ratio, b / Di, of 0.055. The rotating stall pattern with one stall cell was dominant over the pattern with two cells which appeared at flow rates lower than the critical. In addition, the instability in the diffuser was delayed to a lower flow coefficient when rough surfaces were attached to one or both walls of the diffuser with the lowest values achieved by attaching the rough surface to the shroud wall. Results show that the roughness has no significant effect on stall cell frequencies.

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Validation of Numerical Simulation for Rotating Stall in a Transonic Fan

Journal of Turbomachinery ◽

10.1115/1.4006641 ◽

2012 ◽

Vol 135 (2) ◽

Cited By ~ 27

Author(s):

Minsuk Choi ◽

Nigel H. S. Smith ◽

Mehdi Vahdati

Keyword(s):

Mass Flow ◽

Transient Flow ◽

Rotating Stall ◽

Navier Stokes ◽

Implicit Time ◽

Stall Inception ◽

Rotating Speed ◽

Design Speed ◽

Loading Parameter ◽

Transonic Fan

This paper addresses a comparison of numerical stall simulations with experimental data at 60% (subsonic) and 95% (supersonic) of the design speed in a modern transonic fan rig. The unsteady static pressures were obtained with high frequency Kulite transducers mounted on the casing upstream and downstream of the fan. The casing pressure variation was clearly visible in the measurements when a stall cell passed below the transducers. Numerical stall simulations were conducted using an implicit, time-accurate, 3D compressible Reynolds-averaged Navier-Stokes (RANS) solver. The comparisons between the experiment and simulation mainly cover performance curves and time-domain pressure traces of Kulites during rotating stall. At two different fan speeds, the stall characteristics such as the number and rotating speed of the stall cells were well-matched to the experimental values. The mass flow rate and the loading parameter under the fully-developed rotating stall also showed good agreement with the experiment. In both the numerical and experimental results, a large stall cell was eventually formed after stall inception regardless of the fan speed. Based on the validation, the detailed flow has been evaluated to understand rotating stall in a transonic fan. In addition, it was found that the mass flow measurement using casing static pressure might be wrong during transient flow if the Kulites were mounted too close to the fan blade.

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Incipient Surge Analysis in Time and Frequency Domain for Centrifugal Compressors

10.1115/gt2021-59501 ◽

2021 ◽

Author(s):

Paolo Silvestri ◽

Silvia Marelli ◽

Massimo Capobianco

Keyword(s):

Correlation Function ◽

Mass Flow Rate ◽

Mass Flow ◽

Flow Rate ◽

Cross Correlation ◽

Rotating Stall ◽

Cross Correlation Function ◽

Rate Condition ◽

Low Mass ◽

Processing Techniques

Abstract The use of centrifugal compressors has been increasing tremendously in the last decade as they are a key component in the present energy scenario both in the modern internal combustion engine design and in advanced cycles and innovative plant layouts as fuel cell systems. Instability phenomena limit the operating range of the whole compressor system, especially during fast transients. The target is therefore to extend the minimum flow limit in order to improve the operability of each unit, while avoiding compressor surge operation and guaranteeing safe operation. For this reason, it is necessary to develop a monitoring system capable of preventing surge and extending operating range of these machines, their performance, and reliability to allow the integration with the other plant components. The experimental investigation, carried out at the University of Genoa turbocharger test facility and presented in this work, consists of steady state and transient measurements used to characterize and identify compressor behaviour in correspondence of surge inception conditions to determine different techniques which could represent surge precursors. The data analysis concentrates on pressure and vibro-acoustic signals by applying different signal processing techniques in time and frequency domain to classify compressor operation as stable or unstable. The cross correlation function and wavelet analysis have been identified as techniques to define a precursor able to detect incipient surge conditions. Through cross correlation function analysis, it has been possible to identify the presence of propagation phenomena in the system and to evaluate how these events become more significant near an unstable low-mass flow rate condition. At low mass flow rate condition, spikes of significant amplitude are well detectable in the cross correlation function indicating the rise of significant random content in the system responses associated to the rise of incipient surge condition. Additionally, the continuous wavelet transform has been applied to operational signals to show how their time-dependent spectral structure responses can highlight the rise of unstable phenomena, not easily identifiable with traditional signal processing techniques. Exploiting its features in terms of good frequency and time resolution it allowed to identify different contents in system responses regarding phenomena which take place close to surge line and was able to detect their nature in conditions very close to deep surge ones (e.g. rotating stall with its intermitting characteristic nature). Moreover, system response was studied in high frequency range and through a demodulation technique it was found how blade pass frequency energy content change interacting with rotating stall inception, moving close to surge line. The obtained results provide an interesting diagnostic and predictive solution to detect compressor instabilities at low mass flow rate operating conditions and to prevent compressor fails.

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A Dangerous Blade Vibration Phenomenon due to Unsteady Flow in Centrifugal Compressors

Volume 3B: General ◽

10.1115/93-gt-265 ◽

1993 ◽

Author(s):

D. Jin ◽

H. Hasemann ◽

U. Haupt ◽

M. Rautenberg

Keyword(s):

Unsteady Flow ◽

Mass Flow ◽

Reverse Flow ◽

Broad Band ◽

Rotating Stall ◽

High Mass ◽

Resonance Excitation ◽

Blade Vibration ◽

Unsteady Pressure ◽

Vane Diffuser

Dangerous blade excitation caused by unsteady flow in a high pressure/high mass flow compressor running in a low mass flow region has been investigated. Experiments were carried out for compressors with two different types of vaned diffusers. Blade vibration was measured with strain gages while simultaneous unsteady pressure was measured with fast response dynamic transducers. All measured results were analysed in detail so that an in-depth understanding of blade excitation mechanism can be obtained. Firstly, the compressor with a straight-channel vane diffuser at reduced rotational speed of 12,300 rpm in an unstable operation region was considered. The analysis of blade vibration and unsteady pressure showed an unusual excitation phenomenon. Besides a strong blade vibration frequency component near the first blade mode frequency excited by the rotating stall cells existed another dangerous resonance excitation with first blade mode component which dominated the blade vibration spectrum. A detailed pressure signal analysis indicated that this blade vibration was excited by a broad band pressure fluctuation due to a strong reverse flow occurring simultaneously with the rotating stall. Further reducing the compressor mass flow to the operation point shortly before surge, the rotating stall was significantly weakened while the reverse flow kept its intensity until surge occurred. In this operation region blades suffered throughout a violent excitation of resonance because of the strong reverse flow. These blade excitation phenomena were also found in the next experiment for the compressor with a cambered vane diffuser at higher rotational speeds of nred = 13,500 and 14,000 rpm. The maximum strain values of blade vibration were obtained to quantitavely estimate the danger of blade vibration caused by this excitation.

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Numerical Investigation of Diffuser Rotating Stall and System Instability in a Centrifugal Compressor With Vaned Diffuser

Volume 2A: Turbomachinery ◽

10.1115/gt2018-76149 ◽

2018 ◽

Author(s):

Yang Zhao ◽

Guang Xi ◽

Jiayi Zhao

Keyword(s):

Mass Flow Rate ◽

Mass Flow ◽

Flow Rate ◽

Centrifugal Compressor ◽

Flow Instability ◽

Leading Edge ◽

Rotating Stall ◽

Vaned Diffuser ◽

Corner Separation ◽

Lumped Parameter

The operating range of a centrifugal compressor is often limited by the occurrence of the flow instability, such as diffuser rotating stall or system surge. In the paper, the unsteady numerical simulations are performed on a low-speed centrifugal compressor to investigate the characteristic of the rotating stall in the vaned diffuser. And also, the developed model of lumped parameter is used to predict the system instability. The flow field in the diffuser is firstly investigated at near stall condition. It is found that the leading-edge vortex and the secondary flow induce the hub-corner separation at the suction side of the diffuser blade. When the mass flow rate is reduced gradually, the fore part of the volute turns to act as a diffuser from a nozzle. Under the influence of the asymmetry induced by the volute, the hub-corner separation firstly develops into rotating stall in the passage with the lowest mass flow rate when at critical stall point. And then the diffuser rotating stall propagates along the circumferential direction at about 7% of the impeller speed. And also, the model of lumped parameter considering the effect of rotating stall is developed to analyze the system instability of mild surge. The predicted vibration frequency is within 5.8% of the measurement and the predicted transient process in mild surge matches well with the measurement. With different volume of the compressed air, the transient compressor characteristic tends to be stabilized or oscillates in a cycle along the counter-clockwise with different magnitude.

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Phenomenon and Mechanism of Two-Regime-Surge in a Centrifugal Compressor

Journal of Turbomachinery ◽

10.1115/1.4029547 ◽

2015 ◽

Vol 137 (8) ◽

Cited By ~ 23

Author(s):

Xinqian Zheng ◽

Anxiong Liu

Keyword(s):

Mass Flow Rate ◽

Mass Flow ◽

Flow Rate ◽

Leading Edge ◽

Rotating Stall ◽

Transient Pressure ◽

High Rotational Speed ◽

Casing Treatment ◽

Early Occurrence ◽

Stable Flow

“Two-regime-surge” is a special instability behavior of compressors, which was investigated in this paper. When the compressor operates at medium rotor speed, mild surge happens first, where the transient pressure signals show sinusoidal form with Helmholtz frequency of the compressor system. Reducing the mass flow rate, the mild surge vanishes and gets replaced by the local stall. Further reducing the mass flow rate, deep surge breaks out suddenly. During two-regime-surge, two distinct surge patterns exit and vastly narrow stable flow range, which highlights the characteristics of two-regime-surge. It is found that the impeller leading-edge stall is a necessary part of the mild surge, while the diffuser rotating stall incepts the deep surge. At higher speeds, the mild surge oscillation prompts the early occurrence of the diffuser stall so that the mild surge transforms and the deep surge happens in advance. As a result, both regimes of mild surge and deep surge are going to merge, and the stable flow range at high rotational speed is greatly narrowed. Impeller casing treatment is considered as an effective method for flow range extension because the impeller leading-edge stall is removed and the mild surge is avoided as well.

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Bleed Slot Benefits on Turbocharger Centrifugal Compressor Stability

Volume 2D: Turbomachinery ◽

10.1115/gt2014-27185 ◽

2014 ◽

Cited By ~ 2

Author(s):

Matthieu Gancedo ◽

Erwann Guillou ◽

Ephraim Gutmark

Keyword(s):

Mass Flow ◽

Centrifugal Compressor ◽

High Speed ◽

Broad Band ◽

Flow Rates ◽

Mass Flow Rates ◽

Low Mass ◽

Response Pressure ◽

Compressor Map ◽

Speed Response

Bleed slots located in the inducer region of centrifugal compressors have been demonstrated to extend the surge margin with minimal negative impact on performance. This paper describes the investigation of the effect of a bleed slot on map width enhancement of a turbocharger centrifugal compressor used for heavy duty diesel engine application. The goal is to evaluate the overall pressure instabilities on the compressor map and to study the dynamic phenomena occurring at low mass flow rates to better understand the benefits of the bleed slot on the compressor stability and surge line. The obstruction of the bleed slot permitted to compare the compressor behavior with the recirculation feature and without it. The pressure instability levels were measured along the accessible compressor map for the two cases using a high speed response pressure transducer at the compressor outlet. In addition, static and dynamic pressure measurements were conducted within the diffuser using respectively pressure taps and high speed response pressure transducers. The compressor with open bleed slot proved to have lower instability levels at low mass flow rates when not experiencing deep surge. Frequency responses at low mass flow rates showed that the implementation of the bleed slot suppresses broad band frequencies below the rpm frequency, which improves the overall stability. These frequencies are associated with rotating instabilities (RIs) with changing propagation speed depending on the rotational speed. At lower speeds, RIs are propagated with the wheel rotation whereas at higher speeds, they tend to propagate with the speed associated with their characteristic frequency.

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