Elsevier

Journal of Sound and Vibration

Volume 412, 6 January 2018, Pages 270-286
Journal of Sound and Vibration

Investigation of turbocharger compressor surge inception by means of an acoustic two-port model

https://doi.org/10.1016/j.jsv.2017.10.003Get rights and content

Abstract

The use of centrifugal compressors have increased tremendously in the last decade being implemented in the modern IC engine design as a key component. However, an efficient implementation is restricted by the compression system surge phenomenon. The focus in the investigation of surge inception have mainly been on the aerodynamic field while neglecting the acoustic field. In the present work a new method based on the full acoustic 2-port model is proposed for investigation of centrifugal compressor stall and surge inception. Essentially, the compressor is acoustically decoupled from the compression system, hence enabling the determination of sound generation and the quantification of internal aero-acoustic coupling effects, both independently of the connected pipe system. These frequency dependent quantities are indicating if the compressor is prone to self-sustained oscillations in case of positive feedback when installed in a system. The method is demonstrated on experimentally determined 2-port data of an automotive turbocharger centrifugal compressor under a variety of realistic operating conditions.

Introduction

In order to comply with the European Commission Euro 6 emission standard [1], while maintaining the original power output, the concept of engine downsizing (or “rightsizing”) have been introduced and widely implemented by the passenger car industry (See e.g. Refs. [2], [3], [4], [5]). Essentially, the volumetric capacity of the IC engine have been decreased while increasing charge air density, thus leading to a significantly better fuel conversion efficiency with low penalty of NOx emissions. The key component in the concept is a turbocharger that increases the charge air density by means of recovered energy from the high enthalpy exhaust gas. Typically the turbocharger consists of a radial in-flow turbine, and of a centrifugal compressor that are integrated to the gas exchange system of the IC engine.

Substantial challenges related to downsized IC engines involve compressor noise, insufficient mass flow range of the compressor, and reduced transient response. With even more stringent Euro 6c emission standard entering to force in September 2018 [6], consequently requiring further downsizing, these challenges become even more relevant. A recent technology addressing these problems is referred to as 48 V e-boost [7]. Essentially, an additional electric motor driven centrifugal compressor will be added downstream [8] or upstream [9] of the turbocharger compressor. Although such configuration provides many benefits, the fundamental problem of insignificant mass flow range remains and limits the efficient use of centrifugal compressors.

The maximum mass flow of the centrifugal compressor is determined by a choked flow, while the minimum stable mass flow is bounded by the inception of surge phenomenon. When the back pressure of the coupled system becomes too high, the compressor cannot maintain continuous discharge pressure and a local flow reversal occur. This leads to global self-sustained large amplitude pressure and mass flow oscillations across the entire compression system, making surge a system phenomenon [10], [11], [12]. The main focus in the surge investigation has been on the inception process and a root cause of surge occurrence. However, as many different static and dynamic stall scenarios can occur and lead to different types of instable operation, the investigation has proved to be challenging.

In 1976 Greitzer proposed a lumped parameter model for investigation of axial compression system instability [13]. This model was validated on centrifugal compressors by Hansen in 1981 [14]. From this model, it is apparent, that the low mass flow instability in the compression system should occur at the Helmholtz frequency of the system. A global pressure and mass flow oscillations approximately corresponding to the Helmholtz frequency of the compression system have also been observed in a number of different experiments (See e.g. Refs. [15], [16], [17]). Moreover, the oscillations are appearing to be of progressive type respect to the mass flow reduction, and can grow to relatively large amplitudes eventually causing the blow-down of coupled high pressure discharge volume [15]. However, it has been also observed that the oscillations remain relatively small even in case of zero mass flow if the coupled system is sufficiently small [12], [15], thus potentially extending the useful operating range. Based on the previous observation, it has been suggested that the stabilization of the system could be caused by the higher damping within system [17], however, it has not been shown or investigated further. Moreover, as the compressor must be coupled to the system to control its operating conditions, i.e. discharge pressure, the occurrence of surge within the system prevents the separation of damping and excitation.

Flow field changes inside the compressor under reduced mass flow conditions have also been studied in relation to surge inception. If the mass flow is continuously reduced, the compressor casing fixed stalled flow pattern occurs first at the inducer (compressor impeller inlet) annulus [15], [18]. By further reducing the mass flow rotating stalled flow patterns are triggered [15], [18], [19]. In this case one or more stalled flow cells are propagating along the impeller annulus, and thus the resulting frequency of pressure oscillations is a fraction of impeller rotational frequency. A rotating stall can occur at the inducer, in the impeller passages, or at the diffuser with corresponding characteristic frequency ranges [19]. Although associated low frequency elevated noise levels are observed in a number of works [12], [15], [16], [17], [20], the acoustic field have been neglected in the surge inception investigation while the focus have been on the aerodynamic aspects.

In general, the elevated low frequency noise level at low mass flows is a well-recognized problem of the centrifugal compressors. However, an accurate determination of sound generation of the compressor is difficult as it has to be coupled to a system that enables the control of the discharge pressure (throttle valve or IC engine). Such peripheral systems have significant impact on the resulting sound field measured in the compressor inlet and outlet flow channels. Nevertheless, some measurement techniques with different level of simplifications have been proposed in the literature [17], [20], [21], [22], [23], [24]. Although, these methods can be effectively used in comparative measurements performed under different circumstances, they do not provide the “clean” source data necessary for surge inception investigation.

In 1991 Åbom proposed a measurement technique for the determination of passive properties of the acoustic 2-port flow channel element in the scattering matrix (S-matrix) form [25]. This technique have been successfully implemented in the investigation of passive acoustic performance of a wide variety of different flow channel elements, e.g. silencers [26], [27], [28], acoustic flow channel liners [29], vortex mixers [30], heat exchangers [31], turbochargers [32], [37] etc. This technique was extended by Lavrentjev et al. [33] to include the active part, i.e. the source term, thus enabling the measurement of system independent source data of ducted acoustic sources, e.g. axial fans [33], vortex mixers [30], T-junctions [34] etc. Moreover, the whistling phenomena caused by the positive feedback in the flow duct systems [34] can also be predicted by further quantifying the internal coupling effects between aerodynamic and acoustic fields via S-matrix analysis as shown by Aurégan and Starobinski [35].

The scope of the present work is to propose an acoustic method for centrifugal compressor surge investigation. This is achieved by implementing a full acoustic 2-port model that will acoustically de-couple the centrifugal compressor from the compression system. Thereby, enabling detailed characterization of the compressor properties by means of the “clean” or system independent (“reflection-free”) sound generation and the amplification or dissipation of incident sound waves.

Section snippets

Experimental setup

The experimental measurement campaign is carried out at the Competence Center for Gas Exchange (CCGEx) at KTH Royal Institute of Technology [36]. The unique test-rig for turbochargers acoustic characterization was first established in 2008 [32] and is being continuously developed since then. Photos of the current test-rig, used in the experimental investigation herein, is presented in Fig. 1.

The measurement equipment used in the investigation is listed in Table 1 and corresponding location in

Accurate determination of compressor generated sound

In general, the acoustic plane wave propagation in the compressor inlet and outlet channels can conveniently be described by the harmonic solution of the homogeneous linearized wave equation for one-dimensional waves:p(x)=pˆ+ei(ωtk+x)+pˆei(ωt+kx),wherek±=ωc0(1±M),and the x-axis is assumed to point out from the compressor inlet/outlet. The peripheral components of the compression system, e.g., a throttle valve or IC engine inlet valves, cannot be considered as reflection-free terminations (pˆ

Aero-acoustic coupling inside the compressor

The acoustic 2-port with flow can be divided into two different regions: an outer region relatively far upstream and downstream, where acoustic and aerodynamic fields are “fully developed and uncoupled”, and an internal region with interaction between aerodynamic and acoustic fields [35]. The instable flow, associated with flow separations and vorticity inside the compressor, is normally associated to the dissipation of sound but can also feed energy to an incident acoustic wave. This

Conclusion

In the present work, a new method for centrifugal compressor stall and surge investigations is proposed, and experimentally demonstrated on the high-speed centrifugal compressor of a commercial automotive turbocharger. The method is based on the full acoustic 2-port model and enables in-detail investigation of stall and surge by acoustically de-coupling the compressor from the compression system. First, it is shown how the compression system independent (“reflection-free”) sound generation of a

Acknowledgements

This work was accomplished at KTH CCGEx in Stockholm within the framework of the initial training project FlowAirS (http://www.flowairs.eu) funded under the European Commission 7th Framework Program (289352) within People work program (Marie Curie actions). The authors would also like to acknowledge the involvement of Swedish Energy Agency, Borg Warner Inc., Scania, Volvo GTT, Volvo Cars and Audi AG.

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