Compressor Surge Research Articles

Active Control of Surge in Multi-Stage Axial-Flow Compressors

This paper deals with a theoretical study of the active control of purely axial 1D instabilities in multi-stage axial-flow compressors. The paper briefly considers the development of a suitable surge model and continues with the derivation of a controller using linear optimal control theory. The controller is specifically designed to suppress the instabilities predicted by the linearised form of the surge model. The control technique involves a bleed being dynamically varied in response to fluctuations of variables. It is found that a stabilising optimal controller can always be obtained. The second part of the paper presents a non-linear simulation of the surge prediction model with the linear controller, using a 4th order Runge-Kutta scheme. This demonstrates how some non-linearities in the flow model can affect the controller operation. It is also shown that, the amplitudes and the frequency response required of the actuator to retain stability can exceed the practical limitations.Copyright © 1993 by ASME

Active Control of Surge in Multi-Stage Axial-Flow Compressors

Non-linear simulations of a surge model with and without control are investigated using a Runge-Kutta scheme. This shows how some non-linearities in the flow model can affect the normal operation of a linear controller and the problem of actuatop sensor locations. Also, the amplitudes and frequency response required of the actuator to retain stability can sometimes be found to exceed the practical limitations.

Application du contrôle actif aux instabilités aérodynamiques

We describe experimental investigations in which adaptive methods are applied to various structures of active control of flow instabilities: flow excited cavity oscillations, compressor surge, and bursting phenomena in laminar boundary layer. The main results are as follows: the adaptive controller manages to automatically find its optimal characteristics in order to cancel the instability in the three experiments under consideration. In the boundary layer case, despite the non linear behaviour of the bursting process, a periodic train of artificially generated burst is significantly reduced downstream the cancelling source

Developments in Centrifugal Compressor Surge Control—A Technology Assessment

There are a number of surge control schemes in current use for centrifugal compressors employed in natural gas transmission systems. Basically, these schemes consist of a set of detection devices that either anticipate surge or detect it at its inception, and a set of control devices that act to prevent surge from occurring. A patent search was conducted in an attempt to assess the level and direction of technology development over the last 20 years and to define the focus for future R&D activities. In addition, the paper presents the current state of technology in three areas: surge control, surge detection, and surge suppression. Patent data obtained from on-line databases showed that most of the emphasis has been on surge control rather than on detection and control and that the current trend in surge control will likely continue toward incremental improvement of a basic or conventional surge control strategy. Various surge suppression techniques can be grouped in two categories: (i) those that are focused on better compressor interior design, and (ii) others that attempt to suppress surge by external and operational means.

Experimental and Numerical Analysis of Active Suppression of Centrifugal Compressor Surge by Suction-Side Valve Control.

We conducted a series of experiments in order to investigate the feasibility of active surge suppression in life-size industrial machines. The diameter of the impeller used in these experiments was 250 mm. These experiments were performed with closed-loop and open-loop compressor systems with a control valve on the suction pipe, both of which differed from Epstein's model. Active surge suppression was possible in both compressor systems. The action of the control valve was observed to suppress flow fluctuations from the dynamically unstable equilibrium point at every instant. We also numerically analysed the open-loop case. The action of the control valve was modeled by on-off throttle control and the compressor system was analyzed numerically using a lumped parameter model

Active Stabilization of Compressor Instability and Surge in a Working Engine

We have applied feedback control to the compressor of a 45 kW auxiliary power unit. Using this, the engine is able to deliver more than 10 percent extra shaft power before surge occurs. We achieve control by suppressing a nonaxisymmetric flow phenomenon in the diffuser of the centrifugal compressor. Control is effected by modulating a small extra airflow into the impeller. We present results of modeling and analysis suggesting that linear feedback control will be sufficient to stabilize a compression system, even when both axisymmetric surge and rotating stall are present as possible instabilities.

Evaluation of Approaches to Active Compressor Surge Stabilization

Recent work has shown that compression systems can be actively stabilized against the instability known as surge, thereby realizing a significant gain in system mass flow range. Ideally, this surge stabilization requires only a single sensor and a single actuator connected by a suitable control law. Almost all research to date has been aimed at proof of concept studies of this technique, using various actuators and sensor combinations. In contrast, the work reported here can be regarded as a step toward developing active control into a practical technique. In this context, the paper presents the first systematic definition of the influence of sensor and actuator selection on increasing the range of stabilized compressor performance. The results show that proper choice of sensor as well as actuator crucially affects the ability to stabilize these systems, and that, overall, those actuators most closely coupled to the compressor (as opposed to the plenum or throttle) appear most effective. In addition, the source of the disturbances driving the system (for example, unsteady compressor pressure rise or unsteady combustor heat release) has a strong influence on control effectiveness, as would be expected for a controls problem of this type. This paper both delineates general methodologies for the evaluation of active compressor stabilization strategies and quantifies the performance of several approaches that might be implemented in gas turbine engines.

Adaptive Active Control of Instabilities

This article reports the results of an experimental investigation in which adaptive methods are applied to various situations of active control of flow instabilities: combustion instabilities, flow-excited cavity oscillations, and compressor surge. The pres ent work draws attention especially to the adaptive controller behaviour and capacities. The main results are as follows: 1) The adaptive controller manages to automatically find its optimal characteristics in order to cancel the instability in the three experiments under consideration; 2) It is able to cope with time-dependent changes in the operating condi tions such as flow velocity sweeps for the oscillating cavity and air and propane mass flow rate variations in the combustion instability experiment; and 3) In the case of the Helmholtz cavity resonator, the algorithm has also exhibited capacity to recover effective control even when initially set such that artificial closed-loop instabilities induced by the control system occurred.

Dynamic Control of Centrifugal Compressor Surge Using Tailored Structures

A new method for dynamic control of centrifugal compressor surge is presented. The approach taken is to suppress surge by modifying the compression system dynamic behavior using structural feedback. More specifically, one wall of a downstream volume, or plenum, is constructed so as to move in response to small perturbations in pressure. This structural motion provides a means for absorbing the unsteady energy perturbations produced by the compressor, thus extending the stable operating range of the compression system. In the paper, a lumped parameter analysis is carried out to define the coupled aerodynamic and structural system behavior and the potential for stabilization. First-of-a-kind experiments are then conducted to examine the conclusions of the analysis. As predicted by the model and demonstrated by experiment, a movable plenum wall lowered the mass flow at which surge occurred in a centrifugal compression system by roughly 25 percent for a range of operating conditions. In addition, because the tailored dynamics of the structure acts to suppress instabilities in their initial stages, this control was achievable with relatively little power being dissipated by the movable wall system, and with no noticeable decrease in steady-state performance. Although designed on the basis of linear system considerations, the structural control is shown to be capable of suppressing existing large-amplitude limit cycle surge oscillations.

Active Stabilization of Centrifugal Compressor Surge

Active suppression of centrifugal compressor surge has been demonstrated on a centrifugal compressor equipped with a servo-actuated plenum exit throttle controller. The control scheme is fundamentally different from conventional surge control techniques in that it addresses directly the dynamic behavior of the compression system to displace the surge line to lower mass flows. The method used is to feed back perturbations in plenum pressure rise, in real time, to a fast-acting control valve. The increased aerodynamic damping of incipient oscillations due to the resulting valve motion allows stable operation past the normal surge line. For the compressor used, a 25 percent reduction in the surge point mass flow was achieved over a range of speeds and pressure ratios. Time-resolved measurements during controlled operation revealed that the throttle required relatively little power to suppress the surge oscillations, because the disturbances are attacked in their initial stages. Although designed for operation with small disturbances, the controller was also able to eliminate existing, large-amplitude, surge oscillations. Comparison of experimental results with theoretical predictions showed that a lumped parameter model appeared adequate to represent the behavior of the compression system with the throttle controller and, perhaps more importantly, to be used in the design of more sophisticated control strategies.

Lumped-parameter representation of fluid-system transients

Flow and pressure transients in closed pipes are described by non-linear hyperbolic partial differential equations. In many control applications a discrete-space lumped-parameter ordinary differential equation model is required. With such representations, care is needed to ensure that they retain essential physical characteristics of the phenomena described. Intentionally or otherwise, most of these fluid-system models are related to the numerical discretisation technique known as the Method of Lines. Aspects of implementing this are investigated, including the form of the basic equations and variables, representation of pipe friction, choice of interpolating polynomials in space and method of integration in time. These are compared with the standard Method of Characteristics solution for the original partial differential equations to eliminate any potential numerical instabilities, parasitic transient solutions or inconsistencies between steady and transient states which might influence adversely system control or stability studies. The Method of Lines is most appropriate for parabolic problems, and many existing applications to fluid transients are found to be satisfactory only where changes are gradual and continuous. The preferred formulation is free from this restriction, and applied to a stability investigation (prediction of compressor surging) is shown to perform reliably.

Dynamic Simulation of Compressor Station Operation Including Centrifugal Compressor and Gas Turbine

Dynamic simulation of the operation of a compressor station requires mathematical modeling of the dynamic behavior of the compressor unit and various piping elements. Such models consist of large systems of nonlinear partial differential equations describing the pipe flow together with nonlinear algebraic equations describing the quasi-steady flow through various valves, constrictions, and compressors. In addition, the models also include mathematical descriptions of the control system, which consists of mixed algebraic and ordinary differential (mad) equations with some inequalities representing controllers’ limits. In this paper a numerical technique for the solution of the gas dynamics equations is described, based on the transfer matrix formulation relating the state vector time difference at one side of an element to that on the other side. This approach facilitates incorporation of all element transfer matrices into an overall transfer matrix according to the system geometric connectivity. The paper also presents simulation results and comparison with actual field measurements of three case histories: (1) simulation of a compressor surge protection control process; (2) unit startup; and (3) slow transient of a compressor station responding to changes in the discharge pressure set point. Good agreement between simulation results and field measurements is demonstrated.

High-Speed Compressor Surge With Application to Active Control

This paper discusses the mechanics of surge as observed on the high-speed axial compressors of modern aero-engines. It argues that the initial stage of the instability consists of a high-amplitude blast wave that develops nonlinearly from a small-scale disturbance and is thus not correctly described by traditional small perturbation stability theories. It follows from this that active control schemes of the global type may be inappropriate, since to be effective, control would have to be applied in a short time and in a very detailed manner, requiring a large number of transducers and actuators. Active control may, though, be effective in controlling the disturbances that grow into the above blast wave and in the control of other phenomena such as rotating stall, given an adequate number of transducers.

Surge Behavior of Gas Turbines.

Using the lumped parameter Greitzer model, most compressor surge behaviors (for classic or deep surge) can be estimated. However this model is too simple, since, in general, the rotor flow passage length is longer for high-pressure compressors than that for low-pressure ones, and modern high-pressure compressors have a strong axial pressure gradient distribution in the flow passage. Dealing with the above, this paper is aimed towards suggesting a multielement model to which is applied Greitzer model for each element, and explaining the relations between the surge behavior and compressor flow field time constant, by comparing the results of Greitzer model and the experiments of the aircraft gas turbine with a relatively high-pressure compressor.

Active stabilization of compressor surge

This paper describes the stabilization of compressor surge by an active method. It is known that surge follows when small disturbances grow in an unstable compression system, and that small growth can be modelled through a linear stability analysis. An active element is here introduced to counter any tendency to instability and the control law governing the active stabilizer is determined from linear theory. We follow precisely the suggestion put forward by Epstein et al. (1986) and verify that their theory conforms to practice. The theory is verified in an experiment on a compression system whose plenum volume is controlled. Suppression of the flow instability was achieved by switching on the controller and the compressor was made to operate stably on a part of its characteristic beyond the nature stall line. Furthermore the controlled compressor is much more resilient to external disturbances than is the natural case. The controller is even effective on deep surge – a feature of great interest but hardly predictable from the Epstein et al. initiative for this kind of study.

Bidirectional tapered roller thrust bearing for gas turbine engines

The paper discusses a single row, bidirectional tapered roller thrust bearing, which was developed to take advantage of increased life potential and to obtain improved performance by eliminating the thrust balance mechanism in advanced gas turbine engines. This bearing will take the full range of forward thrust without thrust balance and, in addition, will take thrust in the reverse direction caused by compressor surge. The higher load capacity of a tapered roller bearing offers improved reliability over a ball bearing. To verify this concept, a tapered roller bearing was designed, manufactured, and tested at speeds up to 24,000 rpm (2.64 million DN), at loads up to 66.7 kN (15,000 Ibf) in the major thrust direction, and for reverse thrust up to 8.9 kN (2000 Ibf) at 20,000 rpm. Bearing reliability was tested up to 10 times the calculated L10 fatigue. The bearing performed successfully in all phases of testing and proved the concept of reverse thrust load on a high-speed tapered roller bearing. The feasibility of a tapered roller bearing on a gas turbine engine mainshaft was demonstrated both for high speed as well as heavy load.

Determination of compressor in-stall characteristics from engine surge transients

A technique for extracting the in-stall pumping characteristics for an axial flow compressor operating in an engine system environment is developed. The technique utilzes a Hybrid computer simulation of the compressor momentum equation in which actual transient data are used to provide all terms but the desired compressor characteristic. The compressor force characteristic as a function of corrected flow and speed results from the computation. The critical problem of data filtering is addressed. Results for a compressor operating in a turbofan engine are presented and comparison is made with the conventional compressor map. The relationship of the compressor surge charcteristic with its rotating stall characteristic is explored. Initial interpretation of the measured results is presented. Previously announced in STAR as N84-22566

Experimental Evaluation of Heavy Fan-High-Pressure Compressor Interaction in a Three-Shaft Engine: Part II—Analysis of Distortion and Fan Loading

Analysis of a severe rotating-stall phenomenon encountered in the fan core section of a three-shaft low bypass engine revealed interesting insight into the growth and circumferential extension of the stall cells immediately before surge of the high-pressure compressor was induced. The effect of a rotating distortion in contrast to the usually encountered stationary distortion is highlighted. The rotating distortion changes the time and hence the effectively felt distortion sector angle depending on a co- or contra-rotational arrangement of the distorting and the distorted compressor. The analysis shows that a contra-rotational arrangement provides a more stable and tolerant situation against that type of distortion. Analysis of the fan loading shows the static temperature rise coefficient to be the most descriptive for the fan rotor 1 hub section which initiated the rotating-stall mechanism.

Experimental Evaluation of Heavy Fan-High-Pressure Compressor Interaction in a Three-Shaft Engine: Part 1—Experimental Setup and Results

Severe aerodynamic interaction between the fan core stream section and the high-pressure compressor of a three-shaft low-bypass-ratio engine is described. At high fan running lines a heavy single-cell rotating stall was found in the fan core stream even at high aerodynamic speeds between 90–98%. The rotating circumferential distortion with 180–200 deg sector angle is swallowed by the intermediate pressure compressor but erodes the high-pressure compressor surge margin by about 22%, leading to steady-state surges. A remotely mounted transducer in a specific arrangement was used successfully for measurements in the hot environment behind intermediate and high-pressure compressor using a so-called “long-line” system with a closed end at the downstream pipe.

A Discussion of the Factors Affecting Surge in Centrifugal Compressors

The process of surge in a centrifugal compressor has been studied and found to be dependent on a number of complex and often interrelated factors. A major factor defining surge in high-speed, vaned diffuser designs appears to be the flow in the semivaneless space. By developing work initiated by others it has been possible to propose some explanation of previously presented apparently conflicting data. In addition, a more complex mathematical model capable of assessing additional factors is proposed. Using this model good agreement with experimental surge has been obtained. This model takes into consideration pressure losses in the impeller, diffuser, and collector, and by using this model it was possible to identify the stalling elements that are responsible for overall compressor instability.