Work on rotating stall and its related disturbances has been in progress since the Second World War. During this period, certain ‘hot topics’ have come to the fore — mostly in response to pressing problems associated with new engine designs. This paper will take a semi-historical look at some of these fields of study (stall, surge, active control, rotating instabilities etc.) and will examine the ideas which underpin each topic. Good progress can be reported, but the paper will not be an unrestricted celebration of our successes because, after 75 years of research, we are still unable to predict the stalling behaviour of a new compressor or to contribute much to the design a more stall resistant machine. Looking forward from where we are today, it is clear that future developments will come from CFD in the form of better performance predictions, better flow modelling and improved interpretation of experimental results. It is also clear that future experimental work will be most effective when focussed on real compressors with real problems — such as stage matching, large tip clearances, eccentricity and service life degradation. Today’s topics of interest are mostly associated with compressible effects and so further research will require more high speed testing.Copyright © 2015 by ASME

Stall, Surge, and 75 Years of Research

Compressors are widely used for the pressurization of fluids. Applications involve air compression for use in aircraft engines and pressurization and transportation of gas in the process and chemical industries. The article focuses on two commonly used types of continuous flow compressors: the axial compressor, where the gaseous fluid is processed in a direction parallel to the rotational axis, and the radial or centrifugal compressor, where the pressurized fluid leaves the compressor in a direction perpendicular to the rotational axis. In these machines, the entering fluid is pressurized by first accelerating it via the kinetic energy imparted in the rotors and then converting the kinetic energy into potential energy by decelerating the fluid in diverging channels. Toward low mass flows, stable operation of axial and radial compressors is constrained by two aerodynamic flow instabilities: rotating stall and surge. The article gives an overview of the current state of modeling and control of these instabilities.

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Modeling and control of compressor flow instabilities

▪ Abstract The compression system instabilities known as surge and rotating stall are natural limits to the performance of all compressors and are especially troubling in gas turbine engines. In the last 15 years, rapid progress has been made in understanding this complex problem through the application of control technology; in particular, system identification techniques have proven to be useful. New findings include the roles of compressibility and nonlinearity in the stall-inception process. These findings have been used to implement feedback control schemes that have achieved increased stability in a variety of compressors and engines. Approaches fall into one of two categories: (a) operating range extension through active damping of linear disturbances, or (b) manipulation of the nonlinear system dynamics to maintain the operating point close to the instability boundary in the presence of disturbances seen in operation.

Compression system stability and active control

We consider the problem of global stabilization of a semilinear dissipative evolution equation by finite-dimensional control with finite-dimensional outputs. Coupling between the system modes occurs directly through the nonlinearity and also through the control influence functions. Similar modal coupling occurs in the infinite-dimensional error dynamics through the nonlinearity and measurements. For both the control and observer designs, rather than decompose the original system into Fourier modes, we consider Lyapunov functions based on the infinite-dimensional dynamics of the state and error systems, respectively. The inner product terms of the Lyapunov derivative are decomposed into Fourier modes. Upper bounds on the terms representing control and observation spillover are obtained. Linear quadratic regulator (LQR) designs are used to stabilize the state and error systems with these upper bounds. Relations between system and LQR design parameters are given to ensure global stability of the state and error dynamics with robustness with respect to control and observation spillover, respectively. It is shown that the control and observer designs can be combined to yield a globally stabilizing compensator. The control and observer designs are numerically demonstrated on the problem of controlling stall in a model of axial compressors.

Spillover stabilization in finite-dimensional control and observer design for dissipative evolution equations ∗

The effects of circumferentially nonuniform tip clearance on axial compressor performance and stability have been investigated experimentally and analytically. A theoretical model for compressor behavior with nonaxisymmetric tip clearance has been developed and used to design a series of first-of-a-kind experiments on a four-stage, low-speed compressor. The experiments and computational results together show clearly the central physical features and controlling parameters of compressor response to nonaxisymmetric tip clearance. It was found that the loss in stall margin was more severe than that estimated based on average clearance. The stall point was, in fact, closer to that obtained with uniform clearance at the maximum clearance level. The circumferential length scale of the tip clearance (and accompanying flow asymmetry) was an important factor in determining the stall margin reduction. For the same average clearance, the loss in peak pressure rise was 50 percent higher for an asymmetry with fundamental wavelength equal to the compressor circumference than with wavelength equal to one-half the circumference. The clearance asymmetry had much less of an effect on peak efficiency; the measured maximum efficiency decrease obtained was less than 0.4 percent compared to the 8 percent decrease in peak pressure rise due to the asymmetric clearance. The efficiency penalty due to nonaxisymmetric tip clearance was thus close to that obtained with a uniform clearance at the circumferentially averaged level. The theoretical model accurately captured the decreases in both steady-state pressure rise and stable operating range which are associated with clearance asymmetry. It also gave a good description of the observed trends of: (i) increasing velocity asymmetry with decreasing compressor flow, and (ii) decreasing effect of clearance asymmetry with decreasing dominant wavelength of the clearance distribution. The time-resolved data showed that the spatial structure of the prestall propagating disturbances in the compressor annulus was well represented and that the stability limiting process could be linked to the unsteady structure of these disturbance modes. The model was also utilized for parametric studies to define how compressor performance and stability are affected by the circumferential distribution of clearance, steady-state compressor pressure-rise characteristic, and system dynamic parameters. Sensitivity to clearance asymmetry was found to fall off strongly with the (asymmetry-related) reduced frequency and to increase with peak pressure rise and increasing curvature of the characteristic near the peak.

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Effects of Non-Axisymmetric Tip Clearance on Axial Compressor Performance and Stability

Dynamic control of rotating stall in an axial flow compressor has been implemented using aeromechanical feedback. The control strategy developed used an array of wall jets, upstream of a single-stage compressor, which were regulated by locally reacting reed valves. These reed valves responded to the small-amplitude flow-field pressure perturbations that precede rotating stall. The valve design was such that the combined system, compressor plus reed valve controller, was stable under operating conditions that had been unstable without feedback. A 10 percent decrease in the stalling flow coefficient was obtained using the control strategy, and the extension of stable flow range was achieved with no measurable change in the steady-state performance of the compression system. The experiments demonstrate the first use of aeromechanical feedback to extend the stable operating range of an axial flow compressor, and the first use of local feedback and dynamic compensation techniques to suppress rotating stall. The design of the experiment was based on a two-dimensional stall inception model, which incorporated the effect of the aeromechanical feedback. The physical mechanism for rotating stall in axial flow compressors was examined with focus on the role of dynamic feedback in stabilizing compression system instability. As predicted and experimentally demonstrated, the effectiveness of the aeromechanical control strategy depends on a set of nondimensional control parameters that determine the interaction of the control strategy and the rotating stall dynamics.

D. L. Gysling

Papers

Turbomachinery Committee Best 1994 Paper Award: Dynamic Control of Rotating Stall in Axial Flow Compressors Using Aeromechanical Feedback