Pietro Giannattasio

Brief Adaptive control of compressor surge instability

In this paper we propose a simple high-gain type adaptive control scheme for surge stabilization in a compression system. We consider a typical configuration in which the plenum throttle position i...In this paper we propose a simple high-gain type adaptive control scheme for surge stabilization in a compression system. We consider a typical configuration in which the plenum throttle position is the control input, while the measured output is given by the total pressure at the compressor inlet. We prove convergence of the control scheme using a simplified, but widely accepted, second-order model. The system does not satisfy strict minimum-phase assumptions, usually introduced in the high-gain type adaptive control context, and therefore the proof has been tailored to the specific problem. We finally validate the proposed controller, by means of both numerical simulation and experiments on a real plant.

Multistage Centrifugal Compressor Surge Analysis: Part II—Numerical Simulation and Dynamic Control Parameters Evaluation

This paper describes, from a theoretical point of view, the behavior of compression systems during surge and the effect of passive and active control devices on the instability limit of the system. A lumped parameter model is used to simulate the compression system described in Part I of this work (Arnulfi et al., 1999), based on an industrial multistage centrifugal compressor. A comparison with experimental results shows that the model is accurate enough to describe quantitatively all the features of the phenomenon. A movable wall control system is studied in order to suppress surge in the compressor. Passive and active control schemes are analyzed, they both address directly the dynamic behavior of the compression system to displace the surge line to lower flow rates. The influence of system geometry and compressor speed is investigated; the optimum values of the control parameters and the corresponding increase in the extent of the stable operating range are presented in the paper.

Extensive study on the control of centrifugal compressor surge

Abstract This paper reports the results of an extensive study concerning surge instability in an industrial compression system and its dynamic suppression. Both numerical simulations and experiments have been used to analyse the system behaviour under unstable operating conditions and to compare different control devices for surge suppression. First, the characterization of surge is performed over the whole unstable operating range of a compression system based on a four-stage centrifugal compressor. The unsteady energy associated with the surge cycle is employed as an index of the surge intensity to allow for a quantitative approach to the problem. Subsequently, the behaviour and performance of a passive-type control system and of an active-type one are analysed and compared. The former consists of an innovative device based on an oscillating water column, whereas the latter is a high-gain type of feedback device implemented digitally. Experimental data show that the proposed passive device is effective not only in inhibiting surge development, but also in suppressing fully developed instability. The active device turns out to be more effective than the passive one, but under severe conditions it may fail because of actuator limitations and noise amplification. Finally, the relative merits and limitations of the two control systems are discussed with reference to their effectiveness range, flexibility, steady-state performance, installation, maintenance, and costs.

An Innovative Device for Passive Control of Surge in Industrial Compression Systems

The present paper reports a numerical-experimental study on the dynamic behavior of a compression system based on a multistage centrifugal blower and fitted with an innovative device for the dynamic suppression of surge instability. The control device is of passive type and is based on the aeroelastic coupling of the basic compression system with a hydraulic oscillator. The controlled system is modeled at first by using a nonlinear lumped parameter approach. The simulated system dynamics within a wide range of operating conditions allows a parametric analysis to be performed and the optimal values of the control parameters to be singled out. Such optimal values are then used to design the hydraulic oscillator, which results in a technically feasible and very simple configuration. Finally, experimental tests are carried out on the compression plant with and without the passive control device, which demonstrate the effectiveness of the proposed control system in suppressing surge instabilities, at least within the limits predicted by the numerical simulation.

Experimental Evaluation of a High-Gain Control for Compressor Surge Suppression

The present paper considers the suppression of surge instability in compression systems by means of active control strategies based on a high-gain approach. A proper sensoractuator pair and a proportional controller are selected that, in theory, guarantee system stabilization in any operating condition for a sufficiently high value of the gain. Furthermore, an adaptive control strategy is introduced that allows the system automatically to detect a suitable value of the gain needed for stabilization, without requiring any knowledge of the compressor and plant characteristics. The control device is employed to suppress surge in an industrial compression system based on a four-stage centrifugal blower, An extensive experimental investigation has been performed in order to test the control effectiveness in various operating points on the stalled branch of the compressor characteristic and at different compressor speeds. On one hand, the experimental results confirm the good performance of the proposed control strategy; on the other, they show some inherent difficulties in stabilizing the system at high compressor speeds due to the measurement disturbances and to the limited operation speed of the actuator.

Multistage centrifugal compressor surge analysis : Part I. Experimental investigation

This paper reports an experimental investigation on centrifugal compressor surge. The compression system consists of a four-stage blower with vaned diffusers and a large plenum discharging into the atmosphere through a throttle valve. Measurements of unsteady pressure and flow rate in the plant, and of instantaneous velocity in the diffusers of the first and fourth compressor stage, are performed during deep surge, at several valve settings and three different rotation speeds. Additional tests have been carried out on a different system configuration, i.e., without plenum, in order to obtain the steady-state compressor characteristics and to collect reference dato on stall in surge-free conditions. In this configuration, a fully developed rotating stall was detected in the compressor diffusers, while during surge it affects only a limited part of the surge cycle. The goal of the present experimental work was to get a deeper insight into unstable operating conditions of multistage centrifugal compressors and to validate a theoretical model of the system instability to be used for the design of dynamic control systems.

Effects of Rotation and Channel Orientation on the Flow Field Inside a Trailing Edge Internal Cooling Channel

The flow field inside a cooling channel for the trailing edge of gas turbine blades has been numerically investigated with the aim to highlight the effects of channel rotation and orientation. A commercial 3D RANS solver including a SST turbulence model has been used to compute the isothermal steady air flow inside both static and rotating passages. Simulations were performed at a Reynolds number equal to 20000, a rotation number (Ro) of 0, 0.23, and 0.46, and channel orientations of γ = 0, 22.5, and 45, extending previous results towards new engine-like working conditions. The numerical results have been carefully validated against experimental data obtained by the same authors for conditions γ = 0∘ and Ro = 0, 0.23. Rotation effects are shown to alter significantly the flow field inside both inlet and trailing edge regions. These effects are attenuated by an increase of the channel orientation from γ = 0∘ to 45.

Flow field investigations in rotating facilities by means of stationary PIV systems

The flow field inside rotating test sections can be investigated by means of particle image velocimetry (PIV) operated in the phase-locked mode. With this experimental approach, the measurement system is kept fixed and it is synchronized with the periodical passage of the test section. Therefore, the direct output of the PIV measurements is the absolute velocity field, while the relative one is indirectly obtained from proper data processing that relies on accurate knowledge of the peripheral velocity field. This work provides an uncertainty analysis about the evaluation of the peripheral displacement field in phase-locked PIV measurements. The analysis leads to the detection of the levels of accuracy required in the estimation of both the angular velocity and the position of the center of rotation to ensure correct evaluation of the peripheral displacement field. In this regard, a simple methodology is proposed to evaluate the center of rotation position with an accuracy below 1 px. Finally, a procedure to pre-process the PIV images by subtracting the peripheral displacement is described. The advantages of its implementation are highlighted by the comparison with the performance of a more standard methodology where the peripheral field is subtracted from the absolute velocity field and not directly from the PIV raw data.

Airfoil optimization for stall regulated vertical axis wind turbines

Multi-megawatt Vertical Axis Wind Turbines (VAWTs) have inherent design and operational advantages that make them relevant for floating offshore applications. Many offshore VAWT concepts use stall for power regulation. Stall regulation imposes several constrains in the aerodynamic, structural and generator design. Due to the azimuthally varying and unsteady aerodynamics experienced by a VAWT, designing an airfoil for stall regulation is still a significant challenge. In this work, a family of airfoils for stall regulated VAWT is defined through numerical airfoil optimization, based on the original work of Simao Ferreira and Geurts [20]; the optimization is multi-objective, optimizing structural and aerodynamic performance. The control performance is not yet implemented in the function. The optimization is performed at Reynolds numbers representative of multi-megawatt VAWT. The performance of the VAWT, including operation in dynamic stall, is evaluated with an unsteady double-wake viscous-inviscid panel method and CFD simulations. The performance of the optimized airfoils is compared against a conventional airfoil, namely the NACA 0018 airfoil. The stall regulatory performance is assessed to provide insight for future optimizations. The aerodynamic performance is evaluated using three different numerical models: a single wake panel model coupled to airfoil polar data; a viscous-inviscid double wake panel model, capable of simulating dynamic stall; and an eulerian RANS CFD model. The airfoils are also design for a robust performance in the case of surface roughness. The preliminary results show that the design for surface roughness conflicts with the design for dynamic stall control. An extensive study of multi-objective optimisations with different weights of the different elements of aerodynamic performance is presented.

Limits and Trade-Off in the Control of Compressor Surge

The present paper deals with several problems arising in the stabilisation of surge in compression systems. From a theoretical standpoint the system requires a high gain type controller to be stabilised. On the other hand, the real system is affected by persistent measurement noise whose negative effect on stability is strongly amplified by the high gain. Therefore, a suitable filtering of the system output is necessarily required. The bandwidth of the filter is subject to opposite constraints: if it is too large it affects the attenuation property; if it is too narrow, the large phase-lag introduced at low frequencies may compromise the system stabilisation, which requires a fast actuator reaction. This paper analyses all these aspects concurring in the stabilisation problem and the consequent trade-off in the control design. The results of experimental and numerical studies are provided with reference to the active control of surge in a multistage centrifugal compressor.Copyright