Abstract
Turbojets require variable exhaust nozzles to fit high-demanding applications; however, few reports on nozzle control are available. The purpose of this paper is to investigate the possible advantages of an exhaust gas control through a variable exhaust nozzle. The control design method combines successful linear active disturbance rejection control (LADRC) capabilities with a loop shaping controller (LSC) to: (i) allow designing the closed-loop characteristics in terms of gain margin, phase margin and bandwidth, and (ii) increase the LSC disturbance rejection capabilities with an extended state observer. A representation of the nozzle dynamics is obtained from first principles and adapted to achieve a stream-velocity-based control loop. The results show that the resulting controller allows improving the expansion of the exhaust gas to the ambient pressure for the whole operating range of the turbojet, increasing the estimated thrust by 14.23% during the tests with experimental data.
Highlights
Turbojet subsonic–sonic nozzles are devices that accelerate the hot gas incoming from the turbine by reducing the output area, generating more thrust
It is notable that the increased perturbation rejection of the linear active disturbance rejection control (LADRC) + loop shaping controller (LSC) is achieved without increasing the closed loop bandwidth
With this context in mind, the closed loop stability is evaluated with the best-suited controller for this application among the evaluated control schemes, the LADRC + LSC
Summary
Turbojet subsonic–sonic nozzles are devices that accelerate the hot gas incoming from the turbine by reducing the output area, generating more thrust. Maximizing the thrust production requires variable exhaust nozzles that reject the operating disturbances while optimally expanding the exhaust gas to the ambient conditions Most of these difficulties can be assessed through suitable nozzle constriction and an adequate automatic control algorithm. The ADRC schemes showed promising capabilities to improve disturbance rejections in turbojets; a more realistic analysis on the disturbances is required This shows an interesting area of opportunity to develop suitable variable exhaust nozzle controllers considering the particular difficulties of this process, which involve different sources of disturbances and model uncertainty. In principle the application requires a non-linear controller due to the fundamental relationship among static pressure and gas velocity, the proposed design method allows designing the controller with linear control design techniques without compromising the non-linear stability condition This approach allows designing the controller considering the desired robustness margins, model mismatch and input disturbances, ensuring closed-loop stability and safe operation. Simulations performed with real-measured data from turbojet operation show that the proposed approach is able to increase the produced thrust when compared to a fixed-nozzle turbojet for the whole operating range in the presence of input disturbances
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