Control Of Thermoacoustic Instabilities Research Articles

A PINN approach for identifying governing parameters of noisy thermoacoustic systems

Identifying the governing parameters of self-sustained oscillation is crucial for the diagnosis, prediction and control of thermoacoustic instabilities. In this paper, we propose and validate a novel method for computing the parameters of thermoacoustic oscillation in a stochastic environment, which exploits a physics-informed neural network (PINN). Specifically, we introduce a negative log-likelihood loss function that integrates the stochastic samples and the solution of the Fokker–Planck equation. The proposed framework is validated using the numerically generated signal and the experimental data obtained from an annular combustor, both before and after the supercritical Hopf bifurcation. The results of PINN-based system identification show good agreement with the actual system parameters and the original stochastic signal, with improved accuracy compared to established methods. To the best of our knowledge, this study constitutes the first demonstration of the PINN-inverse approach that uses the noise-induced dynamics of thermoacoustic systems, opening up new pathways for diagnosing and predicting the thermoacoustic behaviour of various combustion systems.

Prediction of acoustic pressure of thermoacoustic combustion instability based on Elman neural network

Accurate prediction of thermoacoustic instability is a prerequisite for thermoacoustic control to avoid the damage of combustion chamber, however, this problem has not been completely solved yet. This paper proposes a data-driven method based on the Elman neural network (ENN) to predict the value of acoustic pressure of combustion instability. As a comparison, a model based on support vector machine (SVM) was built. It is proved that ENN has better prediction performance with a certain predicted time horizon compared to the SVM method. What is more, the prediction model based on ENN can adapt to time-varying characteristics of the transition scenario which is characterized by amplitude modulation, multiple frequencies, and irregular bursts. ENN model still maintains enough prediction accuracy for various input training sets, indicating that ENN can fully mine the features of data and has a strong feature extraction ability in combustion oscillation prediction. Hence, it is demonstrated that ENN is a promising prediction tool for thermoacoustic instability under various combustion conditions. These findings are of great significance for the accurate prediction and control of thermoacoustic instability.

Effect of position of radial air injection plane on control of thermo-acoustic instability using active closed-loop method

Thermo-acoustic instability occurs when self-excited oscillations are generated due to the coupling between unsteady heat release and acoustics. This phenomenon can result in an increased rate of vibration, structural damage, and produces unwanted emissions. Thermo-acoustic instability occurs in rocket engines, gas turbines, combustors, and furnaces. When thermo-acoustic instability occurs, many modes are developed naturally at a specific point. Some waves are unstable and some are stable. So, to study this phenomenon the most unstable waves are considered and a technique is developed to suppress these unstable waves. A radial air injector as a closed-loop active control method is used for breaking the coupling between the heat waves and acoustics inside the 1D combustion chamber. The distance between the burner and the air injector is varied for the fixed position of the burner with respect to the Rijke tube, that is, x/L = 0.01125, 0.0075, and 0.00375. This closed-loop method works based on the feedback acquired from a microphone. The control method is built using DAQ and Arduino with the LabVIEW as interface for Arduino (LIFA). An air flow rate controller setup is developed to control and measure air required for suppressing the thermo-acoustic instability. Thermo-acoustic instability is effectively suppressed with the help of radial injection in the form of micro-jets at the downstream of the burner as the closed-loop controlling method. It is concluded that when the radial micro-jet air injection plane is closer to the burner head, the thermo-acoustic instability gets suppressed in a short time and with a lesser quantity of air.

Analysis of a Pseudo-active Approach for the Control of Thermoacoustic Instabilities

ABSTRACT This work describes a first characterization study of a novel approach, named ‘pseudo-active’ instability control (PAIC), which might afford the versatility of active control, but with much simpler hardware requirements. The objective is to spontaneously induce oscillations in pilot fuel injection due to the pressure fluctuations in the combustion chamber, in order to produce effects similar to those obtained with active instability control methods. For optimized designs, this interaction could be exploited to damp pressure fluctuations in gas-turbine combustors. The experimental study was performed for different PAIC configurations and under situations of strong, self-sustained pressure fluctuations. The results obtained show good potentialities of the system proposed, and also revealed purely acoustic interactions between the burner and the pilot line, which superimposed on the interaction between the pilot and main flames. Overall, this work provides novel results on the, previously unexplored, influence of the acoustic properties of the pilot fuel injection on the limit cycle as well as first evidence on the rationale and benefits that could be derived from the so-called PAIC strategy.

Reduced-order modelling of thermoacoustic instabilities in a two-heater Rijke tube

The topic of thermoacoustic instabilities in combustors is well-investigated, as it is important in the field of combustion, primarily in gas-turbine engines. In recent years, much attention has been focused on monitoring, diagnosis, prognosis, and control of high-amplitude pressure oscillations in confined combustion chambers. The Rijke tube is one of the most simple, yet very commonly used, laboratory apparatuses for emulation of thermoacoustic instabilities, which is also capable of capturing the physics of the thermally driven acoustics. A Rijke tube apparatus can be constructed with an electrical heater acting as the heat source, thus making it more flexible to operate and safer to handle than a fuel-burning Rijke tube or a fuel-fired combustor. Augmentation of the heat source of the Rijke tube with a secondary heater at a downstream location facilitates better control of thermoacoustic instabilities. Along this line, much work has been reported on the investigation of thermoacoustics by using computational fluid dynamics (CFD) modelling as well as reduced-order modelling for both single-heater and two-heater Rijke tube systems. However, since reduced-order models are often designed and built upon certain empirical relations, they may not account for the dynamic behaviour of the heater itself, which is a critical factor in the analysis and synthesis of real-time robust control systems. This issue is addressed in the current paper, where modifications have been made to existing models by incorporating heater dynamics. The model results are systematically validated with experimental data, generated from an in-house (electrically heated) Rijke tube apparatus.

Early Detection of Thermoacoustic Instabilities Using Hidden Markov Models

ABSTRACTThis paper presents a dynamic data-driven method for early detection of thermoacoustic instabilities in combustors based on short-length time series of sensor data, where the objective is near-real-time monitoring and active control of pressure oscillations. The main idea is to use the available data at different regimes of the combustion process to train respective hidden-variable models using the concept of Hidden Markov Modeling (HMM) as a statistical learning tool; here, (short-length) time-series data of pressure oscillations are used to infer a Markov chain with unobserved (hidden) states. The proposed HMM-based method has been validated on experimental data collected from an electrically heated Rijke tube apparatus for predicting onset of thermoacoustic instabilities. The results have been compared with those of the current state-of-the-art measurement techniques for instability growth rate and associated computational complexity. The applicability of the proposed method has been demonstrated with respect to anomaly detection and regime identification with limited data requirements, making it a potential candidate for monitoring and active control of thermoacoustic instabilities in commercial-scale combustors.

Detection of Thermoacoustic Instabilities Via Nonparametric Bayesian Markov Modeling of Time-Series Data

Real-time detection and decision and control of thermoacoustic instabilities in confined combustors are challenging tasks due to the fast dynamics of the underlying physical process. The objective here is to develop a dynamic data-driven algorithm for detecting the onset of instabilities with short-length time-series data, acquired by available sensors (e.g., pressure and chemiluminescence), which will provide sufficient lead time for active decision and control. To this end, this paper proposes a Bayesian nonparametric method of Markov modeling for real-time detection of thermoacoustic instabilities in gas turbine engines; the underlying algorithms are formulated in the symbolic domain and the resulting patterns are constructed from symbolized pressure measurements as probabilistic finite state automata (PFSA). These PFSA models are built upon the framework of a (low-order) finite-memory Markov model, called the D-Markov machine, where a Bayesian nonparametric structure is adopted for: (i) automated selection of parameters in D-Markov machines and (ii) online sequential testing to provide dynamic data-driven and coherent statistical analyses of combustion instability phenomena without solely relying on computationally intensive (physics-based) models of combustion dynamics. The proposed method has been validated on an ensemble of pressure time series from a laboratory-scale combustion apparatus. The results of instability prediction have been compared with those of other existing techniques.

Control of thermoacoustic instability with a drum-like silencer

Theoretical investigation is carried out by a novel method of controlling thermoacoustic instability with a drum-like silencer. It is shown that by decreasing the frequency of thermoacoustic system, the instability can be suppressed with the help of drum-like silencer. The purely reactive silencer, which is composed of a flexible membrane and a backing cavity, is usually known as a noise control device that works effectively in low frequency bandwidth without any aerodynamic loss. In present research, the silencer is exploited in a Rijke tube, as a means of decreasing the natural frequency of the system, and consequently changing the resonance period of the system. The “transfer element method” (TEM) is used to consider the interactions between the acoustic waves and the flexible membranes of the silencer. The effects of all possible properties of the silencer on the growth rate and resonance frequency of the thermoacoustic system are explored. According to the calculation results, it is found that for some properties of the silencer, the resonance frequencies are greatly decreased and then the phase difference between the unsteady heat release and the pressure fluctuation is increased. Consequently, the instability is suppressed with some dissipation that can not be able to control its onset in the original system. Therefore, when the damping is low, but not zero, it is effective to control thermoacoustic instability with this technique.

Model-based control of thermoacoustic instabilities in partially premixed lean combustion – a design case study

Self-excited oscillation is becoming a major issue in low-emission, lean partially premixed combustion systems, and active control has been shown to be a feasible method to suppress such instabilities. A number of robust control methods are employed to obtain a feedback controller and it is observed that the robustness to system uncertainty is significantly better for a low complexity controller in spite of the norms being similar. Moreover, we demonstrate that closed-loop stability for such a complex system can be proved via use of the integral quadratic constraint method. Open- and closed-loop nonlinear simulations are provided.

Advanced algorithms for gradient estimation in one- and two-parameter extremum seeking controllers

In extremum seeking control, the gradient estimation is the key enabler for a successful online optimization. For this purpose, the classical algorithm uses a combination of high- and low-pass filters. In this investigation extended Kalman filters (EKF) are used instead. The approach is explained in detail and advantages of Kalman filtering will become apparent. A novel approach for the gradient estimation for dual-input single-output systems is presented. The proposed EKF incorporates the coupling of the output to both inputs, thus, enabling a superior gradient estimate. A simulation study shows that faster convergence of the extremum-seeking controller can be achieved using this estimator. The feasibility of the proposed algorithm in an experimental setup is demonstrated by control of thermoacoustic instabilities in an atmospheric combustor test rig.

Nonlinear Thermoacoustic Instability Dynamics in a Rijke Tube

We present a data-driven nonlinear and chaos theory–based analysis of thermoacoustic instabilities in a simple Rijke tube. Thermoacoustic instability modes in this simple Rijke tube display very rich nonlinear behavior because of the interaction of acoustic modes and unsteady heat-release processes during combustion. This approach of analyzing thermoacoustic instabilities, their evolution, and interactions differs from traditional linear time-series–based approaches, such as the Fourier transform or the autocorrelation function. The objectives of this work are to address the limitations of conventional linear and model-based nonlinear analyses and to describe the potential of data-driven nonlinear and chaos theory–based analyses to gain a deeper understanding of thermoacoustic instability dynamics for reacting flows. The Rijke tube permits investigation of the nonlinear dynamics of thermoacoustic instabilities in a very systematic way by using picosecond time-resolved laser-induced fluorescence (PITLIF) of the OH radical at a rate of 2500 Hz. The 2500-Hz measurement bandwidth is sufficient to capture all thermoacoustic instability modes encountered in the Rijke tube. Through the use of these nontraditional analyses, we find that thermoacoustic instabilities within the Rijke tube contain chaotic behavior. A simple low-order approximate model and a data-adaptive feedback controller are also introduced for the control of thermoacoustic instabilities.

Control of thermoacoustic instabilities by CO2 and N2 jet in cross-flow

Abstract A novel control method for thermoacoustic instabilities with jet in cross-flow (JICF) in a swirl combustor was proposed, and N2 and CO2 were selected as the JICF, respectively, which has not been reported before. The pressure amplitudes, temperature distributions and flue gas components were obtained and analyzed. The influence of stoichiometric ratios was also investigated. The experimental results show that the JICF can control the thermoacoustic instability effectively and the highest control effect ratio could reach 90%. Meanwhile, JICF will not increase the pollutants, and can improve the combustion efficiency. Using CO2 as the JICF is better than N2 in the control effect of thermoacoustic instability.

Active control of combustion instabilities using model-based controllers

An implementation of active control of thermoacoustic instabilities on a swirl-stabilized spray combustor is presented. Loudspeakers were used as control actuators in the closed-loop control scheme. Pressure transducers located in the combustion chamber produced the feedback signal in the control study. Experimental models of the combustor dynamics were developed using a nonparametric identification method. Linear quadratic Gaussian (LQG), LQG/loop transfer recovery, and H ∞ loop-shaping controllers were derived and tested in simulation as well as experimentally. Phase-delay control was used as a baseline method to compare the performance of these different controllers. The advantages of the model-based controllers over the baseline strategy are clearly presented.

FRAMEWORK FOR STUDYING LIMITATIONS OF ACHIEVABLE PERFORMANCE IN CONTROL OF NONLINEAR COMBUSTION PROCESSES

In this paper we propose a framework for studying fundamental limitations in the control of thermoacoustic instabilities described by nonlinear models. More generally, the framework applies to general interconnections of lightly damped linear oscillators and nonlinear static nonlinearities of saturation type driven by broad-band Gaussian disturbances. The key concept is to replace the static nonlinearities with their corresponding random input describing functions. This allows the application of classical fundamental limitations theory to the combustion models. In this paper we interpret the limitations results in the particular case of a model of a combustion process controlled by on-off actuators. We also formulate conservation principles to understand the dynamics associated with this combustion model.

A Model-based Self-tuning Controller forThermoacoustic Instability

Active Control of thermoacoustic instabilities in continuous premixed combustion processes is being increasingly investigated for operating at lean low NOx conditions. Recently, we have developed a model-based approach for active control design which accounts for the underlying acoustics, heat release dynamics, and sensor and actuator dynamics. While this model captures a number of the dominant dynamic features of a premixed laminar combustor, there are a number of uncertainties associated with it as well. In this paper, we study the sensitivity of this model with respect to parametric uncertainties, and the efficacy of a fixed control design for suppressing pressure oscillations. We show that under certain conditions, the fixed controller is inadequate and present a self-tuning controller which is capable of delivering the desired performance in the presence of these uncertainties. The controller proposed is based on a rigorous analytical foundation, and is shown through simulation results to lead to better performance than corresponding fixed controllers. Adaptive algorithms based on the LMS-filter are shown to result in numerical instabilities.

The Origin of Secondary Peaks with Active Control of Thermoacoustic Instability

This paper deals with the generation of new peaks in the pressure spectrum in controlled combustors in experimental investigations of active control of thermoacoustic instabilities. Typically, the reported experiments have demonstrated that the dominant thermoacoustic instability can be suppressed, but secondary peaks at different frequencies which were not excited in the uncontrolled combustor appear. We develop a physically-based model of an actively controlled premixed laminar combustor which takes into account (a) laminar flame kinematics, (b) linear acoustic dynamics with coupling between the acoustic modes, and (c) actuation using side-mounted and end-mounted loudspeakers. Using this model, we analyze some of the experimental controllers proposed in the literature and explain the origin of secondary peaks observed in these studies. Secondary peaks are created while using these controllers due to resonant coupling between various mechanisms in the combustor that are distinct from those responsible for thermoacoustic resonance and mechanisms in the controller other than those that enable suppression of thermoacoustic instability. Other than at acoustic frequencies, premixed laminar combustors respond at lower frequencies due to flame dynamics, and at higher frequencies due to antiresonance. The experimental controllers are usually implemented using analog electronic circuitry whose components are designed so as to provide the functionalities of a phase-shifter, a filter, and an amplifier. Since analog filters tend to provide a phase compensation over a wide range of frequencies and not just at the isolated (unstable) frequency, they can initiate resonances by coupling with various mechanisms present in the combustor. Low frequency secondary peaks are typically due to coupling between the flame dynamics and the filter components in the controller, while high frequency peaks are due to either the interaction between various components of the active controller themselves, or the interaction between the controller components and antiresonance. Such phenomena clearly underscore the need for an active control design which accounts for the combustor dynamics over a range of frequencies with the goal of obtaining the desired performance over this entire range.

Control of thermoacoustic instabilities and emissions in an industrial-type gas-turbine combustor

Unstable thermoacoustic modes were investigated and controlled in an experimental low-emission swirl-stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. The swirl-stabilized combustor was operated in premixed mode as well as with a diffusion-type flame. The combustion structure associated with the different unstable modes was visualized by phase-locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, while the helical modes showed variations in the radial location of maximal heat release. A closed-loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NOx emissions. Microphone and OH emission detection sensors were utilized to monitor the combustion process and provide input to the control system. An acoustic actuation was utilized to modulate the airflow and thus affect the mixing process and the combustion. Suppression levels of up to 5 dB in the pressure oscillations and a concomitant reduction of NOx emissions were obtained using an acoustic power of less than 0.002% of the combustor power. At the optimal control conditions, it was shown that the major effect of the control system was to reduce the coherence of the vortical structures that gave rise to the thermoacoustic instability.