Suppression Of Thermoacoustic Instability Research Articles

Effect of geometrical parameters and use of porous material in a Helmholtz resonator on suppression of thermo-acoustic instabilities

Thermo-acoustic instabilities are mainly formed due to in-phase superposition of non-uniform heat release and pressure variation in the combustors of gas turbines, rocket engines and other acoustically confined spaces. These instabilities not only damage the structural system but also reduce its combustion efficiency and heat transfer rate. Hence suppression of thermo-acoustic instabilities is a prominent requirement for stable and safe heat generation in the combustors. In this work, the Helmholtz resonator has been used to suppress the instability. The efficacy of the resonator has been further increased by the addition of absorptive material to it. This work concentrates on inspecting the influence of cavity volume, neck length and neck diameter of the Helmholtz Resonator and the thickness of the absorptive material in the damping process of thermo-acoustic instabilities. The experimentation was carried out for various combinations of resonator cavity volume, neck diameter and neck length, and the best combination was found to be 6 mm neck diameter with 20 mm neck length at 60% volume which provided an acoustic damping of around 30 dB. Further, it was noticed that the addition of absorptive material is effective at lower volumes of Helmholtz resonator, and with an increase in thickness of absorptive material beyond a certain limit, the damping ability of the resonator reduces.

Unified low-order modeling approach to can-annular combustors

We present a unified, physics-based low-order modeling approach to can-annular combustors, combining a linear stability analysis and a nonlinear Langevin approach to unravel the highly complex, multi-physical dynamics in this type of system. In the linear part, Howe's Rayleigh conductivity model is combined with the projected Helmholtz equation and a Bloch wave ansatz to arrive at a single equation for the frequency spectrum of an N-can combustor. Starting from first principles, we illustrate and give a physical explanation for the coupling-induced amplification and suppression of thermoacoustic instabilities. Adding another layer of complexity, we then take into account nonlinearities in the flame response and the aeroacoustic coupling. The nonlinear dynamics of a symmetric model combustor are explored by exploiting the gradient structure of the averaged slow-flow dynamics. We obtain exact analytical expressions for the steady-state statistics, highlighting the connection between different emergent patterns and the resistive coupling between the cans. We leverage our analysis to explain the intermittent energy transfer between Bloch modes observed in real-world gas turbines.

Effect of helmholtz resonator shape on suppression of thermo-acoustic instability

In combustion chambers thermo-acoustic instabilities arise due to the coupling between the unsteady heat release and pressure fluctuations. The instabilities created cause serious structural vibration, flame blow out, increase in heat transfer and damage, thereby reducing the life span and performance of jet engines, gas turbines and industrial burners. Therefore, there is need to develop a simple technique for effective control of these instabilities. The present work involves experimental study of thermo-acoustic instabilities occurring in a Rijke tube and their suppression by using Helmholtz resonators of different shape (a passive method of suppression). The combustor model is replaced by Rijke tube which is an experimental setup extensively employed for research purpose. Rijke tube is provided with a co-axial pre-mixed gas burner as the source of heat which could be placed at any desired position. When a heat source with proper air to fuel ratio is placed at about one fourth of the tube length, 3rd mode of instability is obtained and a loud noise of frequency 681 Hz is produced. A comparative study of thermo-acoustic instability suppression using cylindrical, spherical and conical Helmholtz resonators has been performed. Spherical resonator has been found to be the most effective one imparting a maximum damping effect of 20 dB and an average damping effect of 10.5 dB.

Suppression of thermoacoustic instabilities by flame-structure interaction

We present here an experimental study of the influence of the aeroelastic coupling between the combustion chamber walls and the acoustic fluid field on the onset and development of thermoacoustic instabilities in stoichiometric propane-air premixed flames. A horizontal quasi-two-dimensional Hele–Shaw chamber formed by two parallel plates separated a small distance h is used. The flames are ignited at the open end, in contact with the atmosphere, and propagate towards the opposite closed end. The experiments reveal three distinct propagation regimes determined by the stiffness of the plates and the evolution of the pressure perturbation generated during ignition: (i) for sufficiently rigid plates, we observed secondary acoustic instabilities with large amplitude oscillations in the direction of propagation of the flame; for flexible enough walls to be compliant with ignition-related pressure changes, (ii) the propagation of the flame undergoes small-amplitude oscillations (primary acoustic instabilities) along the channel or (iii) it is smooth with no oscillations whatsoever. The flexural rigidity of the plate is modified experimentally by changing both the width W and thickness hw of the top plate of the Hele–Shaw cell. The data recorded by the pressure transducer and the accelerometer is used to plot a stability map in the W−hw parametric space to define the combination of structural parameters that triggers the onset of thermoacoustic instabilities. Our experimental measurements, supplemented with results from a theoretical analysis of the walls vibration modes, indicated that deformation-induced volume changes of around 0.1% of the volume of the Hele–Shaw cell are sufficient to suppress thermoacoustic instabilities.

Suppression of thermoacoustic instability and NOx emissions of unsteady swirl premixed flame with upstream microjets

AbstractPassive control methods are widely used in unsteady combustion systems due to their robustness and convenience. This paper used the upstream microjets method for the first time to control thermoacoustic instability and pollutant NOx emissions in unsteady swirl premixed flame. Three variables of CO2 microjets flow rate, upstream microjets distance, and microjets number are investigated. Results demonstrate the upstream microjets distance plays a significant role in restraining the thermoacoustic instability and NOx emissions. The smaller the upstream microjets distance, the more completely the thermoacoustic instability is suppressed while the NOx emissions are the opposite, which means a compromise choice needs to be made between them. The damping ratio of pressure amplitude in the chamber can reach the maximum of 81.3%, and the damping ratio of CH* intensity can reach the maximum of 80.2%. Besides, the frequency of pressure and CH* will transit to a lower mode. NOx emissions can realize a maximum reduction of 12.2 ppm. Meanwhile, the flame mode switches from an enlarged Mode‐A flame to a shrunk Mode‐B flame, and the flame length shows a maximum reduction of 14 mm at the largest upstream distance. However, the specific control mechanism under upstream microjets still needs further study to clarify. This research proved the feasibility of suppressing combustion instability and NOx emissions with upstream microjets in unsteady swirl premixed flame, promoting its practical application in gas turbine systems.

Suppression of thermoacoustic instability by targeting the hubs of the turbulent networks in a bluff body stabilized combustor

Abstract

Experimental investigations of equivalence ratio effect on nonlinear dynamics features in premixed swirl-stabilized combustor

In this work, the dynamics of the pressure oscillations and flame behaviors in a premixed swirl-stabilized combustor were experimentally investigated. As equivalence ratio Φ varied between 0.55 and 1.35, five combustion states were distinguished based on the characteristics of the acoustic pressures and flame dynamics. The permutation entropy (PE) and proper orthogonal decomposition (POD) were used to analyze the fluctuating pressure signal complexities and spatial CH⁎ chemiluminescence images, respectively. The results showed that the magnitude of the PE was at a differentiable level. In addition, the POD modes exhibited distinguishable features when the system was in different combustion states. Further analyses of the state transitions indicated that the system underwent a supercritical Hopf bifurcation and a pair of fold bifurcations when Φ was in the lean condition. The state transition from quasi-periodic oscillation to limit-cycle oscillation was attributed to the movement of the flame location, which enhanced the coupling of the acoustics and combustion by reducing the phase difference between the pressure oscillations and heat release fluctuations. In addition, a hysteresis phenomenon was observed for the trajectory of the fluctuating pressure root-mean-square (RMS) value in the forward and backward processes of the equivalence ratio under the lean condition. A low-order model was used to capture the bifurcations; the theoretical results agreed well with the experimental data. As for the rich condition, a supercritical Hopf bifurcation was captured when the system switched between the dual-frequency oscillation mode and the rich stable state. Furthermore, the influence of combustion chamber length Lc on the combustion stability in relation to Φ was investigated. Parameter Lc was divided into steady and unsteady regions depending on whether the combustion system could reach limit-cycle oscillation. The PE was demonstrated to be effective in distinguishing the stable state and limit-cycle oscillation even for combustors of different lengths. Understanding the nonlinear dynamics by analyzing experimental data could be critical to developing models for the prediction and suppression of thermoacoustic instabilities.

Analytical and Experimental Study on Passive Stabilization of Thermoacoustic Dynamics in a Rijke Tube

This paper deals with passive stabilization of thermoacoustic dynamics in a Rijke tube using a Helmholtz resonator. Thermoacoustic instabilities result from the dynamic coupling between the heat release and pressure in a chamber. Helmholtz resonators are used akin to vibration absorbers to suppress unwanted pressure oscillations in such structures and prevent instabilities. The first contribution of the paper is a state-space representation of the thermoacoustic dynamics for the resonator-mounted Rijke tube. This relationship happens to be in the class of linear time invariant, neutral multiple time delay systems (LTI-NMTDS). Then, benefiting from the cluster treatment of characteristic roots (CTCR) paradigm, we investigate the effect of resonator location on suppression of thermoacoustic instability. CTCR is a mathematical tool that determines the stability of LTI-NMTDS exhaustively and nonconservatively in the parameter space of the system. This capability provides a novel tool for the futuristic design concepts of combustors. These analytically obtained findings are also supported with experimental results from a laboratory-scale Rijke tube. In addition, a conceptual case study is presented where the stabilizing contributions of the resonator to the dynamics are investigated under strong thermoacoustic coupling.

Predicting the secondary dynamic mode interference phenomenon in thermoacoustic instability control

This paper brings a novel mathematical perspective in assessing the rise of the secondary dynamic modes to prominence during the suppression of thermoacoustic instability. This phenomenon is observed by many earlier investigators; however, without a complete analytical reasoning. We consider a Rijke tube with both a passive Helmholtz resonator and an active feedback control to suppress instabilities. The core dynamics is represented as a linear time-invariant multiple time-delay system of neutral type. Parametric stability of the resulting infinite-dimensional dynamics is investigated using a recent analytical tool: cluster treatment of characteristic roots paradigm. This tool reveals the stability outlook of such systems exhaustively and non-conservatively in the parameter space of the system. First, we examine the stability with and without the Helmholtz resonator. We then select an unstable operation for the resonator-mounted Rijke tube, impose a time-delayed integral feedback control over it and reveal the stabilizing controller parameters using the cluster treatment of characteristic roots methodology. When high control gains are inappropriately selected, the new analytical procedure declares how the secondary dynamic modes of the system exhibit instability although the initially unstable mode is now stabilized. All of these stability assessments are cross-validated using experimental results from a laboratory-scale Rijke tube set-up.

Suppression of thermo-acoustic instability using air injection in horizontal Rijke tube

The thermo-acoustic instability arising out of coupling between the pressure fluctuation and the unsteady heat release, when grows sufficiently, is known to cause serious structural damage thereby reducing the life span of systems having combustor for example, jet engines, gas turbines and industrial burners. The present work involves experimental study of thermo-acoustic instabilities occurring in a Rijke tube and their suppression by means of diverting a very small fraction of the incoming air flow in the form of radial injection from the wall of the tube through micro jets. A horizontal quartz tube with pre-mixed burner is used as the test model of Rijke tube. The coefficient Rayleigh Index (RI) is estimated from simultaneous measurement of chemiluminescence using a PMT and pressure in the plane of burner head. Experiments were carried out at the equivalence ratio of Φ = 1 for a range of the burner position inside the tube, velocity of micro jets, and total mass flow rate through the tube. The control technique of micro jets was found to completely suppress the thermo-acoustics in a range of jet velocity depending on the burner position and the total air mass flow rate. For complete suppression of the thermos-acoustics, the phase difference between the pressure and chemiluminescence waves was found to significantly increase and the RI was found to reduce to nearly zero. In such case, the flame luminescence was observed to significantly reduce. The proposed technique of air injection into the flame through radial micro jets is simple yet very effective in controlling the thermos-acoustic instability.

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.