Self-excited Thermoacoustic Oscillations Research Articles

Genetic programing control of self-excited thermoacoustic oscillations

In this experimental study, we use a data-driven machine learning framework based on genetic programing (GP) to discover model-free control laws (individuals) for suppressing self-excited thermoacoustic oscillations in a prototypical laminar combustor. This GP framework relies on an evolutionary algorithm to make decisions based on natural selection. Starting from an initial generation of individuals, we rank their performance based on a cost function that accounts for the trade-off between the state cost (thermoacoustic amplitude) and the input cost (actuator power). We then breed subsequent generations of individuals via a tournament in which the direct forwarding of elite individuals occurs alongside genetic operations such as mutation, replication, and crossover. We implement this GP control framework in both closed-loop and open-loop forms, followed by benchmarking against conventional open-loop control based on time-periodic forcing. We find that while all three control strategies can achieve similarly large reductions in thermoacoustic amplitude, GP closed-loop control consumes the least actuator power, making it the most efficient. It achieves this efficiency by learning an actuation mechanism that exploits the strong heat-release-rate amplification of the open flame at its preferred mode, even though the GP algorithm has never seen the open flame itself. This study demonstrates the feasibility of using GP to discover new and more efficient model-free individuals for suppressing self-excited thermoacoustic oscillations, providing a promising approach to data-driven feedback control of combustion devices.

Forced synchronization of self-excited chaotic thermoacoustic oscillations

We experimentally investigate the forced synchronization of a self-excited chaotic thermoacoustic oscillator with two natural frequencies, $f_1$ and $f_2$ . On increasing the forcing amplitude, $\epsilon _f$ , at a fixed forcing frequency, $f_f$ , we find two different types of synchronization: (i) $f_f/f_1 = 1:1$ or $2:1$ chaos-destroying synchronization (CDS), and (ii) phase synchronization of chaos (PSC). En route to $1:1$ CDS, the system transitions from an unforced chaotic state ( ${\rm {CH}}_{1,2}$ ) to a forced chaotic state ( ${\rm {CH}}_{1,2,f}$ ), then to a two-frequency quasiperiodic state where chaos is destroyed ( $\mathbb {T}^2_{2,f}$ ), and finally to a phase-locked period-1 state ( ${\rm {P1}}_f$ ). The route to $2:1$ CDS is similar, but the quasiperiodic state hosts a doubled torus $(2\mathbb {T}^2_{2,f})$ that transforms into a phase-locked period-2 orbit $({\rm {P2}}_f)$ when CDS occurs. En route to PSC, the system transitions to a forced chaotic state ( ${\rm {CH}}_{1,2,f}$ ) followed by a phase-locked chaotic state, where $f_1$ , $f_2$ and $f_f$ still coexist but their phase difference remains bounded. We find that the maximum reduction in thermoacoustic amplitude occurs near the onset of CDS, and that the critical $\epsilon _f$ required for the onset of CDS does not vary significantly with $f_f$ . We then use two unidirectionally coupled Anishchenko–Astakhov oscillators to phenomenologically model the experimental synchronization dynamics, including (i) the route to $1:1$ CDS, (ii) various phase dynamics, such as phase drifting, slipping and locking, and (iii) the thermoacoustic amplitude variations in the $f_f/f_1$ – $\epsilon _f$ plane. This study extends the applicability of open-loop control further to a chaotic thermoacoustic system, demonstrating (i) the feasibility of using an existing actuation strategy to weaken aperiodic thermoacoustic oscillations, and (ii) the possibility of developing new active suppression strategies based on both established and emerging methods of chaos control.

Compressible Large Eddy Simulation of Thermoacoustic Instabilities in the PRECCINSTA Combustor Using Flamelet Generated Manifolds With Dynamic Thickened Flame Model

Abstract The fully compressible, density-based CFD-solver TRACE has been extended for simulations of turbulent reacting flows in aero engine gas turbine combustors. The flamelet generated manifolds combustion model is utilized to account for detailed chemical kinetics and combined with the dynamically thickened flame model to resolve the flame front on the large eddy simulation (LES) mesh. The chemistry tabulation is coupled with the LES solver by inversion of the transported energy equation using tabulated mixture averaged NASA polynomial coefficients. LES of the PRECCINSTA test case, a lean, partially premixed swirl combustor are performed and the two distinctive regimes are correctly predicted: a stable regime with a “quite” stable flame and an unstable regime with an oscillating flame driven by self-excited thermoacoustic instabilities. Statistics collected from the simulations, mean, and root-mean-square values are in good agreement with the experimental reference data for both operating conditions. The dominant frequency of the unstable flame deviates from the measurement by about 100 Hz and requires further investigation. The results demonstrate the general suitability of the simulation framework for reacting flow simulations in gas turbine combustion systems and the prediction of self-excited thermoacoustic oscillations.

Application of heat transfer at the combustor wall to attenuate self-excited thermoacoustic oscillations

Self-sustained thermoacoustic oscillations (also known as combustion instability) arise in combustors when pressure and heat release fluctuations couple to produce high-energy sound waves. The need for diverse strategies to control these oscillations will increase as future design moves towards highly efficient, lean combustors. In this work, the impact of heat transfer at the combustor wall on the generation of thermoacoustic oscillations is examined through computational fluid dynamics. Characteristics of the thermoacoustic oscillations generated across cases with a range of thermal boundary conditions are compared to an adiabatic reference case. The amplitude of limit cycle oscillations in a Rijke tube is seen to decrease as wall temperatures increase. A stable combustor with a negative growth rate is observed when the full outer wall is heated to 900 K, and a sound pressure level reduction of 75 dB is achieved. The most efficient stable configuration is achieved when only the inlet duct wall is heated to 1000 K, requiring a minimum of 430 W of input power. The present work introduces an alternative stable combustor design and demonstrates that optimizing the temperature distribution of the combustor wall can effectively dampen thermoacoustic oscillations.

Experimental investigation on the effects of a mesh in the downstream region of a combustion-driven Rijke tube on self-excited thermoacoustic oscillations

Self-excited thermoacoustic oscillations are undesirable in most combustion systems due to their negative influence on combustion efficiency and structural vibration. Nonlinear thermoacoustic oscillations driven by combustion can be sensitive to changes in the heat and flow conditions. Therefore, a comprehensive study on self-excited thermoacoustic oscillations was conducted to investigate the effect of incorporating a woven mesh with varying mesh numbers and positions into the downstream region of a Rijke tube. This study determined system frequency and amplitude response and revealed dynamic properties of the system by applying recurrence analysis. To obtain temperature distributions, a thermal imaging system was employed, comprising a short wavelength infrared (SWIR) camera for the mesh and a long wavelength infrared (LWIR) camera for the tube wall. A high-speed schlieren imaging system was utilised for visualising heat and flow conditions at the tube end. The study demonstrated the effect of including a mesh in changing the oscillation eigenfrequency, suppressing the oscillation amplitude by up to 50 % and influencing the system dynamics. Higher mesh number demonstrated greater effectiveness. The captured thermal and schlieren images clearly evidenced the mesh’s significant influence on the downstream heat and flow conditions. The insights gained from this work provide a potential control method of self-excited, nonlinear thermoacoustic oscillations.

Experimental characteristics of adding biogas to premixed self-excited oscillating methane swirling flames

This work is the first to examine the influence of renewable energy biogas addition on the characteristics of methane self-excited thermoacoustic oscillation and pollutant NOx emission. Meantime, the swirl intensity and methane enrichment degree that affect the biogas combustion are also explored. Results indicate that the methane premixed self-excited oscillation swirling flames are effectively suppressed during the biogas addition and present two dynamic combustion states: high-frequency self-excited oscillation and swirl stable combustion. Furthermore, the overall flame temperature and NOx emissions have been reduced, which reveals that the introduction of biogas has effectively restrained thermoacoustic oscillation and limited NOx emissions. Notably, the combustion and emission characteristics of methane/biogas co-firing under low swirl intensity are superior to those under high swirl intensity. However, enriching biogas with more methane results in better flame dynamics of biogas/methane but comes with the cost of higher NOx generation. Regarding the flame and schlieren morphology, with the increase of biogas content in the fuel, the flame shape gradually transforms from the compact and robust M-typed flame to the loose and trumpet V-typed steady flame. The flame schlieren's visualization characterizes the flow field and vortex structures' stretching and deformation in methane-air premixed flame with biogas addition. The distribution appearance of the flame flow field and vortex under lean combustion are more complex and diverse than those under rich combustion. The study's conclusions may contribute to optimizing and improving the combustion and emission performance of premixed combustors primarily based on fossil fuel methane.

Characterisations of self-excited nonlinear oscillations at varied system parameters in a Rijke tube with a premixed laminar flame

Thermoacoustic instability has been one of the major concerns in various industrial combustion systems, such as Rich burn, Quick mix, Lean burn engine and aerospace engines. The complex interactions between heat release and pressure fluctuations could generate thermoacoustic oscillations with high amplitudes and frequencies in a combustion chamber, which causes many combustion problems even severe structural damage. The studies about the effects of various system parameters on thermoacoustic instability would be important due to their crucial roles in characterising the system oscillations. In this study, the effects of different system parameters in a Rijke tube, including equivalence ratio, methane flowrate, burner position and tube inner diameter, have been investigated. The time-domain analysis methods, including both phase space analysis constructed from the time-delayed method and recurrence analysis, have been conducted to explore the system characteristics under different system parameters. Furthermore, the Recurrence Quantitative Analysis (RQA) and Wayland translation error have been processed to quantify the system nonlinearity. The study has shown the effects of system parameters on system frequency response, deterministic nature and nonlinear dynamics. The system oscillation tends to be more complex and unstable when the burner is moved to lower positions or in a large diameter tube. The phase space and recurrence analysis are found to be efficient for characterising the system nonlinearity with useful insights of self-excited thermoacoustic oscillations.

Amplitude death and growth in a pair of nonidentical thermoacoustic oscillators interacting via time-delay and dissipative coupling

We numerically explore the effect of asymmetric limit-cycle amplitudes on the quenching and growth of self-excited thermoacoustic oscillations in two nonidentical Rijke tubes interacting via time-delay and dissipative coupling. When either type of coupling is applied separately, we find that increasing the asymmetry of the (uncoupled) limit-cycle amplitudes can shrink the regions of amplitude death (AD) in both oscillators, while producing a new region in which one of the oscillators becomes amplified, a state we refer to here as amplitude growth (AG). We find that the AG region resembles a 1:1 Arnold tongue when only dissipative coupling is applied, but that it splits into two discrete branches when only time-delay coupling is applied. When both types of coupling are applied simultaneously, we find that the AD and AG regions change in ways that depend sensitively on the detuning between the two oscillators. The findings of this study could be used to gain a better understanding of the can-to-can acoustic interactions in modern gas turbines, facilitating the development of both passive and active strategies to control thermoacoustic instability.

Energy Harvesting Exploiting Self-excited Thermoacoustic Oscillations

Energy Harvesting Exploiting Self-excited Thermoacoustic Oscillations

Equivalence ratio independence and dependence ranges of system responses for a nonlinear thermoacoustic oscillation in a Rijke tube

Thermoacoustic instabilities in a combustion system remains a challengeable problem in industry over decades. It usually involves with the nonlinear interaction between unsteady heat release and pressure fluctuations. The nonlinear oscillation excited by this interaction can be intense enough to result in combustion problems even severe damage of the engine. Therefore, it is important to understand the system response to various parameters. In this study, the system nonlinearity to equivalence ratio change was mainly focused on. A Rijke tube system with a premixed methane-air laminar flame was utilised to trigger the self-excited oscillations with varied equivalence ratios (Φ). The time-domain analysis methods including phase difference, Recurrence Plot (RP) and Recurrence quantification analysis (RQA) were applied to obtain the characteristics of system nonlinearities. Both Φ-independence and Φ-dependence ranges of system frequency responses have been discovered. The results of phase difference and recurrence analysis have also validated the finding of both ranges. The corresponding characteristics of system nonlinearity in both ranges have been investigated. The similarity among the systems in the Φ-independence range under the same burner position and methane flowrate can be highlighted from the system frequency response, phase difference and system dynamics. It is found that the phase difference analysis, RP and RQA utilised in this study are capable of investigating the nonlinearities of the self-excited thermoacoustic oscillation.

Quenching and amplification of thermoacoustic oscillations in two nonidentical Rijke tubes interacting via time-delay and dissipative coupling.

We numerically explore the quenching and amplification of self-excited thermoacoustic oscillations in two nonidentical Rijke tubes interacting via time-delay and dissipative coupling. On applying either type of coupling separately, we find that the presence of nonidentical heater powers can shrink the regions of amplitude death in both oscillators, while producing new regions of amplitude amplification in the weaker oscillator. We find that the magnitude of amplitude amplification grows with the heater power mismatch and with the total power input. These effects are also present when both types of coupling are applied simultaneously. This study highlights the critical role that nonidentical thermal loads can play in determining the amplitude response of coupled thermoacoustic systems, facilitating the design of control strategies for coupled oscillatorlike devices such as gas turbines.

Numerical simulation of nonlinear phenomena in a standing-wave thermoacoustic engine with gas-liquid coupling oscillation

• A CFD model of gas-liquid standing-wave thermoacoustic engine is developed and experimentally verified. • Time-domain impedance boundary condition is employed to truncate the modelling of liquid column. • Entire transient evolution of self-excited thermoacoustic oscillation from onset to saturation is captured. • Three different acoustic streamings are observed through inspecting the time-averaged mass flux field. A computational fluid dynamics (CFD) model based on the compressible, nonlinear Navier-Stokes equations is developed to simulate the nonlinear phenomena in a gas-liquid standing-wave thermoacoustic engine, which employs gas as the working fluid and liquid as the phase-matching element. The CFD model is well validated when comparing the simulated onset and steady-state performances with those measured from a real experimental system. Based on the CFD model, the nonlinear phenomena of the engine are analyzed from three aspects. Firstly, the nonlinear dynamic phenomena of self-excited thermoacoustic oscillation from onset to saturation are investigated, focusing on onset temperature difference and steady-state pressure amplitude. Then, the nonlinear acoustic phenomena of the engine operating at steady state are analyzed. The steady-state fluctuating pressure is decomposed into a superimposition of the acoustic modes with different frequencies, which reveals the existence of high-order harmonics. Finally, the nonlinear hydrodynamic phenomena including multi-dimensional flow effect and mass streaming are observed by examining the steady-state flow fields in the engine. The proposed CFD model provides an effective approach to comprehending the mechanism of nonlinear phenomena in gas-liquid coupling thermoacoustic engines.

Determination of the heat conduction transfer function within the thermoacoustic instability limit cycle in a Rijke tube

In this work, experimental measurements of the heat conduction transfer function (HCTF) within the limit cycle of thermoacoustic instabilities were conducted associated with a feedback controller. The responses of the heat conduction from the heater to the air in the vicinity to the oncoming flow disturbance can be described as the HCTF. When self-excited thermoacoustic instability occurs, it is not possible to measure the HCTF. The measurement experiment is twofold. Firstly, the active feedback control is applied to suppress the self-excited thermoacoustic oscillation in the Rijke tube. Secondly, the two-microphone method is applied to achieve the measurement of the HCTF at different working conditions. Experimental results show that the mean air flow velocity has an effect on the attenuation time of the thermoacoustic oscillation. Both of the mean air flow velocity and the electric-heating power have a significant effect on HCTFs.

Numerical investigation on oscillations characteristics of self-excited thermoacoustic system forced by acoustic wave

Self-excited thermoacoustic oscillations usually occur in many practical systems such as rocket motors, gas turbines and cryogenic distribution systems. We study the dynamics of a self-excited cryogenic thermoacoustic system subjected to acoustic forcing. The effect of the driving phase on the amplitude of the coupled oscillation is analyzed and the results present two states. To provide a guide for the selection of driving signal, the dynamics of a self-excited cryogenic thermoacoustic system subjected to acoustic forcing are investigated. When ϕ=30°, the radius of the limit cycle gradually shrinks, and self-oscillation amplitude is suppressed by 99.5%. For small driving phase, the self-oscillation can be suppressed, while for large driving phase, the phase portraits depict period-1 limit cycle oscillations. The results show that excessive driving phase has unfavorable effect on self-excited oscillation, and small drive phase can achieve oscillation suppression better. To reveal the coupling mechanism of externally forced oscillation and system self-excited oscillation, the phase relationships between self-excited oscillations and coupling pressure fluctuation waves in different periods are analyzed. Hence, the work indicates that under low-frequency disturbances, the system oscillation will have a phase delay, which will cause the hysteresis dissipation and restrain the system oscillation.

Experimental study of transition to instability in a Rijke tube with axially distributed heat source

Self-excited thermoacoustic oscillations are favorable in thermoacoustic engines, but unwanted in many combustion systems due to the detrimental outcomes, including the structure vibration and overloaded thermal flux to combustor wall. To explore some proper methods to modify thermoacoustic oscillations, understanding the mechanism of thermoacoustic instability is needed. In the present work, the transition to instability in a Rijke-type thermoacoustic system with axially distributed heat source is explored experimentally. A silicon/ceramic tube is used as the acoustic resonator, and the distributed heat source is designed by wounding electric wires over two ceramic rings. The influences of the characteristics of heat source (including heating power, heater length and heater location), mass flow rate, and tube materials on the nonlinear dynamic behaviors and stability boundaries of the Rijke-type thermoacoustic system are systematically evaluated. Large-amplitude limit cycle is observed, and subcritical and supercritical bifurcations are present in the thermoacoustic system. For the system with fixed mass flow rates, the critical heat power for transition to instability is found to be approximately linearly proportional to the heater length, while the strength of pressure oscillations responds nonlinearly. In addition, the system with a thin heater is more prone to undergo subcritical bifurcation. As for the influence of the mass flow rate, it has also been found that increasing the mass flow rate would enhance the nonlinearity of the coupling between the unsteady heat release rate and acoustic waves, which leads to a high possibility of occurrence of subcritical bifurcation. For all the heater length studied, there is an optimal mass flow rate where the critical heat power for the transition to instability is minimal. Last, the results show that the thermoacoustic system with ceramic tube has a narrower stability region at small air flow rate and is more prone to subcritical bifurcation than the silicon tube.

Study on Passive Control of the Self-excited Thermoacoustic Oscillations Occurring in Combustion Systems

ABSTRACT Self-excited thermoacoustic oscillations seriously threaten the operation of combustion equipment. Various active and passive control methods for self-excited thermoacoustic oscillations have been proposed, while most existing methods are complex and difficult to implement. A new passive control method was expected to be proposed in the present study. The thermoacoustic oscillations occurring in asymmetrical two-dimensional combustion systems were investigated numerically. The control effects of the asymmetrical fuel-air feed conditions on thermoacoustic oscillations were examined. In addition, experimental tests were performed to further examine the control effects. The results showed that the oscillations would be encouraged in an asymmetrical combustion system if an appropriate standing wave was formed. The asymmetrical fuel-air feed conditions could affect the waveform and consequently influence the occurrence of thermoacoustic oscillations. The oscillations could be eliminated by adjusting the length or the inlet boundary condition for half of the fuel-air feed channels, and the combustion chamber did not need to be retrofitted. The passive control method was simple and easy to implement for eliminating thermoacoustic oscillations.

Numerical investigation on the porosity effect of a bias-flow perforated liner on attenuating self-excited thermoacoustic oscillations

Self-excited thermoacoustic oscillations are unwanted in propulsion and stationary gas turbines due to the undesirable structural and heating damage to the engine systems. To prevent overheating and damping noise, perforated liners are widely applied in engines. It is a cylindrical metal sheet perforated with a number of millimetre-size circle orifices. In this work, numerical studies are conducted on an acoustically open-open thermoacoustic combustor with premixed propane and air fuelled and burnt. A perforated liner with a porosity σ is implemented downstream of the combustor in the presence of a cooling flow (also known as bias flow). The effects of (1) the cooling mass flow rate ṁb and (2) the porosity σp are examined one at a time. It is found that in the absence of the cooling flow, i.e., ṁb = 0, the perforated liner’s acoustic damping effect is quite small. Increasing is found to increase the liners damping performance in terms of attenuating the thermoacoustic oscillations, due to the enhanced vortex shedding generated over the rims of the perforated holes. In addition, there is an optimum cooling flow rate corresponding to the maximum acoustic damping effect of the liner. Furthermore, for a given mass flow rate, there is an optimum porosity.

Nonlinear interaction between clustered unstable thermoacoustic modes in can-annular combustors

A can-annular combustor consists of a set of nominally identical cans, in which the flames burn in an essentially isolated manner. However, adjacent cans are able to communicate acoustically, which provides dynamic coupling of the entire can-annular arrangement. Recently, it was shown that the acoustic coupling is not negligible and can cause clustering of eigenfrequencies. In this study, we present a low-order modeling framework for self-excited thermoacoustic oscillations in generic can-annular combustors consisting of N identical cans. The dynamics of the flames are modeled with the nonlinear G-equation; the acoustic model accounts for plane acoustic waves inside the cans and can-to-can communication. The latter is enabled through a coupling boundary condition that is based on conservation of mass and a Rayleigh conductivity. For weak coupling between adjacent cans, the thermoacoustic feedback cycle shows clusters of linearly unstable modes of different azimuthal order, which are close in frequency and growth rate. Their interaction in the nonlinear regime is investigated using time-domain simulations. Two simulations for generic can-annular combustors consisting of 4 and 6 cans with weak acoustic coupling are discussed in this study. We observe a strong interaction between the modes, which can cause long transition times and allows modes that do not dominate the system dynamics in the linear regime to be dominant in the nonlinear regime. While the N=6 case converges to a periodic oscillation pattern with one dominant frequency, the N=4 case converges to a quasi-periodic oscillation involving modes of different azimuthal order. Moreover, we observe a synchronization of these modes. These results raise the questions whether it is possible to predict which mode(s) will dominate the system in the saturated state and under which conditions synchronization of clustered modes can occur.

Mitigating self-excited thermoacoustic oscillations in a liquid fuel combustor using dual perforated plates

Perforated plates are widely used in many practical burners to attenuate noise emissions. In this study, the acoustic absorption capability of dual perforated plates (DPPs) with different porosities and aperture diameters was evaluated and tested in an impedance tube, and the damping performance of the DPPs located in a liquid fuel combustor inlet section was experimentally studied. The DPPs have an adjustable first cavity and can mitigate the thermoacoustic oscillations within a wide sound absorption bandwidth. The DPPs can absorb 95% of the specified incident sound energy. The combustion results indicated that the installation of DPPs at the inlet section has two effects: sound attenuation and the frequency shift of the combustor. The maximum reduction in dynamic pressure and CH* chemiluminescence intensity inside the chamber were 14 and 19 dB, respectively. When the primary air flow rate deviates from the optimal value, the DPPs can reduce the pressure amplitude in the combustion chamber by almost 80%. In general, this study may promote the application of DPPs under bias flow for the stabilization of spray combustion.

Influence of asymmetric flame transfer functions on can-to-can acoustic interactions between two lean-premixed combustors

In can-annular gas turbines, low-frequency thermoacoustic instabilities can arise from cross-talk interactions between neighboring combustors upstream of the first-stage turbine nozzles. In this experimental study, we investigate the influence of non-identical flame transfer functions (FTFs) between adjacent combustors on the development of self-excited thermoacoustic oscillations. To create different FTFs, we use five different swirl nozzles, one with high swirl (HS) and four with low swirl (LS), all with different porosities. We find that, compared with the LS FTFs, the HS FTF exhibits a smaller and flatter gain as well as a smaller phase difference. We attribute this behavior to differences in the flame structure and the stabilization mechanisms, namely an inner shear layer-stabilized diverging front in the HS case versus a detached reaction zone in the presence of a central jet with an outer swirl flow in the LS cases. Using two tunable lean-premixed combustors connected via a cross-talk section, we show that (i) symmetric FTF combinations (HS + HS or LS + LS) produce in-phase interactions, leading to push-push modes, but that (ii) asymmetric FTF combinations (HS + LS) produce out-of-phase interactions, leading to push-pull modes. Phase-resolved visualization of the asymmetric cases reveals that the inner shear layer-stabilized HS flame exhibits large angle fluctuations, whereas the aerodynamically stabilized LS flame is characterized by the periodic emergence of a bow-shaped front and an oval structure. For all the conditions tested, we find that asymmetry in the FTFs leads to either (i) a completely stable state with negligible amplitude or (ii) a mildly unstable state with an amplitude lower than that of the equivalent symmetric cases. These findings highlight the potential of using asymmetric FTFs for passive control of cross-talk-driven thermoacoustic instabilities in can-annular combustors.