Unsteady Heat Release Research Articles

Synchronization framework for modeling transition to thermoacoustic instability in laminar combustors

We, herein, present a new model based on the framework of synchronization to describe a thermoacoustic system and capture the multiple bifurcations that such a system undergoes. Instead of applying flame describing function to depict the unsteady heat release rate as the flame's response to acoustic perturbation, the new model considers the acoustic field and the unsteady heat release rate as a pair of nonlinearly coupled damped oscillators. By varying the coupling strength, multiple dynamical behaviors, including limit cycle oscillation, quasi-periodic oscillation, strange nonchaos, and chaos can be captured. Furthermore, the model was able to qualitatively replicate the different behaviors of a laminar thermoacoustic system observed in experiments by Kabiraj et al.~[Chaos 22, 023129 (2012)]. By analyzing the temporal variation of the phase difference between heat release rate oscillations and pressure oscillations under different dynamical states, we show that the characteristics of the dynamical states depend on the nature of synchronization between the two signals, which is consistent with previous experimental findings.

Nonlinear response of swirling premixed flames to helical flow disturbances

This paper considers the relationship between nonlinearly interacting helical flow disturbances and flame area response in a swirling premixed flame. The present study was performed to determine whether there are nonlinear mechanisms through which helical modes ( ) can lead to non-zero unsteady heat release rate oscillations. The results show that for single frequency content (at ), helical modes excite unsteady heat release rate response of and that two-frequency excitation (e.g. at and ), leads to a response of at . There are two mechanisms through which this can occur: First, helical flow disturbances can distort the time-averaged flame shape to have an azimuthal component that matches that of the incident disturbance, . Second, multiple helical modes can nonlinearly interact to cause axisymmetric unsteady flame wrinkling. The paper derives the various modal contributions in the incident velocity disturbance that satisfy these criteria. These results suggest that it is only the mode which controls the linear dynamics (e.g. instability inception conditions) of these flames (where ), but that their nonlinear dynamics is also controlled by the helical modes.

A state-space formulation of a discontinuous Galerkin method for thermoacoustic stability analysis

A hybrid approach for thermoacoustic stability analysis is formulated in a state-space framework. The approach distinguishes between regions of the computational domain with and without important interactions between acoustics and mean flow or unsteady heat release, respectively. The former regions are modeled by a discontinuous Galerkin finite element method (DG-FEM) for the linearized Navier-Stokes equations in conservative form. The latter are represented by reduced-order models of acoustic wave propagation or dissipation, and provide complex-valued, frequency dependent impedance boundary conditions for the DG-FEM domain. The flow-flame coupling is modeled by a flame transfer function that governs a volumetric source term for the fluctuating heat release rate.The respective (sub-)models are formulated and interconnected in a state-space framework, which facilitates the monolithic formulation of hybrid thermoacoustic models. Moreover, the state-space interconnect framework makes it possible to formulate thermoacoustic stability analysis as a linear eigenvalue problem – even if flame transfer function or acoustic boundary conditions depend in a non-trivial manner on frequency.The approach is first verified against analytical solutions for a duct with mean flow across a thin heat source, similar to a Rijke tube. Then the thermoacoustic eigenmodes of a premixed, swirl-stabilized combustor are computed in order to validate the method against experimental data for a configuration of applied interest. For this second validation case, a detailed comparison against predictions of a low-order network-model is also presented.

Stochastic properties of thermoacoustic oscillations in an annular gas turbine combustion chamber driven by colored noise

Large-amplitude thermoacoustic oscillations are unwanted in gas turbines due to the detrimental damage to combustors. Thus predicting the onset of such oscillations and a better understanding of the unstable behaviors are important. In this work, the stochastic properties of thermoacoustic oscillations in the subthreshold region of thermoacoustic systems are investigated theoretically and numerically. The energy conversion from the unsteady heat release to sound is mainly achieved via two ways: one is caused by inherent turbulent fluctuations, and the other is due to the flame response to the acoustic waves. The turbulence-induced non-coherent heat release fluctuation is characterized by colored noise, and the coupling between the unsteady heat release and acoustic pressure is characterized by a 3rd order polynomial. Both stochastic averaging and stochastic normal form are utilized to approximate the responses of the system as a Markov process. The result shows that the correlation time τc and intensity D of the colored noise have significant but opposite effects on the dynamics of thermoacoustic oscillations, including the most probable amplitude, autocorrelation of the system as well as the correlation time. In addition, the resonance-like behaviors in signal-to-noise ratio (SNR) are observed, which denotes the emergence of coherence resonance (CR) in this annular combustion system. The optimal noise intensity, at which SNR is maximized, becomes sensitive to the variation of τc, as τc is larger than certain value. Lastly, the extent of the system coherent motions relies significantly on the proximity to the supercritical Hopf bifurcation point. Being closer to the stability boundary is found to increase the strength of system association. Meanwhile, SNR of the colored noise-induced motion becomes more distinguished, and the optimal noise intensity is shifted to a smaller value. This variation can serve as a precursor to predict the onset of thermoacoustic instability, as the correlation time remains unchanged.

Combustion modes and driving mechanisms of pressure fluctuation in a novel vortex-tube combustor with quasi-steady and stratified properties

The operating limit and pressure fluctuations of a localized stratified vortex-tube combustor (LSVC) are investigated experimentally by taking methane as fuel at the inlet temperature of 300 K and the combustor pressure of 1 atm over a range of equivalence ratios (0.1–1.0). The combustion modes are distinguished according to the properties of pressure fluctuations and the driving mechanisms of each combustion mode are explored in combination with numerical simulation. The results show that the LSVC can realize stable combustion with pressure fluctuation amplitudes always less than 4 kPa in a large operating range, and the flame front is always continuous. The operating flammability limit experimental range can be divided into five regimes with different combustion modes. The first and fifth combustion modes are steady combustion with pressure fluctuation amplitudes less than 1 kPa, and the second to fourth ones are quasi-steady combustion with pressure fluctuation amplitudes between 1–4 kPa. The spectrum analyses of pressure fluctuations show that there are one low-frequency peak around 300 Hz and one high-frequency peak around 1500 Hz, which are dominated by the first and third axial natural acoustic modes of resonant oscillation combustion, respectively. In the first and fifth combustion modes, the resonances are both weak due to the influences of low flow rate and laminarization respectively. In the second mode, the thermo-acoustic coupling oscillation and the resonance are excited simultaneously, yielding the highest pressure fluctuation amplitude in the entire operating range. The high-frequency resonance causes the high-frequency pressure fluctuation of the third mode. Both the unsteady heat release and flow field affect the pressure fluctuation in the fourth mode. The former produces the low-frequency fluctuation, which can resonate with the natural frequency and excite a weak thermal-acoustic coupling.

Noise sources of an unconfined and a confined swirl burner

The sound sources of an unconfined and a confined swirl burner are investigated by a two-step approach. The Reynolds number based on the injector diameter downstream of the swirler is ReD = 8800 and the swirl number is S = 0.73. First, the conservation equations of a compressible fluid are solved to determine the flow field under reacting and non-reacting conditions. The solution determines the mean flow field and the source terms of the acoustic perturbation equations (APE). The contributions of the various sound sources are analyzed by solving the acoustic perturbation equations (APE) in a computational aeroacoustics (CAA) simulation for each of the source terms. For the unconfined swirl burner, the precessing vortex core (PVC) of the swirl flow is a dominant sound source in non-reacting flow, whereas in reacting flow the unsteady heat release dominates the sound field. This changes when the burner is confined. A self-excited instability occurs at the quarter-wave mode of the burner and further sources, i.e., the momentum source due to fluctuations of the pressure at the flame front and the energy source due to velocity fluctuations at the flame front, become important to accurately predict the sound pressure amplitude of the limit cycle.

Scaling and prediction of transfer functions in lean premixed H2/CH4-flames

Features of the flame transfer function (FTF) are characterized for turbulent, non-swirled, bluff body stabilized “M” flames for different hydrogen and methane blends including pure hydrogen flames. An increase in the cut-off frequency of the FTF is observed for increasing hydrogen concentration. Modulations in the form of peaks and troughs in the gain and the phase were also observed and are shown to be caused by the interaction of two different flow disturbances, acoustic and convective, originating upstream of the flame. The first mechanism is due to the acoustic velocity fluctuations imposed at the base of the flame. A Strouhal number scaling based on the flame height and bulk velocity is shown to collapse the phase slopes and the cut-off frequencies. The second mechanism is shown to be due to vortex shedding from the grub screws used to align the bluff body in the inlet pipe. The associated convective time-delay is used to define a second Strouhal number which collapses the modulations in the gain and phase.A model is developed that separately considers the impulse response of each mechanism and is interpreted as a distribution of time lags between velocity fluctuations and the unsteady heat release rate. The distributed time lag (DTL) model consists of two distributions that are shown to capture all the features of the FTFs. The distributions show that the acoustic and convective mechanisms behave as a low pass filter and band pass filter, respectively. This results in a band of frequencies where they interact through superposition driving fluctuations of heat release rate. Similar interactions are shown to exist in the forced cold flow revealing that they are of hydrodynamic origin. Further, the band of frequencies are shown to be centered around the natural shedding frequency of the grub screws appearing as peaks in the unforced energy spectra of the velocity at the dump plane.Finally, a generalized model which takes as an input the bulk velocity, flame height and a geometric parameter is derived assuming a linear dependency of the DTL parameters. The model is shown to predict the behavior of the FTFs relatively well and can potentially be used to analyse regions in the operating conditions map which have not been experimentally tested.

Numerical Simulation of Wave Formation and Heat Transfer in Falling Liquid Films at Unsteady Heat Release

The presented mathematical model enables calculation of the wave surface profile, as well as the fields of velocity and temperature, in falling wavy liquid films. Numerical simulation of wave formation and heat transfer intensity was performed for falling films of liquid nitrogen. Different activation functions for input perturbations were checked for films with different parameters. The dependencies of the time till boiling onset and total local evaporation time on the heat flux density were calculated for different inlet Reynolds numbers. Generalization of the simulation results resulted in a regime map, which describes different mechanisms of film flow decay. The presented results of numerical simulation are in satisfactory agreement with the experimental data.

Intrinsic thermoacoustic modes in an annular combustion chamber

Thermoacoustic instabilities originate from the interaction of unsteady heat release rate associated with flames and the acoustic modes of a combustor. A feedback loop not involving the natural acoustic modes has been observed in single-flame configurations with anechoic terminations: an acoustic wave emitted by the flame travels upstream, and the associated velocity fluctuation again excites the flame. This feedback cycle gives rise to thermoacoustic modes intrinsic to the flame. An analytical model for an annular thermoacoustic system is formulated, and the existence of intrinsic modes of various azimuthal orders is demonstrated. The spectrum of an annular combustor is computed with a three-dimensional thermoacoustic Helmholtz solver. The configuration resembles those commonly found in gas turbines. In addition to the observations in previous studies, numerous intrinsic modes are found, with frequencies close to the lowest acoustic modes. All of the intrinsic modes can be grouped into clusters, at frequencies corresponding to multiples of the inverse flame response time delay. It is demonstrated that the newly observed intrinsic modes belong to the same mechanism that has recently been studied in single-sector/flame configurations. An analysis of the evanescent character of cut-off azimuthal modes explains the pattern in the spectrum. The underlying physical mechanism is generically present in any annular combustion chamber and a possible source of instability.

Experimental Investigation on the Route to Vortex-acoustic Lock-In Phenomenon in Bluff Body Stabilized Combustors

ABSTRACT We investigate the phenomenon of vortex-acoustic lock-in in a bluff body stabilized Rijke type combustor, which experiences self-excited thermoacoustic oscillations. The focus is to identify and understand the various regions (in airflow rate) occurring during the transition to lock-in. In this regard, an axisymmetric bluff body stabilized burner is used, which sheds Benard–Von Karman vortices. The dynamics of the combustor is monitored through the measurement of unsteady pressure and heat release rate fluctuations. At low airflow rates, peaks associated with both vortex shedding and acoustic modes are observed in the measured spectra. These peaks are far apart. As the airflow rate is increased, vortex shedding frequency increases. At a certain flow rate, it reaches the acoustic frequency. Beyond this flow rate, large amplitude oscillations occur and only one common peak is observed in the spectra. This common peak has the frequency close to the acoustic mode of the duct. Vortex shedding process locks in to this common frequency and the phenomenon is termed as vortex-acoustic lock-in. We observe a series of well-defined regions that appear as the system progresses toward lock-in. In this paper, we made an attempt to understand the characteristics of these regions in a systematic manner. The study will help in developing lower-order models, which capture the essential dynamics of lock-in observed in vortex shedding combustors.

Lagrangian Analysis of Flame Dynamics in the Flow Field of a Bluff Body-Stabilized Combustor

Abstract Experiments are performed in a partially premixed bluff body-stabilized turbulent combustor by varying the mean flow velocity. Simultaneous measurements obtained for unsteady pressure, velocity, and heat release rate are used to investigate the dynamic regimes of intermittency (10.1 m/s) and thermoacoustic instability (12.3 m/s). Using wavelet analysis, we show that during intermittency, modulation of heat release rate occurring at the acoustic frequency fa by the heat release rate occurring at the hydrodynamic frequency fh results in epochs of heat release rate fluctuations where the heat release rate is phase locked with the acoustic pressure. We also show that the flame position during intermittency and thermoacoustic instability are essentially dictated by saddle point dynamics in the dump plane and the leading edge of the bluff body.

On the mechanism of open-loop control of thermoacoustic instability in a laminar premixed combustor

We identify mechanisms through which open-loop control of thermoacoustic instability is achieved in a laminar combustor and characterize them using synchronization theory. The thermoacoustic system comprises two nonlinearly coupled damped harmonic oscillators – acoustic and unsteady heat release rate (HRR) field – each possessing different eigenfrequencies. The frequency of the preferred mode of HRR oscillations is less than the third acoustic eigenfrequency where thermoacoustic instability develops. We systematically subject the limit-cycle oscillations to an external harmonic forcing at different frequencies and amplitudes. We observe that forcing at a frequency near the preferred mode of the HRR oscillator leads to a greater than 90 % decrease in the amplitude of the limit-cycle oscillations through the phenomenon of asynchronous quenching. Concurrently, there is a resonant amplification in the amplitude of HRR oscillations. Further, we show that the flame dynamics plays a key role in controlling the frequency at which quenching is observed. Most importantly, we show that forcing can cause asynchronous quenching either by imposing out-of-phase relation between pressure and HRR oscillations or by inducing period-2 dynamics in pressure oscillations while period-1 in HRR oscillations, thereby causing phase drifting between the two subsystems. In each of the two cases, acoustic driving is very low and hence thermoacoustic instability is suppressed. We show that the characteristics of forced synchronization of the pressure and HRR oscillations are significantly different. Thus, we find that the simultaneous characterization of the two subsystems is necessary to quantify completely the nonlinear response of the forced thermoacoustic system.

Flame interactions in a stratified swirl burner: Flame stabilization, combustion instabilities and beating oscillations

The present article investigates the interactions between the pilot and main flames in a novel stratified swirl burner using both experimental and numerical methods. Experiments are conducted in a test rig operating at atmospheric conditions. The system is equipped with the BASIS (Beihang Axial Swirler Independently-Stratified) burner fuelled with premixed methane-air mixtures. To illustrate the interactions between the pilot and main flames, three operating modes are studied, where the burner works with: (i) only the pilot flame, (ii) only the main flame, and (iii) the stratified flame (with both the pilot and main flames). We found that: (1) in the pilot flame mode, the flame changes from V-shape to M-shape when the main stage is switched from closed to supplying a pure air stream. Strong oscillations in the M-shape flame are found due to the dilution of the main air to the pilot methane flame. (2) In the main flame mode, the main flame is lifted off from the burner if the pilot stage is supplied with air. The temperature of the primary recirculation zone drops substantially and the unsteady heat release is intensified. (3) In the stratified flame mode, unique beating oscillations exhibiting dual closely-spaced frequencies in the pressure spectrum is found. This is observed within over narrow window of equivalence ratio combinations between the pilot and main stages. Detailed analysis of the experimental data shows that flame dynamics and thermoacoustic couplings at these two frequencies are similar to those of the unstable pilot flame and the attached main flame cases, respectively. Large Eddy Simulations (LESs) are carried out with OpenFOAM to understand the mechanisms of the time-averaged flame shapes in different operating modes. Finally, a simple acoustic analysis is proposed to understand the acoustic mode nature of the beating oscillations.

Coefficient of Rayleigh Index based performance evaluation of radial micro jet injection technique for thermo-acoustic instability control

This study proposes a coefficient of Rayleigh Index (CRI) obtained by normalizing the acquired data which can circumvent the problem of arbitrary scale and unit while quantifying the Rayleigh Index. CRI is estimated from the simultaneous measurements of luminescence of a flame as an indicator of unsteady heat release and pressure fluctuation near the burner head. Experiments were carried out for six different mixture equivalence ratios ranging from 0.75 to 1.0 and a burner positioned at x/L = 0.2. The control technique of micro jets is found to completely suppress the thermo-acoustics by decoupling the pressure waves and unsteady heat release waves as indicated by increasing phase difference between them. The coefficient of Rayleigh Index is found to reduce to nearly zero with the use of radial micro jets for suppressing the thermo-acoustic instability. In such case, the flame luminescence was observed to significantly reduce. The proposed coefficient of Rayleigh index is very useful in quantifying the effectiveness of control technique for the thermo-acoustic instability.

Forced synchronization of quasiperiodic oscillations in a thermoacoustic system

In self-excited combustion systems, the application of open-loop forcing is known to be an effective strategy for controlling periodic thermoacoustic oscillations, but it is not known whether and under what conditions such a strategy would work on thermoacoustic oscillations that are not simply periodic. In this study, we experimentally examine the effect of periodic acoustic forcing on a prototypical thermoacoustic system consisting of a ducted laminar premixed flame oscillating quasiperiodically on an ergodic $\mathbb{T}^{2}$ torus at two incommensurate natural frequencies, $f_{1}$ and $f_{2}$. Compared with that of a classical period-1 system, complete synchronization of this $\mathbb{T}_{1,2}^{2}$ system is found to occur via a more intricate route involving three sequential steps: as the forcing amplitude, $\unicode[STIX]{x1D716}_{f}$, increases at a fixed forcing frequency, $f_{f}$, the system transitions first (i) to ergodic $\mathbb{T}_{1,2,f}^{3}$ quasiperiodicity; then (ii) to resonant $\mathbb{T}_{1,f}^{2}$ quasiperiodicity as the weaker of the two natural modes, $f_{2}$, synchronizes first, leading to partial synchronization; and finally (iii) to a $P1_{f}$ limit cycle as the remaining natural mode, $f_{1}$, also synchronizes, leading to complete synchronization. The minimum $\unicode[STIX]{x1D716}_{f}$ required for partial and complete synchronization decreases as $f_{f}$ approaches either $f_{1}$ or $f_{2}$, resulting in two primary Arnold tongues. However, when forced at an amplitude above that required for complete synchronization, the system can transition out of $P1_{f}$ and into $\mathbb{T}_{1,2,f}^{3}$ or $\mathbb{T}_{2,f}^{2}$. The optimal control strategy is to apply off-resonance forcing at a frequency around the weaker natural mode ($f_{2}$) and at an amplitude just sufficient to cause $P1_{f}$, because this produces the largest reduction in thermoacoustic amplitude via asynchronous quenching. Analysis of the Rayleigh index shows that this reduction is physically caused by a disruption of the positive coupling between the unsteady heat release rate of the flame and the $f_{1}$ and $f_{2}$ acoustic modes. If the forcing is applied near the stronger natural mode ($f_{1}$), however, resonant amplification can occur. We then phenomenologically model this $\mathbb{T}_{1,2}^{2}$ thermoacoustic system as two reactively coupled van der Pol oscillators subjected to external sinusoidal forcing, and find that many of its synchronization features – such as the three-step route to $P1_{f}$, the double Arnold tongues, asynchronous quenching and resonant amplification – can be qualitatively reproduced. This shows that these features are not limited to our particular system, but are universal features of forced self-excited oscillators. This study extends the applicability of open-loop control from classical period-1 systems with just a single time scale to ergodic $\mathbb{T}^{2}$ quasiperiodic systems with two incommensurate time scales.

Propagation speed of inertial waves in cylindrical swirling flows

Thermo-acoustic combustion instabilities arise from feedback between flow perturbations and the unsteady heat release rate of a flame in a combustion chamber. In the case of a premixed, swirl stabilized flame, an unsteady heat release rate results from acoustic velocity perturbations at the burner inlet on the one hand, and from azimuthal velocity perturbations, which are generated by acoustic waves propagating across the swirler, on the other. The respective time lags associated with these flow–flame interaction mechanisms determine the overall flame response to acoustic perturbations and therefore thermo-acoustic stability. The propagation of azimuthal velocity perturbations in a cylindrical duct is commonly assumed to be convective, which implies that the corresponding time lag is governed by the speed of convection. We scrutinize this assumption in the framework of small perturbation analysis and modal decomposition of the Euler equations by considering an initial value problem. The analysis reveals that azimuthal velocity perturbations in swirling flows should be regarded as dispersive inertial waves. As a result of the restoring Coriolis force, wave propagation speeds lie above and below the mean flow bulk velocity. The differences between wave propagation speed and convection speed increase with increasing swirl. A linear, time invariant step response solution for the dynamics of inertial waves is developed, which can be approximated by a concise analytical expression. This study enhances the understanding of the flame dynamics of swirl burners in particular, and contributes physical insight into the inertial wave dynamics in general.

Noise Sources of Lean Premixed Flames

The thermoacoustic sound generation mechanisms of lean premixed laminar and turbulent flames are investigated by a two-step approach. First, the conservation equations of a reacting compressible fluid are solved. This solution is used to determine the acoustic source terms of the acoustic perturbation equations (APE). Second, the contributions of the different source terms to the amplitude and phase of the acoustic pressure signal are analyzed by solving the APE in a computational aeroacoustics (CAA) simulation. The results show that it is not sufficient to only consider the unsteady heat release rate fluctuations which occur in the substantial pressure-density relation. The acceleration of density gradients occurring at the flame front is a significant contributor to the overall sound emission. For the investigated laminar and turbulent flames the amplitude and phase of the acoustic pressure signal can only be predicted accurately if both source terms are included in the acoustic analysis.

Design and development of thin wire sensor for transient temperature measurement

The temperature measurement is an important parameter in industrial process equipments. Various techniques were developed to measure the transient temperature of the fluid and the solid surface. The precise and accurate measurement of transient fluid temperature is also important in combustors of various power producing devices. During the combustion process in the combustor, when the coupling between heat release and pressure fluctuations takes place, thermo-acoustic instabilities are developed. These instabilities have adverse effect on the combustor. For predicting the instabilities Rayleigh index is very much essential, which can be estimated from unsteady heat release and pressure fluctuation measurement. In this study, the development of sensor, based on hot wire method, to measure unsteady heat release in combustion chamber is presented. For transient heat release measurement, fast response of a sensor is very much important. For fast response, the wire diameter of probe should be as small as possible. To develop a sensor, wire of different diameters and materials are used as probe materials. The stainless steel wire is used as prongs. The constructed sensor is kept in one of the arm of Wheatstone bridge to measure the resistance of sensor. For this NI LabVIEW software and NI-DAQ 6211 is used. From different results, it is observed that the resistance of sensor varies linearly with rise in temperature. The plunge test method is used to measure the response time of sensors. A 0.1 mm Pt wire sensor is checked for different diameter of SS prongs i.e. 0.5, 0.55 and 0.9 mm. A modified 0.1 mm Pt wire sensor with 0.5 mm SS prongs has response time of 50 ms, which is calibrated using FLUKE-9144 bath-tub from 0 °C to 500 °C temperature. Then it is tested in combustion chamber to measure unsteady temperature, in the presence of thermo-acoustic instability with J-Type thermocouple and variation in measured temperature was found to be 8.11%.

An Efficient Method to Reproduce the Effects of Acoustic Forcing on Gas Turbine Fuel Injectors in Incompressible Simulations

Previous studies have highlighted the importance of both air mass flow rate and swirl fluctuations on the unsteady heat release of a swirl stabilised gas turbine combustor. The ability of a simulation to correctly resolve the heat release fluctuations or the flame transfer function (FTF), important for thermoacoustic analysis, is therefore dependent on the ability of the method to correctly include both the swirl number and mass flow rate fluctuations which emerge from the multiple air passages of a typical lean-burn fuel injector. The fuel injector used in this study is industry representative and has a much more complicated geometry than typical premixed, lab-scale burners and the interaction between each flow passage must be captured correctly. This paper compares compressible, acoustically forced, CFD (computational fluid dynamics) simulations with incompressible, mass flow rate forced simulations. Incompressible mass flow rate forcing of the injector, which is an attractive method due to larger timesteps, reduced computational cost and flexibility of choice of combustion model, is shown to be incapable of reproducing the swirl and mass flow fluctuations of the air passages given by the compressible simulation as well as the downstream flow development. This would have significant consequences for any FTF calculated by this method. However, accurate incompressible simulations are shown to be possible through use of a truncated domain with appropriate boundary conditions using data extracted from a donor compressible simulation. A new model is introduced based on the Proper Orthogonal Decomposition and Fourier Series (PODFS) that alleviates several weaknesses of the strong recycling method. The simulation using this method is seen to be significantly computationally cheaper than the compressible simulations. This suggests a methodology where a non-reacting compressible simulation is used to generate PODFS based boundary conditions which can be used in cheaper incompressible reacting FTF calculations. In an industrial context, this improved computational efficiency allows for greater exploration of the design space and improved combustor design.

Control of self-excited thermoacoustic oscillations using transient forcing, hysteresis and mode switching

In many combustion devices, strong self-excited flow oscillations can arise from feedback between unsteady heat release and acoustics, resulting in increased vibration and pollutant emissions. Open-loop acoustic forcing has been shown to be effective in weakening such thermoacoustic oscillations, but current implementations of this control strategy require the forcing to be continuously applied. In this proof-of-concept study, we experimentally demonstrate an alternative method of weakening thermoacoustic oscillations in a self-excited combustion system – a laminar premixed flame in a double open-ended tube. Unlike existing methods, the proposed method combines the use of transient forcing with hysteresis and mode switching, thus avoiding the need to continuously supply energy to the control system. Control is achieved by exploiting the fact that most combustors have a multitude of natural thermoacoustic modes, some of which are linearly unstable but some are nonlinearly unstable. By applying open-loop acoustic forcing at an off-resonance frequency and at an amplitude higher than that required for synchronization, we find that the combustor can switch to one of the nonlinearly unstable natural modes (f2) and remain there, even after the forcing is removed. Dynamic mode decomposition of high-speed chemiluminescence videos shows that this mode switching occurs because the flame structure at f2 is more robust than that at the original linearly unstable natural mode. The final unforced state has a thermoacoustic amplitude of just half that of the initial unforced state, even though the Rayleigh index of the former is higher than that of the latter. Although this 50% reduction in thermoacoustic amplitude is not as large as the 95% reduction achieved with asynchronous quenching, it is achieved without the use of continuous forcing. This is a distinct advantage over existing control strategies as it allows the complexity and power requirements of the control system to be reduced. With further development and testing, particularly on turbulent swirling combustors, the proposed control strategy could pave the way for a new class of open-loop control techniques based on transient forcing rather than continuous forcing.