Flame Response Research Articles

Effect of Natural Agents on the Mechanical and Flame Retardant Properties of Cotton Fabric–Dyeing Technique

Abstract Home textile products manufactured and exported in huge quantities from Karur, Namakkal, Erode, and Salem are the established foremost important geographical locations centers in Southern India. The cotton fibers are less resistant to fire and that leads to unexpected accidents, thus there is an importance of the flame retardance tendency for home textiles. Eco-friendly herbal extract–dyed cotton has been investigated for tensile strength and flame retardant properties. The current research elaborately studies the antibacterial dyed cotton fabric materials in terms of improved flame retardant property through eco-friendly herbal extracts to cotton fabrics with simultaneous analysis on their tensile strength properties. The mangosteen-dyed cotton fabrics showed better results with specifications of 140 g/m2, plain weave with 40 % dye concentration, 240 g/m2, terry weave with 40 % dye concentration and untreated cotton fabrics of 140 g/m2 and 240 g/m2 had the tensile strength of 237.87 kg/cm2, 288.69 kg/cm2 and 238.38 kg/cm2, 288.47 kg/cm2. The flame response studies revealed the the burning time increases from 35 s for untreated 140 g/m2 plain cotton fabric to 50 s and 45 s for the fabrics dyed with mangosteen and borage. Correspondingly, the burning rate changes from 320 mm/min for untreated 140 g/m2 plain cotton fabrics to 290 mm/min and 300 mm/min for mangosteen- and borage-dyed fabrics, respectively. The limited oxygen index (LOI) test results showed that the borage-dyed fabrics exhibited little increased LOI values of 20 over the other treated samples on an average of LOI of 19 in comparison with the untreated cotton fabrics of LOI 18.

Comparison of flame response characteristics between Non‐premixed and premixed flames under acoustic excitation

A comparative study was conducted on the response characteristics according to the increase in velocity perturbation intensity (u'/u) under acoustic excitation conditions of non-premixed and premixed flames. In addition, the flame dynamic characteristics and flame structure according to acoustic excitation were analyzed, and the response characteristics were compared according to the two flame types through the flame transfer function (FTF) and flame height perturbation analysis. In a non-premixed flame, the number of waveforms (undulations) on the flame surface increased according to the forcing frequency in the flame structure according to the acoustic excitation. On the other hand, as the velocity perturbation intensity increased, the flame height decreased. Thus, in the low forcing frequency range, the magnitude of the flame fluctuation increases, and in the high forcing frequency, the magnitude of the flame fluctuation tends to decrease. Therefore, the Strouhal number and FTF gain are nonlinear properties, and the nonlinear effect depends on the magnitude of the gain and the critical Strouhal number. In a premixed flame, various flame structures are shown according to acoustic excitation. The average flame height does not change much with increasing (u'/u¯). The correlation between the Strouhal number and FTF gain is linear. The flame height perturbation parameters were introduced to examine the flame dynamic characteristics according to the acoustic excitation, and non-premixed and premixed flames were analyzed. As a result, it shows the trend and similarity of the FTF gain, suggesting its potential as a parameter for simple analysis of the heat-release rate. In the correlation between the Strouhal number and FTF gain, the non-premixed flame has a nonlinear characteristic and agrees with the analytical study. On the other hand, the premixed flame is not consistent with analytic studies when the Strouhal number is greater than 1. The reason for various flame structures is to consider parameters such as the unusual flame structure and laminar flame speed by external perturbation.

The effect of mean flow on the intrinsic thermoacoustic instabilities in the duct and annular combustion chambers

Thermoacoustic instability in the combustion chamber of gas turbines, aero and rocket engines has become a topic concern. In this paper, a new instability phenomenon in the field of the thermoacoustic instability - Intrinsic ThermoAcoustic instabilities (ITA) is studied. The classic thermoacoustic instability is caused by the coupling among acoustic disturbances, flow fluctuations and heat release pulsations. When the influence of acoustic disturbance is removed, there is still a thermoacoustic instability phenomenon in the combustion chamber, which is known as ITA. In the previous research on ITA, the scholars found that the ITA is mainly determined by the flame response, but many scholars neglected the influence of the mean flow and entropy waves. In this paper, the influence of the mean flow on the ITA mode in the duct and annular combustion chambers is studied by constructing a low-order acoustic network model. At the same time, it also explores the influence of other parameters on the ITA mode. The mean flow greatly affects the growth rate of the ITA mode, and has little effect on its frequency in the duct combustion chamber model. This is also the main reason for neglecting the effect of mean flow on the ITA mode in previous studies. In addition, the influence of other parameters on the ITA mode is also reflected in the growth rate. In the annular combustion chamber, a new ITA mode, which does not meet the −π criterion, is found. And the increase of Mach number makes all ITA modes more unstable. The circumferential mode number has a great influence on the high frequency ITA mode. In addition, the ITA mode has a significant effect on the thermoacoustic mode of the system in the annular combustion chamber.

Understanding the difference in combustion and flame propagation characteristics between ammonia and methane using an optical SI engine

As a carbon-free hydrogen-carrier fuel with a high content of hydrogen, ammonia (NH3) exhibits unfavorable combustion properties such as low flammability and large cyclic variations, requiring further investigation in actual engines. In this work, for the first time, using a single-cylinder optical spark ignition (SI) engine with a high compression ratio, engine performance as well as combustion and flame propagation characteristics were investigated. Synchronization measurement of in-cylinder pressure and high-speed photography was performed, and the combustion characteristics were compared between ammonia and methane under the same operating conditions. The results show that the combustion stability and power capability of ammonia are significantly lower than that of methane. Further analysis combining combustion phasing and flame propagation characteristics indicates that the burning time loss mainly comes from the initial flame development process rather than the main combustion duration which results in a deficit in engine performance. The average probability maps of the flame location indicate that the flame front propagation of the two fuels emerges with obviously different characteristics. Quantitative flame solving shows that NH3 flame propagation features a higher flame stretch sensitivity which is macroscopically reflected in the cyclic variations. Furthermore, the quantification of flame response to turbulence was solved and observed much higher effects for NH3 flame than methane, which indicates NH3 flame propagation is mainly driven by the flame response to turbulence while the flame propagation of methane exhibits higher sensitivity to temperature than ammonia. The current study shall provide insights into the application of ammonia in the internal combustion engine.

Do flame describing functions suitably represent combustion dynamics under self-sustained oscillations?

Transfer function concepts that appear in many areas and most notably in control systems have been extensively used to represent the flame response in low-order models of combustion instability. Much of the theoretical work is based on flame transfer functions (FTFs). In recent years, its nonlinear extension, namely the flame describing function (FDF), has been used to get a more accurate representation of the flame response when the level of oscillation becomes large and the system reaches a limit cycle. Despite their wide and reasonably successful use in predicting instabilities, the direct validity of using FTF/FDFs to represent the flame response still remains to be experimentally substantiated. This article is aimed at providing a direct assessment of the capacity of the FDF to suitably describe the flame behavior under self-sustained oscillations (SSOs) for a spray-swirl flame anchored by an injector that is weakly-transparent to acoustic waves. This is accomplished by making use of an experimental combustion configuration that not only exhibits unstable oscillations but also features a set of driver units to modulate the flame (namely stable flame modulation or SFM). The flame is modulated at the frequency of SSO, and the amplitude of incident velocity modulations is then progressively varied until it coincides with that found under SSO. The injector dynamics is shown to be different between SSO and SFM for an injector that is weakly-transparent to acoustic waves and imposes a certain degree of decoupling between plenum and chamber. For such injectors, the FDF built with the upstream velocity would not suitably represent SSO, as this lumps the injector and flame dynamics together. It is then important to use velocity measurement at the injector outlet, at a point where the relative velocity fluctuation matches the relative volumetric flow rate fluctuation. The describing function with velocity reference at the injector outlet is measured for various input levels and found to approximately match those measured under SSO. The best match is obtained when the amplitude of external modulation induces a level of velocity oscillations that comes closest to that prevailing under SSO. This demonstrates that the FDF may suitably capture the nonlinearity of the flame response, at least in the configuration investigated in this research.

Influence of radial fuel staging on combustion instabilities and exhaust emissions from lean-premixed multi-element hydrogen/methane/air flames

The utilization of hydrogen-blended hydrocarbon fuels, or even completely carbon-free fuels like pure hydrogen and ammonia, will be indispensable in the effort to reduce anthropogenic carbon dioxide emissions from heavy-duty land-based gas turbines. In order to make this energy transition, however, low emissions, low dynamics fuel-flexible gas turbine combustion systems will be required. To address some of the key technological obstacles in the development of such systems, we explore a prototype multi-element injector assembly devised to prevent flashback in extremely fast premixed hydrogen flames, paying particular attention to the impact of radial dual-fuel staging on the generation of self-induced instabilities and the formation of nitrogen oxides and carbon monoxide. In conjunction with phase-averaged OH*/CH* chemiluminescence and OH PLIF flame imaging, we carry out extensive measurements over the full range of 0 to 100% H2/CH4 fuel staging conditions, including even/uneven blends of H2/CH4 fuels between inner and outer nozzle groups, partial/complete fuel split cases, and pure H2 or CH4 fuel for all constituent flames, under a constant thermal power condition of 78 kW. Our measurements demonstrate that whereas carbon monoxide concentrations are largely unaffected by the radial fuel staging conditions except under high and pure hydrogen percentage conditions, there exists a strong correlation between total nitrogen oxides emissions and overall adiabatic flame temperature. Integrated analyses of isocontour instability maps reveal that near XH2,g = 0.50 a discontinuous mode transition takes place, where the intermediate-amplitude lower frequency oscillations associated with relatively low hydrogen concentration conditions give way to stronger, higher frequency instabilities under high hydrogen content conditions. While even-blend (non-staging) conditions are characterized by coherent oscillations of the constituent flames, the responses of clustered flames to radial fuel staging conditions are eccentric and complex, manifested as large-scale asynchronous modulations between inner- and outer-stage heterogeneous reaction zones. This observation suggests that in a multi-element injector environment spatiotemporal incoherence driven by inhomogeneous heat release can be used to neutralize self-excited pressure oscillations, by disrupting pressure-heat release coupling processes.

The effect of hydrogen enrichment, flame-flame interaction, confinement, and asymmetry on the acoustic response of a model can combustor

To maximise power density practical gas turbine combustion systems have several injectors which can lead to complex interactions between flames. However, our knowledge about the effect of flame-flame interactions on the flame response, the essential element to predict the stability of a combustor, is still limited. The present study investigates the effect of hydrogen enrichment, flame-flame interaction, confinement, and asymmetries on the linear and non-linear acoustic response of three premixed flames in a simple can combustor. A parametric study of the linear response, characterised by the flame transfer function (FTF), is performed for swirling and non-swirling flames. Flame-flame interactions were achieved by changing the injector spacing and the level of hydrogen enrichment by power from 10 to 50%. It was found that the latter had the most significant effect on the flame response. Asymmetry effects were investigated by changing one of the flames by using a different bluff-body to alter both the flame shape and flow field. The global flame response showed that the asymmetric cases can be reconstructed using a superposition of the two symmetric cases where all three bluff-bodies and flames are the same. Overall, the linear response characterised by the flame transfer function (FTF) showed that the effect of increasing the level of hydrogen enrichment is more pronounced than the effect of the injector spacing. Increasing hydrogen enrichment results in more compact flames which minimises flame-flame interactions. More compact flames increase the cut-off frequency which can lead to self-excited modes at higher frequencies. Finally, the non-linear response was characterised by measuring the flame describing function (FDF) at a frequency close to a self-excited mode of the combustor for different injector spacings and levels of hydrogen enrichment. It is shown that increasing the hydrogen enrichment leads to higher saturation amplitude whereas the effect of injector spacing has a comparably smaller effect.

Forcing of a flame by a periodic flow in a Hele-Shaw burner

Understanding how time-dependent flows affect the propagation of a flame is of prime importance for engineering applications. We experimentally study the case of a quasi-two-dimensional flame propagating in a flow that varies periodically in time. The flow acts as a parametric forcing and induces parametric restabilization and parametric destabilization of the flame front. The comparison of our experimental measurements of these destabilization and restabilization thresholds with a low frequency theory indicates how the flame response varies with the time scale associated with the dynamics of the flow.

Investigation on flame dynamic characteristics to air-inlet excitation in a gas turbine model combustor

The combustion characteristics for a gas turbine (GT) model combustor in air-inlet excitation are studied by Large Eddy Simulation (LES). Time series of heat release rate (HRR) fluctuation is captured during simulation. The precessing vortex core (PVC) structure is shown by pressure iso-surface, and the single/double helix branches of PVC alternately evolve in one forcing cycle. With the rotation of PVC, the high-speed ring zone (HSRZ) is periodically squeezed, and large-scale vortex roll-up results from that. The flame root structure is also significantly affected by PVC, which is one of the reasons for the HRR fluctuation. With the increase of the forcing amplitude, the flame response is divided into three parts. The linear growth zone and secondary rising zone are separated by saturation plateau. The flame dynamics analysis shows that the change of the flame structures and flame front oscillation modes are the apparent mechanism of the nonlinear flame response appearance.

Linear instability of a premixed slot flame: Flame transfer function and resolvent analysis

The response to forcing of a 2D laminar premixed slot flame is investigated by means of linear analysis, based on the compressible flow equations with a two-step reaction scheme for methane combustion. The flame transfer function (FTF) is computed from this linear model, in excellent agreement with reference nonlinear calculations. The input-output gain between externally applied forcing and the global heat release rate response is computed, and peaks in the gain are related to intrinsic thermoacoustic (ITA) modes. The receptivity of the flame to arbitrary flow forcing is characterised by the resulting amplitude of global heat release rate fluctuations. Linear resolvent analysis is used to identify optimal forcing structures and their associated flame response, leading to a discussion of the dominant mechanisms for the amplification of flow perturbations, which trigger flame oscillations. These seem to involve a resonance with ITA instability modes.

Plasma-assisted combustion with nanosecond discharges. I: Discharge effects characterization in the burnt gases of a lean flame

The prediction of a flame response to plasma assistance requires extensive knowledge of discharge-induced plasma kinetics. Detailed studies of nanosecond discharges are common in N2/O2 and fresh combustible mixtures but are still lacking in burnt gases. To fill this gap, we define a reference test case and investigate the effects of nanosecond repetitively pulsed (NRP) discharges placed in the recirculation zone of a lean (Φ = 0.8) CH4-air bluff-body stabilized flame at atmospheric pressure. In this zone, the plasma discharge is created in a mixture of burnt gases. Quantitative optical emission spectroscopy, coupled with measurements of electrical energy deposition, is performed to provide temporally (2 ns) and spatially (0.5 mm) resolved evolutions of the temperatures and concentrations of N2(B), N2(C), N2 +(B), OH(A), NH(A), and CN(B) in the discharge. At steady state, the 10-ns pulses deposit 1.8 mJ at a repetition frequency of 20 kHz. Spatially resolved temperature profiles are measured during the discharge along the interelectrode gap. The temperature variations are more pronounced near the electrodes than in the middle of the gap. On average, the gas temperature increases by approximately 550 K. The heat release corresponds to about 20% of the total deposited electric energy. The electron number density, measured by Stark broadening of H α , increases up to about 1016cm−3. These characteristics allow to classify the discharge as a non-equilibrium NRP spark, as opposed to the thermal NRP spark where the temperature can reach 40 000 K and the degree of ionization is close to 100%. These measurements will serve (i) as a reference for future studies in the mini-PAC burner at the same conditions, (ii) to test discharge kinetic models, and (iii) to derive a simplified model of plasma-assisted combustion, which will be presented in companion paper.

Numerical Prediction and Suppression of Thermoacoustic Longitudinal Mode in Annular Combustor

Many industrial applications, such as gas turbines or aeroengines, are subject to thermoacoustic instability, especially in the lean combustion regime. In this paper, the linearized Navier–Stokes equations (LNSEs) framework is applied to predict the acoustic liner damping effects with real geometry and different bias flow velocities. The results show that the LNSEs can capture the tilt angle effect of the orifice or grazing and bias flow coupling phenomenon. The forced large-eddy simulation results of premixed methane flame present the same trend in gain and phase of the flame transfer function with the equation. Then, the flame response model is coupled into the Helmholtz and LNSE solvers. The mean flow from Reynolds-averaged Navier–Stokes calculation is used in the LNSEs, and no assumption of liner geometry is made. The linear stability analysis reveals that the velocity fluctuation in the combustion is also damped when a stable acoustic mode is achieved with the acoustic liner. The flame dynamic, the hydrodynamic, and the acoustic phenomena can all be considered when using the LNSEs. This indicates that the LNSEs may allow us to consider a more complex mechanism that can affect the thermoacoustic inability in the combustor system and help us to design better dampers to control.

The effects of injector size on the dynamics and instabilities of lean premixed swirling flame

The external- and self-excited premixed swirling flames are experimentally studied to understand the thermoacoustic instabilities under different injector sizes. We use high-speed CH* chemiluminescence, formaldehyde laser-induced fluorescence and particle image velocimetry to measure the dynamic characteristics of the flame surface and flow field. Reducing the size of the injector from 20 mm to 16 mm changes the thermoacoustic system's stability. The flame transfer function shows that reducing the size of the injector significantly reduces the phase delay, enhances the response amplitude, and annihilates the local gain extrema. The low-order network model analysis is performed, and it is found that the phase variation caused by changing injector size is the main reason for the induced thermoacoustic instability. The pixel-by-pixel decomposition of CH* images is used to extract the weighted gain and phase maps of local heat release oscillation. The swirling flame with a small injector size of 16 mm has a short time delay and a slow phase velocity, which results in strong in-phase interference of the local heat release oscillators and enhanced gains. Reducing the size of the injector strengthens the inner and outer shear layers, thereby suppressing the heat release fluctuation of the flame base. The enhanced vorticity supply of the shear layer increases the size and strength of the outer vortex ring, resulting in strong heat release fluctuations at the flame tip, which is responsible for its large unadulterated fluctuation intensity. These changes in the flow field change the relative heat release intensity and fluctuation amplitude between the flame base and tip. The out-of-phase interference among multiple vortices causes weak flame response at high frequencies.

A nonlinearly kinematic model of the asymmetrically turbulent premixed slit flame subjected to two-way harmonic disturbances

Transversal and longitudinal combustion instabilities typically occur in gas turbines and rocket engines. Quantifying the nonlinear flame dynamic response of turbulent premixed flames subjected to the two-way disturbances and mean bulk flows is significant to understand the complex nonlinear combustion instability phenomena. A Two-way Nonlinear Unforced Flame Speed Modeling (2W-NUFSM) approach was proposed in this study. The turbulent effects were accounted for in the flame propagation speed term and the steady flame term in the kinematic equation of the flame front; an expansion method was then used to obtain the nonlinear solutions corresponding to the transverse and longitudinal acoustic disturbances. The turbulent effect on the flame consumption speed was also considered in the heat release rate calculation of the perturbed flame. Validations of this approach were conducted by comparing its results to the former credible results obtained from other methods and good agreements were obtained. The influence of turbulent effects on the flame dynamic response in an asymmetrical premixed flame was investigated in detail. The coupling effect of the turbulence and harmonic flow disturbances on the flame nonlinear dynamic response was also quantified.

The flame displacement speed: A key quantity for turbulent combustion and combustion instability

Future combustion power and propulsion systems may operate in premixed regime enabling reduced fuel burn and reduced pollutant emissions. The turbulent premixed regime in those future combustion systems is likely to be in the corrugated regime where modeling the flame as a thin interface propagating into the fresh gas is made possible. The flame displacement speed is thus a key quantity for turbulent combustion in this regime. This quantity is also important for combustion instabilities. Indeed, the flame displacement speed [Formula: see text] combined to the flow speed [Formula: see text] determines the flame surface location by determining the flame surface speed [Formula: see text]. The flame surface location has shown to play a major role on combustion instabilities. Research work have also demonstrated the role of the flame displacement speed on the flame response which is used for subsequent combustion instability prediction. In this context, the derivation of flame speed models and flame transfer function models based on this quantity are required. This paper presents the theoretical derivation of flame transfer function coefficients for swirling premixed flames in this context. The derivation is based on the definition of the flame speed for turbulent flame, its perturbed form for oscillating flow, and the kinematic flame-flow speed budget. The obtained results are compared to previous literature data and discussed. The effect of the flame angle, id est the effect of the swirl number on the flame response is also investigated. This works motivates detailed local measurements and simulations to evaluate flow-flame speed budget terms.

Nonlinear flame response modelling by a parsimonious set of ordinary differential equations

In this work we present a parsimonious set of ordinary differential equations (ODEs) that describes with satisfactory precision the linear and non-linear dynamics of a typical laminar premixed flame in time and frequency domain. The proposed model is characterized by two ODEs of second-order that can be interpreted as two coupled mass-spring-damper oscillators with a symmetric, nonlinear damping term. This non-linear term is identified as function of the rate of displacement following [Formula: see text]. The model requires only four constants to be calibrated. This is achieved by carrying out an optimization procedure on one input and one output broadband signal obtained from high-fidelity numerical simulations (CFD). Note that the Transfer Function (FTF) or describing function (FDF) of the flame under investigation are not known a-priori, and therefore not used in the optimization procedure. Once the model is trained on CFD input and output time series, it is capable of recovering with quantitative accuracy the impulse response of the laminar flame under investigation and, hence, the corresponding frequency response (FTF). If fed with harmonic signals of different frequency and amplitude, the trained model is capable of retrieving with qualitative precision the flame describing function (FDF) of the studied flame. We show that the non-linear term [Formula: see text] is essential for capturing the gain saturation for high amplitudes of the input signal. All results are validated against CFD data.

Autoignition flame transfer matrix: Analytical model versus large eddy simulations

Modern gas turbines need to fulfil increasingly stringent emission targets on the one hand and exhibit outstanding operational and fuel flexibility on the other. Ansaldo Energia GT26 and GT36 gas turbine models address these requirements by employing a combustion system in which two lean premixed combustors are arranged in series. Due to the high inlet temperatures from the first stage, the second combustor stage predominantly relies on autoignition for flame stabilization. In this paper, the response of autoignition flames to temperature, pressure and velocity excitations is investigated. The gas turbine combustor geometry is represented by a backward-facing step. Based on the conservation equations an analytical model is derived by solving the linearized Rankine-Hugoniot conditions. This is a commonly used analytical approach to describe the relation of thermodynamic quantities up- and downstream of a propagation stabilized flame. In particular, the linearized Rankine-Hugoniot jump conditions are derived taking into account the presence of a moving discontinuity as well as upstream entropy inhomogeneities. The unsteady heat release rate of the flame is modelled as a linear superposition of flame transfer functions, accounting for velocity, pressure, and entropy disturbances, respectively. This results in a 3[Formula: see text]3 flame transfer matrix relating both primitive acoustic variables and the temperature fluctuations across the flame. The obtained analytical expression is compared to large eddy simulations with excellent agreement. A discussion about the contribution of the single terms to the modelling effort is provided, with a focus on autoignition flames.

Investigation into the coherence of flame intensity oscillations in a model multi-element rocket combustor using complex networks

Capturing the complex spatiotemporal flame dynamics inside a rocket combustor is essential to validate high-fidelity simulations for developing high-performance rocket engines. Utilizing tools from a complex network theory, we construct positively and negatively correlated weighted networks from methylidyne (CH*) chemiluminescence intensity oscillations for different dynamical states observed during the transition to thermoacoustic instability (TAI) in a subscale multi-element rocket combustor. We find that the distribution of network measures quantitatively captures the extent of coherence in the flame dynamics. We discover that regions with highly correlated flame intensity oscillations tend to connect with other regions exhibiting highly correlated flame intensity oscillations. This phenomenon, known as assortative mixing, leads to a core group (a cluster) in the flow-field that acts as a “reservoir” for coherent flame intensity oscillations. Spatiotemporal features described in this study can be used to understand the self-excited flame response during the transition to TAI and validate high-fidelity simulations essential for developing high-performance rocket engines.

Modal Decomposition Analysis of Combustion Instability Due to External Perturbation in a Mesoscale Burner Array

AbstractIn this work, the growth regime of combustion instability was studied by analyzing 10 kHz OH planar laser-induced fluorescence (PLIF) images through a combination of dynamic mode decomposition (DMD) and spectral proper orthogonal decomposition (SPOD) methods. Combustion instabilities were induced in a mesoscale burner array through an external speaker at an imposed perturbation frequency of 210 Hz. During the transient growth phase of combustion instability, 10 kHz OH PLIF imaging was employed to capture spatially and temporally resolved flame dynamics. Increased acoustic perturbations prevented flame reignition in the central recirculation zone and eventually led to the flame being extinguished inward from the outer burner array elements. Coherent modes and their growth rates were obtained from DMD spectral analyses of high-speed OH PLIF images. Positive growth rates were observed at the forcing frequency during the growth regime. Coherent structures, closely associated with thermoacoustic instability, were extracted using an appropriate SPOD filter operation to identify mode structures that correlate to physical phenomena such as shear layer instability and flame response to longitudinal acoustic forcing. Overall, a combination of DMD and SPOD was shown to be effective at analyzing the onset and propagation of combustion instabilities, particularly under transient burner operations.

Non-premixed counterflow methane flames in DC/AC/NS electric fields

The response of counterflow diffusion flames to sub-breakdown DC and AC electric fields, as well as their superposition with ns pulse discharge waveforms, is studied in the plane-to-plane electrode geometry. Sub-breakdown DC and low-frequency AC electric fields cause the flame displacement toward the grounded electrode, in the direction of the applied field, indicating that the body force on the positive ions exceeds that on the electrons and negative ions. As the AC frequency increases, the flame response becomes less pronounced, due to the reduction of the electrohydrodynamic (EHD) body force impulse over the AC half-period. The electric field in the electrode gap is determined by ps Electric Field Induced Second Harmonic (E-FISH) generation, with absolute calibration using sub-breakdown ns pulses overlapped with the measured electric field waveform. The results show that the electric field distribution across the flame in the current saturation regime follows the Laplacian field. This indicates that the space charge density in the gap is too low to distort the applied DC or AC field, consistent with the kinetic modeling predictions. Combining a nanosecond pulse discharge with a sub-breakdown DC field generates a diffuse plasma across the entire gap. Time-resolved and spatially resolved measurements of the electric field in the discharge indicate the ionization wave propagation between the electrodes. The present results do not exhibit a detectable flame displacement enhancement by ns discharge pulses combined with a sub-breakdown field, observed previously. Kinetic modeling calculations show that the absence of this effect in the plane-to-plane geometry is due to the rapid plasma self-shielding. This indicates that alternative electrode geometries limiting the self-shielding would be more effective for the plasma / electric field enhanced flameholding and flame stabilization applications.