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• Direction numerical simulation of transient flame-wall interaction. • Transient vs steady wall heat fluxes in side-wall a quenching configuration. • Markstein numbers under instationary conditions for flame-wall interaction. • Fluctuations cause different Markstein lengths depending on phase angle. This work focuses on sidewall quenching (SWQ) for premixed methane/air flames that are forced by a periodic oscillatory inflow with excitation frequency f = 100 Hz. The effects of steady-state and transient flame stretch on the near-wall flame dynamics are evaluated using two-dimensional direct numerical simulation (2D-DNS) and the GRI 3.0 reaction mechanism. The velocity fluctuations lead to significant changes in flame speed and flame stretch, as well as the associated Markstein numbers. The phenomenon of SWQ is analyzed using flame quenching distance, wall heat flux and heat release rate. For steady-state conditions, there is a strong correlation between the maximum wall heat flux WHF max and the flame quenching Peclet Number (Pe q), as well as between the flame speed and the flame stretch; for transient conditions, the flame quenching distance (d q) increases continuously from phase angles of 1/4 f −1 to 3/4 f −1 in one cycle as time progresses, and the fluctuation of the quenching distance (Δd q) decreases gradually with increasing equivalence ratio (∅). The flame stretch changes from negative to positive in the process from 1/4 f −1 to 3/4 f −1, while the heat release rate and fuel reaction rate near the wall gradually decrease. Furthermore, the FWI region is dominated by negative flame stretch while positive flame stretch is present at the base of the flame. Moreover, the methane/air flame has a nearly twofold increase in the consumption speed during the oscillation from phase angle 3/4 f −1 to the next cycle at 1/4 f −1 at ∅ = 0.5 and ∅ = 1.0. These results show that flow field perturbations are not negligible in elucidating the effects of flame-wall interactions. [ABSTRACT FROM AUTHOR]

The effects of flow structure and flame dynamics on combustion instabilities in a lean premixed swirl combustor were numerically investigated using Large eddy simulation (LES) by varying the inlet geometry of combustor. The dynamic k -equation and G-equation flamelet models were respectively employed as the LES subgrid models of turbulence and combustion. The divergent half angle (α) in the combustor inlet was varied systematically from 30° to 90° to quantify the effect of inlet geometry on the combustion instabilities. This variation caused considerable deformation in recirculation zones in terms of their size and location, leading to significant changes in flame dynamics. Analysis of unsteady pressure distributions in the combustor showed that the largest damping caused by combustion instabilities takes place at α = 45°, and the amplitude of acoustic pressure oscillation is largest at α = 30°. Examination of local Rayleigh parameters indicated that controlling flame-vortex interactions by modifying inlet geometry can change the local characteristics of combustion instabilities in terms of their amplification and suppression, and thus serve as a useful approach to reduce the instabilities in a lean premixed swirl combustor. These phenomena were studied in detail through unsteady analysis associated with flow and flame dynamics. [ABSTRACT FROM AUTHOR]

For the numerical evaluation of the thermoacoustic stability of rocket engines often hybrid methods are applied, which separate the computation of wave propagation in the combustor from the analysis of the flame response to acoustic perturbations. Closure requires a thermoacoustic feedback model which provides the heat release fluctuation in the source term of the employed wave transport equations. The influence of the acoustic fluctuations in the combustion chamber on the heat release fluctuations from the modulation of the atomization of the propellants in a hypergolic upper stage rocket engine is studied. Numerical modeling of a single injector provides the time mean reacting flow field. A network of transfer functions representing all aspects relevant for the feedback model is presented. Analytical models for the injector admittances and for the atomization transfer functions are provided. The dynamics of evaporation and combustion are studied numerically and the numerical results are analyzed. An analytical approximation of the computed flame transfer function is combined with the analytical models for the injector and the atomization quality to derive the feedback model for the wave propagation code. The evaluation of this model on the basis of the Rayleigh index reveals the thermoacoustic driving potential originating from the fluctuating spray quality. [ABSTRACT FROM AUTHOR]

The structure and dynamics of laminar nonpremixed dimethyl ether (DME)/air coflow flames were investigated at elevated temperatures and pressures. Computations with detailed chemistry were performed for DME and heated coflow air at 30 atm with uniform but sinusoidally oscillating inlet velocities. These unsteady cases were compared with the steady results from Deng et al. (2015)[14] to elucidate the effect of oscillation frequency on the flame dynamics. To benchmark the unsteady cases, a normalized displacement velocity was defined to differentiate flame propagation from autoignition, and this definition was validated against the steady cases. In the oscillating reacting flow, transition between a multibrachial autoignition front and a tribrachial flame occurs periodically. However, unlike the harmonic velocity oscillation, the combustion mode transition is hysteretic. The oscillation cycle starts with the largest inlet velocity, with the multibrachial thermal structure, located downstream, being governed by autoignition chemistry. As flow velocity decreases, the autoignition front moves upstream and transitions to a tribrachial flame near the lower velocity limit, similar to the steady flow, as autoignition chemistry becomes weaker with decreasing upstream residence time. As the flow velocity increases again, the tribrachial flame is convected downstream, and, ultimately, due to the radical and heat accumulation in time, autoignition eventually occurs and becomes the dominant pathway. The finite induction time for autoignition results in the hysteretic behavior during the decreasing- and increasing-velocity cycles, which diminishes at lower oscillation frequency as there is more time for chemistry to respond to the hydrodynamic changes and consequently approach steady state. At the relatively low oscillation frequencies investigated in the current study, first-stage NTC chemistry is less affected by flow dynamics with only second-stage autoignition and flame chemistry, which accounts for the majority heat release, coupled with the flow oscillation. [ABSTRACT FROM AUTHOR]

We report on the linear stability of micro-combustion in pipe, where two instabilities manifest at high and low flow-rates. The combustion of a stoichiometric methane/air premixed mixture has been numerically investigated within a 3D reduced model. This model reproduces decently the flame dynamics in the range of speed between 5–100 cm/s. The flame position, the stability thresholds of the Flame with Repetitive Extinction and Ignition (FREI) and the flame shape are in accordance with the experiments. Furthermore, an analysis of the integral values of all mechanisms involved in the flame evolution has been carried out near the two stability thresholds. The phase shift between the reaction term and the radial diffusion has been identified as the source of instability in both cases. The global behavior has been then investigated with a linear stability analysis. The 2D and 3D temperature and concentration perturbations have been found by solving the eigenvalue problem obtained by linearizing the model around the basic state. Only one unstable axisymmetric mode has been found. This is in agreement with the direct numerical simulation of the model. [ABSTRACT FROM AUTHOR]

In this paper the dynamics of rich hydrogen–air flames in a microflow reactor with controlled temperature of the walls is investigated numerically using the thermal-diffusion model with two-step kinetics in one spatial dimension. It is found that as the parameters of the system are varied the sequence of bifurcation occurs leading to the formation of complex spatio-temporal patterns. These include pulsating, chaotic, mixed-mode and FREI (Flames with Repetitive Extinction and Ignition) oscillations. The critical parameter values for the existence of different dynamical regimes are found in terms of equivalence ratio and flow velocity. [ABSTRACT FROM PUBLISHER]

The structure and dynamics of premixed flames in narrow channels in the presence of a highly conductive wall segment are investigated in this work. It is shown that these systems present a multiple steady-state regimes. A linear stability analysis applied to these steady solutions reveals that several steady-state solutions may be simultaneously stable. The stability analysis results are confirmed by time-dependent simulations, that show that the actual realization of one or another stable regime depends on the initial conditions. Finally, we show that nonlinear periodic and chaotic dynamics may appear under certain conditions in these system. [ABSTRACT FROM AUTHOR]

In this study, the heat release rate (HRR) response during self-excited azimuthal thermoacoustic instabilities in a pressurized annular combustor is investigated for hydrogen/methane blended flames. The equivalence ratio and hydrogen power fraction were varied for a constant air mass flow rate, resulting in self-excited azimuthal instabilities of varying amplitudes. Although significant harmonic components were observed in the pressure response, the spatially integrated HRR response was dominated by the fundamental component of the mode. Despite the wide range of operating conditions, an approximately linear relationship was observed between limit cycle velocity and HRR oscillation amplitudes for both the fundamental and first harmonic components, and the phase varied slowly as a function of amplitude. As the phase lag between the HRR and pressure oscillations reduced, the instability amplitude increased due to the increased driving through the Rayleigh criterion. Fuel blends with higher hydrogen fractions produced larger phase differences and lower amplitude responses. High-speed flame imaging was used to analyze the spatial distribution of the HRR oscillations which exhibited significant harmonic content and only provided small contributions to the integrated HRR. The high frequencies associated with these harmonics resulted in the presence of multiple structures with opposing phases simultaneously on the flame brush. This explains why the integrated contributions to the HRR were small. Fourier mode decomposition was employed to better understand the observed reduction in instability amplitude with increasing hydrogen power fraction. When the hydrogen content is higher, multiple fluctuations of the HRR are present simultaneously on the flame, resulting in global cancellation. In contrast, for lower hydrogen content, higher amplitude instabilities are generated, resulting in different flame dynamics. At high amplitudes, the response is more asymmetric as a result of the azimuthal nature of the instability. [ABSTRACT FROM AUTHOR]

Laser interferometric vibrometry (LIV) has recently been proposed as an alternative mean to obtain time-resolved density and heat release rate measurements at relatively low cost and experimental effort. This technique is sensitive to fluctuations of the refractive index of gases resulting from density and composition changes along the laser beam intersecting the reacting flow. It yields a line-of-sight integrated signal of the probed flow from which density and heat release rate disturbances may be inferred. The link between these signals with chemiluminescence is examined in the present study by first considering a theoretical analysis to determine the relationships between the LIV, density and heat release rate perturbation signals in a multi-species reactive mixture of gases. For air combustion systems interacting with sound waves, low frequency density perturbations in the flame zone, result mainly from heat release rate fluctuations below a certain frequency threshold. An experimental analysis is then conducted with confined conical laminar premixed flames submitted to harmonic flow modulations. Measurements are presented for methane–air mixtures at different equivalence ratios 0.8 ≤ ϕ ≤ 1.2 and thermal powers. It is shown that fluctuations of the chemiluminescence signal examined in different wavelength bands, including the OH * , CH * or the entire visible emission bands, always capture the same dynamics. This indicates that heat release rate fluctuations can be deduced without specific filters for the laminar premixed methane–air flames investigated. It is then shown that heat release rate measurements deduced from LIV and chemiluminescence data match well. A proportional relation is found that does not depend on the measurement position, modulation frequency and modulation level for fixed injection conditions. This linear relation slightly depends on the mean flow operating conditions partly due to the difficulty to interpret chemiluminescence emission for rich flames. [ABSTRACT FROM AUTHOR]

Combustion instability is a nonlinear process with interaction between combustion and acoustics. The nonlinearity in the combustion process is observed in the flame dynamics. In our study of a ducted laminar premixed V-flame, we varied the distance between the flame anchor and the duct exit. We observed that the thermoacoustic system bifurcates from a stable state to a frequency locked state, followed by quasi-periodicity, period 3 oscillations, and finally chaotic oscillations. During the occurrence of these dynamical states, the role of flame dynamics is investigated using high speed imaging. We observed wrinkle formation, its propagation and growth along flame front, rollup of the flame, separation, and annihilation of flame elements during instability. From the present study it is found that each dynamical state is characterized by a particular sequence of flame behavior, highlighting the role of nonlinear flame dynamics in establishing the observed dynamical state. [ABSTRACT FROM PUBLISHER]

We report on numerical simulations of stoichiometric methane/air premixed flames into a micro-tube at atmospheric pressure. These simulations result from numerical resolutions of reduced-order models. Former experimental and numerical studies reported the occurrence of a Flame Repetitive Extinction/Ignition (FREI) phenomenon provided that a temperature gradient is sustained at the walls. Conducting unsteady one-dimensional simulations including complex chemistry, a late numerical study tried to explain the occurrence of this phenomenon in terms of chemical balance. The present study aims to point out the main mechanisms underlying the arising of this instability. Provided a calibration of some empirical constants, an unsteady two-dimensional model including one-step chemical reaction is shown to quantitatively reproduce the FREI regime all along the range of flow rates investigated by the experimental studies. Interestingly, the stability diagram of the configuration investigated can be decently captured combining a very simple reaction model with an elementary hydrodynamical feature consisting of non-uniform velocity profile. Complementing the aforementioned numerical study, it is shown that, when the tube diameter is varied, the two-dimensional model unveils an unstable regime that a one-dimensional model cannot capture. As two-dimensional hydrodynamics appears to play a key role into the flame׳s dynamics, one-dimensional models are not believed to deliver in general an adequate strategy of combustion control into such microchannels. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study into the structure and dynamics of the phase-averaged heat release rate during self-excited spinning and standing azimuthal modes in an annular combustion chamber. The flame response was characterised using two methods: high-speed OH ∗ chemiluminescence imaged above the annulus to investigate the structure of the phase-averaged fluctuations in heat release rate, and high-speed OH-PLIF measured across the centreline of two adjacent flames to investigate phase-averaged flame dynamics. Two-microphone measurements were obtained at three circumferential locations to determine the modes and the amplitude of the velocity fluctuations. It was found that the flame responds differently to spinning and standing wave modes. During standing wave modes, the amplitude of the unsteady heat release rate of each flame (sector) varied according to its location in the mode shape with maximum fluctuations occurring at the pressure anti-nodes and minimum fluctuations occurring at the pressure nodes. At the pressure anti-nodes, peak fluctuations result from the production of flame surface area by axisymmetric flame motions caused by the modulation of flow at the burner inlet by the pressure fluctuations. However, at the pressure nodes, an anti-symmetric, transverse flapping motion of the flame occurred producing negligible unsteady heat release rate over the oscillations cycle via the mechanism of cancellation. During spinning modes, the structure of the heat release rate was found to be asymmetric and characterised by the preferential suppression of shear layer disturbances depending on the spin direction. [ABSTRACT FROM AUTHOR]

Abstract: The dynamics of premixed hydrogen/air flame ignited at different locations in a finite-size closed tube is experimentally studied. The flame behaves differently in the experiments with different ignition positions. The ignition location exhibits an important impact on the flame behavior. When the flame is ignited at one of the tube ends, the heat losses to the end wall reduce the effective thermal expansion and moderate the flame propagation and acceleration. When the ignition source is at a short distance off one of the ends, the tulip flame dynamics closely agrees with that in the theory. And both the tulip and distorted tulip flames are more pronounced than those in the case with the ignition source placed at one of the ends. Besides, the flame–pressure wave coupling is quite strong and a second distorted tulip flame is generated. When the ignition source is in the tube center, the flame propagates in a much gentler way and the tulip flame can not be formed. The flame oscillations are weaker since the flame–pressure wave interaction is weaker. [Copyright &y& Elsevier]

Abstract: The effect of the equivalence ratio on the stability and dynamics of a premixed flame in a planar micro-channel with a step-wise wall temperature profile is numerically investigated using the thermo-diffusive approximation. To characterize the stability behavior of the flame, we construct the stability maps delineating the regions with different flame dynamics in the inlet mass flow rate m vs. the equivalence ratio parametric space. The flame stability is analyzed for fuels with different diffusivity by changing the Lewis numbers in the range . On the other hand, the Lewis number of the oxidizer is kept constant and equal to unity . Our results show that, for very diffusive fuels, the stability of the flame varies significantly with the equivalence ratio, transitioning from stable flames for lean mixtures to highly unstable flames when . As the fuel Lewis number approaches unity, the stability behavior of the flame for lean and rich mixtures becomes more similar to give, in the equidiffusional case , a symmetric stability map around the stoichiometric mixture . In all cases considered, the most stable flames are always found around the stoichiometric mixtures , when the flame instabilities are completely suppressed for very diffusive fuels , or are reduced to a narrow range of inflow velocities for fuel Lewis numbers equal or greater than unity. The ratio between the size of the channel and the flame thickness d turns out to be of great importance in the stability behavior of the flame. Keeping the rest of parameters constant, an increase in d for lean flames makes the flame considerably more unstable, confirming the findings of previous works. Nevertheless, as the stoichiometric ratio approaches , that trend is reversed to give flames that become more stable as the size of the channel is increased. [Copyright &y& Elsevier]

Abstract: The dynamics and stabilization of fuel lean premixed /air atmospheric pressure flames in meso-scale channels were investigated numerically, using detailed gas phase chemistry and transport. Experiments in a channel flow reactor by means of chemiluminescence detection of the excited OH radical allowed for model validation at steady conditions and identification of the conditions at which unsteady flame dynamics were present. A detailed parametric study of the influence of wall temperature and ratio on the ensuing flame dynamics was performed. The numerical results revealed different flame modes which included oscillatory ignition, random ignition spots, as well as steady weak and V-shaped flames. The wall temperature stability intervals of these modes changed with the ratio. The richest variety was found for molar ratios between 4 and 10, while at lower ratios the random and the weak modes were absent. At higher ratios all the dynamic modes were suppressed. The Computational Singular Perturbation (CSP) method was used to obtain insights into the physicochemical processes responsible for the weak flames, which were found at relatively high inflow velocities compared to previous studies, and V-shaped flames. A kinetic explanation of the phenomena was supported by the CSP analysis. [Copyright &y& Elsevier]

The effects of Soret diffusion (SD) on the hydrogen-air edge flame propagation and the diffusion-chemistry interaction are investigated through simulation facilitated by the numerical code MultiDiffFOAM. The edge flames in this study gradually develop from a flame kernel into a tri-brachial structure in a hydrogen-air mixing layer that temporally evolves due to transverse reactant concentration gradient. We demonstrate that the responses of flame displacement speed S d to flame curvature K , stretch rate κ and scalar dissipation rate χ are distinctly influenced by SD. For the linear S d - K and S d - κ correlations, SD would result in a smaller Markstein length. Moreover, SD is shown to lead to shifting of the S d - χ curve towards the regime with larger χ. Compared with the weak influences of SD on the tangential diffusion component S d , t and normal diffusion component S d , n , the chemical reaction component S d , r is significantly weakened by SD. The important chemical reactions for edge flame propagation are identified based on sensitivity analysis and their rates are found to be smaller when SD is considered. For the local composition at the flame marker, the mass fraction of H 2 is slightly larger and that of H is obviously smaller when SD is considered. The SD flux of H 2 j H 2 S D and that of H j H S D are both coupled with the driving force ∇ (l n T) along the mixture fraction coordinate. However, the j H 2 S D is mainly concentrated on the unburnt side while the j H S D is on the burnt side. The analyses on decomposed fluxes of H 2 and H along the flame normal direction further suggest that SD would enhance the H 2 mass diffusion but weaken the H mass diffusion. Such opposite effects stem from the distribution features that H 2 is mainly on the unburnt side while H on the burnt side. • Soret diffusion decreases flame displacement speed S d through chemical reaction component S d , r. • Soret diffusion leads to smaller Markstein length through lateral "compensating" flux of H 2 for curved flame front. • Rates of important reaction are decreased by Soret diffusion due to obviously smaller local mass fraction of H. • Soret diffusion has opposite effects on H 2 and H transport due to different species distribution features relative to flame front. [ABSTRACT FROM AUTHOR]

Abstract: Swirling flame dynamics is investigated using a radial swirler equipped with movable blades. This device allows to continuously change the swirl number and to examine various flame configurations while keeping fixed values of mass flow rate, equivalence ratio, and other system parameters. This is used to reveal two types of thermo-acoustic instabilities, the first giving rise to a low frequency bulk oscillation while the second corresponds to a higher frequency mode. Because the swirl number plays a central role in this analysis, it is here deduced from measured axial and azimuthal velocity profiles under stable operating conditions. It is first found that values deduced from experimental profiles are lower than those estimated from standard expressions relying on geometrical parameters of the swirler. A modified expression derived for swirlers without central insert is proposed that improves swirl number estimates. Systematic variations of the swirl number indicate that the flow features a central recirculation zone when this number is sufficiently large and that a precessing vortex core (PVC) exists under cold flow and reactive conditions for measured swirl numbers exceeding 0.45. Velocity, chemiluminescence, and pressure signals are then used to identify frequencies corresponding to the PVC, self-sustained thermo-acoustic oscillations, and the difference frequency of the previous components. The flame transfer function when the burner is submitted to harmonic flow rate modulations is then determined by progressively increasing the swirler blade angle. As the swirl number is augmented, it is found that the phase lag decreases for a fixed forcing frequency while the transfer function gain covers a broader frequency range. These data are finally used in a linear stability analysis to identify the origin of combustion instabilities observed in the present experiment. It is shown that the time lag between velocity perturbations at the burner outlet and resulting heat release rate disturbances strongly depends on the swirler blade angle. It is also demonstrated that the adjustable blade angle swirler could be used in principle to control the flame dynamics and avoid regions of instability. [Copyright &y& Elsevier]

Abstract: Heat release rate fluctuations cause recurrent problems in many steady operating combustors. Direct measurements are difficult and these fluctuations are generally inferred from optical diagnostics. An alternative acoustic method was recently developed and is validated here in the case of unconfined laminar premixed flames submitted to harmonic flow modulations. The technique relies on determining the transmission time of frequency-modulated ultrasounds propagating through the perturbed flow. The transmission of ultrasounds is altered by the perturbed interface between the burned gases and ambient air and by the perturbed flame front. A theoretical link between the dynamics of these two interfaces is derived for small low frequency perturbations. The predicted shapes taken by these interfaces are compared with two-color Schlieren images. This model is then used to determine the resulting heat release rate fluctuations and corresponding disturbances in the transmission time of ultrasounds. An analytical expression is derived for the transfer function between heat release rate and sound transmission time disturbances as a function of frequency when these conical flames are submitted to harmonic flow disturbances. Measurements made with this acoustic technique are compared to analytical predictions and to optical measurements exploiting the chemiluminescence emission from the flame. Effects of the forcing frequency and modulation level are examined. Results indicate a good match between predictions and measurements for the gain and phase of the transfer functions of the lean and stoichiometric unconfined flames investigated when the input level is not too large. Issues and perspectives are then briefly discussed to extend this heat release rate measurement technique to practical configurations. [Copyright &y& Elsevier]

Abstract: Introducing a new measurand, namely the rate of change of density in line-of-sight, can be of great interest for people working on thermoacoustics and combustion noise. In this work the relationship between the radiated acoustic pressure, the time rate of change of unsteady heat release and of density fluctuations in a laminar pulsated premixed flame is presented. The burner produced a ‘M’-shaped flame which is excited by harmonic modulations of the flow at two forcing frequencies f =51 and 101Hz. Measurements were performed by a microphone to record the sound pressure radiated by the flame in the far field, a photomultiplier to collect the light emission from which the heat release rate was estimated, and a laser vibrometer to detect the rate of change of density fluctuations within the reaction region. As the latter technique is a novel approach in combustion analysis a detailed description of the fundamentals of laser interferometric vibrometry is presented. Laser vibrometry offers the advantage to be low-demanding in terms of instrumentation and settings and to be relatively straightforward to process for a broad frequency range. The different signals recorded are correlated. An interpretation of the measurement data is given and the potential of laser vibrometry to obtain time resolved data on density and heat release rate fluctuations in the flame region is demonstrated. [Copyright &y& Elsevier]

The results of modeling of the combustion of a gas-air mixture in a closed volume filled with high-porosity medium have been given. A comparison of the calculation results to experimental data has shown their qualitative agreement. It has been established that a quasistationary regime of propagation of the flame is possible in the case of fairly large relative length of the system and specific surface of the porous medium. Rapid deceleration of the flame is attributed to the decrease in the area of its surface because of the quenching on the reactor walls and the intense heat exchange between the hot combustion products and the porous medium. Stabilization of the flame propagation occurs when the heat release in gaseous combustion and the heat loss during the heat exchange between the combustion products and the porous medium are approximately equal. [ABSTRACT FROM AUTHOR]

Abstract: In this study large eddy simulation (LES) technique has been used to predict the fuel variability effects and flame dynamics of four hydrogen-enriched turbulent nonpremixed flames. The LES governing equations are solved on a structured non-uniform Cartesian grid with the finite volume method, where the Smagorinsky eddy viscosity model with the localised dynamic procedure is used to model the subgrid scale turbulence. The conserved scalar mixture fraction based thermo-chemical variables are described using the steady laminar flamelet model. The Favre filtered scalars are obtained from the presumed beta probability density function approach. Results are discussed for the instantaneous flame structure, time-averaged flame temperature and combustion product mass fractions. In the LES results, significant differences in flame temperature and species mass fractions have been observed, depending on the amount of H2, N2and CO in the fuel mixture. Detailed comparison of LES results with experimental measurements showed that the predicted mean temperature and mass fraction of species agree well with the experimental data. The high diffusivity and reactivity of H2 largely affect the flame temperature and formation of combustion products in syngas flames. The study demonstrates that LES together with the laminar flamelet model is capable of predicting the fuel variability effects and flame dynamics of turbulent nonpremixed hydrogen-enriched combustion including syngas flames. [Copyright &y& Elsevier]

Abstract: This paper describes measurements and analysis of the unsteady characteristics of a turbulent reacting fuel jet in a vitiated cross-flow. This problem involves coupling between the hydrodynamic stability of the jet–wall system, combustion induced gas expansion and interactions of the autoigniting jet with the wake flow. Analysis of the unsteady jet motions shows that the reacting jet flaps in a sinuous manner with an amplitude that increases with downstream distance but is relatively independent of jet-to-cross-flow momentum flux ratio, J. This study particularly focuses on unsteady flame–wall interactions in these systems, showing that even though the time averaged flame trajectory may be well removed from wall regions, its instantaneous trajectory may be very near or even attached to the wall. Specifically, for lower J jets (J <∼20), the flame intermittently attaches to the wall for some fraction of time that decreases with downstream distance and increasing J. From a practical point of view, intermittent flame–wall interactions cause an increase in the time averaged wall temperature, raising durability concerns. We suggest that this unsteady jet–wall attraction can be understood from analogies to the dynamics of co-flowing jet or wake pairs, as low J jets deflect into the cross-flow direction quickly. Side-by-side jets or wakes are “attracted” toward each other and exhibit global instabilities that are not present when they are isolated. In support of this argument, we show that data taken over the 1.7< J <7.2 range and a range of axial locations can be collapsed using the time averaged distance of the reacting jet from the wall. [Copyright &y& Elsevier]

Abstract: In this paper the global flame dynamics of a model annular gas turbine combustor undergoing strong self-excited circumferential instabilities is presented. The combustor consisted of either 12, 15 or 18 turbulent premixed bluff-body flames arranged around an annulus of fixed circumference so that the effect of flame separation distance, S, on the global heat release dynamics could be investigated. Reducing S was found to produce both an increase in the resonant frequency and the limit-cycle amplitudes of pressure and heat release for the same equivalence ratio. The phase-averaged global heat release, obtained from high-speed chemiluminescence imaging from above, showed that these changes are caused by large-scale modifications to the flame structure around the annulus. For the largest S studied (12 flame configuration) the azimuthal instability produced a helical-like global heat release structure for each flame. When S was decreased, large-scale merging or linking between adjacent flames occurred spanning approximately half of the annulus with the peak heat release concentrated at the outer annular wall. The circumferential nature of the instability was evident from both the pressure measurements and the phase-averaged chemiluminescence showing the phase of the heat release on either side of the annulus to be ≈180° apart and spinning in the counter clockwise direction. Both spinning and standing modes were found but only spinning modes are considered in this paper. To the best of the authors knowledge, these are the first experiments to provide a phase-averaged picture of self-excited azimuthal instabilities in a laboratory-scale annular combustor relevant to gas turbines. [Copyright &y& Elsevier]

Abstract: The influence of confinement side walls on the response of premixed conical flames submitted to velocity disturbances is investigated experimentally and theoretically. When the flame is sufficiently confined, the burnt gases cannot fully expand at the nozzle outlet. In that case, the confinement ratio C r = R 0/R 1 between the burner and flame tube radii and the burnt to unburnt gas volume expansion ratio E = ρ u /ρ b need to be taken into account in the description of the flame transfer function (FTF). The main effect of confinement is an acceleration of the fresh stream of reactants induced by the over-pressure of the confined burnt gases. Experiments on steady flames reveal that the flame height increases for increasing confinement ratios C r , leading to a gain shift and a phase drop for the FTF. A theoretical analysis is conducted to model this acceleration and examine its impact on the steady flame shape and flame response to flow disturbances. The change in the FTF phase is shown to be related to a reduction of the mean time lag between heat release rate perturbations and velocity modulations induced by both the velocity acceleration in the fresh reactants and a change in the location of the steady flame front. An expression to scale the FTF of confined flames is derived based on a modification of the classical reduced frequency ω ∗ used to scale the response of unconfined flames. This expression includes explicitly the confinement ratio C r and the volume expansion ratio E. This new dimensionless number enables to plot FTF measured for different confinements with reasonable collapse. It may also be used to transpose FTF gathered on single burners to multiple injection configurations when the burnt gases cannot fully expand due to the presence of neighboring flames. [Copyright &y& Elsevier]

In this study, the influence of the negative velocity field formed ahead of an abruptly deformed flame tip on the propagation behaviour of a laminar premixed flame is numerically investigated. A strong deformation in the flame front is induced by imposing a very narrow, in-line pre-heating zone in the unburned region. The simulation is performed under low Mach number approximation by using a multi-scale multi-physics Computational Fluid Dynamics (CFD) solver FrontFlow/Red with one-step finite rate chemistry in order to track the time-dependent flame dynamics. The computed results unveil that the flame front is deformed significantly within a short time due to the narrow in-line pre-heating effect. The flame deformation induces a strong negative velocity field ahead of the deformed flame tip, acting in the direction of propagation, which gives rise to a strong pair vortex. This strong pair vortex interacts with the flame tip and then slides down along the flame surface in the upstream direction during propagation. This flame-vortex interaction causes further deformation in the flame surface in the upstream direction, and consequently, the flame exhibits a wave-like surface, which enhances the flame propagation speed. The auto-generation of a strong pair vortex ahead of the flame front due to the localised thermal input could be applied as one of the methods to control the combustion externally. It is also expected that the results obtained in this study could have a significant impact on the detailed understanding of the local thermo-fluid dynamical interaction process of turbulent combustion in practical combustors. [ABSTRACT FROM PUBLISHER]

Abstract: This paper describes experimental investigation of co-flow air velocity effects on the flickering behaviour of laminar non-lifted methane diffusion flames. Chemiluminescence, high-speed photography, schlieren and Particle Imaging Velocimetry (PIV), have been used to study the changes in the flame/vortex interactions as well as the flame flickering frequency and magnitude by the co-flow air. Four cases of methane flow rates at different co-flow air velocities are investigated. It has been observed that the flame dynamics and stability of co-flow diffusion flames are strongly affected by the co-flow air velocity. When the co-flow velocity has reached a certain value the buoyancy driven flame oscillation was completely suppressed. The schlieren and PIV imaging have revealed that the co-flow of air is able to push the initiation point of the outer toroidal vortices beyond the visible flame to create a very steady laminar flow region in the reaction zone. Then the buoyancy driven instability is only effective in the plume of hot gases above the visible flame. It is observed that a higher co-flow rate is needed in order to suppress the flame flickering at a higher fuel flow rate. Therefore the ratio of the air velocity to the fuel velocity, γ, is a stability controlling parameter. The velocity ratio, γ, was found to be 0.72 for the range of tested flow rates. The dominant flickering frequency was observed to increase linearly with the co-flow rate (a) as; f =0.33a +11. The frequency amplitudes, however, were observed to continuously decrease as the co-flow air was increasing. [Copyright &y& Elsevier]

Abstract: An analytical model is derived for the linear response of swirling flames submitted to velocity disturbances. The flame dynamics is represented by a linearized version of the G-equation. Turbulent fluctuations are first averaged in time to obtain a kinematic equation in which the flame is represented by a wrinkled sheet. The variables are then phase averaged to describe acoustic perturbations and obtain a perturbed G-equation. It is first concluded that the flame motion results from the combined effects of axial and azimuthal velocity perturbations. The latter disturbances formed at the swirler outlet when this element is submitted to axial velocity fluctuations are convected by the flow and impinge on the flame. In this disturbance field the swirl number is perturbed and this is effectively modeled by assuming that the turbulent burning velocity is modulated by the axial and azimuthal velocity perturbations. It is then shown that the response of swirling flames can be deduced from the transfer function of inverted conical flames submitted to axial velocity perturbations. It is however important to account for the phase shift resulting from the propagation of axial and azimuthal disturbances on the downstream side of the swirler. This phase shift, due to the difference in propagation velocity of acoustic and convective perturbations, is determined experimentally. Theoretical transfer functions are compared with measurements corresponding to two bulk velocities at a constant swirl number . A good agreement is obtained. It is shown in particular that minima and maxima of the flame response are suitably retrieved and the Strouhal number can be used to collapse the data. [ABSTRACT FROM AUTHOR]

Abstract: The propagation characteristics of a laminar premixed flame front in meso-scale straight and diverging channels of 5°, 10° and 15° with inlet dimension of 25mm×2mm are reported in this paper. The downstream part of the channels was heated with an external heat source, to maintain a positive wall temperature gradient along the direction of fluid flow. These investigations show that planar flames are observed near flash back limits. Negatively stretched flames were observed for moderate flow rates and rich mixtures and for high flow rates, flames were positively stretched. These flames were either symmetric or asymmetric in nature. Partially stable flames were observed at high velocities for rich mixtures, whereas for lean mixture partially stable flames were observed for all flow rates. All the divergent channels showed an improvement in high velocity limits compared with the straight channel for the same mixture. Planar flames observed in the experiments helped in determining the laminar burning velocities for these mixtures at different preheat temperatures. A co-relation of laminar burning velocity with mixture preheat temperature is also obtained for a stoichiometric methane–air mixture. This co-relation Su/Su,o =(Tu /Tu,o )1.558 is in good agreement with the earlier co-relations. [Copyright &y& Elsevier]

Abstract: In the present article we present computational and experimental investigations of a turbulent asymmetric vortex flame. Such flame was created in a novel asymmetric combustor, which is described for the first time in this article. The three dimensional isothermal and reacting flow fields have been described using a computational methodology that impalements the R ε /k − ε and the eddy dissipation turbulence and combustion models, respectively. The computational model is validated for both isothermal and reacting flows. Additionally, the visible flame structure was captured by direct photography at a wide range of equivalence ratios in order to emphasize the exceptional stability of such flame. The mechanism of flame stability and interaction with the forced vortex field is preliminarily discussed. Finally, the basic characteristics of the asymmetric vortex flames are concluded. [Copyright &y& Elsevier]

Abstract: Carbon black has diverse industrial applications such as a reinforcing agent in tires and as a black pigment in printing inks. Flame aerosol synthesis is the commonly employed route for large scale production of carbon black. A coupled flame dynamics – monodisperse population balance model for the synthesis of carbon black in an aerosol flame reactor is presented here. The population balance model was incorporated into the commercial computational fluid dynamics software CFX to simulate the effect of flame dynamics on particle synthesis. The model was tested with published experimental data for acetylene black synthesis through oxidative thermal decomposition of acetylene with oxygen in a premixed flame reactor. The predicted axial temperature profiles at different equivalence ratios (actual acetylene/oxidizer ratio divided by stoichiometric acetylene/oxidizer ratio) were reasonably close to experimental data. The effect of the equivalence ratio on process parameters like maximum temperature of the flame, specific surface area of particles and flame structure was investigated. It was observed that the maximum temperature of the flame and specific surface area of carbon black particles decrease upon increasing the equivalence ratio which is in agreement with published experimental studies. Simulation results are also presented for carbon black synthesis in a diffusion flame reactor with different burner designs. [Copyright &y& Elsevier]

Abstract: The flame regimes of ignition and flame propagation as well as transitions between different flame regimes of n-heptane-air mixtures in a one-dimensional, cylindrical, spark assisted homogeneously charged compression ignition (HCCI) reactor are numerically modeled using a multi-timescale method with reduced kinetic mechanism. It is found that the initial mixture temperature and pressure have a dramatic impact on flame dynamics. Depending on the initial temperature gradient, there exist at least six different combustion regimes, an initial single flame front propagation regime, a coupled low temperature and high temperature double-flame regime, a decoupled low temperature and high temperature double-flame regime, a low temperature ignition regime, a single high temperature flame regime, and a hot ignition regime. The results show that the low temperature and high temperature flames have distinct kinetic and transport properties as well as flame speeds, and are strongly influenced by the low temperature chemistry. The pressure and heat release rates are affected by the appearance of different flame regimes and the transitions between them. Furthermore, it is found that the critical temperature gradient for ignition and acoustic wave coupling becomes singular at the negative temperature coefficient (NTC) region. The results show that both the NTC effect and the acoustic wave propagation in a closed reactor have a dramatic impact on the ignition front and acoustic interaction. [ABSTRACT FROM AUTHOR]

Abstract: The dynamics of premixed propane–air flame in a meso-scale ceramic combustor has been examined here. The flame characteristics in the combustor were examined by measuring the acoustic emissions and preheat temperatures together with high-speed cinematography. For the small-scale combustor, the volume to surface area ratio is small and hence the walls have significant effect on the global flame structure, flame location and flame dynamics. In addition to the flame–wall thermal coupling there is a coupling between flame and acoustics in the case of confined flames. Flame–wall thermal interactions lead to low frequency flame fluctuations (∼100Hz) depending upon the thermal response of the wall. However, the flame–acoustic interactions can result in a wide range of flame fluctuations ranging from few hundred Hz to few kHz. Wall temperature distribution is one of the factors that control the amount of reactant preheating which in turn effects the location of flame stabilization. Acoustic emission signals and high-speed flame imaging confirmed that for the present case flame–acoustic interactions have more significant effect on flame dynamics. Based on the acoustic emissions, five different flame regimes have been identified; whistling/harmonic mode, rich instability mode, lean instability mode, silent mode and pulsating flame mode. [ABSTRACT FROM AUTHOR]

Dynamical behaviour of the premixed flame propagating in the inert high-porosity micro-fibrous porous media has been studied numerically. Effects of mixture filtration velocity, equivalence ratio and burner transverse size on the flame structure have been investigated and the regions of existence of different combustion regimes have been determined. It was found that the influence of the hydrodynamic instability on the flame dynamics is significant in the case of the moderate and high filtration velocities and this effect is negligible at the low velocities. At the moderate filtration velocities the effect of hydrodynamic instability manifests in the flame front deformation and in particular in the flame inclination. It was found that the flame can be stabilized within the whole interval of the filtration gas velocity, whereas in the ordinary porous media the standing wave is settled only at fixed value of gas filtration velocity. This finding is in line with recent experimental results on combustion in micro-fibrous porous media (Yang et al., Combust. Sci. Tech. 181 (2009), 1-16). Possible physical interpretation of the flame anchoring effect may be given on the base of present numerical analysis. At the high filtration velocities the hydrodynamic instability manifests itself in periodical appearance of the moving wrinkles on the flame front surface which forms non stationary high temperature trailing spots behind the leading part of the flame front. Such dynamics may be associated with splitting wave structures which were revealed in previous experiments (Yang et al., Combust. Sci. Tech. 181 (2009), 1-16). [ABSTRACT FROM AUTHOR]

Abstract: Temporally resolved measurements of turbulence–flame interaction were used to experimentally determine relationships for the strain-rate and curvature stretch-rate exerted on a premixed flame surface. These relationships include a series of transfer functions that are analogous to, but not equal to, stretch-efficiency functions. The measurements were obtained by applying high-repetition-rate particle image velocimetry in a turbulent slot Bunsen flame and were able to resolve the range of turbulent scales that cause flame surface straining and wrinkling. Fluid control masses were tracked in a Lagrangian manner as they interacted with the flame surface. From each interaction, the spatially and temporally filtered subgrid strain-rate and curvature stretch-rate were measured. By analyzing the statistics of thousands of turbulence–flame interactions, relationships for the strain-rate and curvature stretch-rate were determined that are appropriate for Large Eddy Simulation. It was found that the strain-rate exerted on the flame during these interactions was better correlated with the strength of the subgrid fluid-dynamic strain-rate field than with previously used characteristic strain-rates. Furthermore, stretch-efficiency functions developed from simplified vortex–flame interactions significantly over-predict the measurements. Hence, the proposed relationship relates the strain-rate on the flame to the filtered subgrid fluid-dynamic strain-rate field during real turbulence–flame interactions using an empirically determined Strain-Rate Transfer function. It was found that the curvature stretch-rate did not locally balance the strain-rate as has been proposed in previous models. A geometric relationship was found to exist between the subgrid flame surface wrinkling factor and subgrid curvature stretch-rate, which could be expressed using an empirically determined wrinkling factor transfer function. Curve fits to the measured relationships are provided that could be implemented in numerical simulations of turbulent premixed combustion. [Copyright &y& Elsevier]

The chamber pressure and fuel flow rate effects on the flickering behavior of methane-air diffusion flames was studied over the pressure range of 1 to 10 bar. Photomultipliers and high-speed imaging techniques have been used to study the frequency and magnitude of the flame oscillation and the change in global flame shape. The instability behavior of the flame was observed to be sensitive to both the fuel flow rate and pressure. Particularly, it has been observed that the flame responds to the change of pressure more when the pressure is relatively low. High-speed imaging has shown that the periodical break-up of the methane flame at higher flow rates is almost symmetric. However, the methane flames at lower flow rates oscillate in a more waving manner due to the alternating lateral nature of the outer vortices. The average flame luminosity was observed to increase with pressure up to 6 bar and then starts to decrease with the further increase of pressure. The flame oscillation magnitude (Lf) and oscillation wavelength (λ) were obtained from the high-speed imaging database. It has been observed that the trends of these parameters correlate well with the standard deviation (σ) of mean pixel intensity (MPI), measured from the flame high-speed images. The increase in fuel flow rate was observed to increase the magnitude of oscillation. The dominant flickering frequency of a methane diffusion flame varies with the chamber pressure as a function of Pn(f = 15.7P0.17). [ABSTRACT FROM AUTHOR]