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Publié le 20 juillet 2024

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1

Ayoobi, Mohsen, et Ingmar Schoegl. « Numerical analysis of flame instabilities in narrow channels : Laminar premixed methane/air combustion ». International Journal of Spray and Combustion Dynamics 9, no 3 (5 juin 2017) : 155–71. http://dx.doi.org/10.1177/1756827717706009.

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Premixed flames propagating within small channels show complex combustion phenomena that differ from flame propagation at conventional scales. Available experimental and numerical studies have documented stationary, non-stationary, or asymmetric modes that depend on properties of the incoming reactant flow as well as channel geometry and wall temperatures. This work seeks to illuminate mechanisms leading to symmetry breaking and limit cycle behavior that are fundamental to these combustion modes. Specifically, four cases of lean premixed methane/air combustion—two equivalence ratios (0.53 and 0.7) and two channel widths (2 mm and 5 mm)—are investigated in a 2D configuration with constant channel length and bulk inlet velocity, where numerical simulations are performed using detailed chemistry. External wall heating is simulated by imposing a linear temperature gradient as a boundary condition on both walls. In the 2 mm channel, both equivalence ratios produce flames that stabilize with symmetric flame fronts after propagating upstream. In the 5 mm channel, flame fronts start symmetrically, although symmetry is broken almost immediately after ignition. Further, 5 mm channels produce non-stationary combustion modes with dramatically different limit cycles: in the leaner case ( φ = 0.53), the asymmetric flame front flops periodically, whereas in the richer case ( φ = 0.7), flames with repetitive extinctions and ignitions (FREI) are observed. To further understand the flame dynamics, reaction fronts and flame fronts are captured and differentiated. Results show that the loss of flame front symmetry originates in a region close to the flame cusp, where flow and chemical characteristics exhibit large gradients and curvatures. Limit cycle behavior is illuminated by investigating flame edges that are formed along the wall, and accompany local or global ignition and extinction processes. In the flopping mode ( φ = 0.53), local ignition and extinction in regions adjacent to the wall result in oblique fronts that advance and recede along the wall and redirect the flow ahead of the flame. In the FREI mode, asymmetric flames propagate much farther upstream, where they experience global extinction due to heat losses, and re-ignite far downstream with opposite flame front orientation. In both cases, an interaction of flow and chemical effects drives the asymmetric limit cycles. The lack of instabilities and asymmetries for the 2mm cases is attributed to insufficient wall separation, which is of the same order of magnitude as the flame thickness.
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2

Xia, Yongfang, Tingyong Fang, Haitao Wang, Erbao Guo et Jinwei Ma. « Numerical investigation of low-velocity filtration combustion instability based on the initial preheating non-uniformity ». E3S Web of Conferences 136 (2019) : 02040. http://dx.doi.org/10.1051/e3sconf/201913602040.

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The effects of the initial preheating perturbation on the dynamical behaviors of FGC wave propagation instability for low-velocity FGC in packed bed are studied numerically. The behaviors of the flame front inclination, break, and shrinking instabilities are always observed in experiments. Based on the experimental phenomena, an initial thermal perturbation model is numerically proposed as to predict the deformation behaviors of the flame front instabilities. The typical flame shapes are obtained depending on filtration velocity, equivalence ratio, and initial preheating temperature difference. It is demonstrated that the development of flame front inclination instability is proportional to the magnitude of initial preheating perturbation. At a lower equivalence ratio, the initial thermal perturbation of 300 K leads to the evolution of flame front break. Increasing filtration velocity leads to the appearance of flame front break, due to the intensification of the hydrodynamic instability. In addition, a perculiar instability of flame front shifting is also confirmed with the initial thermal perturbation of 400 K, which results in a fuel leakage of incomplete combustion.
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3

Yang, Sheng, Abhishek Saha, Zirui Liu et Chung K. Law. « Role of Darrieus–Landau instability in propagation of expanding turbulent flames ». Journal of Fluid Mechanics 850 (10 juillet 2018) : 784–802. http://dx.doi.org/10.1017/jfm.2018.426.

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In this paper we study the essential role of Darrieus–Landau (DL), hydrodynamic, cellular flame-front instability in the propagation of expanding turbulent flames. First, we analyse and compare the characteristic time scales of flame wrinkling under the simultaneous actions of DL instability and turbulent eddies, based on which three turbulent flame propagation regimes are identified, namely, instability dominated, instability–turbulence interaction and turbulence dominated regimes. We then perform experiments over an extensive range of conditions, including high pressures, to promote and manipulate the DL instability. The results clearly demonstrate the increase in the acceleration exponent of the turbulent flame propagation as these three regimes are traversed from the weakest to the strongest, which are respectively similar to those of the laminar cellularly unstable flame and the turbulent flame without flame-front instability, and thus validating the scaling analysis. Finally, based on the scaling analysis and the experimental results, we propose a modification of the conventional turbulent flame regime diagram to account for the effects of DL instability.
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4

Palies, Paul, Milos Ilak et Robert Cheng. « Transient and limit cycle combustion dynamics analysis of turbulent premixed swirling flames ». Journal of Fluid Mechanics 830 (5 octobre 2017) : 681–707. http://dx.doi.org/10.1017/jfm.2017.575.

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Premixed low swirling flames (methane–air and hydrogen–methane–air) are experimentally investigated for three different regimes. Stable, local transient to instability and limit cycle regimes corresponding to three distinct equivalence ratios are considered. Dynamic mode decomposition is applied to the hydrogen–air–methane flame to retrieve the modes frequencies, growth rates and spatial distributions for each regime. The results indicate that a vortical wave propagating along the flame front is associated with the transition from stability to instability. In addition, it is shown that a key effect on stability is the location of the non-oscillating (0 Hz) flame component. The phase-averaged unsteady motion of the flames over one cycle of oscillation shows the vortical wave rolling up the flame front. The Rayleigh index maps are formed to identify the region of driving and damping of the self-sustained oscillation, while the flame transfer function phase leads to the propagation mode of the perturbations along the flame front. The second mechanism identified concerns the swirl number fluctuation induced by the mode conversion. By utilizing hypotheses for the flow field and the flame structure, it is pointed out that those mechanisms are at work for both flames (methane–air and hydrogen–methane–air) and their effects on the unsteady heat release are determined. Both unsteady heat release contributions, the vortical wave induces flame surface fluctuations and swirl number oscillation induces unsteady turbulent burning velocity, are in phase opposition and of similar amplitudes.
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5

Yu, Rixin. « Deep learning of nonlinear flame fronts development due to Darrieus–Landau instability ». APL Machine Learning 1, no 2 (1 juin 2023) : 026106. http://dx.doi.org/10.1063/5.0139857.

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The Darrieus–Landau instability is studied using a data-driven, deep neural network approach. The task is set up to learn a time-advancement operator mapping any given flame front to a future time. A recurrent application of such an operator rolls out a long sequence of predicted flame fronts, and a learned operator is required to not only make accurate short-term predictions but also reproduce characteristic nonlinear behavior, such as fractal front structures and detached flame pockets. Using two datasets of flame front solutions obtained from a heavy-duty direct numerical simulation and a light-duty modeling equation, we compare the performance of three state-of-art operator-regression network methods: convolutional neural networks, Fourier neural operator (FNO), and deep operator network. We show that, for learning complicated front evolution, FNO gives the best recurrent predictions in both the short and long term. A consistent extension allowing the operator-regression networks to handle complicated flame front shape is achieved by representing the latter as an implicit curve.
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6

JOULIN, GUY, HAZEM EL-RABII et KIRILI A. KAZAKOV. « On-shell description of unsteady flames ». Journal of Fluid Mechanics 608 (11 juillet 2008) : 217–42. http://dx.doi.org/10.1017/s0022112008002140.

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The problem of a non-perturbative description of unsteady premixed flames with arbitrary gas expansion is addressed in the two-dimensional case. Considering the flame as a surface of discontinuity with arbitrary local burning rate and gas velocity jumps, we show that the flame-front dynamics can be determined without having to solve the flow equations in the bulk. On the basis of the Thomson circulation theorem, an implicit integral representation of the downstream gas velocity is constructed. It is then simplified by a successive stripping of the potential contributions to obtain an explicit expression for the rotational component near the flame front. We prove that the unknown potential component is left bounded and divergence-free by this procedure, and hence can be eliminated using the dispersion relation for its on-shell value (i.e. the value along the flame front). The resulting system of integro-differential equations relates the on-shell fresh-gas velocity and the front position. As limiting cases, these equations contain all the theoretical results on flame dynamics established so far, including the linear equation describing the Darrieus–Landau instability of planar flames, and the nonlinear Sivashinsky–Clavin equation for flames with weak gas expansion.
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7

Hicks, E. P. « A shear instability mechanism for the pulsations of Rayleigh–Taylor unstable model flames ». Journal of Fluid Mechanics 748 (6 mai 2014) : 618–40. http://dx.doi.org/10.1017/jfm.2014.198.

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AbstractPrevious studies have shown that the behaviour of Rayleigh–Taylor (RT) unstable flames depends on the boundary conditions. If the boundary conditions at the domain walls are impermeable/adiabatic or reflecting then the flame assumes a stable parabolic shape. On the other hand, periodic boundary conditions can produce unstable pulsating solutions. In this paper, we explore why periodic boundary conditions allow unstable solutions by showing the results of two-dimensional direct numerical simulations of model flames. We show that RT unstable premixed model flames pulsate at low gravity because of a shear instability of the vorticity layers behind the flame front. The resulting vortex shedding is controlled by a region of absolute-like instability which moves closer to the flame front as gravity is increased, ultimately disturbing the flame and leading to pulsations. We demonstrate that this region is ‘absolutely unstable’ by showing that the wake is dominated by pure frequency oscillations. In addition, the shear instability can be described by the Landau equation and can be represented dynamically by a Hopf bifurcation. The applicability of the Landau equation allows the apparently complex spatio-temporal behaviour of the vortex shedding to be described by a simple temporal model with only a secondary spatial dependence. We show that the flame behaviour is analogous to the initial instability downstream of a circular cylinder, which leads to the von Kármán vortex street for large enough values of the Reynolds number.
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8

Altantzis, C., C. E. Frouzakis, A. G. Tomboulides, M. Matalon et K. Boulouchos. « Hydrodynamic and thermodiffusive instability effects on the evolution of laminar planar lean premixed hydrogen flames ». Journal of Fluid Mechanics 700 (18 mai 2012) : 329–61. http://dx.doi.org/10.1017/jfm.2012.136.

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AbstractNumerical simulations with single-step chemistry and detailed transport are used to study premixed hydrogen/air flames in two-dimensional channel-like domains with periodic boundary conditions along the horizontal boundaries as a function of the domain height. Both unity Lewis number, where only hydrodynamic instability appears, and subunity Lewis number, where the flame propagation is strongly affected by the combined effect of hydrodynamic and thermodiffusive instabilities are considered. The simulations aim at studying the initial linear growth of perturbations superimposed on the planar flame front as well as the long-term nonlinear evolution. The dispersion relation between the growth rate and the wavelength of the perturbation characterizing the linear regime is extracted from the simulations and compared with linear stability theory. The dynamics observed during the nonlinear evolution depend strongly on the domain size and on the Lewis number. As predicted by the theory, unity Lewis number flames are found to form a single cusp structure which propagates unchanged with constant speed. The long-term dynamics of the subunity Lewis number flames include steady cell propagation, lateral flame movement, oscillations and regular as well as chaotic cell splitting and merging.
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9

Jiang, Xiaozhen, Jingxuan Li et Lijun Yang. « Nonlinear response of laminar premixed flames to dual-input harmonic disturbances ». INTER-NOISE and NOISE-CON Congress and Conference Proceedings 265, no 4 (1 février 2023) : 3408–19. http://dx.doi.org/10.3397/in_2022_0484.

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In gas turbines, aero-engines and rocket engines, flames are always disturbed by perturbations of dual or multiple harmonic frequencies, resulting in corresponding combustion instability. This paper analyses the nonlinear response of laminar premixed flames to dual-input harmonic disturbances to further understand those associated combustion instability. Nonlinear results of flame dynamics were derived from analytical and numerical solutions of the G-equation. The spatial front-tracking of premixed flames was obtained, where types of nonlinear behaviors were classified and related mechanisms of that were elucidated. A dual-input flame description function (DIFDF) was proposed to separately determine global nonlinearities of flame responses of fundamental and higher harmonics frequencies under dual-input disturbances. The fundamental frequency response consists of linear and nonlinear components, and the higher harmonic frequency one is purely nonlinear. The DIFDF properties of conical and "V" flames were compared, with particular emphasis on their differences in nonlinear behavior. The spatial and global effects of the second input frequency on the flame kinematics perturbed by the first frequency were also clarified. Furthermore, the roles of perturbation amplitude and flame height in spatial flame dynamics and DIFDF were quantified.
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10

Mokrin, Sergey, R. V. Fursenko et S. S. Minaev. « Thermal-Diffusive Stability of Counterflow Premixed Flames at Low Lewis Numbers ». Advanced Materials Research 1040 (septembre 2014) : 608–13. http://dx.doi.org/10.4028/www.scientific.net/amr.1040.608.

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Dynamics of radiative, near-limit, stretched premixed flames is investigated analytically and numerically. Investigation of counterflow premixed flames stability is important for the development of new combustion technologies such as those associated with low-NOx emission, lean burn and material synthesis. Emphasis is paid on the linear stability of multiple flame regimes. The present analysis, for the first time, gives out a dispersion equation describing growth rate of small spatial perturbations of the flame front. The stability diagram is obtained and the region of instability is distinguished.
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11

CRETA, F., et M. MATALON. « Propagation of wrinkled turbulent flames in the context of hydrodynamic theory ». Journal of Fluid Mechanics 680 (1 juin 2011) : 225–64. http://dx.doi.org/10.1017/jfm.2011.157.

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We study the propagation of premixed flames in two-dimensional homogeneous isotropic turbulence using a Navier–Stokes/front-capturing methodology within the context of hydrodynamic theory. The flame is treated as a thin layer separating burnt and unburnt gases, of vanishingly small thickness, smaller than the smallest fluid scales. The method is thus suitable to investigate the flame propagation in the wrinkled flamelet regime of turbulent combustion. A flow-control system regulates the mean position of the flame and the incident turbulence intensity. In this context we study the individual effects of turbulence intensity, turbulence scale, thermal expansion, hydrodynamic strain and hydrodynamic instability on the propagation characteristics of the flame. Results are obtained assuming positive Markstein length, corresponding to lean hydrocarbon–air or rich hydrogen–air mixtures. For stable planar flames we find a quadratic dependence of turbulent speed on turbulence intensity. Upon onset of hydrodynamic instability, corrugated structures replace the planar conformation and we observe a greater resilience to turbulence, the quadratic scaling being replaced by scaling exponents less than one. Such resilience is also confirmed by the observation of a threshold turbulence intensity below which the propagation speed of corrugated flames is indistinguishable from the laminar speed. Turbulent speed is found to increase and later plateau with increasing thermal expansion, this affecting the average flame displacement but not the mean flame curvature. In addition, turbulence integral scale is also observed to affect the propagation of the flame with the existence of an intermediate scale maximizing the turbulent speed. This maximizing scale is smaller for corrugated flames than it is for planar flames, implying that small eddies that will be unable to significantly perturb a planar front could be rather effective in perturbing a corrugated flame. Turbulent planar flames, and more so corrugated flames, were observed to experience a positive mean hydrodynamic strain, which was explained in terms of the overwhelming mean contribution of the normal component of strain. The positive straining causes a decrease in the mean laminar propagation speed which in turn can decrease the turbulent speed. The effect of the flame on the incident turbulent field was examined in terms of loss of isotropy and vorticity destruction by thermal expansion. The latter can be mitigated by a baroclinic vorticity generation which is enhanced for corrugated flames.
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12

Searby, G., et D. Rochwerger. « A parametric acoustic instability in premixed flames ». Journal of Fluid Mechanics 231 (octobre 1991) : 529–43. http://dx.doi.org/10.1017/s002211209100349x.

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We present an experimental and theoretical investigation of some aspects of the coupling between a premixed laminar quasi-planar flame front and acoustic standing waves in tubes. A multidimensional instability of the front arises from its interaction with the oscillating field of acceleration. This instability can be described by the Clavin–Williams laminar wrinkled flame theory in which the periodic acceleration created by the acoustic field is added to the acceleration due to gravity. As first suggested by Markstein, the resulting equation can be reduced to the Mathieu equation for a parametric oscillator. A cellular instability appears with a finite excitation threshold. This instability is responsible for the spontaneous generation of intense acoustic oscillations observed elsewhere. The value of the acoustic field at the threshold of instability and the wavelength of the cellular structures are measured experimentally for propane flames and are found to be in good agreement with the calculated values. It is also seen, both experimentally and theoretically, that for certain amplitudes of pumping, the parametric mechanism can also stabilize an initially unstable system.
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Heckl, Maria. « Advances by the Marie Curie project TANGO in thermoacoustics ». International Journal of Spray and Combustion Dynamics 11 (janvier 2019) : 175682771983095. http://dx.doi.org/10.1177/1756827719830950.

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This paper gives an overview of the research performed by the project TANGO – an Initial Training Network (ITN) with an international consortium of seven academic and five industrial partners. TANGO is the acronym for ‘Thermoacoustic and Aeroacoustic Nonlinearities in Green combustors with Orifice structures’). The researchers in TANGO studied many of the intricate physical processes that are involved in thermoacoustic instabilities. The paper is structured in such a way that each section describes a topic investigated by one or more researchers. The topics include: - transition from combustion noise to thermoacoustic instability - development of an early-warning system by detecting the precursor of an instability - analytical flame models based on time-lags - Green's function approach for stability predictions from nonlinear flame models - intrinsic thermoacoustic modes - transport phenomena in swirl waves - model of the flame front as a moving discontinuity - development of efficient numerical codes for instability predictions - heat exchanger tubes inside a combustion chamber A substantial amount of valuable new insight was gained during this four-year project.
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14

Röpke, F. K., et W. Hillebrandt. « On the Stability of Thermonuclear Burning Fronts in Type Ia Supernovae ». International Astronomical Union Colloquium 192 (2005) : 333–38. http://dx.doi.org/10.1017/s0252921100009386.

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SummaryThe propagation of cellularly stabilized thermonuclear flames is investigated by means of numerical simulations. In Type Ia supernova explosions the corresponding burning regime establishes at scales below the Gibson length. The cellular flame stabilization - which is a result of an interplay between the Landau-Darrieus instability and a nonlinear stabilization mechanism - is studied for the case of propagation into quiescent fuel as well as interaction with vortical fuel flows. Our simulations indicate that in thermonuclear supernova explosions stable cellular flames develop around the Gibson scale and that a deflagration-to-detonation transition is unlikely to be triggered from flame evolution effects here.
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15

Pereira, Carlos Alvarez, et José M. Vega. « On the pulsating instability of two-dimensional flames ». European Journal of Applied Mathematics 3, no 1 (mars 1992) : 55–73. http://dx.doi.org/10.1017/s0956792500000681.

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We consider a well-known thermo-diffusive model for the propagation of a premixed, adiabatic flame front in the large-activation-energy limit. That model depends only on one nondimensional parameter β, the reduced Lewis number. Near the pulsating instability limit, as β↓β0= 32/3, we obtain an asymptotic model for the evolution of a quasi-planar flame front, via a multi-scale analysis. The asymptotic model consists of two complex Ginzburg–Landau equations and a real Burgers equation, coupled by non-local terms. The model is used to analyse the nonlinear stability of the flame front.
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CLAVIN, P., L. MASSE et F. A. WILLIAMS. « COMPARISON OF FLAME-FRONT INSTABILITIES WITH INSTABILITIES OF ABLATION FRONTS IN INERTIAL-CONFINEMENT FUSION ». Combustion Science and Technology 177, no 5-6 (avril 2005) : 979–89. http://dx.doi.org/10.1080/00102200590926950.

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17

Krikunova, Anastasia. « Numerical simulation of combustion instabilities under the alternating gravity conditions ». MATEC Web of Conferences 209 (2018) : 00005. http://dx.doi.org/10.1051/matecconf/201820900005.

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The work is devoted to the analysis of the methane-air conical flame behaviour under conditions of an alternating gravitational field. Numerical simulation based on the software package FlowVision, has shown the possibility of modeling the flame front instabilities during the transition from the normal gravitational conditions to zero gravity. The appearance of the flame front oscillations is demonstrated under the such conditions. Further studies will provide a complete picture of the behavior of the flame in an alternating gravitational field.
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Dulin, Vladimir, Leonid Chikishev, Dmitriy Sharaborin, Aleksei Lobasov, Roman Tolstoguzov, Zundi Liu, Xiaoxiang Shi, Yuyang Li et Dmitriy Markovich. « On the Flow Structure and Dynamics of Methane and Syngas Lean Flames in a Model Gas-Turbine Combustor ». Energies 14, no 24 (8 décembre 2021) : 8267. http://dx.doi.org/10.3390/en14248267.

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The present paper compares the flow structure and flame dynamics during combustion of methane and syngas in a model gas-turbine swirl burner. The burner is based on a design by Turbomeca. The fuel is supplied through injection holes between the swirler blades to provide well-premixed combustion, or fed as a central jet from the swirler’s centerbody to increase flame stability via a pilot flame. The measurements of flow structure and flame front are performed by using the stereo particle image velocimetry and OH planar laser-induced fluorescence methods. The measurements are performed for the atmospheric pressure without preheating and for 2 atm with the air preheated up to 500 K. The flow Reynolds numbers for the non-reacting flows at these two conditions are 1.5 × 103 and 1.0 × 103, respectively. The flame dynamics are analyzed based on a high-speed OH* chemiluminescence imaging. It is found that the flame dynamics at elevated conditions are related with frequent events of flame lift-off and global extinction, followed by re-ignition. The analysis of flow structure via the proper orthogonal decomposition reveals the presence of two different types of coherent flow fluctuations, namely, longitudinal and transverse instability modes. The same procedure is applied to the chemiluminescence images for visualization of bulk movement of the flame front and similar spatial structures are observed. Thus, the longitudinal and transverse instability modes are found in all cases, but for the syngas at the elevated pressure and temperature the longitudinal mode is related to strong thermoacoustic fluctuations. Therefore, the present study demonstrates that a lean syngas flame can become unstable at elevated pressure and temperature conditions due to a greater flame propagation speed, which results in periodic events of flame flash-back, extinction and re-ignition. The reported data is also useful for the validation of numerical simulation codes for syngas flames.
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Peracchio, A. A., et W. M. Proscia. « Nonlinear Heat-Release/Acoustic Model for Thermoacoustic Instability in Lean Premixed Combustors ». Journal of Engineering for Gas Turbines and Power 121, no 3 (1 juillet 1999) : 415–21. http://dx.doi.org/10.1115/1.2818489.

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Lean premixed combustors, such as those used in industrial gas turbines to achieve low emissions, are often susceptible to the thermoacoustic combustion instabilities, which manifest themselves as pressure and heat release oscillations in the combustor. These oscillations can result in increased noise and decreased durability due to vibration and flame motion. A physically based nonlinear parametric model has been developed that captures this instability. It describes the coupling of combustor acoustics with the rate of heat release. The model represents this coupling by accounting for the effect of acoustic pressure fluctuations on the varying fuel/air ratio being delivered to the flame, causing a fluctuating heat release due to both fuel air ratio variations and flame front oscillations. If the phasing of the fluctuating heat release and pressure are proper, an instability results that grows into a limit cycle. The nonlinear nature of the model predicts the onset of the instability and additionally captures the resulting limit cycle. Tests of a lean premixed nozzle run at engine scale and engine operating conditions in the UTRC single nozzle rig, conducted under DARPA contract, exhibited instabilities. Parameters from the model were adjusted so that analytical results were consistent with relevant experimental data from this test. The parametric model captures the limit cycle behavior over a range of mean fuel air ratios, showing the instability amplitude (pressure and heat release) to increase and limit cycle frequency to decrease as mean fuel air ratio is reduced.
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MATALON, MOSHE, et PHILIPPE METZENER. « The propagation of premixed flames in closed tubes ». Journal of Fluid Mechanics 336 (10 avril 1997) : 331–50. http://dx.doi.org/10.1017/s0022112096004843.

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A nonlinear evolution equation that describes the propagation of a premixed flame in a closed tube has been derived from the general conservation equations. What distinguishes it from other similar equations is a memory term whose origin is in the vorticity production at the flame front. The two important parameters in this equation are the tube's aspect ratio and the Markstein parameter. A linear stability analysis indicates that when the Markstein parameter α is above a critical value αc the planar flame is the stable equilibrium solution. For α below αc the planar flame is no longer stable and there is a band of growing modes. Numerical solutions of the full nonlinear equation confirm this conclusion. Starting with random initial conditions the results indicate that, after a short transient, a at flame develops when α>αc and it remains flat until it reaches the end of the tube. When α<αc, on the other hand, stable curved flames may develop down the tube. Depending on the initial conditions the flame assumes either a cellular structure, characterized by a finite number of cells convex towards the unburned gas, or a tulip shape characterized by a sharp indentation at the centre of the tube pointing toward the burned gases. In particular, if the initial conditions are chosen so as to simulate the elongated finger-like flame that evolves from an ignition source, a tulip flame evolves downstream. In accord with experimental observations the tulip shape forms only after the flame has travelled a certain distance down the tube, it does not form in short tubes and its formation depends on the mixture composition. While the initial deformation of the flame front is a direct result of the hydrodynamic instability, the actual formation of the tulip flame results from the vortical motion created in the burned gas which is a consequence of the vorticity produced at the flame front.
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KAZAKOV, KIRILL A., et MICHAEL A. LIBERMAN. « NONLINEAR THEORY OF FLAME FRONT INSTABILITY ». Combustion Science and Technology 174, no 7 (juillet 2002) : 129–51. http://dx.doi.org/10.1080/00102200208984090.

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Kodakoglu, Furkan, Sinan Demir, Damir Valiev et V’yacheslav Akkerman. « Analysis of Gaseous and Gaseous-Dusty, Premixed Flame Propagation in Obstructed Passages with Tightly Placed Obstacles ». Fluids 5, no 3 (17 juillet 2020) : 115. http://dx.doi.org/10.3390/fluids5030115.

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A recent predictive scenario of premixed flame propagation in unobstructed passages is extended to account for obstructions that can be encountered in facilities dealing with explosive materials such as in coalmines. Specifically, the theory of globally-spherical, self-accelerating premixed expanding flames and that of flame acceleration in obstructed conduits are combined to form a new analytical formulation. The coalmining configuration is imitated by two-dimensional and cylindrical passages of high aspect ratio, with a comb-shaped array of tightly placed obstacles attached to the walls. It is assumed that the spacing between the obstacles is much less or, at least, does not exceed the obstacle height. The passage has one extreme open end such that a flame is ignited at a closed end and propagates to an exit. The key stages of the flame evolution such as the velocity of the flame front and the run-up distance are scrutinized for variety of the flame and mining parameters. Starting with gaseous methane-air and propane-air flames, the analysis is subsequently extended to gaseous-dusty environments. Specifically, the coal (combustible, i.e., facilitating the fire) and inert (such as sand, moderating the process) dust and their combinations are considered, and the impact of the size and concentration of the dust particles on flame acceleration is quantified. Overall, the influence of both the obstacles and the combustion instability on the fire scenario is substantial, and it gets stronger with the blockage ratio.
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Yang, Sheng, Abhishek Saha, Fujia Wu et Chung K. Law. « Morphology and self-acceleration of expanding laminar flames with flame-front cellular instabilities ». Combustion and Flame 171 (septembre 2016) : 112–18. http://dx.doi.org/10.1016/j.combustflame.2016.05.017.

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Steinbacher, Thomas, et Wolfgang Polifke. « Convective Velocity Perturbations and Excess Gain in Flame Response as a Result of Flame-Flow Feedback ». Fluids 7, no 2 (31 janvier 2022) : 61. http://dx.doi.org/10.3390/fluids7020061.

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Convective velocity perturbations (CVPs) are known to play an important role in the response of flames to acoustic perturbations and in thermoacoustic combustion instabilities. In order to elucidate the flow-physical origin of CVPs, the present study models the response of laminar premixed slit flames to low amplitude perturbations of the upstream flow velocity with a reduced order flow decomposition approach: A linearized G-equation represents the shape and heat release rate of the perturbed flame, while the velocity perturbation field is decomposed into irrotational and solenoidal contributions. The former are determined with a conformal mapping from geometry and boundary conditions, whereas the latter are governed by flame front curvature and flow expansion across the flame, which generates baroclinic vorticity. High-resolution CFD analysis provides values of model parameters and confirms the plausibility of model results. This flow decomposition approach makes it possible to explicitly evaluate and analyze the respective contributions of irrotational and solenoidal flows to the flame response, and conversely the effect of flame perturbations on the flow. The use of the popular ad hoc hypothesis of convected velocity perturbation is avoided. It is found that convected velocity perturbations do not result from immediate acoustic-to-hydrodynamic mode conversion, but are generated by flame-flow feedback. In this sense, models for flame dynamics that rely on ad-hoc models for CVPs do not respect causality. Furthermore, analysis of the flame impulse response reveals that for the configuration investigated, flame-flow feedback is also responsible for “excess gain” of the flame response, that is, the magnitude of the flame frequency response above unity.
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CLANET, CHRISTOPHE, GEOFFREY SEARBY et PAUL CLAVIN. « Primary acoustic instability of flames propagating in tubes : cases of spray and premixed gas combustion ». Journal of Fluid Mechanics 385 (25 avril 1999) : 157–97. http://dx.doi.org/10.1017/s0022112099004231.

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This paper is concerned with the coupling mechanisms leading to the spontaneous generation of sound during flame propagation in a tube open at one end. We consider the cases of premixed gaseous combustion and of premixed spray combustion of decane droplets in air. The flame front propagates from the open to the closed end of a tube and, for a particular position, starts to amplify a longitudinal acoustic mode of the tube. We call this mode the primary acoustic instability and focus our study on the physical mechanisms responsible for sound amplification. Measured amplification rates are compared to calculated values. In the gaseous case, it is shown that the instability results from a coupling between the acoustic acceleration field and the geometry of the flame front separating the burnt gases from the denser unburnt mixture. The situation is quite different in the spray case. The primary acoustic instability is much stronger and results from a modification of the inner structure of the flame. This modification arises from the velocity lag of the droplets in the acoustic velocity field, leading to a modulation of the fuel flux at the flame.
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26

Harouadi, Farid, et Salim Boulahrouz. « Etude et Analyse de la Combustion Turbulente dans un Moteur Alimenté en Gaz Naturel ». Journal of Renewable Energies 3, no 2 (31 décembre 2000) : 93–103. http://dx.doi.org/10.54966/jreen.v3i2.914.

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L'analyse aérodynamique de la combustion turbulente dans un moteur alimenté en gaz naturel, dépend fortement des caractéristiques de la turbulence et celles des flammes laminaires. La structure de la flamme turbulente à haut régime de fonctionnement du moteur est située entre la flamme plissée pour un mélange relativement riche, et celui de flammelettes avec formation de poches de gaz frais pour un mélange pauvre. Cette structure déchiquetée du front de flamme révèle le caractère instable de la combustion en mélange pauvre, et suggère une forte interaction entre la turbulence et la combustion dans les moteurs alimentés en gaz naturel.
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Golovastov, Sergey, Grigory Bivol, Fyodor Kuleshov et Victor Golub. « The Formation of a Flame Front in a Hydrogen–Air Mixture during Spark Ignition in a Semi-Open Channel with a Porous Coating ». Fire 6, no 12 (28 novembre 2023) : 453. http://dx.doi.org/10.3390/fire6120453.

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An experimental study of ignition and flame front propagation during spark initiation in a hydrogen–air mixture in a semi-open channel with a porous coating is reported. The bottom surface of the channel was covered with a porous layer made of porous polyurethane or steel wool. The measurements were carried out for a stoichiometric mixture (equivalence ratio ER = 1.0) and for a lean mixture (ER = 0.4) of hydrogen with air, where ER is the molar excess of hydrogen. The flame front was recorded with a high-speed camera using the shadow method. Depending on the pore size, the velocity of the flame front and the sizes of disturbances generated on the surface of the flame front were determined. Qualitative features of the deflagration flame front at ER = 0.4, consisting of disturbances resembling small balls of flame, were discovered. The sizes of these disturbances significantly exceed the analytical values for the Darrieus–Landau instability. The effect of coatings made of porous polyurethane or steel wool is compared with the results obtained for an empty smooth channel. Depending on the hydrogen concentration in the hydrogen–air mixture, the velocity of the flame front compared to a smooth channel was three times higher when the channel was covered with steel wool and five times higher when the channel was covered with porous polyurethane.
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KUSKE, R., et P. MILEWSKI. « Modulated two-dimensional patterns in reaction–diffusion systems ». European Journal of Applied Mathematics 10, no 2 (avril 1999) : 157–84. http://dx.doi.org/10.1017/s095679259800360x.

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New modulation equations for hexagonal patterns in reaction–diffusion systems are derived for parameter régimes corresponding to the onset of patterns. These systems include additional nonlinearities which are not present in Rayleigh–Bénard convection or Swift–Hohenberg type models. The dynamics of hexagonal and roll patterns are studied using a combination of analytical and computational approaches which exploit the hexagonal structure of the modulation equations. The investigation demonstrates instabilities and new phenomena not found in other systems, and is applied to patterns of flame fronts in a certain model of burner stabilized flames.
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Yuan, Jiao, Yiguang Ju et Chung K. Law. « On flame-front instability at elevated pressures ». Proceedings of the Combustion Institute 31, no 1 (janvier 2007) : 1267–74. http://dx.doi.org/10.1016/j.proci.2006.07.180.

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Yuan, Jiao, Yiguang Ju et Chung K. Law. « Effects of turbulence and flame instability on flame front evolution ». Physics of Fluids 18, no 10 (octobre 2006) : 104105. http://dx.doi.org/10.1063/1.2359744.

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Zhang, Xinyi, Chenglong Tang, Huibin Yu et Zuohua Huang. « Flame-Front Instabilities of Outwardly Expanding Isooctane/n-Butanol Blend–Air Flames at Elevated Pressures ». Energy & ; Fuels 28, no 3 (10 mars 2014) : 2258–66. http://dx.doi.org/10.1021/ef4025382.

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NOVICK-COHEN, A., et G. I. SIVASHINSKY. « Hydrodynamic Instabilities in Flame Fronts : Breathing Solutions ». Combustion Science and Technology 46, no 1-2 (avril 1986) : 109–11. http://dx.doi.org/10.1080/00102208608959795.

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Kinugawa, Hikaru, Kazuhiro Ueda et Hiroshi Gotoda. « Chaos of radiative heat-loss-induced flame front instability ». Chaos : An Interdisciplinary Journal of Nonlinear Science 26, no 3 (mars 2016) : 033104. http://dx.doi.org/10.1063/1.4941854.

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van Wonterghem, J., et A. van Tiggelen. « L'épaisseur et la vitesse de propagation du front de flamme ». Bulletin des Sociétés Chimiques Belges 63, no 5-6 (1 septembre 2010) : 235–60. http://dx.doi.org/10.1002/bscb.19540630503.

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Wang, Yue, Minqi Zhang, Shuhang Chang, Shengji Li et Xuefeng Huang. « Laser-Induced Ignition and Combustion Behavior of Individual Graphite Microparticles in a Micro-Combustor ». Processes 8, no 11 (19 novembre 2020) : 1493. http://dx.doi.org/10.3390/pr8111493.

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Microscale combustion has potential application in a micro power generator. This paper studied the ignition and combustion behavior of individual graphite microparticles in a micro-combustor to explore the utilization of carbon-based fuels at the microscale system. The individual graphite microparticles inside the micro-combustor were ignited by a highly focused laser in an air flow with natural convection at atmospheric temperature and pressure. The results show that the ignition of graphite microparticles was heterogeneous. The particle diameter had a small weak effect on ignition delay time and threshold ignition energy. The micro-combustor wall heat losses had significant effects on the ignition and combustion. During combustion, flame instability, photophoresis, repetitive extinction and reignition were identified. The flame structure was asymmetric, and the fluctuation of flame front and radiation intensity showed combustion instability. Photophoretic force pushed the graphite away from the focal point and resulted in extinction. Owing to large wall heat loss, the flame quickly extinguished. However, the graphite was inductively reignited by laser.
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Rajamanickam, Kuppuraj, Franck Lefebvre, Carole Gobin, Gilles Godard, Corine Lacour, Bertrand Lecordier, Armelle Cessou et David Honoré. « Effect of H2 addition on the local extinction, flame structure, and flow field hydrodynamics in non-premixed bluff body stabilized flames ». Physics of Fluids 35, no 4 (avril 2023) : 047110. http://dx.doi.org/10.1063/5.0142921.

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We examined the effect of hydrogen (H2) enrichment on the primary fuel methane (CH4) in a canonical non-premixed bluff-body stabilized burner operating under typical central jet-dominated flame mode. In the chosen mode of operation, globally, the flow field and flame feature three important successive spatial zones: the recirculation zone, the neck zone, and the jet-like flame zone. The flame is exposed to a higher stretch rate in the neck zone in such a configuration and eventually undergoes local extinction. Such local extinction and subsequent re-ignition/reconnection of broken flame branches have substantial implications for the hydrodynamic instability of the coaxial annular air shear layer. It is well known that H2 addition increases the flame extinction strain rate ([Formula: see text] and thus alters the local extinction phenomenon. To understand this, we performed experiments at 0%, 10%, 20%, 30%, 50%, 80%, and 100% hydrogen proportion in the H2-CH4 blend. High repetition rate (5 kHz) Particle Image Velocimetry and OH Planar Laser Induced Fluorescence (PLIF) measurements are simultaneously implemented to gain quantitative insight into the flow field and flame structure. A detailed analysis performed over the instantaneous OH–PLIF datasets reveals the absence of local extinctions in flames with H2 enrichment >30% due to an increased extinction strain rate ([Formula: see text]. Furthermore, it is found that H2 enrichment plays a significant role in the reconnection/re-ignition of the broken flame branches formed during the local extinction. For instance, a high reconnection probability is observed in flames with an H2 addition of ≥20%. Consequently, variations in the mean reaction zone height are witnessed for different H2 enrichment levels. Further analysis of the influence of variation in reaction zone height on flow field hydrodynamics is explored using Proper Orthogonal Decomposition (POD) and Continuous Wavelet Transform (CWT). The results obtained from POD and CWT indicated the suppression of vortex shedding at the annular air shear layer for H2 addition greater than 20% and irregular wrinkling of flame fronts. Thus, they quantified the beneficial effect of H2 addition in turbulent flame stabilization.
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Ohyagi, Shigeharu, Jun Matsui et Teruo Yoshihashi. « Instabilities of Flame Front Propagating in a Constant-Volume Chamber. Hydrogen-Air, Methane-Air, and Propane-Air Flames. » Transactions of the Japan Society of Mechanical Engineers Series B 60, no 569 (1994) : 300–307. http://dx.doi.org/10.1299/kikaib.60.300.

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KADOWAKI, Satoshi. « Asymptotic Analysis on High-temperature Premixed Flames : Instability of Flame Fronts under the Constant-enthalpy Conditions ». Journal of Thermal Science and Technology 5, no 1 (2010) : 1–10. http://dx.doi.org/10.1299/jtst.5.1.

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Xia, Yongfang, Lu Chen, Junrui Shi et Benwen Li. « Flame Front Deformation Instabilities of Filtration Combustion for Initial Thermal Perturbation ». Chemical Engineering & ; Technology 43, no 8 (13 mai 2020) : 1608–17. http://dx.doi.org/10.1002/ceat.201900649.

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Bychkov, Vitaliy V., et Michael A. Liberman. « Thermal Instability and Pulsations of the Flame Front in White Dwarfs ». Astrophysical Journal 451 (octobre 1995) : 711. http://dx.doi.org/10.1086/176257.

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41

Luo, Liezhao, Ting Li, Jiangge Deng, Runzhou Zhao, Jinkui Wang et Lijun Xu. « Experimental Investigation on Self-Excited Thermoacoustic Instability in a Rijke Tube ». Applied Sciences 12, no 16 (11 août 2022) : 8046. http://dx.doi.org/10.3390/app12168046.

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The experimental investigations into the thermoacoustic instability in a Rijke tube are presented. In order to capture the dynamics of the temperature, a single-ended tunable diode laser absorption spectroscopy (TDLAS) technique was developed, with a measurement rate of 5 kHz. The temperature was found to fluctuate periodically at a dominant frequency of 230 Hz, corresponding to the fundamental frequency of the Rijke tube used in the experiment. The flame chemiluminescence was detected by a high-speed camera to demonstrate flame response to thermoacoustic instability. It was evident that the flame front stretched regularly and had jagged edges. To quantitate the fluctuations of chemiluminescence intensity, the relative area was defined. According to the result, the intensity also oscillated at 230 Hz. Furthermore, the same feature was found in regard to pressure at the exit of the Rijke tube. Compared with temperature and chemiluminescent intensity, the pressure oscillations presented the most approximate standard waveform, as they suffered the least disruptions. The results indicated that the dominant frequencies of temperature, chemiluminescent intensity and pressure were consistent, in accordance with the fundamental frequency of the Rijke tube in the experiment. In addition, etalon effects on the TDLAS signals were mitigated efficiently by a lowpass filter.
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42

SUBRAMANIAN, PRIYA, et R. I. SUJITH. « Non-normality and internal flame dynamics in premixed flame–acoustic interaction ». Journal of Fluid Mechanics 679 (13 mai 2011) : 315–42. http://dx.doi.org/10.1017/jfm.2011.140.

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This paper investigates the non-normal nature of premixed flame–acoustic interaction. The thermoacoustic system is modelled using the acoustic equations for momentum and energy, together with the equation for the evolution of the flame front obtained from the kinematicG-equation. As the unsteady heat addition acts as a volumetric source, the flame front is modelled as a distribution of monopole sources. Evolutions of the system are characterized with a measure of energy due to fluctuations. In addition to the acoustic energy, the energy due to fluctuations considered in the present paper accounts for the energy of the monopole sources. The linearized operator for this thermoacoustic system is non-normal, leading to non-orthogonality of its eigenvectors. Non-orthogonal eigenvectors can cause transient growth even when all the eigenvectors are decaying. Therefore, classical linear stability theory cannot predict the finite-time transient growth observed in non-normal systems. In the present model, the state space variables include the monopole source strengths in addition to the acoustic variables. Inclusion of these variables in the state space is essential to account for the transient growth due to non-normality. A parametric study of the variation in transient growth due to change in parameters such as flame location and flame angle is performed. In addition to projections along the acoustic variables of velocity and pressure, the optimal initial condition for the self-evolving system has significant projections along the strength of the monopole distribution. Comparison of linear and corresponding nonlinear evolutions highlights the role of transient growth in subcritical transition to instability. The notion of phase between acoustic pressure and heat release rate as an indicator of stability is examined.
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Blinnikov, Sergei Iv, et Pavel V. Sasorov. « Landau-Darrieus instability and the fractal dimension of flame fronts ». Physical Review E 53, no 5 (1 mai 1996) : 4827–41. http://dx.doi.org/10.1103/physreve.53.4827.

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44

Tao, Chengfei, Rongyue Sun, Yichen Wang, Yang Gao, Lin Meng, Liangbao Jiao, Shaohua Liang et Ling Chen. « Dynamic Response Mechanism of Ethanol Atomization–Combustion Instability under a Contrary Equivalence Ratio Adjusting Trend ». Aerospace 11, no 2 (17 février 2024) : 163. http://dx.doi.org/10.3390/aerospace11020163.

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This study experimentally explored the effects of equivalence ratio settings on ethanol fuel combustion oscillations with a laboratory-scale combustor. A contrary flame equivalence ratio adjusting trend was selected to investigate the dynamic characteristics of an ethanol atomization burner. Research findings denote that optimizing the equivalence ratio settings can prevent the occurrence of combustion instability in ethanol burners. In the combustion chamber, the sound pressure amplitude increased from 138 Pa to 171 Pa and eventually dropped to 38 Pa, as the equivalence ratio increased from 0.45 to 0.90. However, the sound pressure amplitude increased from 35 Pa to 199 Pa and eventually dropped to 162 Pa, as the equivalence ratio decreased from 0.90 to 0.45. The oscillation frequency of the ethanol atomization burner presents a migration characteristic; this is mainly due to thermal effects associated with changes in the equivalence ratio that increase/decrease the speed of sound in burnt gases, leading to increased/decreased oscillation frequencies. The trend of the change in flame heat release rate is basically like that of sound pressure, but the time-series signal of the flame heat release rate is different from that of sound pressure. It can be concluded that the reversible change in equivalence ratio will bring significant changes to the amplitude of combustion oscillations. At the same time, the macroscopic morphology of the flame will also undergo significant changes. The flame front length decreased from 25 cm to 18 cm, and the flame frontal angle increased from 23 to 42 degrees when the equivalence ratio increased. A strange phenomenon has been observed, which is that there is also sound pressure fluctuation inside the atomized air pipeline, and it presents a special square waveform. This study explored the equivalence ratio adjusting trends on ethanol combustion instability, which will provide the theoretical basis for the design of ethanol atomization burners.
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45

OHASHI, Hiroshi, et Satoshi KADOWAKI. « Instability of CH4/O2/CO2 Premixed Flames (Front Shape and Fluctuation of Cellular Flames) ». TRANSACTIONS OF THE JAPAN SOCIETY OF MECHANICAL ENGINEERS Series B 79, no 799 (2013) : 477–81. http://dx.doi.org/10.1299/kikaib.79.477.

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46

Elyanov, A., V. Golub et V. Volodin. « Conditions for the development of Rayleigh-Taylor instability on the spherical flame front ». Journal of Physics : Conference Series 1129 (novembre 2018) : 012011. http://dx.doi.org/10.1088/1742-6596/1129/1/012011.

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47

Meng, Yu, Hongbin Gu et Fang Chen. « Influence of Plasma on the Combustion Mode in a Scramjet ». Aerospace 9, no 2 (28 janvier 2022) : 73. http://dx.doi.org/10.3390/aerospace9020073.

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To examine the plasma-assisted combustion of a scramjet, a microwave-enhanced gliding arc plasma method was proposed in this study, and the flame structure and combustion instability were observed. The mechanism of plasma-assisted combustion was obtained via a Bunsen experiment, and then the influence on supersonic combustion was obtained on a direct-connected scramjet. The active species of the flame was determined via optical emission spectroscopy, and the flame temperature was measured with a thermocouple. The luminous intensity of the OH radicals in the flame increased ninefold when the flame temperature was increased to 1573 K, but the luminous intensity of CH* and C2 was not obviously changed with the excitation of arc plasma. Moreover, the DC arc plasma had no effect on the rotation and the vibration temperature of OH radicals under these experimental conditions. In the range of microwave energy less than 800 W, there was no typical change in the intensity of the radicals; however, when the microwave power was up to 1000 W, the effect became obvious. When plasma was applied to the scramjet, the plasma caused the pre-combustion shock train to move forward, and the initial and stable position of the flame was transferred from the cavity shear layer to the front of the fuel jet. These results clearly show that plasma free radical mechanisms cause changes to combustion modes.
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48

Gárzon Lama, Luis Fernando Marcondes, Loreto Pizzuti, Julien Sotton et Cristiane A. Martins. « Experimental investigation of hydrous ethanol/air flame front instabilities at elevated temperature and pressures ». Fuel 287 (mars 2021) : 119555. http://dx.doi.org/10.1016/j.fuel.2020.119555.

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49

Greenberg, J., A. McIntosh et J. Brindley. « Instability of a flame front propagating through a fuel-rich droplet–vapour–air cloud ». Combustion Theory and Modelling 3, no 3 (septembre 1999) : 567–84. http://dx.doi.org/10.1088/1364-7830/3/3/308.

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50

Chen, Lu, Yong-Fang Xia, Ben-Wen Li et Jun-Rui Shi. « Flame front inclination instability in the porous media combustion with inhomogeneous preheating temperature distribution ». Applied Thermal Engineering 128 (janvier 2018) : 1520–30. http://dx.doi.org/10.1016/j.applthermaleng.2017.09.085.

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