Articles de revues : « Flamme front instabilities »

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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, n^o 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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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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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, n^o 5-6 (avril 2005) : 979–89. http://dx.doi.org/10.1080/00102200590926950.

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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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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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KUSKE, R., et P. MILEWSKI. « Modulated two-dimensional patterns in reaction–diffusion systems ». European Journal of Applied Mathematics 10, n^o 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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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, n^o 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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NOVICK-COHEN, A., et G. I. SIVASHINSKY. « Hydrodynamic Instabilities in Flame Fronts : Breathing Solutions ». Combustion Science and Technology 46, n^o 1-2 (avril 1986) : 109–11. http://dx.doi.org/10.1080/00102208608959795.

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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, n^o 3 (10 mars 2014) : 2258–66. http://dx.doi.org/10.1021/ef4025382.

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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, n^o 8 (13 mai 2020) : 1608–17. http://dx.doi.org/10.1002/ceat.201900649.

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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, n^o 569 (1994) : 300–307. http://dx.doi.org/10.1299/kikaib.60.300.

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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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Dowd, Cody, et Joseph Meadows. « The effects of ring-shaped porous inert media on equivalence ratio oscillations in a self-excited thermoacoustic instability ». International Journal of Spray and Combustion Dynamics 13, n^o 1-2 (27 mai 2021) : 3–19. http://dx.doi.org/10.1177/1756827721991776.

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Gas turbine operation increasingly relies on lean premixed (LPM) combustion to reduce harmful emissions, which is susceptible to thermoacoustic instabilities. Most combustion systems are technically premixed and exhibit a degree of equivalence ratio inhomogeneity. Thermoacoustic pressure oscillations can couple with the heat release oscillations through the generation of equivalence ratio fluctuations at fuel injection sites, which are then convected to the flame front. Previous experimental studies have shown that porous inert media (PIM) can passively mitigate these instabilities by adding acoustic damping and by reducing the thermoacoustic feedback mechanism. To understand the role of PIM on these equivalence ratio oscillations, spatially resolved, phased averaged equivalence ratio fluctuations are measured using the ratio of OH*/CH* chemiluminescence. Spatial imaging of OH* or CH* radicals produce integrated line of sight intensity values and an Abel transformation is used to obtain spatially resolved values. Phase averaged images are synced with dynamic pressure measurements, and an axisymmetric atmospheric burner is used to study the effects of ring-shaped PIM on the spatially resolved equivalence ratio field with self-excited thermoacoustic instabilities. The results show that PIM significantly reduces these fluctuations, and the effects on the stability of the system are discussed.

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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, n^o 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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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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Clavin, P., et L. Masse. « Instabilities of ablation fronts in inertial confinement fusion : A comparison with flames ». Physics of Plasmas 11, n^o 2 (février 2004) : 690–705. http://dx.doi.org/10.1063/1.1634969.

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Cai, Pin, Shigeharu Ohyagi et Teruo Yoshihashi. « Instabilities of Flame Front Propagating in a Constant-Volume Chamber. Effects of Dilution by Inert Gases. » Transactions of the Japan Society of Mechanical Engineers Series B 60, n^o 580 (1994) : 4267–72. http://dx.doi.org/10.1299/kikaib.60.4267.

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KADOWAKI, Satoshi, Taisuke WASHIO, Thwe Thwe Aung, Wataru YAMAZAKI, Toshiyuki KATSUMI et Hideaki KOBAYASHI. « The effects of unburned-gas temperature on the characteristics of cellular premixed flames generated by hydrodynamic and diffusive-thermal instabilities in large space : fractal dimension of cellular-flame fronts ». Journal of Thermal Science and Technology 12, n^o 1 (2017) : JTST0015. http://dx.doi.org/10.1299/jtst.2017jtst0015.

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Mohammad Nurizat Rahman, Mohd Fairus Mohd Yasin et Mohd Shiraz Aris. « Reacting Flow Characteristics and Multifuel Capabilities of a Multi-Nozzle Dry Low NOx Combustor : A Numerical Analysis ». CFD Letters 13, n^o 11 (11 novembre 2021) : 21–34. http://dx.doi.org/10.37934/cfdl.13.11.2134.

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The fluctuating quality of natural gas (NG) in Peninsular Malaysia (PM) makes it challenging for the gas turbine (GT) combustor to meet the combustion performance requirements from the Original Equipment Manufacturer (OEM). Moreover, the gas quality sensitivity is more apparent in modern dry low NOx (DLN) combustors. Many of the prior combustion investigations were conducted on a modest scale in the laboratory. In actuality, combustion characterizations in complicated DLN combustors are more valuable to the power generation sector. Hence, the current numerical analysis utilized the RANS formulation and a detailed chemistry to examine the impact of ethane (C2H6), carbon dioxide (CO2), and nitrogen (N2) proportions in NG on combustion characteristics in a multi-nozzle DLN (MN-DLN) combustor, with the support of Modified Wobbe Index (MWI) calculations. Validations were performed using the combustor outlet temperature (COT) from the power plant where the actual MN-DLN combustor is operated, which revealed less than 10 % discrepancy. Qualitative validations were carried out by comparing the burn trace from the actual combustor wall to the predicted results, revealing an adequate Structural Similarity Index (SSIM) of 0.43. From numerical results of flame fronts and COTs, the addition of 20 % diluents (CO2 and N2) to NG demonstrated the blowoff risk. When MWIs of Kerteh and the JDA (major NG resources) were used as baselines, MWI ranges of all NG compositions under study surpassed the OEM’s ± 5 % limit. The increase in CO2 proportion results in a wide MWI range, especially when Kerteh is used as the baseline. Therefore, any GTs in PM that have previously been calibrated to use Kerteh's NG are more likely to have combustion instabilities if CO2 levels in NG suddenly increase. The higher MWI range backs up the current numerical results that showed the deleterious effects of a high CO2 composition throughout the combustor firing process. However, increasing the amount of C2H6 by up to 20 % is predicted to have minor effects on combustion characteristics. Overall, the validated numerical model of the MN-DLN combustor provided critical information about combustion characteristics and multifuel capabilities in respect to the NG quality in PM.

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Katzy, Peter, Josef Hasslberger, Lorenz R. Boeck et Thomas Sattelmayer. « The Effect of Intrinsic Instabilities on Effective Flame Speeds in Under-Resolved Simulations of Lean Hydrogen–Air Flames ». Journal of Nuclear Engineering and Radiation Science 3, n^o 4 (31 juillet 2017). http://dx.doi.org/10.1115/1.4036984.

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The presented work aims to improve computational fluid dynamics (CFD) explosion modeling for lean hydrogen–air mixtures on under-resolved grids. Validation data are obtained from an entirely closed laboratory-scale explosion channel (GraVent facility). Investigated hydrogen–air concentrations range from 6 to 19 vol %. Initial conditions are p = 0.1 MPa and T = 293 K. Two highly time-resolved optical measurement techniques are applied simultaneously: (1) 10 kHz shadowgraphy captures line-of-sight integrated macroscopic flame propagation and (2) 20 kHz planar laser-induced fluorescence of the OH radical (OH-PLIF) resolves microscopic flame topology without line-of-sight integration. This paper presents the experiment, measurement techniques, data evaluation methods, and simulation results. The evaluation methods encompass the determination of flame tip velocity over distance and a detailed time-resolved quantification of the flame topology based on OH-PLIF images. One parameter is the length of wrinkled flame fronts in the OH-PLIF plane obtained through automated postprocessing. It reveals the expected enlargement of flame surface area by instabilities on a microscopic level. A strong effect of mixture composition is observed. Simulations based on the new model formulation, incorporating the microscopic enlargement of the flame front, show a promising behavior, where the impact of the augmented flame front on the observed flame front velocities can be detected.

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Shi, Shuguo, Adrian Breicher, Robin Schultheis, Sandra Hartl, Robert S. Barlow, Dirk Geyer et Andreas Dreizler. « Structures of Laminar Lean Premixed H2/CH4/Air Polyhedral Flames : Effects of Flow Velocity, H2 Content and Equivalence Ratio ». Flow, Turbulence and Combustion, 25 juin 2024. http://dx.doi.org/10.1007/s10494-024-00561-3.

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AbstractPolyhedral Bunsen flames, induced by hydrodynamic and thermo-diffusive instabilities, are characterized by periodic trough and cusp cellular structures along the conical flame front. In this study, the effects of flow velocity, hydrogen content, and equivalence ratio on the internal cellular structure of premixed fuel-lean hydrogen/methane/air polyhedral flames are experimentally investigated. A high-spatial-resolution one-dimensional Raman/Rayleigh scattering system is employed to measure the internal scalar structures of polyhedral flames in troughs and cusps. Planar laser-induced fluorescence of hydroxyl radicals and chemiluminescence imaging measurements are used to quantify the flame front morphology. In the experiments, stationary polyhedral flames with varying flow velocities from 1.65 to 2.50 m/s, hydrogen contents from 50 to 83%, and equivalence ratios from 0.53 to 0.64 are selected and measured. The results indicate that the positively curved troughs exhibit significantly higher hydrogen mole fractions and local equivalence ratios compared to the negatively curved cusps, due to the respective focusing/defocusing effect of trough/cusp structure on highly diffusive hydrogen. The hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are first increased and then decreased with increasing measurement height from 5 to 13 mm, due to the three-dimensional effect of the flame front. With increasing flow velocity from 1.65 to 2.50 m/s, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps decrease, which is attributed to the overall decreasing curvatures in troughs and cusps due to the decreased residence time and increased velocity-induced strain. With increasing hydrogen content from 50 to 83%, the hydrogen mole fraction and local equivalence ratio differences between troughs and cusps are amplified, due to the enhanced effects of the flame front curvature and the differential diffusion of hydrogen. With increasing equivalence ratio from 0.53 to 0.64, a clear increasing trend in hydrogen mole fraction and equivalence ratio differences between troughs and cusps is observed at constant flow velocity condition, which is a trade-off result between increasing effective Lewis number and increasing curvatures in troughs and cusps.

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Shrivastava, Sourabh, Ishan Verma, Rakesh Yadav et Pravin Nakod. « Solution-based Mesh Adaption Criteria Development for Accelerating Flame Tracking Simulations ». Journal of Engineering for Gas Turbines and Power, 23 septembre 2022. http://dx.doi.org/10.1115/1.4055751.

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Abstract Solution-based mesh adaption approaches have been widely studied and tested by different research groups to generate the required finer meshes in the critical regions on the fly while keeping the overall mesh count to a manageable level. However, these approaches are typically applicable for a set of problems, and therefore, there is a need for a generic approach suitable for a broader range of problems. This work explores various parameters and specific weightage factors to predict correct flame-tracking outcomes for different types of flames. The selections of flow quantities (flow-variables, their gradients, curvatures) are performed using simple flames and flow configurations. The functions based on selected flow-quantities derived from these studies are then tested to predict the results for the more complex set of published flames like the Engine Combustion Network (ECN) spray flame and Knowledge for Ignition, Acoustics and Instabilities (KIAI) five-burner configuration (liquid and gas fuel). Derived adaption criteria are found to predict the correct flame tracking behavior in terms of transient evolution of flame front, flame propagation, and ignition timing of burners. The parameters used for the study are identified keeping genericity as the key point, and thus making sure that the derived adaption functions can be applied across different types of fuel blends, combustion systems (gaseous or liquid fuel-based systems) and combustion models, for example species transport or mixture fraction-based models.

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Greiffenhagen, Felix, Jakob Woisetschläger, Johannes Gürtler et Jürgen Czarske. « Quantitative measurement of density fluctuations with a full-field laser interferometric vibrometer ». Experiments in Fluids 61, n^o 1 (28 novembre 2019). http://dx.doi.org/10.1007/s00348-019-2842-y.

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Abstract Modern, lean and premixed gas turbine combustion concepts for low NOx emissions are prone to combustion instabilities. In a previous work it was shown that laser interferometric vibrometry (LIV) can be used to record global as well as local heat release fluctuations in swirl-stabilized premixed methane flames quantitatively, if other effects influencing density are small. In this work a newly developed camera-based full-field LIV system (CLIV) was applied to a lean, confined, premixed and swirl-stabilized methane flame under atmospheric conditions. Instead of time-consuming pointwise scanning of the flame, CLIV records full-field line-of-sight density fluctuations with high spatio-temporal resolution. With a recording rate of 200 kHz, CLIV enables the visualization of highly unsteady processes in fluid dynamics and combustion research. As an example for an unsteady process, the propagation of the flame front through a lean, premixed gas volume is visualized during an ignition process. A discussion of algorithms and assumptions necessary to calculate heat release oscillations from density oscillations is presented and applied to phase-averaged data recorded with CLIV for this type of flame. As reference, OH* chemiluminescence data were recorded simultaneously. While density gradients travelling with the flow are recorded by LIV and CLIV, chemiluminescence imaging will show nothing in the absence of chemical reaction. Graphic abstract a Time-averaged density gradient within the combustor in lateral direction. b Density fluctuations along line-of-sight 7 ms after ignition. c Phase-averaged and local heat release fluctuations at 225 Hz perturbation frequency

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Lai, ShuYue, Chao Xu, Martin Howard Davy et XiaoHang Fang. « Flame acceleration and transition to detonation in a pre-/main- chamber combustion system ». Physics of Fluids, 10 octobre 2022. http://dx.doi.org/10.1063/5.0122240.

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Numerical simulations are performed to study the mechanism of deflagration to detonation transition (DDT) in a pre-/main- chamber combustion system. The fully compressible Navier-Stokes equations, coupled with a chemical-diffusive model in a stoichiometric ethylene-oxygen mixture, are solved with a high-order numerical algorithm on a dynamically adapting mesh. The two-dimensional simulation shows that a laminar flame grows in the pre-chamber and then develops into a jet flame as it passes through the orifice. A strong shock forms immediately ahead of the flame, reflecting off the walls, and interacting with the flame front. The shock-flame interactions are crucial for the development of flame instabilities, which trigger the subsequent turbulent flame development. The DDT arises due to an energy-focusing mechanism, where multiple shocks collide at the flame front. A chemical explosive mode analysis (CEMA) criteria is developed to study the DDT ignition mode. Preliminary one-dimensional computations for a laminar propagating flame, a fast flame deflagration, and a Chapman-Jouguet detonation are conducted to demonstrate the validity of CEMA on the chemical-diffusive model, as well as to determine the proper conditioning value for CEMA diagnostic. The two-dimensional analysis with CEMA indicates that the DDT initiated by the energy focusing mechanism can form a strong thermal expansion region that features large positive eigenvalues for the chemical explosive mode and dominance of the local autoignition mode. Thus, the CEMA criterion proposed in this study provides a robust diagnostic for identifying autoignition-supported DDT of which emergence of excessive local autoignition mode is found to be a precursor.

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Duva, B. C., L. E. Chance et E. Toulson. « Effect of CO2 Dilution on the Laminar Burning Velocities of Premixed Methane/Air Flames at Elevated Temperature ». Journal of Engineering for Gas Turbines and Power 142, n^o 3 (3 février 2020). http://dx.doi.org/10.1115/1.4044641.

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Abstract With increased interest in reducing emissions, the staged combustion concept for gas turbine combustors is gaining in popularity. For this work, the effect of CO2 dilution on laminar burning velocities of premixed methane/air flames was investigated at elevated temperature through both experiments and numerical simulations. Validation of the experimental setup and methodology was completed through experimental testing of methane/air mixtures at 1 bar and 298 K. Following validation, high temperature experiments were conducted in an optically accessible constant volume combustion chamber at 1 bar and 473 K. Laminar burning velocities of premixed methane/air flames with 0%, 5%, 10%, and 15% CO2 dilution were determined using the constant pressure method enabled via schlieren visualization of the spherically propagating flame front. Results show that laminar burning velocities of methane/air mixtures at 1 bar increase by 106–145% with initial temperature increases from 298 K to 473 K. Additions of 5%, 10%, and 15% CO2 dilution at 1 bar and 473 K cause a 30–35%, 51–54%, and 66–68% decrease in the laminar burning velocity, respectively. Numerical results were obtained with CHEMKIN (Kee et al., 1985, “PREMIX: A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flames,”) using the GRI-Mech 3.0 (Smith et al., 2019) and the San Diego (“Chemical-Kinetic Mechanisms for Combustion Applications,” San Diego Mechanism Web Page, Mechanical and Aerospace Engineering (Combustion Research), University of California at San Diego, San Diego, CA) mechanisms. It is concluded that the GRI-Mech 3.0 (Smith et al.., 2019) better captures the general overall trend of the experimental laminar flame speeds of methane/air/CO2 mixtures at 1 bar and 473 K. Additionally, the dilution, thermal-diffusion, and chemical effects of CO2 on the laminar burning velocities of methane/air mixtures were investigated numerically by diluting the mixtures with both chemically active and inactive CO2 following the determination of the most important elementary reactions on the burning rate through sensitivity analysis. Finally, it was shown that CO2 dilution suppresses the flame instabilities during combustion, which is attributable to the increase in the burned gas Markstein length (Lb) with the addition of diluent.

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Bauerheim, M., T. Jaravel, L. Esclapez, E. Riber, L. Y. M. Gicquel, B. Cuenot, M. Cazalens, S. Bourgois et M. Rullaud. « Multiphase Flow Large-Eddy Simulation Study of the Fuel Split Effects on Combustion Instabilities in an Ultra-Low-NOx Annular Combustor ». Journal of Engineering for Gas Turbines and Power 138, n^o 6 (17 novembre 2015). http://dx.doi.org/10.1115/1.4031871.

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This paper describes the application of a coupled acoustic model/large-eddy simulation approach to assess the effect of fuel split on combustion instabilities in an industrial ultra-low-NOx annular combustor. Multiphase flow LES and an analytical model (analytical tool to analyze and control azimuthal modes in annular chambers (ATACAMAC)) to predict thermoacoustic modes are combined to reveal and compare two mechanisms leading to thermoacoustic instabilities: (1) a gaseous type in the multipoint zone (MPZ) where acoustics generates vortex shedding, which then wrinkle the flame front, and (2) a multiphase flow type in the pilot zone (PZ) where acoustics can modify the liquid fuel transport and the evaporation process leading to gaseous fuel oscillations. The aim of this paper is to investigate these mechanisms by changing the fuel split (from 5% to 20%, mainly affecting the PZ and mechanism 2) to assess which mechanism controls the flame dynamics. First, the eigenmodes of the annular chamber are investigated using an analytical model validated by 3D Helmholtz simulations. Then, multiphase flow LES are forced at the eigenfrequencies of the chamber for three different fuel split values. Key features of the flow and flame dynamics are investigated. Results show that acoustic forcing generates gaseous fuel oscillations in the PZ, which strongly depend on the fuel split parameter. However, the correlation between acoustics and the global (pilot + multipoint) heat release fluctuations highlights no dependency on the fuel split staging. It suggests that vortex shedding in the MPZ, almost not depending on the fuel split, is the main feature controlling the flame dynamics for this engine.

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Berger, Frederik M., Tobias Hummel, Michael Hertweck, Jan Kaufmann, Bruno Schuermans et Thomas Sattelmayer. « High-Frequency Thermoacoustic Modulation Mechanisms in Swirl-Stabilized Gas Turbine Combustors—Part I : Experimental Investigation of Local Flame Response ». Journal of Engineering for Gas Turbines and Power 139, n^o 7 (14 février 2017). http://dx.doi.org/10.1115/1.4035591.

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This paper presents the experimental approach for determination and validation of noncompact flame transfer functions of high-frequency, transverse combustion instabilities observed in a generic lean premixed gas turbine combustor. The established noncompact transfer functions describe the interaction of the flame's heat release with the acoustics locally, which is necessary due to the respective length scales being of the same order of magnitude. Spatiotemporal dynamics of the flame are measured by imaging the OH⋆ chemiluminescence signal, phase-locked to the dynamic pressure at the combustor's front plate. Radon transforms provide a local insight into the flame's modulated reaction zone. Applied to different burner configurations, the impact of the unsteady heat release distribution on the thermoacoustic driving potential, as well as distinct flame regions that exhibit high modulation intensity, is revealed. Utilizing these spatially distributed transfer functions within thermoacoustic analysis tools (addressed in this joint publication's Part II) allows then to predict transverse linear stability of gas turbine combustors.

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Li, Fangyan, Xiaotao Tian, Minglong Du, Lei Shi et Jiashan Cui. « Effects of Intrinsic Flame Instabilities on Thermoacoustic Oscillations in Lean Premixed Gas Turbines ». Journal of Engineering for Gas Turbines and Power 144, n^o 5 (21 février 2022). http://dx.doi.org/10.1115/1.4053421.

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Abstract Thermoacoustic instabilities are commonly encountered in the development of aeroengines and rocket motors. Research on the fundamental mechanism of thermoacoustic instabilities is beneficial for the optimal design of these engine systems. In the present study, a thermoacoustic instability model based on the lean premixed gas turbines (LPGT) combustion system was established. The longitudinal distribution of heat release caused by the intrinsic instability of flame front was considered in the model. Effects of different heat release distributions and characteristic parameters (Lewis number Le, Zeldovich Number β, and Prandtl number Pr) of the premixed gas on thermoacoustic instability behaviors of the LPGT system were investigated based on the established model. Results show that the LPGT system features have two kinds of unstable thermoacoustic modes. The first one corresponds to the natural acoustic mode of the plenum and the second one corresponds to that of the combustion chamber. The characteristic parameters of the premixed gases have a large impact on the stability of the system and can even change the system from stable to unstable state.

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Gövert, Simon, Jonathan T. Lipkowicz et Bertram Janus. « Compressible Large Eddy Simulation of Thermoacoustic Instabilities in the PRECCINSTA Combustor Using Flamelet Generated Manifolds with Dynamic Thickened Flame Model ». Journal of Engineering for Gas Turbines and Power, 13 septembre 2023, 1–19. http://dx.doi.org/10.1115/1.4063419.

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

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Zhao, Wandong, Jianhan Liang, Ralf Deiterding, Xiaodong Cai et Xinxin Wang. « Flame-turbulence interactions during the flame acceleration using solid and fluid obstacles ». Physics of Fluids, 13 septembre 2022. http://dx.doi.org/10.1063/5.0118091.

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A combination of solid and transverse jet obstacles is proposed to trigger flame acceleration and deflagration-to-detonation transition (DDT). A numerical study of this approach is performed by solving the reactive Navier-Stokes equations deploying an adaptive mesh refinement technique. A detailed hydrogen-air reaction mechanism with 12 species and 42 steps is employed. The efficiency and mechanisms of the combined obstacles on the flame acceleration are investigated comprehensively. The effects of multiple jets, jet start time, and jet stagnation pressure on the DDT process are studied. Results show that there is a 22.26% improvement in the DDT run-up time and a 33.36% reduction in the DDT run-up distance for the combined obstacles compared to that having only solid obstacles. The jet acts as an obstruction by producing a suitable blockage ratio and introducing an intense turbulent region due to the Kelvin-Helmholtz instability. This leads to dramatic flame-turbulence interactions, increasing the flame surface area dramatically. The dual jet produces mushroom-like vortices, leading to a significantly stretched flame front and intensive Kelvin-Helmholtz instabilities, and therefore these features produce a high flame acceleration. As the jet operation time decreases, the jet obstacle almost changes its role from both physical blockage ratio as well as turbulence and vorticity generator to a physical blockage ratio. There is a moderate jet stagnation pressure that reduces the run-up time to detonation and run-up distance to detonation in the obstacle-laden chamber. While further increasing the jet stagnation pressure, it does not have a positive effect on shortening the detonation transition.

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Desai, Ajinkya, Scott Goodrick et Tirtha Banerjee. « Investigating the turbulent dynamics of small-scale surface fires ». Scientific Reports 12, n^o 1 (22 juin 2022). http://dx.doi.org/10.1038/s41598-022-13226-w.

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AbstractHigh frequency (30 Hz) two-dimensional particle image velocimetry data recorded during a field experiment exploring fire spread from point ignition in hand-spread pine needles under calm ambient wind conditions are analysed in this study. In the initial stages, as the flame spreads approximately radially away from the ignition point in the absence of a preferred wind-forcing direction, it entrains cooler ambient air into the warmer fire core, thereby experiencing a dynamic pressure resistance. The fire-front, comprising a flame that is tilted inward, is surrounded by a region of downdraft. Coherent structures describe the initial shape of the fire-front and its response to local wind shifts while also revealing possible fire-spread mechanisms. Vortex tubes originating outside the fire spiral inward and get stretched thinner at the fire-front leading to higher vorticity there. These tubes comprise circulation structures that induce a radially outward velocity close to the fuel bed, which pushes hot gases outward, thereby causing the fire to spread. Moreover, these circulation structures confirm the presence of counter-rotating vortex pairs that are known to be a key mechanism for fire spread. The axis of the vortex tubes changes its orientation alternately towards and away from the surface of the fuel bed, causing the vortex tubes to be kinked. The strong updraft observed at the location of the fire-front could potentially advect and tilt the kinked vortex tube vertically upward leading to fire-whirl formation. As the fire evolves, its perimeter disintegrates in response to flow instabilities to form smaller fire “pockets”. These pockets are confined to certain points in the flow field that remain relatively fixed for a while and resemble the behavior of a chaotic system in the vicinity of an attractor. Increased magnitudes of the turbulent fluxes of horizontal momentum, computed at certain such fixed points along the fire-front, are symptomatic of irregular fire bursts and help contextualize the fire spread. Most importantly, the time-varying transport terms of the turbulent kinetic energy budget equation computed at adjacent fixed points indicate that local fires along the fire-front primarily interact via the horizontal turbulent transport term.

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Viswamithra, Varun Nanjunda Rao, et Shyam Menon. « A Distributed Fuel Injection Approach to Suppress Lean Blow-Out and NOx Emissions in a Methane-Ammonia-Fueled Premixed Swirl Combustor ». Journal of Engineering for Gas Turbines and Power, 16 mars 2022. http://dx.doi.org/10.1115/1.4054105.

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Abstract In an effort to reduce harmful effects of green house gas emissions, ammonia is being pursued as a fuel for power generation as it is a carbon-free energy source. However, the use of ammonia-air mixtures in premixed swirl combustors poses challenges due to low flame speed, reactivity, and high Nitrous oxide emissions. This study attempts to overcome lean blowout limits of methane-ammonia-air mixtures by a novel, multi-point injection strategy, whereby micron-sized holes on the swirler vanes generate a co-flowing stream of fuel and air, which is then injected into swirling air cross-flow. The resulting improvement in mixing facilitated by increases in momentum flux ratio and fine-scale turbulence is found to reduce LBO limits to equivalence ratios between 0.65-0.7 for mixtures containing ammonia as high as 80-90% by volume. The measurements carried out using a model-swirl combustor setup are analyzed further using 0-D chemical kinetic models as well as CH* and OH* chemiluminescence. Chemiluminescence imaging shows the heat release zone to move downstream and broaden with increase in ammonia content, as a result of decreasing flame speed. This forms precursor to lean blow out through action of instabilities at the flame front, which is potentially alleviated by the improved mixing achieved through the multi-point injection strategy. The resulting ultra-short mixing length can lead to compact combustor design with the ability to lower LBO limits and improve Nitrous oxide emissions, while utilizing carbon-free ammonia.

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Mersinligil, Mehmet, Jean-François Brouckaert et Julien Desset. « Unsteady Pressure Measurements With a Fast Response Cooled Probe in High Temperature Gas Turbine Environments ». Journal of Engineering for Gas Turbines and Power 133, n^o 8 (7 avril 2011). http://dx.doi.org/10.1115/1.4002276.

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This paper presents the first experimental engine and test rig results obtained from a fast response cooled total pressure probe. The first objective of the probe design was to favor continuous immersion of the probe into the engine to obtain a time series of pressure with a high bandwidth and, therefore, statistically representative average fluctuations at the blade passing frequency. The probe is water cooled by a high pressure cooling system and uses a conventional piezoresistive pressure sensor, which yields, therefore, both time-averaged and time-resolved pressures. The initial design target was to gain the capability of performing measurements at the temperature conditions typically found at high pressure turbine exit (800–1100°C) with a bandwidth of at least 40 kHz and in the long term at combustor exit (2000 K or higher). The probe was first traversed at the turbine exit of a Rolls-Royce Viper turbojet engine at exhaust temperatures around 750°C and absolute pressure of 2.1 bars. The probe was able to resolve the high blade passing frequency (≈23 kHz) and several harmonics of up to 100 kHz. Besides the average total pressure distributions rom the radial traverses, phase-locked averages and random unsteadiness are presented. The probe was also used in a virtual three-hole mode yielding unsteady yaw angle, static pressure, and Mach number. The same probe was used for measurements in a Rolls-Royce intermediate pressure burner rig. Traverses were performed inside the flame tube of a kerosene burner at temperatures above 1600°C. The probe successfully measured the total pressure distribution in the flame tube and typical frequencies of combustion instabilities were identified during rumble conditions. The cooling performance of the probe is compared with estimations at the design stage and found to be in good agreement. The frequency response of the probe is compared with cold shock-tube results and a significant increase in the natural frequency of the line-cavity system formed by the conduction cooled screen in front of the miniature pressure sensor were observed.

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