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1
CRETA, F., and M. MATALON. "Propagation of wrinkled turbulent flames in the context of hydrodynamic theory." Journal of Fluid Mechanics 680 (June 1, 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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Robin, Vincent, Arnaud Mura, and Michel Champion. "Direct and indirect thermal expansion effects in turbulent premixed flames." Journal of Fluid Mechanics 689 (November 3, 2011): 149–82. http://dx.doi.org/10.1017/jfm.2011.409.
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AbstractThe thermal expansion induced by the exothermic chemical reactions taking place in a turbulent reactive flow affects the velocity field so strongly that the large-scale velocity fluctuations as well as the small-scale velocity gradients can be governed by chemistry rather than by turbulence. Moreover, thermal expansion is well known to be responsible for counter-gradient turbulent diffusion and flame-generated turbulence phenomena. In the present study, by making use of an original splitting procedure applied to the velocity field, we establish the occurrence of two distinct thermal expansion effects in the flamelet regime of turbulent premixed combustion. The first is referred to as the direct thermal expansion effect. It is associated with a local flamelet crossing contribution as previously considered in early analyses of turbulent transport in premixed flames. The second, denoted herein as the indirect thermal expansion effect, is an outcome of the turbulent wrinkling processes that increases the flame surface area. Based on a splitting procedure applied to the velocity field, the respective influences of the two effects are identified and analysed. Furthermore, the theoretical analysis shows that the thermal expansion induced through the local flames can be treated separately in the usual continuity and momentum equations. This description of the turbulent reactive velocity field, leads also to relate all of the usual turbulent quantities to the reactive scalar field. Finally, algebraic closures for the turbulent transport terms of mass and momentum are proposed and successfully validated through comparison with direct numerical simulation data.
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Chakraborty, Nilanjan. "Influence of Thermal Expansion on Fluid Dynamics of Turbulent Premixed Combustion and Its Modelling Implications." Flow, Turbulence and Combustion 106, no. 3 (March 2021): 753–848. http://dx.doi.org/10.1007/s10494-020-00237-8.
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AbstractThe purpose of this paper is to demonstrate the effects of thermal expansion, as a result of heat release arising from exothermic chemical reactions, on the underlying turbulent fluid dynamics and its modelling in the case of turbulent premixed combustion. The thermal expansion due to heat release gives rise to predominantly positive values of dilatation rate within turbulent premixed flames, which has been shown to have significant implications on the flow topology distributions, and turbulent kinetic energy and enstrophy evolutions. It has been demonstrated that the magnitude of predominantly positive dilatation rate provides the measure of the strength of thermal expansion. The influence of thermal expansion on fluid turbulence has been shown to strengthen with decreasing values of Karlovitz number and characteristic Lewis number, and with increasing density ratio between unburned and burned gases. This is reflected in the weakening of the contributions of flow topologies, which are obtained only for positive values of dilatation rate, with increasing Karlovitz number. The thermal expansion within premixed turbulent flames not only induces mostly positive dilatation rate but also induces a flame-induced pressure gradient due to flame normal acceleration. The correlation between the pressure and dilatation fluctuations, and the vector product between density and pressure gradients significantly affect the evolutions of turbulent kinetic energy and enstrophy within turbulent premixed flames through pressure-dilatation and baroclinic torque terms, respectively. The relative contributions of pressure-dilatation and baroclinic torque in comparison to the magnitudes of the other terms in the turbulent kinetic energy and enstrophy transport equations, respectively strengthen with decreasing values of Karlovitz and characteristic Lewis numbers. This leads to significant augmentations of turbulent kinetic energy and enstrophy within the flame brush for small values of Karlovitz and characteristic Lewis numbers, but both turbulent kinetic energy and enstrophy decay from the unburned to the burned gas side of the flame brush for large values of Karlovitz and characteristic Lewis numbers. The heat release within premixed flames also induces significant anisotropy of sub-grid stresses and affects their alignments with resolved strain rates. This anisotropy plays a key role in the modelling of sub-grid stresses and the explicit closure of the isotropic part of the sub-grid stress has been demonstrated to improve the performance of sub-grid stress and turbulent kinetic energy closures. Moreover, the usual dynamic modelling techniques, which are used for non-reacting turbulent flows, have been shown to not be suitable for turbulent premixed flames. Furthermore, the velocity increase across the flame due to flame normal acceleration may induce counter-gradient transport for turbulent kinetic energy, reactive scalars, scalar gradients and scalar variances in premixed turbulent flames under some conditions. The propensity of counter-gradient transport increases with decreasing values of root-mean-square turbulent velocity and characteristic Lewis number. It has been found that vorticity aligns predominantly with the intermediate principal strain rate eigendirection but the relative extents of alignment of vorticity with the most extensive and the most compressive principal strain rate eigendirections change in response to the strength of thermal expansion. It has been found that dilatation rate almost equates to the most extensive strain rate for small sub-unity Lewis numbers and for the combination of large Damköhler and small Karlovitz numbers, and under these conditions vorticity shows no alignment with the most extensive principal strain rate eigendirection but an increased collinear alignment with the most compressive principal strain rate eigendirection is obtained. By contrast, for the combination of high Karlovitz number and low Damköhler number in the flames with Lewis number close to unity, vorticity shows an increased collinear alignment with the most extensive principal direction in the reaction zone where the effects of heat release are strong. The strengthening of flame normal acceleration in comparison to turbulent straining with increasing values of density ratio, Damköhler number and decreasing Lewis number makes the reactive scalar gradient align preferentially with the most extensive principal strain rate eigendirection, which is in contrast to preferential collinear alignment of the passive scalar gradient with the most compressive principal strain rate eigendirection. For high Karlovitz number, the reactive scalar gradient alignment starts to resemble the behaviour observed in the case of passive scalar mixing. The influence of thermal expansion on the alignment characteristics of vorticity and reactive scalar gradient with local principal strain rate eigendirections dictates the statistics of vortex-stretching term in the enstrophy transport equation and normal strain rate contributions in the scalar dissipation rate and flame surface density transport equations, respectively. Based on the aforementioned fundamental physical information regarding the thermal expansion effects on fluid turbulence in premixed combustion, it has been argued that turbulence and combustion modelling are closely interlinked in turbulent premixed combustion. Therefore, it might be necessary to alter and adapt both turbulence and combustion modelling strategies while moving from one combustion regime to the other.
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Massey, James C., Ivan Langella, and Nedunchezhian Swaminathan. "A scaling law for the recirculation zone length behind a bluff body in reacting flows." Journal of Fluid Mechanics 875 (July 22, 2019): 699–724. http://dx.doi.org/10.1017/jfm.2019.475.
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The recirculation zone length behind a bluff body is influenced by the turbulence intensity at the base of the body in isothermal flows and also the heat release and its interaction with turbulence in reacting flows. This relationship is observed to be nonlinear and is controlled by the balance of forces acting on the recirculation zone, which arise from the pressure and turbulence fields. The pressure force is directly influenced by the volumetric expansion resulting from the heat release, whereas the change in the turbulent shear force depends on the nonlinear interaction between turbulence and combustion. This behaviour is elucidated through a control volume analysis. A scaling relation for the recirculation zone length is deduced to relate the turbulence intensity and the amount of heat release. This relation is verified using the large eddy simulation data from 20 computations of isothermal flows and premixed flames that are stabilised behind the bluff body. The application of this scaling to flames in an open environment and behind a backward facing step is also explored. The observations and results are explained on a physical basis.
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Zurbach, Stephan, Danièle Garreton, Mohamed Kanniche, and Sébastien Candel. "Calcul de flammes turbulentes non prémélangées à l'aide d'une approche probabiliste et d'une cinétique chimique réduite." Comptes Rendus de l'Académie des Sciences - Series IIB - Mechanics-Physics-Astronomy 327, no. 10 (September 1999): 997–1004. http://dx.doi.org/10.1016/s1287-4620(00)87010-6.
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Schmidt-Laine, C., and A. Ben Taïb. "Sur un algorithme en volumes finis non structurés pour la simulation des flammes turbulentes en chimie infiniment rapide." ESAIM: Mathematical Modelling and Numerical Analysis 32, no. 6 (1998): 681–97. http://dx.doi.org/10.1051/m2an/1998320606811.
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Sabelnikov, V. A., A. N. Lipatnikov, S. Nishiki, and T. Hasegawa. "Investigation of the influence of combustion-induced thermal expansion on two-point turbulence statistics using conditioned structure functions." Journal of Fluid Mechanics 867 (March 20, 2019): 45–76. http://dx.doi.org/10.1017/jfm.2019.128.
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The second-order structure functions (SFs) of the velocity field, which characterize the velocity difference at two points, are widely used in research into non-reacting turbulent flows. In the present paper, the approach is extended in order to study the influence of combustion-induced thermal expansion on turbulent flow within a premixed flame brush. For this purpose, SFs conditioned to various combinations of mixture states at two different points (reactant–reactant, reactant–product, product–product, etc.) are introduced in the paper and a relevant exact transport equation is derived in the appendix. Subsequently, in order to demonstrate the capabilities of the newly developed approach for advancing the understanding of turbulent reacting flows, the conditioned SFs are extracted from three-dimensional (3-D) direct numerical simulation data obtained from two statistically 1-D planar, fully developed, weakly turbulent, premixed, single-step-chemistry flames characterized by significantly different (7.53 and 2.50) density ratios, with all other things being approximately equal. Obtained results show that the conditioned SFs differ significantly from standard mean SFs and convey a large amount of important information on various local phenomena that stem from the influence of combustion-induced thermal expansion on turbulent flow. In particular, the conditioned SFs not only (i) indicate a number of already known local phenomena discussed in the paper, but also (ii) reveal a less recognized phenomenon such as substantial influence of combustion-induced thermal expansion on turbulence in constant-density unburned reactants and even (iii) allow us to detect a new phenomenon such as the appearance of strong local velocity perturbations (shear layers) within flamelets. Moreover, SFs conditioned to heat-release zones indicate a highly anisotropic influence of combustion-induced thermal expansion on the evolution of small-scale two-point velocity differences within flamelets, with the effects being opposite (an increase or a decrease) for different components of the local velocity vector.
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Chakraborty, Nilanjan, Sanjeev Kumar Ghai, and Hong G. Im. "Anisotropy of Reynolds Stresses and Their Dissipation Rates in Lean H2-Air Premixed Flames in Different Combustion Regimes." Energies 17, no. 21 (October 25, 2024): 5325. http://dx.doi.org/10.3390/en17215325.
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The interrelation between Reynolds stresses and their dissipation rate tensors for different Karlovitz number values was analysed using a direct numerical simulation (DNS) database of turbulent statistically planar premixed H2-air flames with an equivalence ratio of 0.7. It was found that a significant enhancement of Reynolds stresses and dissipation rates takes place as a result of turbulence generation due to thermal expansion for small and moderate Karlovitz number values. However, both Reynolds stresses and dissipation rates decrease monotonically within the flame brush for large Karlovitz number values, as the flame-generated turbulence becomes overridden by the strong isotropic turbulence. Although there are similarities between the anisotropies of Reynolds stress and its dissipation rate tensors within the flame brush, the anisotropy tensors of these quantities are found to be non-linearly related. The predictions of three different models for the dissipation rate tensor were compared to the results computed from DNS data. It was found that the model relying upon isotropy and a linear dependence between the Reynolds stress and its dissipation rates does not correctly capture the turbulence characteristics within the flame brush for small and moderate Karlovitz number values. In contrast, the models that incorporate the dependence of the invariants of the anisotropy tensor of Reynolds stresses were found to capture the components of dissipation rate tensor for all Karlovitz number conditions.
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Jaseliūnaitė, Justina, Mantas Povilaitis, and Ieva Stučinskaitė. "RANS- and TFC-Based Simulation of Turbulent Combustion in a Small-Scale Venting Chamber." Energies 14, no. 18 (September 10, 2021): 5710. http://dx.doi.org/10.3390/en14185710.
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A laboratory-scale chamber is convenient for combustion scenarios in the practical analysis of industrial explosions and devices such as internal combustion engines. The safety risks in hazardous areas can be assessed and managed during accidents. Increased hydrogen usage in renewable energy production requires increased attention to the safety issues since hydrogen produces higher explosion overpressures and flame speed and can cause more damage than methane or propane. This paper reports numerical simulation of turbulent hydrogen combustion and flame propagation in the University of Sydney's small-scale combustion chamber. It is used for the investigation of turbulent premixed propagating flame interaction with several solid obstacles. Obstructions in the direction of flow cause a complex flame front interaction with the turbulence generated ahead of it. For numerical analysis, OpenFOAM CFD software was chosen, and a custom-built turbulent combustion solver based on the progress variable model—flameFoam—was used. Numerical results for validation purposes show that the pressure behaviour and flame propagation obtained using RANS and TFC models were well reproduced. The interaction between larger-scale flow features and flame dynamics was obtained corresponding to the experimental or mode detailed LES modelling results from the literature. The analysis revealed that as the propagating flame reached and interacted with obstacles and the recirculation wake was created behind solid obstacles, leaving traces of an unburned mixture. The expansion of flames due to narrow vents generates turbulent eddies, which cause wrinkling of the flame front.
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Robin, Vincent, Arnaud Mura, Michel Champion, and Tatsuya Hasegawa. "Modeling the Effects of Thermal Expansion on Scalar Turbulent Fluxes in Turbulent Premixed Flames." Combustion Science and Technology 182, no. 4-6 (June 10, 2010): 449–64. http://dx.doi.org/10.1080/00102200903462896.
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Akkerman, V. B., and V. V. Bychkov. "Flames with Realistic Thermal Expansion in a Time-Dependent Turbulent Flow." Combustion, Explosion, and Shock Waves 41, no. 4 (July 2005): 363–74. http://dx.doi.org/10.1007/s10573-005-0044-9.
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Wang, Siyuan, Haiou Wang, Kun Luo, and Jianren Fan. "The Effects of Differential Diffusion on Turbulent Non-Premixed Flames LO2/CH4 under Transcritical Conditions Using Large-Eddy Simulation." Energies 16, no. 3 (January 18, 2023): 1065. http://dx.doi.org/10.3390/en16031065.
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In this paper, a large-eddy simulation (LES) of turbulent non-premixed LO2/CH4 combustion under transcritical conditions is performed based on the Mascotte test rig from the Office National d’Etudes et de Recherches Ae´rospatiales (ONERA), and the aim is to understand the effects of differential diffusion on the flame behaviors. In the LES, oxygen was injected into the environment above the critical pressure while the temperature was below the critical temperature. The flamelet/progress variable (FPV) approach was used as the combustion model. Two LES cases with different species diffusion coefficient schemes—i.e., non-unity and unity Lewis numbers—for generating the flamelet tables were carried out to explore the effects of differential diffusion on the flame and flow structures. The results of the LES case with non-unity Lewis numbers were in good agreement with the experimental data. It was shown that differential diffusion had evident impacts on the flame structure and flow dynamics. In particular, when unity Lewis numbers were used to evaluate the species diffusion coefficient, the flame length was underestimated and the flame expansion was more significant. Compared to laminar counterflow flames, turbulence in jet flames allows chemical reactions to take place in a wider range of mixture fractions. The density distributions of the two LES cases in the mixture fraction space were very similar, indicating that differential diffusion had no significant effects on the phase transition under transcritical conditions.
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Joo, S. H., K. M. Chun, Y. Shin, and K. C. Lee. "An Investigation of Flame Expansion Speed With a Strong Swirl Motion Using High-Speed Visualization." Journal of Engineering for Gas Turbines and Power 125, no. 2 (April 1, 2003): 485–93. http://dx.doi.org/10.1115/1.1564067.
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In this study, a simple linear supposition method is proposed to separate the flame expansion speed and swirl motion of a flame propagating in an engine cylinder. Two series of images of flames propagating in the cylinder with/without swirl motion were taken by a high frame rate digital video camera. A small tube (4 mm ID) was installed inside the intake port to deliver the fuel/air mixture with strong swirl motion into the cylinder. An LDV was employed to measure the swirl motion during the compression stroke. Under the assumption that flame propagates spherically from the each point of the flame front, a diameter of small spherical flames can be calculated from the two consecutive images of the flame without swirl motion in the cylinder. Using the normalized swirl motion of the mixture during the compression stroke and the spherical flame diameters, the flame expansion speed and swirl ratio of combustion propagation in the engine cylinder can be obtained. This simple linear superposition method for separating the flame expansion speed and swirl motion can be utilized to understand the flow characteristics, such as swirl and turbulence, during the combustion process.
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Sabelnikov, Vladimir A., and Andrei N. Lipatnikov. "Recent Advances in Understanding of Thermal Expansion Effects in Premixed Turbulent Flames." Annual Review of Fluid Mechanics 49, no. 1 (January 3, 2017): 91–117. http://dx.doi.org/10.1146/annurev-fluid-010816-060104.
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Lu, Xiaoyi, and Carlos Pantano. "Linear stability analysis of a premixed flame with transverse shear." Journal of Fluid Mechanics 765 (January 19, 2015): 150–66. http://dx.doi.org/10.1017/jfm.2014.728.
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AbstractOne-dimensional planar premixed flames propagating in a uniform flow are susceptible to hydrodynamic instabilities known (generically) as Darrieus–Landau instabilities. Here, we extend that hydrodynamic linear stability analysis to include a lateral shear. This generalization is a situation of interest for laminar and turbulent flames when they travel into a region of shear (such as a jet or shear layer). It is shown that the problem can be formulated and solved analytically and a dispersion relation can be determined. The solution depends on a shear parameter in addition to the wavenumber, thermal expansion ratio, and Markstein lengths. The study of the dispersion relation shows that perturbations have two types of behaviour as wavenumber increases. First, for small shear, we recover the Darrieus–Landau results except for a region at small wavenumbers, large wavelengths, that is stable. Initially, increasing shear has a stabilizing effect. But, for sufficiently high shear, the flame becomes unstable again and its most unstable wavelength can be much smaller than the Markstein length of the zero-shear flame. Finally, the stabilizing effect of low shear can make flames with negative Markstein numbers stable within a band of wavenumbers.
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Zhu, Yuejin, Lei Yu, Gang Dong, Jianfeng Pan, and Zhenhua Pan. "Flow Topology of Three-Dimensional Spherical Flame in Shock Accelerated Flows." Advances in Materials Science and Engineering 2016 (2016): 1–12. http://dx.doi.org/10.1155/2016/3158091.
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The flow topologies of compressible large-scale distorted flames are studied by means of the analysis of the invariants of the velocity gradient tensor (VGT). The results indicate that compressibility plays a minor role in the distorted flame zone. And the joint probability density function (p.d.f.) of the Q-R diagram appears as a teardrop shape, which is a universal feature of turbulence. Therefore, the distorted flame exhibits the characteristic of large-scale turbulence combustion, especially behind the reflected shock wave, while the p.d.f. of the QS⁎-QW diagram implies that the dissipation is enhanced in the compression and expansion regions, where it is higher than that when P=0. Furthermore, we identify that the flame evolution is dominated by rotation by means of a quantitative statistical study, and the SFS topology is the predominant flow pattern. Not surprisingly, negative dilatation could suppress the unstable topologies, whereas positive dilatation could suppress the stable topologies.
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GHOSAL, SANDIP, and LUC VERVISCH. "Theoretical and numerical study of a symmetrical triple flame using the parabolic flame path approximation." Journal of Fluid Mechanics 415 (July 25, 2000): 227–60. http://dx.doi.org/10.1017/s0022112000008685.
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In non-premixed turbulent combustion the reactive zone is localized at the stoichiometric surfaces of the mixture and may be locally approximated by a diffusion flame. Experiments and numerical simulations reveal a characteristic structure at the edge of such a two-dimensional diffusion flame. This ‘triple flame’ or ‘edge flame’ consists of a curved flame front followed by a trailing edge that constitutes the body of the diffusion flame. Triple flames are also observed at the edge of a lifted laminar diffusion flame near the exit of burners. The speed of propagation of the triple flame determines such important properties as the rate of increase of the flame surface in non-premixed combustion and the lift-off distance in lifted flames at burners. This paper presents an approximate theory of triple flames based on an approximation of the flame shape by a parabolic profile, for large activation energy and low but finite heat release. The parabolic flame path approximation is a heuristic approximation motivated by physical considerations and is independent of the large activation energy and low heat release assumptions which are incorporated through asymptotic expansions. Therefore, what is presented here is not a truly asymptotic theory of triple flames, but an asymptotic solution of a model problem in which the flame shape is assumed parabolic. Only the symmetrical flame is considered and Lewis numbers are taken to be unity. The principal results are analytical formulas for the speed and curvature of triple flames as a function of the upstream mixture fraction gradient in the limit of infinitesimal heat release as well as small but finite heat release. For given chemistry, the solution provides a complete description of the triple flame in terms of the upstream mixture fraction gradient. The theory is validated by comparison with numerical simulation of the primitive equations.
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Sattelmayer, T., W. Polifke, D. Winkler, and K. Do¨bbeling. "NOx-Abatement Potential of Lean-Premixed GT Combustors." Journal of Engineering for Gas Turbines and Power 120, no. 1 (January 1, 1998): 48–59. http://dx.doi.org/10.1115/1.2818087.
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The influence of the structure of perfectly premixed flames on NOx formation is investigated theoretically. Since a network of reaction kinetics modules and model flames is used for this purpose, the results obtained are independent of specific burner geometries. Calculations are presented for a mixture temperature of 630 K, an adiabatic flame temperature of 1840 K, and 1 and 15 bars combustor pressure. In particular, the following effects are studied separately from each other: • molecular diffusion of temperature and species; • flame strain; • local quench in highly strained flames and subsequent reignition; • turbulent diffusion (no preferential diffusion); • small scale mixing (stirring) in the flame front. Either no relevant influence or an increase in NOx production over that of the one-dimensional laminar flame is found. As a consequence, besides the improvement of mixing quality, a future target for the development of low-NOx burners is to avoid excessive turbulent stirring in the flame front. Turbulent flames that exhibit locally and instantaneously near laminar structures (“flamelets”) appear to be optimal. Using the same methodology, the scope of the investigation is extended to lean-lean staging, since a higher NOx-abatement potential can be expected in principle. As long as the chemical reactions of the second stage take place in the boundary between the fresh mixture of the second stage and the combustion products from upstream, no advantage can be expected from lean-lean staging. Only if the primary burner exhibits much poorer mixing than the second stage can lean-lean staging be beneficial. In contrast, if full mixing between the two stages prior to afterburning can be achieved (lean-mix-lean technique), the combustor outlet temperature can in principle be increased somewhat without NO penalty. However, the complexity of such a system with a larger flame tube area to be cooled will increase the reaction zone temperatures, so that the full advantage cannot be realized in an engine. Of greater technical relevance is the potential of a lean-mixlean combustion system within an improved thermodynamic cycle. A reheat process with sequential combustion is perfectly suited for this purpose, since, first, the required low inlet temperature of the second stage is automatically generated after partial expansion in the high pressure turbine, second, the efficiency of the thermodynamic cycle has its maximum and, third, high exhaust temperatures are generated, which can drive a powerful Rankine cycle. The higher thermodynamic efficiency of this technique leads to an additional drop in NOx emissions per power produced.
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Champion, Michel, Vincent Robin, and Arnaud Mura. "A simple strategy to model the effects of thermal expansion on turbulent transports in premixed flames." Comptes Rendus Mécanique 340, no. 11-12 (November 2012): 769–76. http://dx.doi.org/10.1016/j.crme.2012.10.025.
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Ahmed, Umair, Sanjeev Kumar Ghai, and Nilanjan Chakraborty. "Direct Numerical Simulation Analysis of the Closure of Turbulent Scalar Flux during Flame–Wall Interaction of Premixed Flames within Turbulent Boundary Layers." Energies 17, no. 8 (April 18, 2024): 1930. http://dx.doi.org/10.3390/en17081930.
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The statistical behaviour and modelling of turbulent fluxes of the reaction progress variable and non-dimensional temperature in the context of Reynolds-Averaged Navier–Stokes (RANS) simulations have been analysed for flame–wall interactions within turbulent boundary layers. Three-dimensional Direct Numerical Simulation (DNS) databases of two different flame–wall interaction configurations—(i) statistically stationary oblique wall quenching (OWQ) of a V-flame in a turbulent channel flow and (ii) unsteady head-on quenching (HOQ) of a statistically planar flame propagating across a turbulent boundary layer—have been considered for this analysis. Scalar fluxes of both the temperature and reaction progress variable exhibit counter-gradient behaviour at all times during unsteady HOQ of statistically planar turbulent premixed flames considered here. In the case of statistically stationary V-flame OWQ, the scalar fluxes of both reaction progress variable and temperature exhibit counter-gradient behaviour before quenching, but gradient behaviour has been observed close to the wall once the flame begins to quench. The weakening of the effects of thermal expansion close to the wall as a result of flame quenching gives rise to a gradient type of transport for the streamwise component in the oblique quenching of the V-flame. It has been found that the relative orientation of the flame normal vector with respect to the wall normal vector needs to be accounted for in the algebraic scalar flux closure, which can be applied to different flame/flow configurations. An existing algebraic scalar flux model has been modified in this analysis for flame–wall interaction within turbulent boundary layers, and it has been demonstrated to capture the turbulent fluxes of the reaction progress variable and non-dimensional temperature reasonably accurately for both configurations considered here based on a priori DNS analysis.
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WENZEL, HOLGER, and NORBERT PETERS. "Direct Numerical Simulation and Modeling of Kinematic Restoration, Dissipation and Gas Expansion Effects of Premixed Flames in Homogeneous Turbulence." Combustion Science and Technology 158, no. 1 (September 2000): 273–97. http://dx.doi.org/10.1080/00102200008947337.
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Giannattasio, Pietro, Marco Pretto, and Enrico De Betta. "A phenomenological model for predicting the early development of the flame kernel in spark-ignition engines." Journal of Physics: Conference Series 2648, no. 1 (December 1, 2023): 012070. http://dx.doi.org/10.1088/1742-6596/2648/1/012070.
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Abstract This work presents a simple and effective phenomenological model for the prediction of the early growth of the flame kernel in SI engines, including its initiation as a result of the electrical breakdown of the fuel/air mixture between the spark plug electrodes. The present model aims to provide an improved description of the ignition-affected early phases of flame kernel development compared to the majority of models currently available in literature. In particular, these models focus on electrical energy supply and turbulence, whereas the stretch-induced kernel growth slowdown is quantified with linear models that are inconsistent with the small kernel radius. For the flame kernel initiation, this model replaces the current methods that rely on 1D heat diffusion within a plasma column with a more consistent analysis of post-breakdown conditions. Concerning the kernel growth, the present model couples the mass and energy conservation equations of a spherical kernel with the species and temperature profiles outside of it. This combination leads to a non-linear description of the flame stretch, according to which the kernel development is controlled by the Lewis-number-dependent balance between the heat gained via combustion and the heat lost via thermal diffusion. As a result, the kernel temperature differs from the adiabatic flame temperature, causing the laminar flame speed to change from its adiabatic value and ultimately affecting the overall kernel development. Kernel growth predictions are conducted for laminar flames and compared to literature data, showing a satisfactory agreement and highlighting the ability to describe the stretch-induced kernel slowdown, up to its possible extinction. A good agreement with literature data is also obtained for kernel expansions under moderately turbulent conditions, typical of internal combustion engines. The simple formulation of the present model enables swift integration into phenomenological combustion models for sparkignition engines, while simultaneously offering useful insight into the early kernel development even for CFD-based approaches.
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Чугуев, А. П., А. В. Мордвинова, А. Н. Сычев, and И. А. Мартынова. "STUDY OF FAN GAS JETS AND DIFFUSIVE FAN FLAMES." Pozharnaia bezopasnost`, no. 4(113) (December 13, 2023): 30–35. http://dx.doi.org/10.37657/vniipo.pb.2023.113.4.003.
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Основным способом газосброса (дренажа) различных газов из технологического или иного оборудования является сброс через специально оборудованную вертикальную трубу, по которой газ отводится на большую высоту с дожиганием или без него. Подробный анализ преимуществ и недостатков различных способов сброса технологических горючих газов показал, что наиболее эффективными с точки зрения сокращения длины факелов и зон опасных концентраций являются устройства, обеспечивающие сброс газа в виде веерных струй или пламени, формирующихся на выходе из кольцевой щели. Результаты экспериментов по оценке пожароопасных параметров при веерном сбросе в виде струй или пламени таких газов, как водород, метан и пропан, а также их смесей с инертными разбавителями позволили получить аналитические выражения, оценка которых проведена в настоящей работе. The research is devoted to the study of the effectiveness of the method of technological and emergency discharge (drainage) of various gases, including flammable, toxic or other hazardous gaseous media at petrochemical facilities, based on the use of fan gas discharge, the effectiveness of which is shown in this paper. Currently, facilities utilize a method of drainage from process equipment using a vertical pipe that drains gas to a higher elevation with or without afterburning. The disadvantage of this method is the formation of significant sizes of zones with dangerous concentrations of burning torches in the surrounding space. For example, many studies have shown that the flame length during the discharge of hydrogen or methane is 220–250 diameters of the drainage pipe in the turbulent mode of gas discharge. The use of drainage devices of this type for pipes of large diameter (400 mm or more) leads to material costs while ensuring safe conditions for technological drainage. The experimental and analytical data obtained in the work on the size of the flames of such combustible gases as hydrogen, methane, propane, as well as hazardous zones and the size of fan jets of gases found a good generalization by the Froude criterion. This makes it possible to use this analytical generalization in the calculations and creation of fan devices for real technological objects. According to the results of the study, a very important indicator of the effective fan discharge of the tested gases should be noted in comparison with drainage through a cylindrical pipe. The study showed that the fan flames of hydrocarbon gases, similar to hydrogen, turned out to be 4 times smaller than cylindrical ones. For example, reducing the flame size by 2 times is achieved by diluting propane with nitrogen by half. With a similar dilution of propane with freon 13B1, the flame size is reduced by 3 times. As a result of the research, data on the parameters of fan jets and flames of hydrocarbon gases were obtained, allowing them to be recommended during technological operations for the discharge of various gases using drainage devices in the form of annular slits. The relevance of experimental determination of the sizes of fan flames of various hydrocarbon gases, in particular LNG, and obtaining generalizing analytical dependencies for their assessment is due to the expansion of the current scale of their production and application and the requirements to increase the level of fire and environmental safety when handling combustible or other gases. The lack of necessary data in the scientific literature on the parameters of LNG fan flames also indicates the relevance of such studies
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Kim, Seung Hyun. "A Method to Simulate an Outwardly Propagating Turbulent Premixed Flame at Constant Pressure." Flow, Turbulence and Combustion, April 13, 2024. http://dx.doi.org/10.1007/s10494-024-00544-4.
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AbstractAn outwardly propagating premixed flame in homogeneous isotropic turbulence at constant pressure is considered one of canonical configurations to study turbulent premixed flames. In this paper, a surface forcing method to prevent the undesirable influence of the boundary-condition-induced backflow on the flame evolution, while maintaining the constant pressure, in the simulation of the outwardly propagating flame is presented. The method is validated for laminar and turbulent flames. The results show that the present method well preserves the characteristics of turbulence and of an outwardly propagating flame, without the undesirable influence of the boundary condition, by feeding the homogeneous turbulence relative to the velocity field induced by the volume expansion due to heat release to the domain in which the flame develops.
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Sabelnikov, Vladimir Anatolievich, Andrei Lipatnikov, Nikolay Nikitin, Francisco Hernandez Perez, and Hong G. Im. "Conditioned structure functions in turbulent hydrogen/air flames." Physics of Fluids, July 10, 2022. http://dx.doi.org/10.1063/5.0096509.
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Direct numerical simulation data obtained from two turbulent, lean hydrogen-air flames propagating in a box are analyzed to explore the influence of combustion-induced thermal expansion on turbulence in unburned gas. For this purpose, Helmholtz-Hodge decomposition is applied to the computed velocity fields. Subsequently, the second-order structure functions conditioned to unburned reactants are sampled from divergence-free solenoidal velocity field or irrotational potential velocity field, yielded by the decomposition. Results show that thermal expansion significantly affects the conditioned potential structure functions not only inside the mean flame brushes, but also upstream of them. Upstream of the flames, firstly, transverse structure functions for transverse potential velocities grow with distance r between sampling points more slowly when compared to the counterpart structure functions sampled from the entire or solenoidal velocity field. Secondly, the former growth rate depends substantially on the distance from the flame-brush leading edge, even at small r. Thirdly, potential root-mean-square (rms) velocities increase with decreasing distance from the flame-brush leading edge and are comparable with solenoidal rms velocities near the leading edge. Fourthly, although the conditioned axial and transverse potential rms velocities are always close to one another, thus, implying isotropy of the potential velocity field in unburned reactants; the potential structure functions exhibit a high degree of anisotropy. Fifthly, thermal expansion effects are substantial even for the solenoidal structure functions and even upstream of a highly turbulent flame. These findings call for development of advanced models of turbulence in flames, which allow for the discussed thermal expansion effects.
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Sabelnikov, Vladimir Anatolievich, Andrei Lipatnikov, Nikolay Nikitin, Francisco Hernandez Perez, and Hong G. Im. "Effects of thermal expansion on moderately intense turbulence in premixed flames." Physics of Fluids, October 20, 2022. http://dx.doi.org/10.1063/5.0123211.
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The study aims at analytically and numerically exploring the influence of combustion-induced thermal expansion on turbulence in premixed flames. In the theoretical part, contributions of solenoidal and potential velocity fluctuations to the unclosed component of the advection term in the Reynolds-averaged Navier-Stokes equations are compared and a new criterion for assessing the importance of the thermal expansion effects is introduced. The criterion highlights a ratio of the dilatation in the laminar flame to the large-scale gradient of root-mean-square (rms) velocity in the turbulent flame brush. To support the theoretical study, direct numerical simulation (DNS) data obtained earlier from two complex-chemistry, lean H2-air flames are analyzed. In line with the new criterion, even at sufficiently high Karlovitz numbers, results show significant influence of combustion-induced potential velocity fluctuations on the second moments of the turbulent velocity upstream of and within the flame brush. In particular, the DNS data demonstrate that (i) potential and solenoidal rms velocities are comparable in the unburnt gas close to the leading edge of the flame brush and (ii) potential and solenoidal rms velocities conditioned to unburnt gas are comparable within the entire flame brush. Moreover, combustion-induced thermal expansion affects not only the potential velocity, but even the solenoidal one. The latter effects manifest themselves in a negative correlation between solenoidal velocity fluctuations and dilatation or in the counter-gradient behavior of the solenoidal scalar flux. Finally, a turbulence-in-premixed-flame diagram is sketched to discuss the influence of combustion-induced thermal expansion on various ranges of turbulence spectrum.
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Ahmed, Umair, Nilanjan Chakraborty, and Markus Klein. "Influence of Flow Configuration and Thermal Wall Boundary Conditions on Turbulence During Premixed Flame-Wall Interaction within Low Reynolds Number Boundary Layers." Flow, Turbulence and Combustion, July 6, 2023. http://dx.doi.org/10.1007/s10494-023-00437-y.
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AbstractThe influence of flow configuration on flame-wall interaction (FWI) of premixed flames within turbulent boundary layers has been investigated. Direct numerical simulations (DNS) of two different flow configurations for flames interacting with chemically inert isothermal and adiabatic walls in fully developed turbulent boundary layers have been performed. The first configuration is an oblique wall interaction (OWI) of a V-flame in a turbulent channel flow and the second configuration is a head-on interaction (HOI) of a planar flame in a turbulent boundary layer. These simulations are representative of stoichiometric methane-air mixture under atmospheric conditions and the non-reacting turbulence for these simulations corresponds to the friction velocity based Reynolds number of $$Re_{\tau }=110$$ R e τ = 110 . It is found that the mean wall shear stress, mean wall friction velocity and the mean velocity statistics are affected during FWI and the behaviour for these quantities varies under the different flow configurations as well as for the different thermal wall boundary conditions. The behaviour of the quenching distance and mean wall heat flux under isothermal wall conditions is found to be significantly different between the two flow configurations. The variation of the non-dimensional temperature in wall units for cases with isothermal walls suggests that the temperature in the log-layer region is significantly altered by the evolving wall heat flux in both flow configurations. Statistics of the mean Reynolds stresses and turbulence dissipation rate show that the flame significantly alters the behaviour of turbulence due to thermal expansion effects and flow configuration plays an important role.
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Qian, Xiang, Hao Lu, Chun Zou, and Hong Yao. "On the inverse kinetic energy cascade in premixed isotropic turbulent flames." International Journal of Modern Physics C, September 4, 2021, 2250015. http://dx.doi.org/10.1142/s0129183122500152.
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The understanding of energy transfer in fluids is important for the accurate modeling of turbulent reacting flows. In this study, we investigate interscale kinetic energy transfer and subgrid-scale (SGS) backscatter using data from direct numerical simulations (DNSs) of premixed isotropic turbulent flames. Results reveal that in the examined premixed flames, the pressure transfer term appearing in the transport equation of turbulent kinetic energy dominates the nonlinear advection and the dissipation at large scales, and noticeably contributes to the inverse kinetic energy cascade. Filtered DNS data show that SGS backscatter is correlated with the appearance of positive pressure-dilatation work, i.e. thermal expansion. A priori test results of three SGS stress models reveal that the Smagorinsky stress model is unable to capture SGS backscatter, but that two nonlinear structural stress models are able to predict SGS backscatter.
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Robin, Vincent, Arnaud Mura, and Michel Champion. "Direct and indirect thermal expansion effects in turbulent premixed flames." Journal of Fluid Mechanics, November 3, 2011, 1–34. http://dx.doi.org/10.1017/jfm.h2011.409.
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Sabelnikov, V. A., A. N. Lipatnikov, N. V. Nikitin, F. E. Hernández Pérez, and H. G. Im. "Backscatter of scalar variance in turbulent premixed flames." Journal of Fluid Mechanics 960 (March 30, 2023). http://dx.doi.org/10.1017/jfm.2023.195.
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To explore the direction of inter-scale transfer of scalar variance between subgrid scale (SGS) and resolved scalar fields, direct numerical simulation data obtained earlier from two complex-chemistry lean hydrogen–air flames are analysed by applying Helmholtz–Hodge decomposition (HHD) to the simulated velocity fields. Computed results show backscatter of scalar (combustion progress variable $c$ ) variance, i.e. its transfer from SGS to resolved scales, even in a highly turbulent flame characterized by a unity-order Damköhler number and a ratio of Kolmogorov length scale to thermal laminar flame thickness as low as 0.05. Analysis of scalar fluxes associated with the solenoidal and potential velocity fields yielded by HHD shows that the documented backscatter stems primarily from the potential velocity perturbations generated due to dilatation in instantaneous local flames, with the backscatter being substantially promoted by a close alignment of the spatial gradient of mean scalar progress variable and the potential-velocity contribution to the local SGS scalar flux. The alignment is associated with the fact that combustion-induced thermal expansion increases local velocity in the direction of $\boldsymbol {\nabla } c$ . These results call for development of SGS models capable of predicting backscatter of scalar variance in turbulent flames in large eddy simulations.
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Velez, Carlos, Scott Martin, Aleksander Jemcov, and Subith Vasu. "Large Eddy Simulation of an Enclosed Turbulent Reacting Methane Jet With the Tabulated Premixed Conditional Moment Closure Method." Journal of Engineering for Gas Turbines and Power 138, no. 10 (April 12, 2016). http://dx.doi.org/10.1115/1.4032846.
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The tabulated premixed conditional moment closure (T-PCMC) method has been shown to provide the capability to model turbulent, premixed methane flames with detailed chemistry and reasonable runtimes in Reynolds-averaged Navier–Stokes (RANS) environment by Martin et al. (2013, “Modeling an Enclosed, Turbulent Reacting Methane Jet With the Premixed Conditional Moment Closure Method,” ASME Paper No. GT2013-95092). Here, the premixed conditional moment closure (PCMC) method is extended to large eddy simulation (LES). The new model is validated with the turbulent, enclosed reacting methane backward facing step data from El Banhawy et al. (1983, “Premixed, Turbulent Combustion of a Sudden-Expansion Flow,” Combust. Flame, 50, pp. 153–165). The experimental data have a rectangular test section at atmospheric pressure and temperature with an inlet velocity of 10.5 m/s and an equivalence ratio of 0.9 for two different step heights. Contours of major species, velocity, and temperature are provided. The T-PCMC model falls into the class of table lookup turbulent combustion models in which the combustion model is solved offline over a range of conditions and stored in a table that is accessed by the computational fluid dynamic (CFD) code using three controlling variables: the reaction progress variable (RPV), variance, and local scalar dissipation rate. The local scalar dissipation rate is used to account for the affects of the small-scale mixing on the reaction rates. A presumed shape beta function probability density function (PDF) is used to account for the effects of subgrid scale (SGS) turbulence on the reactions. SGS models are incorporated for the scalar dissipation and variance. The open source CFD code OpenFOAM is used with the compressible Smagorinsky LES model. Velocity, temperature, and major species are compared to the experimental data. Once validated, this low “runtime” CFD turbulent combustion model will have great utility for designing the next generation of lean premixed (LPM) gas turbine combustors.
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Briones, Alejandro M., Balu Sekar, and Timothy Erdmann. "Effect of Centrifugal Force on Turbulent Premixed Flames." Journal of Engineering for Gas Turbines and Power 137, no. 1 (August 5, 2014). http://dx.doi.org/10.1115/1.4028057.
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The effect of centrifugal force on flame propagation velocity of stoichiometric propane–, kerosene–, and n-octane–air turbulent premixed flames was numerically examined. The quasi-turbulent numerical model was set in an unsteady two-dimensional (2D) geometry with finite length in the transverse and streamwise directions but with infinite length in the spanwise direction. There was relatively good comparison between literature-reported measurements and predictions of propane–air flame propagation velocity as a function of centrifugal force. It was found that for all mixtures the flame propagation velocity increases with centrifugal force. It reaches a maximum, then falls off rapidly with further increases in centrifugal force. The results of this numerical study suggest that there are no distinct differences among the three mixtures in terms of the trends seen of the effect of centrifugal force on the flame propagation velocity. There are, however, quantitative differences. The numerical model is set in a noninertial, rotating reference frame. This rotation imposes a radially outward (centrifugal) force. The ignited mixture at one end of the tube raises the temperature and its heat release tends to laminarize the flow. The attained density difference combined with the direction of the centrifugal force promotes Rayleigh–Taylor instability. This instability with thermal expansion and turbulent flame speed constitute the flame propagation mechanism towards the other tube end. A wave is also generated from the ignition zone but propagates faster than the flame. During propagation the flame interacts with eddies that wrinkle and/or corrugate the flame. The flame front wrinkles interact with streamtubes that enhance Landau–Darrieus (hydrodynamic) instability, giving rise to a corrugated flame. Under strong stretch conditions the stabilizing equidiffusive-curvature mechanism fails and the flame front breaks up, allowing inflow of unburned mixture into the flame. This phenomenon slows down the flame temporarily and then the flame speeds up faster than before. However, if corrugation is large and the inflow of unburned mixture into the flame is excessive, the latter locally quenches and slows down the flame. This occurs when the centrifugal force is large, tending to blowout the flame. The wave in the tube interacts continuously with the flame through baroclinic torques at the flame front that further enhances the above mentioned flame–eddy interactions. Only at low centrifugal forces, the wave intermingles several times with the flame before the averaged flame propagation velocity is determined. The centrifugal force does not substantially increase the turbulent flame speed as commented by previous experimental investigations. The results also suggest that an ultracompact combustor (UCC) with high-g cavity (HGC) will be limited to centrifugal force levels in the 2000–3000 g range.
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Latifi, Mojtaba, and Mohammad Mahdi Salehi. "Numerical simulation of turbulent premixed flames with the conditional source-term estimation model using Bernstein polynomial expansion." Combustion Theory and Modelling, September 25, 2023, 1–21. http://dx.doi.org/10.1080/13647830.2023.2261895.
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Jiang, Lei, Gang Li, Xi Jiang, Hongbin Hu, Bo Xiao, Yanji Xu, and Zhijun Lei. "Experimental investigation of non-premixed and partially premixed methane lifted flames established on a lobed swirl injector." Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, September 9, 2020, 095765092095500. http://dx.doi.org/10.1177/0957650920955004.
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A lobed swirl injector was tested to examine its potential in combustion control for non-premixed and partially premixed flames. It was found in the experiment that the flame derived from the injector changed between attached and detached flames at different conditions, demonstrating a promising way to control combustion. When air is supplied through the external channel of the lobed swirl injector and fuel passes through the internal channel, a stable lifted flame that is partially premixed was established above the injector exit. With the increase of airflow rate, the flame lift-off height decreases gradually until it is reattached to the injector, forming a diffusion flame. When increasing the fuel flow rate, the lift-off height increases gradually until the flame is blown out. Flow fields of the partially premixed lifted flames were investigated using stereoscopic particle image velocimetry. Streamlines located in the near field of the injector exit do not expand but bend inward, which is quite different from the expansion motion at the exit of the traditional vane swirler. The trough structure on the lobed swirler leads to the outer air flowing inward. Although the crest structure should make the inside gas flow outward, the strong entrainment of the surrounding air would restrain the radial outward motion of the inner gas, thus causing a contracted motion. After the streamline develops to an axial position further away from the injector exit, the swirling jet begins to expand under effects of both the centrifugal force and the development of shear layer to form turbulence. This flow pattern affects both the flame stabilization position and the neighboring reaction zone structure significantly. The measurements also show that the lobed swirl injector is very capable of entraining the ambient air that is sucked into the mainstream from the downward direction.
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Versailles, Philippe, Antoine Durocher, Gilles Bourque, and Jeffrey M. Bergthorson. "Measurements of the reactivity of premixed, stagnation, methane-air flames at gas turbine relevant pressures." Journal of Engineering for Gas Turbines and Power 141, no. 1 (October 17, 2018). http://dx.doi.org/10.1115/1.4041125.
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The adiabatic, unstrained, laminar flame speed, SL, is a fundamental combustion property, and a premier target for the development and validation of thermochemical mechanisms. It is one of the leading parameters determining the turbulent flame speed, the flame position in burners and combustors, and the occurrence of transient phenomena, such as flashback and blowout. At pressures relevant to gas turbine engines, SL is generally extracted from the continuous expansion of a spherical reaction front in a combustion bomb. However, independent measurements obtained in different types of apparatuses are required to fully constrain thermochemical mechanisms. Here, a jet-wall, stagnation burner designed for operation at gas turbine relevant conditions is presented, and used to assess the reactivity of premixed, lean-to-rich, methane–air flames at pressures up to 16 atm. One-dimensional (1D) profiles of axial velocity are obtained on the centerline axis of the burner using particle tracking velocimetry, and compared to quasi-1D flame simulations performed with a selection of thermochemical mechanisms available in the literature. Significant discrepancies are observed between the numerical and experimental data, and among the predictions of the mechanisms. This motivates further chemical modeling efforts, and implies that designers in industry must carefully select the mechanisms employed for the development of gas turbine combustors.
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Ahmed, Umair, Sanjeev Kumar Ghai, and Nilanjan Chakraborty. "Relations between Reynolds stresses and their dissipation rates during premixed flame–wall interaction within turbulent boundary layers." Physics of Fluids 36, no. 4 (April 1, 2024). http://dx.doi.org/10.1063/5.0204038.
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A direct numerical simulation (DNS) database for head-on quenching of premixed flames propagating across turbulent boundary layers representative of friction Reynolds numbers, Reτ, of 110 and 180 has been utilized to analyze the interrelation between Reynolds stresses and their dissipation rates during flame–wall interaction. The Reynolds stresses and their dissipation rates exhibit significant deviations from the corresponding non-reacting flow profiles within the flame brush and in the burned gas region. This behavior is prominent for the components in the wall-normal direction because the mean direction of flame normal acceleration due to thermal expansion aligns with the wall-normal direction in this configuration. The anisotropy of Reynolds stresses and their dissipation rate tensors have been found to be qualitatively similar, but the anisotropic behavior weakens with increasing Reτ. However, the components of the anisotropy tensors of Reynolds stresses and viscous dissipation rate are not related according to a linear scaling, and thus, the models based on this assumption do not successfully capture the viscous dissipation rate components obtained from the DNS data. By contrast, a model, which includes the invariants of the anisotropy tensor of Reynolds stresses and satisfies the limiting conditions, has been found to capture especially the diagonal components of the viscous dissipation rate tensor more successfully for both non-reacting and reacting cases considered in this work. However, the quantitative prediction of this model suffers for the components in the wall-normal direction for lower values of Reτ, but the performance of this model improves with an increase in Reτ.
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Li, Qinyuan, Bo Yan, Mingbo Sun, Yifu Tian, Minggang Wan, Zhongwei Wang, Xueni Yang, Tao Tang, and Jiajian Zhu. "Spatiotemporal visualization of instantaneous flame structure in a hydrogen-fueled axisymmetric supersonic combustor." Physics of Fluids 36, no. 12 (December 1, 2024). https://doi.org/10.1063/5.0235001.
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Spatiotemporal visualization of instantaneous flame structures in a hydrogen-fueled axisymmetric supersonic combustor was investigated using multiview planar laser-induced fluorescence of the hydroxyl radical, coupled with high-speed photography and pressure measurement. The axisymmetric cavity generates a loop-shaped recirculation flow and shear layer that sustains the flame. An irregular and wrinkled flame loop with a central hole is formed near the loop-shaped region. Due to turbulent disturbances, multiple small-scale holes and fragmented flames are randomly distributed in the flame loop or near the wrinkled flame front. The combustion near the cavity shear layer is more likely to be stronger and sustained. As the thickness of the cavity shear layer increases along the axial direction, the flame loop is expanded toward the core flow and the cavity. The flame base anchors near the cavity leading edge with a low global equivalence ratio (GER). The increased GER expands the flame loop to compress the high-speed core flow dramatically, promoting the flame base to propagate upstream along the hydrogen jet wake. The flame base is unable to anchor near the thin boundary layer. Consequently, it propagates reciprocally to enhance the combustion oscillation that disturbs the flame structure dramatically. The flame structure becomes more complex and tendentially fragmented, which increases the fractal dimension, especially near the middle part of the combustor. In comparison, the flame structure near the ramp is more resistant to disturbances due to the dramatic expansion of local flame loop, extending the favorable combustion environment. Despite the instantaneous flame structure being severely wrinkled and even tendentially fragmented, it is primarily sustained within a relatively regular loop region near the cavity recirculation flow and the cavity shear layer.