The structure of lean hydrogen-air flame balls

Fernández-Tarrazo, Eduardo; Sánchez, Antonio L.; Liñán, A.; Williams, Forman A.

doi:10.1016/j.proci.2010.05.086

Cited by 20 publications

(18 citation statements)

References 13 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Numerical studies, including detailed-chemistry mechanisms and different models for radiation [17,18,19], have shown that radiative heat losses are very significant, keeping the peak temperature of hydrogen-air flame balls below 1200 K along the whole branch of stable solutions. As emphasized recently [20], under those near-crossover conditions, all chemical intermediates appear in very small concentrations and obey chemical-kinetic steady-state approximations. Consequently, a reduced chemical-kinetic mechanism involving a single global reaction 2H 2 þ O 2 / 2H 2 O, previously derived for planar deflagrations near the lean flammability limit [21,22], can be employed to describe hydrogen-air flame balls with excellent accuracy.…”

Section: Introductionmentioning

confidence: 74%

“…where W represents the mean molecular weight of the gas mixture, and s and k H 2 O denote the StefaneBoltzmann constant and the Plank-mean absorption coefficient, respectively, with the latter being a function of the local temperature [34] that in the range of temperatures of interest (270 < T < 1300 K) can be shown to be accurately represented by the expression k H 2 O ¼ 5:72 Â 10 À4 ðT=298Þ À2 s 2 /kg [20].…”

Section: Problem Formulationmentioning

confidence: 99%

“…Also, numerical simulations of three-dimensional flame-ball dynamics, previously attempted with a generic Arrhenius chemistry model [24,25], could make use of the one-step H 2 eO 2 reduced mechanism for a more realistic description, enabling the flame-ball response to be linked directly to the ambient conditions. It was seen in [20] that, because of the large value of b present in the one-step rate, at leading order the flame-ball size is determined by the condition that radiation removes enough of the chemical heat for the flame temperature to remain close to the crossover value. The leading-order analytic solution computed in this way describes with excellent accuracy the variation of the flame-ball radius r f with equivalence ratio f along the upper branch of stable solutions, except near the lean limit, where it fails to give the turning point of the r f À f curve.…”

Section: Introductionmentioning

confidence: 99%

“…Simplifications to the SNB description of radiative heat losses are also needed to enable analytical results to be developed. Assuming the characteristic absorption length to be much larger than the characteristic flame-ball size leads to the well-known optically thin approximation, used in previous studies [17,18,19,20]. The results obtained with the one-step approximation for the chemistry combined with the optically thin approximation for radiation are shown as dashed curves in Figs.…”

mentioning

confidence: 97%

“…In fuel-lean hydrogenair flame balls, the molecular transport is dominated by the abundant presence of nitrogen and oxygen, so that the thermal conductivity and the molecular diffusivities of the different species can be computed with the simple temperature-dependent laws suggested in [26] for methaneair flames. As shown previously [20], Soret diffusion of light species needs to be included if accurate results are desired. Besides, the computation requires specification of the chemical-reaction scheme, an appropriate choice being the so-called San Diego Mechanism of 21 steps [27,28].…”

mentioning

confidence: 99%

See 4 more Smart Citations

Flammability conditions for ultra-lean hydrogen premixed combustion based on flame-ball analyses

Fernández-Tarrazo

Sánchez

Liñán³

et al. 2012

International Journal of Hydrogen Energy

View full text Add to dashboard Cite

Keywords:Hydrogen Flammability limit Ultra-lean combustion Reduced chemistryFlame balls a b s t r a c t It has been reasoned that the structures of strongly cellular flames in very lean mixtures approach an array of flame balls, each burning as if it were isolated, thereby indicating a connection between the critical conditions required for existence of steady flame balls and those necessary for occurrence of self-sustained premixed combustion. This is the starting assumption of the present study, in which structures of near-limit steady sphericosymmetrical flame balls are investigated with the objective of providing analytic expressions for critical combustion conditions in ultra-lean hydrogen-oxygen mixtures diluted with N 2 and water vapor. If attention were restricted to planar premixed flames, then the lean-limit mole fraction of H 2 would be found to be roughly ten percent, more than twice the observed flammability limits, thereby emphasizing the relevance of the flame-ball phenomena.Numerical integrations using detailed models for chemistry and radiation show that a onestep chemical-kinetic reduced mechanism based on steady-state assumptions for all chemical intermediates, together with a simple, optically thin approximation for water-vapor radiation, can be used to compute near-limit fuel-lean flame balls with excellent accuracy.The previously developed one-step reaction rate includes a crossover temperature that determines in the first approximation a chemical-kinetic lean limit below which combustion cannot occur, with critical conditions achieved when the diffusion-controlled radiation-free peak temperature, computed with account taken of hydrogen Soret diffusion, is equal to the crossover temperature. First-order corrections are found by activation-energy asymptotics in a solution that involves a near-field radiation-free zone surrounding a spherical flame sheet, together with a far-field radiation-conduction balance for the temperature profile. Different scalings are found depending on whether or not the surrounding atmosphere contains water vapor, leading to different analytic expressions for the critical conditions for flame-ball existence, which give results in very good agreement with those obtained by detailed numerical computations. The one-step chemistry employed in the present work, which involves a non-Arrhenius rate having a cutoff at the crossover temperature, applies with excellent accuracy to the description of lean premixed hydrogen-air combustion, i.e, for f(0:5 at atmospheric pressure, and could be used for instance in the numerical simulation of the propagation of curved or cellular flames in ultra-lean reactive atmospheres, of interest for safety analyses related to the storage, transport, and handling of hydrogen.

show abstract

Section: Introductionmentioning

confidence: 74%

Section: Problem Formulationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 97%

mentioning

confidence: 99%

See 3 more Smart Citations

Flammability conditions for ultra-lean hydrogen premixed combustion based on flame-ball analyses

Fernández-Tarrazo

Sánchez

Liñán³

et al. 2012

International Journal of Hydrogen Energy

View full text Add to dashboard Cite

show abstract

Modeling Combustion Chemistry in Large Eddy Simulation of Turbulent Flames

Fiorina

Veynante

Candel

2014

Flow Turbulence Combust

View full text Add to dashboard Cite

Flame ignition, stabilization and extinction or pollutant predictions are crucial issues in Large Eddy Simulations (LES) of turbulent combustion. These phenomena are strongly influenced by complex chemical effects. Unfortunately, despite the rapid increase in computational power, performing turbulent simulations of industrial configurations including detailed chemical mechanisms will still remain out of reach for a long time. This article proposes a review of commonly-used approaches to address fluid/chemistry interactions at a reduced computational cost. Several chemistry modeling routes are first examined with a focus on tabulated chemistry techniques. The problem of coupling chemistry with LES is considered in a second step. Examples of turbulent combustion simulations are presented in the final part of the article. Three LES applications are analyzed: a lean swirled combustor, a non-adiabatic turbulent stratified flame and a combustion chamber where internal recirculations promote the dilution of fresh gases by burnt gases. Keywords Turbulent combustion • Large Eddy Simulation • Tabulated chemistry • Complex chemistry 1 Introduction Combustion accounts for about 90 % of the global consumption of primary energy [1]. Despite its environmental impact this share will probably not fall in the near future. Combustion delivers energy which is immediately available through the exothermic conversion of gaseous, liquid or solid fuels. There are no other way to provide energy which are as convenient and as effective. In automotive or aerospace applications, combustion provides

show abstract

Theory of the propagation dynamics of spiral edges of diffusion flames in von Kármán swirling flows

Urzay

Nayagam

Williams

2011

Combustion and Flame

View full text Add to dashboard Cite

a b s t r a c tThis analysis addresses the propagation of spiral edge flames found in von Kármán swirling flows induced in rotating porous-disk burners. In this configuration, a porous disk is spun at a constant angular velocity in an otherwise quiescent oxidizing atmosphere. Gaseous methane is injected through the disk pores and burns in a flat diffusion flame adjacent to the disk. Among other flame patterns experimentally found, a stable, rotating spiral flame is observed for sufficiently large rotation velocities and small fuel flow rates as a result of partial extinction of the underlying diffusion flame. The tip of the spiral can undergo a steady rotation for sufficiently large rotational velocities or small fuel flow rates, whereas a meandering tip in an epicycloidal trajectory is observed for smaller rotational velocities and larger fuel flow rates. A formulation of this problem is presented in the equidiffusional and thermodiffusive limits within the framework of one-step chemistry with large activation energies. Edge-flame propagation regimes are obtained by scaling analyses of the conservation equations and exemplified by numerical simulations of straight two-dimensional edge flames near a cold porous wall, for which lateral heat losses to the disk and large strains induce extinction of the trailing diffusion flame but are relatively unimportant in the front region, consistent with the existence of the cooling tail found in the experiments. The propagation dynamics of a steadily rotating spiral edge is studied in the large-core limit, for which the characteristic Markstein length is much smaller than the distance from the center at which the spiral tip is anchored. An asymptotic description of the edge tangential structure is obtained, spiral edge shapes are calculated, and an expression is found that relates the spiral rotational velocity to the rest of the parameters. A quasiestatic stability analysis of the edge shows that the edge curvature at extinction in the tip region is responsible for the stable tip anchoring at the core radius. Finally, experimental results are analyzed, and theoretical predictions are tested.

show abstract

The structure of lean hydrogen-air flame balls

Cited by 20 publications

References 13 publications

Flammability conditions for ultra-lean hydrogen premixed combustion based on flame-ball analyses

Flammability conditions for ultra-lean hydrogen premixed combustion based on flame-ball analyses

Modeling Combustion Chemistry in Large Eddy Simulation of Turbulent Flames

Theory of the propagation dynamics of spiral edges of diffusion flames in von Kármán swirling flows

Contact Info

Product

Resources

About