Detailed numerical simulation of flame ball structure and dynamics

A computational study has been conducted to determine the critical conditions for the transition from localized flame ignition and propagation to the establishment of a flame ball. Lean H 2 /air mixtures are investigated using a time-dependent, spherically symmetric code with detailed chemistry, transport, and radiation submodels. Results show that outwardly propagating spherical flames can be ignited for hydrogen mole fractions larger than ϳ3.5%. Furthermore, assuming optically thin radiative heat loss, flame balls X H 2 can be established from centrally ignited premixed spherical flames only within a narrow range of mixture compositions (i.e., ϳ3.5% Ͻ Ͻϳ6.5%). For ϳ6.5% Ͻ Ͻϳ11%, flames propagate until radiativeextinction, never evolving into flame balls, while for Ͼ ϳ11%, the expanding spherical flames develop X H 2 asymptotically into planar propagating flames. These findings corroborate the experimental result that the range of mixtures within which flame balls have been observed is much narrower than that predicted by previous one-dimensional instability analysis of the flame ball, where it was shown that steady flame balls exist for 3.5% Ͻ Ͻ 10.7%. The present simulation also shows that the dynamic transition from a X H 2 spherically propagating flame to the flame ball controls the range of mixtures for which flame balls can be reached, with radiative loss being both the requisite mechanism for and the limiting mechanism against the dynamic transformation. Additional calculations show that the size of the flame ball is noticeably enlarged when radiative reabsorption is incorporated.

Section: Introductionmentioning

confidence: 69%

Section: Introductionmentioning

confidence: 99%

A computational study of the transition from localized ignition to flame ball in lean hydrogen/air mixtures

Tse

Law

2000

“…The enhancement in the flame temperature through the Lewis-number effect would be important in such descriptions (and has been obtained in hydrogen-air flame-ball computations with detailed chemistry [10,11]). It is simpler, however, in investigating flame-ball modes to use a one-step chemistry model with high activation energy and then to apply activation-energy asymptotics (AEA), as has been done for planar deflagrations [7] (as well as for most of the original flame-ball analyses).…”

Section: The Chemistrymentioning

confidence: 95%

“…There have been many observations and measurements of flame balls in lean hydrogen mixtures [9], and calculations of their structures have been performed with radiative transfer and detailed molecular transport and chemistry included [10,11]. Their use in approximations for propagating flames would, of course, be limited to very lean conditions where well-developed cells occur.…”

Section: Introductionmentioning

confidence: 99%

A hypothetical burning-velocity formula for very lean hydrogen–air mixtures

Williams¹,

Grcar²

2009

Very lean hydrogen-air mixtures experience strong diffusive-thermal types of cellular instabilities that tend to increase the laminar burning velocity above the value that applies to steady, planar laminar flames that are homogeneous in transverse directions. Flame balls constitute an extreme limit of evolution of cellular flames. To account qualitatively for the ultimate effect of diffusive-thermal instability, a model is proposed in which the flame is a steadily propagating, planar, hexagonal, close-packed array of flame balls, each burning as if it were an isolated, stationary, ideal flame ball in an infinite, quiescent atmosphere. An expression for the laminar burning velocity is derived from this model, which theoretically may provide an upper limit for the experimental burning velocity.

“…Computational methods are sufficiently advanced that flame-ball structures can be addressed numerically [4,5]. While numerical approaches can include many phenomena, they often are not well suited to specifically identifying the most important ones.…”

Section: Introductionmentioning

confidence: 99%

The structure of lean hydrogen-air flame balls

Fernández-Tarrazo

Sánchez

Liñán

et al. 2011

An analysis of the structure of flame balls encountered under microgravity conditions, which are stable due to radiant energy losses from H 2 O, is carried out for fuel-lean hydrogen-air mixtures. It is seen that, because of radiation losses, in stable flame balls the maximum flame temperature remains close to the crossover temperature, at which the rate of the branching step H + O 2 ? OH + O equals that of the recombination step H + O 2 +M ? HO 2 + M. Under those conditions, all chemical intermediates have very small concentrations and follow the steady-state approximation, while the main species react according to the overall step 2H 2 + O 2 ? 2H 2 O; so that a one-step chemical-kinetic description, recently derived by asymptotic analysis for near-limit fuel-lean deflagrations, can be used with excellent accuracy to describe the whole branch of stable flame balls. Besides molecular diffusion in a binary-diffusion approximation, Soret diffusion is included, since this exerts a nonnegligible effect to extend the flammability range. When the large value of the activation energy of the overall reaction is taken into account, the leading-order analysis in the reaction-sheet approximation is seen to determine the flame ball radius as that required for radiant heat losses to remove enough of the heat released by chemical reaction at the flame to keep the flame temperature at a value close to crossover. The results are relevant to burning velocities at lean equivalent ratios and may influence fire-safety issues associated with hydrogen utilization.