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

Abstract

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.

Unexpected Propagation of Ultra-Lean Hydrogen Flames in Narrow Gaps

“…This result then identifies mass diffusivity as the mechanism that counteracts heat losses [14] and thermodiffusive instabilities triggered by the high diffusivity of hydrogen become the survival mechanism that enables local flame quenching under nonadiabatic conditions and gives birth to flame cells within which the temperature is high enough to sustain combustion [32]. The instantaneous high concentration gradient across the front triggers the fast diffusion of hydrogen from the unburned region toward the surroundings of the flame, increasing the local availability of H 2 and keeping the gas above the crossover temperature ∼1000 K, the temperature below which the chemical reaction cannot proceed [33][34][35]. The additional energy released by the burning of this extra fuel is used to counteract conductive heat losses, extending hydrogen combustion toward ultralean mixtures below %H 2 < 5.…”

Characterization of unconventional hydrogen flame propagation in narrow gaps

“…[10][11][12] support that diffusivity determines the distance of unburned pockets between the flame fingers. The diffusivity should counteract heat losses in Hele-Shaw cells making the flame hotter by the diffusion transport of deficient components towards the reaction zone [10,11,33].…”

A one-step reduced mechanism for near-limit hydrogen combustion with general stoichiometry

“…The associated H 2 -oxidation rate was found to be equal to that of the elementary three-body reaction 4f , involving the concentration of H, which was determined by solving exactly the radical steady-state equations stemming from the 7-step mechanism, thereby providing an explicit expression for the global oxidation rate in terms of the concentration of main species and temperature [1]. The resulting one-step model has been successfully applied to H 2 -O 2 combustion problems involving very lean mixtures, including computations of burning rates [2], flame ball structures [3], and predictions of flammability conditions [4]. This brief communication extends these previous ideas, developed for lean combustion, to systems of general stoichiometry, under conditions such that, as a result of dilution of the reactants with inert species (e.g., by exhaust-gas recirculation), the peak temperatures are not far above the crossover value at which the rate vanishes according to the reducedchemistry description, the calculated concentrations of all radicals going to zero because of the absence of low-temperature chemistry.…”