Hydrogen–oxygen Induction Times Above Crossover Temperatures

Williams

2012

Self Cite

This paper addresses homogeneous ignition of hydrogen-oxygen mixtures when the initial conditions of temperature and pressure place the system below the crossover temperature associated with the second explosion limit.A three-step reduced mechanism involving H 2 , O 2 , H 2 O, H 2 O 2 and HO 2 , derived previously from a skeletal mechanism of eight elementary steps by assuming O, OH and H to follow steady state, is seen to describe accurately the associated thermal explosion. At sufficiently low temperatures, HO 2 consumption through HO 2 + HO 2 → H 2 O 2 + O 2 is fast enough to place this intermediate in steady state after a short build-up period, thereby reducing further the chemistry description to the two global steps 2H 2 + O 2 → 2H 2 O and 2H 2 O → H 2 O 2 + H 2 . The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be used in describing the resulting thermal runaway, yielding an explicit expression that predicts with excellent accuracy the ignition time for different conditions of initial temperature, composition, and pressure.

Section: Reduced-chemistry Descriptionmentioning

confidence: 99%

Section: Reduced-chemistry Descriptionmentioning

confidence: 99%

See 1 more Smart Citation

Explicit analytic prediction for hydrogen–oxygen ignition times at temperatures below crossover

Boivin

Williams

2012

Self Cite

“…As shown in [2,3], hydrogen ignition below crossover takes place as a thermal explosion, controlled by slow reactions involving hydroperoxyl and hydrogen peroxide, whereas the radicals H, O, and OH, which dominate the branched-chain explosion that characterizes ignition above crossover [4], are consumed at a fast rate, with the result that these species follow closely a steadystate approximation. The most important elementary reactions are 2HO 2 M !…”

Section: Introductionmentioning

confidence: 99%

Hydrogen-air mixing-layer ignition at temperatures below crossover

Fernández-Tarrazo

Williams

2013

Self Cite

a b s t r a c tThis paper addresses ignition histories of diffusion flames in unstrained hydrogen-air mixing layers for initial conditions of temperature and pressure that place the system below the crossover temperature associated with the second explosion limit of hydrogen-oxygen mixtures. It is seen that a two-step reduced chemical-kinetic mechanism involving as main species H 2 , O 2 , H 2 O, and H 2 O 2 , derived previously from a detailed mechanism by assuming all radicals to follow a steady-state approximation, suffices to describe accurately the ignition process. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be employed for the analysis, following the ideas developed for mixing-layer ignition by Liñán and Crespo in 1976 on the basis of one-step Arrhenius model chemistry. When the initial temperatures of both reactants differ by a relative amount that is of the order of or smaller than the ratio of this temperature to the effective activation temperature, the chemical reaction is seen to occur at a significant rate all across the mixing layer. The ignition time is then determined as a thermal runaway in a parabolic problem describing the evolution of the temperature increment and the H 2 O 2 concentration, with local accumulation, chemical reaction, and transverse convection and diffusion, all being important. By way of contrast, when the air side is sufficiently hotter than the hydrogen side, as often occurs in applications, ignition occurs in a thin layer close to the air-side boundary, enabling a simplified description to be developed in which the ignition time is determined by analyzing the existence of solutions to a two-point boundary-value problem involving quasi-steady diffusion-reaction ordinary differential equations.

“…5, one can expect the steady-state approximation for all four intermediates to provide a very accurate description for φ = 0.3 and less accurate results for φ = 0.5. This situation is different from that encountered in autoignition, in which HO 2 is not in steady state, OH and O obey good steady states only under fuel-rich conditions, and the H steady state is accurate only for φ 0.05 [18].…”

Section: One-step Reduced Kineticsmentioning

confidence: 77%

One-step reduced kinetics for lean hydrogen–air deflagration

Fernández-Galisteo

Liñán³

et al. 2009

A short mechanism consisting of seven elementary reactions, of which only three are reversible, is shown to provide good predictions of hydrogen-air lean-flame burning velocities. This mechanism is further simplified by noting that over a range of conditions of practical interest, near the lean flammability limit all reaction intermediaries have small concentrations in the important thin reaction zone that controls the hydrogen-air laminar burning velocity and therefore follow a steady state approximation, while the main species react according to the global irreversible reaction 2H 2 + O 2 → 2H 2 O. An explicit expression for the non-Arrhenius rate of this one-step overall reaction for hydrogen oxidation is derived from the seven-step detailed mechanism, for application near the flammability limit. The one-step results are used to calculate flammability limits and burning velocities of planar deflagrations. Furthermore, implications concerning radical profiles in the deflagration and reasons for the success of the approximations are clarified. It is also demonstrated that adding only two irreversible direct recombination steps to the seven-step mechanism accurately reproduces burning velocities of the full detailed mechanism for all equivalence ratios at normal atmospheric conditions and that an eight-step detailed mechanism, constructed from the seven-step mechanism by adding to it the fourth reversible shuffle reaction, improves predictions of O and OH profiles. The new reduced-chemistry descriptions can be useful for both analytical and computational studies of lean hydrogen-air flames, decreasing required computation times.