Ignition Of Hydrogen-air Mixtures Research Articles

Inhibition of hydrogen–oxygen/air gaseous detonations using CF3I, H2O, and CO2

The ignition of hydrogen-air mixtures can cause intense explosions. To ensure the safe utilization of hydrogen fuel and prevent catastrophic explosion accidents, the inhibiting effect of flame inhibitors must be understood since they have important consequences for hazard analyses. In the present study, the detonability of hydrogen mixtures diluted with CF3I, H2O, CO2, and N2 is investigated using numerical computations. The inhibiting effect of chemically active (CF3I, H2O, and CO2) and chemically inert (N2) diluents on the chemical length and time scales of a ZND detonation structure was studied using a detailed chemical kinetic mechanism for hydrogen oxidation. The inhibition efficiency of a halogenated compound, CF3I, towards quenching of hydrogen-air/oxygen gaseous detonations was also studied. As an additive, CF3I was found to promote detonation at lower concentrations. However, at higher concentrations, CF3I suppresses gaseous detonations by acting as a radical-scavenging agent. In the present study, a given retardant's effectiveness for suppressing a given detonation wave was evaluated for CF3I, H2O, CO2, and N2. The combined suppression effects of CF3I, H2O, and CO2 were also investigated. The efficacy of a composite inhibitor towards the quenching of a detonation wave was also evaluated and compared.

Ignition of hydrogen-air mixtures under volumetric expansion

A better understanding of chemical kinetics under volumetric expansion is important for a number of situations relevant to industrial safety including detonation diffraction and direct initiation, reflected shock-ignition at obstacles, ignition behind a decaying shock, among others. The ignition of stoichiometric hydrogen-air mixtures was studied using 0D numerical simulations with time-dependent specific volume variations. The competition between chemical energy release and expansion-induced cooling was characterized for different cooling rates and mathematical forms describing the shock decay rate. The critical conditions for reaction quenching were systematically determined, and the thermo-chemistry dynamics were analyzed near the critical conditions.

Self Ignition of Hydrogen-Air Mixtures with Inclined Porthole Injection in Supersonic Flows

Experiments were performed in the T4 shock tunnel to investigate the self ignition of hydrogen in a supersonic air stream. Hydrogen was injected into the flow over an inclined flat plate for oncoming Mach numbers of 7.9 to 8.0. The nozzle-supply enthalpy was kept between 3.1 and 3.4 MJ/kg and two different pressure levels were used in the tests. Measurements of surface pressures were used to infer the location of ignition but only small pressure increases were obtained when combustion occurred. Therefore multiple tests at nominally the same condition were used so that statistical methods could be used to identify the ignition lengths. The ignition lengths of the hydrogen air mixture directly behind a strong leading edge shock indicate that Pergament's method is able to predict the ignition length to within 35% for the observed autoignition over the range of conditions tested.

Boundary layer ignition of hydrogen-air mixtures in supersonic flows

Due to viscous heating spontaneous ignition of a supersonic flow of premixed combustible gases can occur in boundary layers. This process is studied numerically for a hydrogen-air mixture in the case of a laminar boundary layer over a flat plate. In a previous study the main structure of the reacting flow was given as well as a first mapping of the ignition conditions versus boundary conditions. In the present work computations are performed in order to further specify the controlling mechanisms and parameters of such a boundary layer ignition. We emphasize more precisely i) the elementary steps of the chemical process which effectively control the ignition ii) the unusual role played by the equivalence ratio of the mixture iii) the influence of the Soret effect (species transport due to temperature gradients).

Analysis of the thermal diffusion effects on the ignition of hydrogen-air mixtures in the boundary layer of a hot flat plate

In this article the steady-state process leading to ignition of a combustible mixture of hydrogen, oxygen, and nitrogen by a hot flat plate in a boundary layer flow is studied, including the effect of thermal diffusion. For sufficiently large values of the plate temperature, the ignition event corresponds to a typical chain-branching explosion with negligible heat release, in a first approximation. In the framework of the reduced kinetic mechanism appropriate for this regime, the boundary layer equations are solved by using the fact that the chain branching reaction H + O 2 → OH + O has a relatively large activation energy. Assuming a three-layer structure, the governing equations reduce to a single integrodifferential equation for the concentration of atomic hydrogen. Close to the plate leading edge, the concentration of atomic hydrogen around the plate surface is higher when thermal diffusion is included, but, later on, this effect enhances the evacuation of atomic hydrogen by the external convective/diffusive layer. Thus, in spite of the initial enrichment in molecular hydrogen by thermal diffusion, the final ignition distance is shown to be enlarged by a factor of about 2, under usual conditions. On the other hand, for low plate temperature ignition, thermal diffusion produces the expected reduction in the ignition distance.

Effect of multipoint ignition of hydrogen-air mixtures on the development of an explosion in a closed vessel

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