Hydrogen-air mixing-layer ignition at temperatures below crossover

Abstract

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.

Weak-Shock Interactions with Transonic Laminar Mixing Layers of Fuels for High-Speed Propulsion

“…7and (8) and subject to the boundary conditions U 1, T 1, and Y 0 as η → ∞ and U U 2 , T T 2 , and Y 1 as η → −∞, together with the additional boundary condition M M 1 UR 1∕2 1 at η 0, stating that the arbitrary origin of the transverse coordinate η is selected to be the sonic point. The resulting description is similar to that presented in a previous analysis of transient hydrogen-air mixing [30]. A distinguishing feature of the present analysis is the inclusion of the last two terms in Eq.…”

Autoignition dynamics of DME/air and EtOH/air homogeneous mixtures

“…Since the conclusions drawn with this methodology are limited to the neighborhood of the specific operating conditions considered, the analysis has to be assessed or even repeated for any significantly different set of operating conditions. Although these mathematical tools are still successfully employed [7][8][9], their usefulness is questionable when they have to deal with the very large chemical kinetics mechanisms that have lately been of interest. The development of algorithmic methodologies for the construction of reduced models facilitated considerably the analysis of autoignition processes.…”

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.…”