The chemistry involved in the third explosion limit of H2–O2 mixtures

Abstract

a b s t r a c tThe third explosion limit of hydrogen oxidation in closed vessels has always been thought to be the result of the competition between homogeneous gas-phase reactions and diffusion of hydroperoxyl radicals to the walls, where they are destroyed. It has recently been observed that this species actually follows a chemical-kinetic steady state in this regime, with the consequence that its diffusive rate toward the catalytic walls becomes irrelevant. Here we show that the critical explosion conditions are determined instead by the fate of hydrogen peroxide, which emerges as the controlling reactant for the resulting gas-phase chemistry. A simple, accurate analytic expression for the third explosion limit follows from identification of the critical conditions for existence of weakly reactive, diffusion-reaction solutions, thereby providing the answer to a long-standing problem that in early work was characterized as being hopelessly difficult.

Thermal explosions in spherical vessels at large Rayleigh numbers

“…Using the value of Da c together with the definition of the Damkhöhler number leads to explicit expressions for critical explosion sizes, giving results in close agreement with experimental observations, a remarkable achievement of the early theory, given the many different simplifying assumptions involved in its derivation [2]. This success has motivated recent extensions of the early theory incorporating realistic chemistry in descriptions of hydrogen-oxygen systems that have been shown to predict explosion conditions in spherical vessels in excellent agreement with experiments [3], including critical pressures along the so-called third-explosion limit [4]. The FK problem was recently revisited in [5] to investigate influences on ignition times of initial conditions and of temporal pressure variations in spherical vessels with fixed walls.…”

Explicit formulation of the reactivity of hydrogen, methane and decane

“…Surely a different choice of reactions would be required for lower temperature conditions. For instance, as shown in [20,21], H 2 ignition processes below the second explosion limit are governed by a different set of reactions. Nonetheless, this study suggests that whichever the choice of reactions, the positive eigenvalue of the chemical Jacobian may be approximated as the solution of a low-order polynomial, so that deriving explicit expressions for characteristic times in different conditions, or even different fuels should be straight-forward.…”

“…Reciprocally, below crossover, the chainbranching remains ''frozen'' by the termination step which consumes the H radical. The reaction path is then fundamentally different, and consideration of HO 2 and H 2 O 2 becomes necessary [20,21]. The characteristic polynomial of matrix A reads:…”

Effects of heterogeneous–homogeneous interaction on the homogeneous ignition in hydrogen-fueled catalytic microreactors

“…Note that the latter (high pressures) is far in excess of the second explosion limit of hydrogen-air mixtures [79,80]. In addition, in the previous works of Mantzaras et al [64] and Ghermay et al [66] who computed the homogeneous ignition delays for hydrogen-air mixtures in a constant pressure batch reactor using the SENKIN package of CHEMKIN for initial temperatures ranging from 950 to 1250 K, the homogeneous reactivity of hydrogen is not a monotonic quantity of the pressure, but there is a monotonic increase of homogeneous reactivity or a monotonic decrease of homogeneous ignition delay with increasing pressure at equivalence ratio 4 ¼ 0.3 and initial temperatures above 1200 K. Therefore, for the stoichiometric mixture in the range of higher temperatures discussed in the present work, a simple assumption is made that the homogeneous reactivity of hydrogen is enhanced or the homogeneous ignition delay is decreased with increasing pressure.…”