Heat release and radical recombination in premixed fuel-lean flames of H2+ O2+ N2. Rate constants for H + OH + M → H2O + M and HO2+ OH → H2O + O2

Abstract

The temperatures along the burnt gases of several oxygen-rich flames of H₂+ O₂+ N₂ have been measured. They show there is a rise of some 300–1200 K in the early part of the burnt gases. A detailed evaluation of all the chemical reactions proceeding in such fuel-lean flames has been carried out. In addition, the kinetics of these reactions, especially of radical recombination, have been formulated mathematically. In these flames an equilibrated pool of the minor species O, OH, H and H₂ is rapidly established. Early in the burnt gases radical recombination proceeds via H + OH + M → H₂O + M (V) and removes species from this pool. Later in the burnt gases, recombination also occurs in the series of reactions: H + O₂+ M ⇄ HO₂+ M (VI), OH + HO₂→ H₂O + O₂(X) the net overall effect of which is identical to reaction (V) . The fact that heat is released by reaction (V) proceeding either directly or by this catalytic sequence of reaction (VI) followed by reaction (X) enables the total concentration of species in the pool of recombining radicals to be deduced from the measured temperature profiles along each flame. In addition, concentrations of each individual member (i.e. O, OH, H and H₂) of the pool are obtained, both directly and in terms of Sugden's disequilibrium parameters. The measured rates of disappearance of species from the pool are consistent with rate constants for reactions (V) and (X), respectively, of k₅= 1.0 × 10^–28/T cm⁶ molecule^–2s^–1 and k₁₀= 1.0 × 10^–10 cm³ molecule^–1 s^–1. In each case there are associated errors of 50%.