A kinetic criterion of flammability limits: The C-H-O-inert system

An experimental and theoretical investigation has been conducted on the determination of the flammability limits of the C-H-O-inert system and on the understanding of limit phenomena in general. Experimentally, flammability limits have been determined by first measuring the extinction limits of stretched, counterflow flames and extrapolating the results to zero stretch. Consequently, lean and rich flammability limits have been determined for mixtures of methane, ethane, ethylene, acetylene, and propane with air, for mixtures of H₂, H₂/CH₄, and H₂/CO with O₂/N₂, and for the effects of dilution, inert substitution chemical additives such as CH₃Br and H₂, and radiative heat loss due to flame broadening. By further hypothesizing that the limit phenomena are primarily controlled by the kinetic processes of chain branching gersus termination, a predictive theory has been, advanced for the a priori determination of flammability limits. Calculated results largely agree with the experimental data, for both the lean and rich limits, except for excessively thick flames for which the limits could be qualitatively affected by radiative heat loss. The study further show that H+O₂O+OH is the dominant branching, reaction for all lean and rich limits, that H+O₂+MHO₂+M is the dominant termination reaction for all lean limits, that the dominant termination reaction for rich limits can be mixture specific, and that as the flammability limit is approached the maximum termination rate occurs in the same physical region as that of the maximum branching rate, thereby allowing for the most efficient radical scavenging. Pressure effects on rich limits and the concept of limit temperature are also interpreted based on the present theory and understanding.