Study of ignition and detonation of hydrocarbon-air mixtures with detailed and reduced chemical mechanisms

Uncertainties in the chemical-kinetic processes that take place in detonations lead to difficulties in obtaining fundamental knowledge about detonations and in facilitating investigations of practical devices like Pulse Detonation Engines. This research is focused on reducing the chemical-kinetic uncertainties and developing simplified chemical-kinetic descriptions for use in detonation studies. The fuels investigated are acetylene, ethylene and JP-10. Conditions addressed cover initial (post-shock) temperatures between 1000 K and 2500 K, pressures between 0.5 bar and 100 bar and equivalence ratios between 0.5 and 2. An existing detailed mechanism is extended to 175 steps among 37 chemical species by evaluating the rates of several additional reactions relevant for acetylene, ethylene and JP-10 combustion. This mechanism is tested extensively with data from shock-tube studies and flame-speed measurements.; Based on the detailed mechanism, short mechanisms are derived for ignition and detonation of acetylene and ethylene in air. Application of steady-state and partial-equilibrium approximations leads to further systematic reduction. A seven-step reduced mechanism is obtained for acetylene detonations, four of which are important during the induction stage and the remaining three are important for the slower carbon-monoxide oxidation and radical-recombination processes that follow the induction stage. The theory of chain-branching thermal explosions is developed using activation-energy asymptotics and is applied for acetylene ignition, leading to an expression for ignition time. For ethylene, the strong dependence of the chemistry on initial temperatures and pressures complicates analysis and leads to identification of separate reduced mechanisms for high and low temperatures. Expressions for ignition time in terms of the elementary reaction rates are also derived. For JP-10 ignition, a reaction set including 27 additional reactions is proposed, which involves overall approximations in breaking the large hydrocarbon into smaller hydrocarbons. Reduced-chemistry descriptions are also explored for JP-10 ignition.; Based on such detailed studies, a generalized two-step mechanism is proposed for ignition and detonation of acetylene, ethylene, propane and JP-10. The first step in this mechanism corresponds to the induction stage and the second step to the slower radical-recombination stage. The rates of these two steps are given in terms of the elementary reaction-rate parameters. Although deficient for addressing some details of detonation processes, this mechanism is expected to be useful in understanding the interplay between gas dynamics and chemical kinetics and in obtaining quantitative predictions for detonation applications.