Modeling study of the acceleration of ignition in ethane-air and natural gas-air mixtures via photochemical excitation of oxygen molecules

The paper addresses the study of the influence of laser-induced excitation of O2 molecules to the b1Δg+ electronic state and their photodissociation by laser photons on the ignition of ethane and natural gas in air. The extended reaction mechanism for the oxidation of CH4-C2H6-air mixture comprising excited O2(a1Δg), O2(b1Σg+) molecules and O(1D) atoms has been built. This mechanism includes recent ab initio data on the interaction of C2H6 with O2(a1Δg) molecule. The cross section of laser radiation absorption at given wavelength was calculated in a detailed manner by taking into account the effects of temperature, pressure and contribution of several nearest spectral lines. The computations have shown that both considered photochemical methods accelerated the ignition and decreased the ignition temperature. They are much more effective in the ignition enhancement than mere heating the mixture by equivalent input energy. It has been revealed that the photodissociation of O2 molecules by laser photons with λI = 193.3 nm was somewhat more effective in accelerating the ignition of the C2H6-air mixture, especially, at low temperatures (T0 < 850 K) and high pressure (P0 = 10 atm). However, at smaller pressure (P0 = 1 atm) both for ethane and for natural gas, the excitation of O2 molecules to the singlet sigma state provides higher reduction in the induction time and ignition temperature compared to their photodissociation.