Ion Precursors, chemi-on formation and electron densities in shock-induced methane combustion

Abstract

The reflected shock region of a conventional shock tube was used to study chemi-ion formation and related processes during homogeneous methane combustion in the temperature range of 1600°K to 2300°K at reactant pressures of one atmosphere. Absolute concentrations and induction times of positive ions and electrons were measured in addition to time-resolved profiles of ions and ion-precursors. Langmuir probe measurements revealed that positive-ion concentrations in this uniform system were considerably higher than observed in burner flames. Microwave measurements of electron concentrations indicated an excess of positive ions over electrons in the shock tube. Part of the observed electron deficiency is attributed to diffusion of the mobile electrons out of the observation region although negative ions may also play a significant, but as yet not well-defined role. The temperature dependencies of the experimentally determined positive ion and electron induction times were nearly identical. Time-resolved spectrographic measurements of CH, CH * , OH, and OH * taken simultaneously with the probe ion-current indicated that CH and not CH * is the probable ion precursor. In a numerical analysis of the kinetics, generally accepted ionization steps were combined with independently established methane reaction mechanisms and a parametric analysis of the ion precursor reactions was made. The combined reaction scheme matches the experimental data of species profiles and concentrations.