A Microwave Investigation of the Ionization of Hydrogen-Oxygen and Acetylene-Oxygen Flames

Abstract

The ionization of premixed hydrogen‐oxygen and acetylene‐oxygen flames burning at atmospheric pressure was studied by a microwave method. The attenuation of electromagnetic radiation by the flames was employed to determine the free electron concentration. To obtain a high sensitivity for these measurements, a microwave bridge operating slightly off balance was employed, with the flames positioned between two electromagnetic horns in one of the bridge arms. This method permits the determination of electron densities down to about 5·10⁸ electrons per cm³ at atmospheric pressure. In interpreting the data obtained in this study it was found necessary to take a quantitative account of the ``background ionization'' resulting from the presence of alkali salts as contaminants in the air and the flame gases. After the appropriate background measurements and corrections were made, it was found that the ionization of the hydrogen‐oxygen flames and of lean and stoichiometric oxy‐acetylene flames was thermal and agreed well with the electron concentration values obtained from equilibrium calculations using the adiabatic flame temperatures. A complete equilibration and equipartition of the internal energy for H₂–O₂ flames is indicated by the good agreement between the effective ionization temperature and the rotational, vibrational, and electronic temperatures determined previously. The background corrected ionization of the rich acetylene‐oxygen flames (C₂H₂/O₂=1.5/1 to 4/1) was found to be higher by a factor of 10 to 100 than that calculated for a purely thermal flame ionization. This can very plausibly be accounted for on the basis that the high electron concentration in these rich hydrocarbon flames is primarily due to the thermal evaporation of electrons from the carbon atom aggregates (soot particles) formed in these flames. A calculation showed that a work function of 8.5 ev for the small carbon particles in the flame accounts well for all the observed electron concentrations. This value stands in a reasonable relationship to the ionization potential of gaseous carbon atoms (11.265 ev) and the work function of graphite (4.35 ev). It is suggested on the basis of these results that the ionization in hydrogen‐oxygen and acetylene‐oxygen flames, at least as far as the hot combustion gases are concerned, is an equilibrium process which can be accounted for by purely thermal considerations. Whether or not this holds true also for the reaction zone proper must remain open to question for the present, since the measurements described here pertain by necessity to an ionization density averaged over a rather extensive region of the flame.