Hydroxyl radical concentrations and reactant temperature profiles during oscillatory ignition of hydrogen: experimental measurements by laser resonance absorption spectroscopy and comparisons with numerical calculations

Abstract

Laser-induced absorption at wavelengths in the vicinity of 306 nm, at which electronic excitations of the hydroxyl radical occur, has been used to measure absolute concentrations of the hydroxyl radicals and the gas temperature during the course of the oscillatory ignition of hydrogen in a jet-stirred, flow reactor. Experiments were carried out on stoichiometric mixtures of hydrogen and oxygen at a mean residence time of 1.2 s and a total reactant pressure of 16 Torr. Oscillatory ignition occurs at vessel temperatures in excess of 710 K. Supplementary numerical studies were also carried out to compare predicted free radical concentrations and gas temperatures with the experimental measurements. A microcomputer-controlled system was used to trigger the pulsed laser each time an ignition occurred. From the study of very long trains of oscillations (greater than 1000), either the absorption spectrum could be scanned at a constant reaction time with respect to the trigger signal, or, alternatively, the temporal variation of the absorption signal at a fixed wavelength could be studied. The hydroxyl radical concentrations and the gas temperature were deconvoluted from these signals via the relative population densities of different absorption bands of the spectrum. Under the present experimental conditions the highest measured hydroxyl radical concentrations corresponded to 7.3 x 10 15 cm -3 . This is within a factor of two of the calculated concentration. The measured hydroxyl radical concentration not only rose to a maximum within 1.5 ms of the onset of ignition, but also decayed rapidly, falling to below 20% of the maximum absorption signal within ca 10 ms. The gas temperature rose simultaneously with the free radical concentration, to reach a measured maximum in excess of 2000 K, but its decay was very much slower than that of the radicals. The measured temperatures were approximately 500 K lower than those predicted numerically. The shapes of the measured temporal profiles for both concentration and temperature were in excellent agreement with the numerical calculations, and we believe that improvements of experimental technique could lead to closer quantitative accord between the absolute magnitudes.