Measurement of OH Density Profiles in Atmospheric-Pressure Propane-Air Flames

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Abstract
Hydroxyl radical (OH) density profiles as a function of height above the surface of a flat-flame burner have been determined for propane-air flames at three fuel-air equivalence ratios (0.63, 1.17, and 1.45) and for two total gas flow velocities at the richest equivalence ratio. The experiments have been performed using both absorption spectroscopy and laser-induced-fluorescence (LIF) imaging. The absorption and fluorescence profiles are in good agreement. Superequilibrium OH densities are observed near the flame zone with decay toward equilibrium occurring in the post-flame gases. An apparent second order kinetic dependence of this OH decay has been verified for the 𝚽 = 1.17 flame. The rate constant for the irreversible consumption of acetylene by OH has been determined to be 1.2(+0.3)x 10-12cm3/molecule-sec in the burned gas from the richest flame for temperatures of 1620-1700 K. The data suggest that the methyl radical is an important ultimate product formed in this reaction. The observed CO decay rate in the burned gas, measured by a quartz probe, is consistent with the OH density determination in the lean (𝚽= 0.63) flame.