OH and CH₂O measurement and simulation through local extinction of nonpremixed flames

A local extinction point in a nonpremixed flame sheet was stabilized using a co-annular counterflow flame geometry that has been previously described. Measurements of hydroxyl planar laser-induced fluorescence (PLIF) and formaldehyde PLIF were made at the local extinction point. These measurements were used to estimate the OH+CH₂O forward reaction rate and to assess a method of approximating scalar dissipation rate based on CH₂O PLIF width in the presence of local extinction. In addition, two-dimensional numerical simulations of the flame and extinction point were completed using a comprehensive chemical kinetic model. Since the local temperature at the extinction point decreases strongly, the measured PLIF profiles include significant dependence on Boltzmann fraction, particularly for CH₂O. The simulated species concentrations were modified to include the temperature-dependent variations that could not be removed from the measurements and to assess the impact of these corrections on interpretation of the measured data. The unknown effects of CH₂O fluorescence quenching in the measurements are also assessed by comparing a range of quenching models in the simulations at the extinction point. The simulated and measured profile shapes are in good agreement, but the simulations show a stronger quantitative increase in the OH+CH₂O reaction prior to extinction and a thinner reaction rate at the centerline than observed in the measurements.