OH and CH2O Measurement and Simulation Through Local Extinction in Nonpremixed Flames

Abstract

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.