Quantitative atomic hydrogen measurements in premixed hydrogen tubular flames

Quantitative measurements of atomic hydrogen are reported in laminar premixed tubular flames using femtosecond two-photon laser-induced fluorescence. The H-atom fluorescence is corrected for collisional quenching by using local values of major-species concentrations and temperature measured by spontaneous Raman scattering. Lean hydrogen flames are sustained with two different diluents (N-2, CO2) to investigate low-Lewis number flames under high curvature. When compared to planar stretched flames, the curved tubular flames enhance the H-atom concentration and temperature through increased preferential diffusion. Peak H-atom number densities on the order of 10(15) per cm(3) are measured, and absolute H-atom profiles show differences up to 40% in peak number density and flame radial position when compared to the pseudo one-dimensional flame model with detailed chemistry and transport. Although the overall agreement in absolute H-atom profiles is good considering the relative uncertainties in the model and experiment, the differences suggest the ability for this flame geometry to provide evidence for revision of molecular transport and chemical kinetic modeling in flames with substantial preferential diffusion. Peak absolute H-atom concentrations vary up to 30% depending on the assumed temperature dependency for the collisional quenching factors pointing to the need for high-temperature data for H-atom collisional quenching. Relative H-atom profiles in the flames are minimally affected by the collisional quenching. (c) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.