Femtosecond LIF Imaging of Reactive Radicals Near Impinging Flame Walls at Low Pressure

O-atom is a key species for many combustion reactions. Hydrogen-air flames are utilized in many fields for generating high flame temperatures, producing zero carbon emissions, and material synthesis. Accordingly, there is a necessity for methodologies for experimentally determining quantitative O concentrations in these flames using non-intrusive measurement techniques. Additionally, prioritizing methods that allow for spatial determination of O concentrations can be useful for understanding reaction chemistry and species production near flame walls and substrates. A common quantitative method of comparing the fluorescence signal of a known quantity of a noble gas, such as Xe, to that of an unknown amount of O, has previously been applied to flames. However, a pivotal aspect of that method requires the measurement of the LIF decay rate of the species being compared, which is typically performed in inert environments or plasmas. To avoid using decay rates measured in these non-representative conditions, experiments have been performed in this study to measure decay rates directly within methane flames by using a femtosecond two-photon laser-induced fluorescence method. These measurements have been made previously with a nanosecond laser system, which significantly limits the pressure range at which decay rates can be measured due to its long pulse durations. In this work, a femtosecond laser is used to avoid these limitations and directly measure decay rates within H2-Air flames and subsequently spatially map O-atom concentration. Peak O mole fractions of over 0.4% were detected 4.2 mm above the burner surface. An impinging disk situated above the flame showed a near-linear decrease in concentration up to the surface of the disk, where non-uniform distributions of around 0.2% O mole fraction were noted.