Atomic hydrogen concentration profile measurements in stagnation-flow diamond-forming flames using three-photon excitation laser-induced fluorescence

Gas-phase atomic hydrogen concentration profiles were measured near the deposition substrate in atmospheric-pressure, stagnation-flow diamond-forming flames. In these flames, an acetylene-oxygen-hydrogen mixture accelerates through a nozzle and impinges on a water-cooled molybdenum substrate, stabilizing a flat-flame approximately 1 mm below the substrate. A thin, polycrystalline diamond film is deposited on the substrate under appropriate conditions of flame stoichiometry and substrate temperature. Three-photon-excitation laser-induced fluorescence (LIF) was used to determine the H-atom concentration at various points between the substrate and the incoming premixed jet. The estimated H-atom LIF accuracy is ±40–50% in the diamond-forming flames, and the estimated spatial resolution is ±100 μm perpendicular to the deposition surface. The LIF measurements show that the peak atomic hydrogen mole fraction is approximately 5%, significantly less than the calculated adiabatic equilibrium concentration. This subequilibrium mole fraction results from the slow rate of acetylene dissociation for the fuel-rich conditions in the post-flame gases and the creation of superequilibrium concentrations of CO2 and H2O in the reaction zone. The measured subequilibrium H-atom concentrations are consistent with our previous measurements of superequilibrium temperatures in the post-flame region of these flames. We present numerical calculations of species and temperature profiles in our diamond-forming flames. The measured subequilibrium H-atom concentration profiles are in good agreement with theoretical calculations of the profile. Measured peak flame temperatures are in general 100–200 K lower than calculated peak temperatures, but the calculated and measured profiles are in excellent agreement in the high-gradient region near the deposition substrate.