Temperature measurements in low-pressure, diamond-forming, premixed flames

Measurements of temperature in low-pressure, diamond-forming, premixed flames are described. Optical emission spectroscopy was used to determine the rotational temperature of CH radicals in the flame by analyzing the spectra of the R-branch of the A 2Δ→X 2Π(0,0) CH electronic transition near 430 nm. Emission spectroscopic measurements were taken in stagnation-point flames of three different fuel types; CH4–O2, C2H4–O2, and C2H2–O2 flames, all of which have been previously used in the synthesis of diamond films. The emission-based rotational temperatures are found to be in good agreement with temperature measurements from thermocouples for the CH4–O2 flame, when a correction is made to the thermocouple measurements for radiation losses. The measurements show that peak flame temperatures are higher than the adiabatic flame temperatures for both C2H4–O2, and C2H2–O2 flames; a result that is also predicted by a numerical simulation of the stagnation-point strained flame that accounts for detailed finite-rate gas-phase chemistry and heterogeneous surface reactions.