Temperature profile measurements in the near-substrate region of low-pressure diamond-forming flames

Coherent anti-Stokes Raman scattering (CARS) spectroscopy of diatomic hydrogen was used to measure gas-phase temperature profiles in low-pressure, stagnation-flow, diamond-forming flames. The objectives of the study were to measure detailed temperature profiles for comparison with a numerical flame model and to investigate the presence and magnitude of the temperature jump at the deposition substrate surface. At distances of several hundred microns or greater from the deposition substrate, the measured temperatures were in excellent agreement with the results of a numerical flame model incorporating elementary chemical kinetic and multi-species transport models. The estimated hydrogen CARS temperature accuracy was +/-4% near the deposition substrate and was +/-5% away from the substrate. The estimated spatial resolution of the measurements was 30 mum normal to the surface, and the gradient in temperature was resolved fully up to approximately 25 mum from the surface. The excellent spatial resolution for the near-surface measurements allowed the investigation of the presence and the magnitude of the discontinuity between the gas and surface temperature at the deposition substrate, a phenomenon known as the temperature jump. Temperature jumps of approximately 100 K were observed in these rich, premixed oxy-acetylene flames ranging from 30 Torr. to 125 Torr. The influence of gas flow rate and pressure on the temperature jump was investigated. The observed temperature jumps are consistent with an accommodation co-efficient in the range of 0.1 to 0.2 for the exchange of energy between the diamond-forming flame gases and the molybdenum surface or the diamond film on the deposition substrate. In these low-pressure flames, the ability to fully resolve the near-substrate temperature profiles will be extremely useful for the validation and improvement of surface chemistry models. (C) 2002 by The Combustion Institute.