Two-color, two-photon laser-induced polarization spectroscopy (LIPS) measurements of atomic hydrogen in near-adiabatic, atmospheric pressure hydrogen/air flames

摘要

Two-color, two-photon laser-induced polarization spectroscopy (LIPS) of atomic hydrogen has been demonstrated and applied in atmospheric pressure hydrogen/air flames. Fundamental and frequency-doubled beams from a single 486-nm dye laser were used in the experiments. The 243-nm pump beam in the measurements was tuned to the two-photon n = 1 → n = 2 resonance of the hydrogen atom. The 486-nm probe beam was tuned to the single-photon n = 2 → n = 4 resonance of the hydrogen atom. Measurements were performed in an atmospheric pressure H_2/air flame stabilized on a near-adiabatic, flat-flame calibration burner (the Hencken burner). For the range of pump beam intensities used, the LIPS signal was found to be nearly proportional to the square of the pump beam intensity over a wide range of flame equivalence ratios. Spectral lineshapes were recorded at flame equivalence ratios ranging from 0.85 to 2.10. Vertical H-atom number density distribution profiles were measured in the Hencken burner. The vertical H-atom number density profiles measured along the burner centerline for various flame equivalence ratios were compared with the results of a numerical flame calculation using the UNICORN (Unsteady Ignition and Combustion with Reactions) code. Good agreement between theory and experiment was obtained for stoichiometric and rich flame conditions. For flames with equivalence ratios greater than 1.5, the H-atom concentration was substantially above the adiabatic equilibrium value, even at 50 mm above the burner surface. The slow approach to the adiabatic equilibrium H-atom concentration value can be explained by assuming partial equilibrium in the postflame gases; the H-atom concentration is proportional to the O_2 concentration which requires significant residence time to decrease to its very low equilibrium concentration. These results suggest that the use of the Hencken burner as a radical measurement technique calibration source may be of questionable value for equivalence ratios greater than 1.5 and less than 0.8.