Spectroscopic measurement of nitric oxide in a diffusion flame
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Conventional measurements of NO and NO₂ produced by a diffusion flame around a cotton ball wetted by heptane have been performed and prove that NO is oxidized to NO₂ on a mole for mole basis when the air of the flame is doped with hydrogen and that the NO to NO₂ mechanism does not require carbon atoms in the dopant. In-situ spectroscopic measurements of NO in a laminar H₂-air diffusion flame were performed and compared to data obtained with probe sampling procedures. Ultraviolet absorption of the (1,0) gamma bands of nitric oxide near 214.8 nm were used for the spectroscopy. Spectroscopic measurements were possible only when the air stream was seeded with ca. 100 ppm NO. A conventional sampling system was operated at a probe pressure of 0.3 atmosphere and was used to sample from both the high temperature combustion zone and relatively cool regions on both sides of the flame. Spectroscopic and probe measurements of NO agree to within 30%, with probe concentrations being greater. The air of the flame was doped to give 1200 ppm methane and the NO concentrations were measured again, using probe and spectroscopic techniques. Both techniques confirm that even small unburned hydrocarbon concentrations cause the disappearance of NO on the air side of the visible reaction zone.
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