| The majority of natural gas, composed primarily of methane (CH;Thus, the concentration and temperature of various molecules (indicated in parenthesis) as they exist in Bunsen flames and lean-premixed combustors burning mixtures of CH;The different diagnostic techniques each have their advantages and disadvantages. The absorption techniques were versatile at measuring both low- and high-level concentrations of CO, and when used in conjunction with computer tomographic reconstruction techniques, they also provide spatially resolved measurements. Raman and Rayleigh scattering proved to be perhaps the most powerful techniques due to their ability to simultaneously measure a wealth of different molecules and to provide an extremely accurate measurement of temperature while providing excellent temporal and spatial resolution. Raman scattering, however, is a weak process, thus the detection of anything much less than a percent mole fraction at flame temperatures is difficult. For species which require a higher degree of sensitivity, we employ laser induced fluorescence (LIF), which proved to be highly sensitive for OH and NO, but requires the simultaneous measurement of the temperature and species concentration using Raman-Rayleigh scattering to correct for the effects of collisional quenching. The feasibility of two-photon LIF (TPLIF) for the measurement of CO was also examined. TPLIF proved to be a very promising diagnostic, with the potential for sensitive point measurements without the problems associated with collisional quenching that OH and NO have. TPLIF however, requires large laser irradiances to promote high rates of photo-ionization which permit operation in a linear fluorescence regime; as a consequence, the detection sensitivity of the TPLIF of CO is not as high for the single photon LIF of OH and NO. |
