Application of electronic-resonance-enhanced coherent anti-Stokes Raman scattering to nitric oxide measurements in flames.

摘要

Nitrogen oxides (NOx) are important air pollutants generated primarily from combustion of fossil fuels for energy and heat production. Nitric oxide accounts for the majority of total nitrogen oxides emissions because of the high temperatures involved in the combustion processes. Nitric oxide is also an important molecule in biology where it acts as a signaling molecule in the cardiovascular system. The governments of many countries are enforcing stricter regulations on NO emissions. In-situ measurements of NO are crucial for testing advanced combustor designs and pollution control strategies. Non-intrusive optical techniques such as LIF and CARS stand out as viable methods for NO measurements in combustion systems at high pressures. LIF has been applied successfully as a combustion diagnostic, along with a powerful variant, PLIF. Recently, quantitative LIF measurements have been performed at pressures up to &sim15 atm. CARS, on the other hand, provides accurate flame temperature measurements and also quantitative concentration measurements for major species.Combustion in modern gas turbines normally occurs at high pressure and temperature with a high luminescent background, thus posing special difficulties for measurements of temperature and concentrations. LIF suffers from line-broadening, attenuation of the excitation beam and signal owing to absorption, and interferences from other species (O2 for fuel-lean conditions, hydrocarbons or soot for fuel-rich conditions). These effects have been identified and investigated in depth. CARS, however, is a third order non-linear optical process. The CARS signal is proportional to the square of number density of the molecule under investigation. However, the detection limit for ns-CARS is restricted to 1%, even with a polarization-sensitive technique, owing to the small cross-sections for Raman scattering. By tuning one or more laser beams into resonance with appropriate electronic transitions, the CARS signal can be enhanced by orders of magnitude. This feature makes electronic-resonance-enhanced (ERE) CARS a promising technique for trace species measurements.The study conducted in this thesis focuses on ERE-CARS measurements of NO in atmospheric flames and under high pressure conditions. ERE-CARS measurements of acetylene(C2H2) are also discussed as another demonstration of this technique. Two different ERE-CARS systems for probing ro-vibrational Raman transitions are developed including their applications to flame measurements.The time-dependent density matrix equations for the nonlinear ERE-CARS of NO are derived and manipulated into a form suitable for direct-numerical-integration(DNI). Collisional effects including collisional dephasing of the coherence and collisional quenching of the A2Sigma+ electronic level are considered in the model. Two types of saturation are examined: (1) saturation of the two-photon Raman-resonant Q-branch transitions and (2) saturation of the one-photon electronic-resonant P-, or Q- or R-branch probe transition in the A2Sigma+ - X2pi electronic system. The interaction between the Raman transition and the electronic transition shows that they are coupled and that the saturation of either transition depends on the level of saturation of the other transition. The calculations show that the integrated ERE-CARS signal is insensitive to the rates of collisional electronic quenching and collisional dephasing. The insensitivity to collisional dephasing rates is more evident under saturation conditions. Future research efforts are proposed for further understanding and evaluation of the ERE-CARS technique.