Fundamental Studies Of Flame Structure Through Laser Plasma Diagnostics

Increasing concerns about air pollution and global climate change are drawing attention to the need for efficiency improvements and emission reductions for combustion processes, which account for more than 85% of energy production in United States. Combustion efficiency and emissions are affected by the mixing and reacting of fuel and oxidizer. Understanding such behavior plays a critical role in flame structure studies and combustion optimization. However, experimentally obtaining mixture fraction, which is a widely used quantity to describe the mixing behavior, has proven to be a challenge, especially for heavier hydrocarbon fuels or fuel rich flames. Moreover, measuring flame temperature simultaneously with mixture fraction adds complexity into the experimental setup. In this dissertation, laser plasma diagnostics techniques were developed to provide a straightforward method to simultaneously obtain composition and temperature measurements. The capability of these novel techniques is applicable to more complex fuels and a broader range of equivalence ratios than has heretofore been possible, and facilitates a better understanding of flame structure. Laser-induced breakdown spectroscopy (LIBS) is proposed as an alternative method of measuring mixture fraction. A back-scattering setup is utilized to mitigate the beam steering effects in non-uniform and unsteady flames. The calibration for the LIBS system was completed in an ethylene-air premixed flame under a broad range of equivalence ratios. The elemental species distributions for H, C, N, O were measured in a counter-flow diffusion flame. The measured mixture fraction compared favorably with the numerical results from OPPDIF flame code. On the basis of LIBS measured elemental species profile, the preferential diffusion effect was analyzed. Utilizing the sound emission from laser-induced plasmas, acoustic-based laser induced breakdown thermometry (LIBT) was developed as a novel method for flame temperature measurement. The established correlation between the optical emission and acoustic emission in a premixed flame demonstrated that the acoustic signal can serve as an internal standard in the gas phase LIBS measurement. The influences of flame temperature and composition on the acoustic signal were investigated independently. The composition effect was found to be second order comparing to the temperature effect. The statistics of the LIBT measurement were also analyzed to better understand the distribution of samples. Furthermore, the temperature and gas density distributions in a counter-flow diffusion flame were measured using LIBT and were found to compare favorably with numerical results. To evaluate the possibility of simultaneous composition and temperature measurement using laser plasma diagnostics, the spatial and temporal resolutions of LIBS and LIBT were carefully examined. The accuracy of LIBT technique was analyzed as a function of sample size from a statistical perspective. The results demonstrated that LIBT has spatial and temporal resolutions comparable to that of LIBS. Finally, a preliminary study using a Burke- Schumann flame and a Hencken burner was performed to understand the influence of turbulent flow. Measuring composition and temperature simultaneously using laser plasma diagnostics provides substantial benefits over traditional measurement technique. However, in exchange for such benefits, information on major species concentrations can no longer be directly measured. To infer the molecular species profile from the elemental species profile, the underlying partial-equilibrium assumption was examined. Among partially-equilibrated reactions, the water-gas shift (WGS) reaction is most often assumed to be in equilibrium because of its important role in the high temperature zone. Thus, the equilibrium domain of WGS reaction was systematically studied in different hydrocarbon flames under varying strain rates to evaluate the validity of partial equilibrium assumption. The underlying mechanism for WGS-equilibrium was also examined. The results suggested that even though the WGS reaction has a broad partial-equilibrium domain in syngas, methane, ethylene and propane flames, the mechanisms responsible for partial equilibrium were very different. In hydrocarbon flames, the water-gas-shift reaction can achieve partial equilibrium even though the two elementary reactions behind it are not equilibrated