Advanced Laser-based Multi-scalar Imaging for Flame Structure Visualization towards a Deepened Understanding of Premixed Turbulent Combustion

Abstract: Popular Abstract in English Nowadays, the majority of energy production comes from combustion which is universally linked to daily human activities. Public awakening of the environmental effects of combustion pollution evokes extensive research activities in improving combustion efficiency and reducing pollutant production. Development of combustion processes also includes the use of renewable biomass-derived fuels has emerged to replace the foreseeable limited energy supply from fossil fuels. However, in spite of the long history over which man has tried to control combustion, our knowledge of combustion processes is still limited. One of the major reasons is the multi-scale nature of combustion process. Practical combustion processes typically consist of thousands of chemical reactions and species interacting with turbulence within a continuous spectrum of time and length scales. Therefore, a comprehensive picture of complex combustion process will typically involve two parts, i.e. the understanding of (1) chemical kinetics of fuels and (2) their interactions with various levels of turbulence. Experimental investigations of both parts can be resorted to the employment of advanced laser-based diagnostic techniques which have proved valuable in providing in-situ, non-intrusive measurements with high spatial and temporal resolution. In the present thesis, such laser-based techniques have been developed for single-shot based visualization of key species in flames of hydrocarbon and nitrogen-containing fuels. The experimental results in laminar flames give insights into chemical kinetics of fuels to better understand the detailed processes how fuels are converted into the final products, release heat, and how pollutants like nitrogen oxides (NOx) are produced. Furthermore, it needs to be answered how combustion chemistry couples with turbulence. In our daily life, one might see a thin chemiluminescent bluish layer in non-sooty laminar flames. This thin layer is called the reaction zone where the primary chemical reactions with major heat release are believed to take place. In the context of low-to-medium turbulent flows, the reaction zone layer has been shown to be wrinkled but remains thin. Contrarily, the existence of a combustion mode with a broadened reaction zone layer has been theoretically predicted under highly turbulent flows as the small length scales of turbulence might start to penetrate into the layer. This could be experimentally verified by the observation of gradually broadened layers of reaction-zone scalars. The present work provides the first experimental evidences that the reaction zone layer can be broadened and even distributed through rapid turbulent mixing. A broadened/distributed reaction zone layer implies a homogeneous heat release during combustion and therefore uniform temperature increase with less maximum temperature. This particular combustion mode has the potential benefits to achieve lower global fuel consumption rate with less NOx emissions. Investigations of the criteria for achieving distributed reactions are also presented in this thesis. The results will provide significant input into the development of numerical combustion models for the high-turbulence regime.