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

Abstract: The work presented in the thesis concerns the developments of laser-based diagnostics and its application to the study of turbulent premixed combustion. The diagnostic developments, mainly concerned with planar laser-induced fluorescence (PLIF), are intended to provide instantaneous visualization of the key species originating from the combustion processes involved in the burning of hydrocarbons and nitrogen-containing fuels under turbulent conditions. For the burning of hydrocarbons, these species include HCO, hot O2 etc. and for the burning of nitrogen-containing fuels, these include NH3, NH and CN. In connection with this, the potential for instantaneous temperature mapping using two-line atomic LIF (TLIF) with a novel seeding system are also demonstrated. The study of turbulent premixed combustion involves simultaneous imaging of such scalars as HCO, CH, CH2O and OH as well as temperature. Laboratory-scale premixed CH4/air flames stabilized on the Lund University Pilot Jet burner (LUPJ) and the Low-Swirl Burner (LSB) were investigated over a wide operational range and various combustion regimes. The results from both the LUPJ and the LSB flames provide the first experimental evidences for its being possible to appreciably broaden the reaction zone of premixed flames can be significantly broadened through rapid turbulence mixing, the results being verified by observations of broadened/distributed short-lived radicals, HCO and/or CH. The observations obtained for the two clearly different burner configurations suggest that distributed reactions can be a common combustion mode. For the LUPJ flames, the dependence of the reaction zone broadening on the jet speed and the equivalent ratio was investigated systematically. Spatial correlations between the scalars that were measured were investigated, and the detailed local flame structures with and without the presence of distributed reactions were analyzed and compared. It was found that having a temperature above ~ 1000 K is important for sustaining the distributed reactions. The build-up of radical pools through rapid turbulence transport in regions containing (intermediate) reactants was found to be responsible for the distributed reactions occurring. In addition, a study of Mild combustion that has similarities with the distributed reaction concept was performed using optical diagnostics. Certain insights concerning the reaction zone structure of Mild combustion are discussed.