Reactive flow visualization through high-speed optical laser diagnostics

Turbulent combustion is a complex phenomenon that involves the interaction between fluid mechanics and chemical kinetics. Turbulence plays an important role in combustion because it ensures a thorough mixing between the oxidizer and fuel at a molecular level. Improper mixing can lead to incomplete combustion, resulting in increased emissions and decreased flame stability. Hence, understanding and controlling the coupling between chemical kinetics and turbulence can lead to significant improvements in the design of combustion systems, improving their stability, efficiency and emissions characteristics. In this study, a method was demonstrated using temporally-resolved laser diagnostics to visualize: 1) The turbulent flow field with particle image velocimetry (PIV); and 2) the distribution of combustion radicals, by planar laser-induced fluorescence (PLIF). This allows for the visualization of the flame surface behavior, while at the same time linking it to inherent flow and combustion instabilities. Characterizing turbulence requires high temporal resolution and combining it with high-speed combustion radical imaging adds an additional diagnostics challenge. Trying to extend the capabilities of current laser diagnostics techniques to higher repetition rates to achieve the desired temporal resolution in order to resolve turbulence is a major issue due to limitations in terms of laser power scalability with repetition rate. In this work, a new approach is investigated that reduces the overall laser power required to visualize the flame structure (CH and OH radicals) while allowing for simultaneous flow field imaging at high repetition rates (10 kHz). A comprehensive study of CH-PLIF imaging using the of the CH molecule to visualize the flame reaction zone is presented here. The CH-PLIF imaging effectiveness and its suitability for use in conjunction with PIV were quantified using a laminar Bunsen flame. Due to the high Einstein coefficients, the Q-branch rotational excitation strategy has the lowest laser power requirements and hence is best suited for high-speed imaging. However, isolating the fluorescence signal from the excitation wavelength is a major concern especially in high-scattering environments, due to the inherent resonant transition that is observed in the Q-branch excitation strategy. To address this, the excitation scheme was switched to the weaker (by a factor of 5) R-branch transition, whose fluorescence signal proved to be sufficient for CH-imaging, with separation of the fluorescence and excitation wavelengths through the use of a custom-made sharp cut-off edge filter. The low laser pulse energy requirements of this transition combined with the use of the custom edge filter allowed for simultaneous 10 kHz CH-PLIF and PIV imaging in a highly turbulent Hi-Pilot burner (ReT ~ 7900). Reaction layer thicknesses were estimated from the CH-PLIF images, and their interaction with the flow field was observed. Folding of the flame sheet caused an interaction between the out-of-plane and in-plane flame sheets, manifesting in the presence of products well upstream of the flame. Additionally, preheat zone broadening effects were observed, suggesting the existence of eddies smaller than the laminar flame thickness, able to penetrate into the preheat zone and therefore enhance scalar transport, through a purely hydrodynamic straining mechanism. Finally, the ability to image three major combustion radicals (OH, CH and CH2O), using a reduced experimental setup was demonstrated in mesoscale burner array, by making use of the increased efficiency of the C-X CH transition, as well as the presence of OH lines (from the A-X (0,0) band) in the vicinity.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste