Reaction rate imaging

Abstract

It has long been recognized that common experimental observables such as species concentrations are not generally reliable markers of flame front topology, heat release rate, or reaction rate. We show that with proper selection of excitation transitions, the planar laser-induced fluorescence (PLIF) signals of two spatially correlated species can be used to determine a bimolecular reaction rate in flames. This measurement technique provides a spatial marker of a species-specific bimolecular reaction region as well as a measure of the relative forward rate of that reaction. Our primary focus is the CO+OH reaction due to its significance in H−C−O mechanisms and the feasibility of simultaneous OH and CO PLIF imaging. The reaction rate diagnostic was applied to methane/air-based Bunsen flames and an acoustically forced vortex-flame interaction. Measured reaction rate, CO LIF, and OH LIF profiles extracted from the Bunsen flame images agreed reasonably well with theoretical estimates, and the measured reaction rate was observed to intensify near the flame tip. Measurements in a vortex-flame system indicated entrainment of CO directly into the vortex core, but only minor reaction rates occurred in the core due to low oxidizer (OH) concentration. On the primary flame surface, the forward reaction rate initially increased threefold due to enhanced reactive scalar mixing resulting from the applied strain field. The vortex passage caused the base of the CO+OH reaction surface to detach and lift similar to OH, with the base of the CO surface detaching later in time. For the majority of cases considered, the topologies of the reaction rate fields were distinctly different than those of the individual reactive scalars, suggesting that insight into CO oxidation by OH can be improved by the use of rate imaging diagnostics.