Atomic Oxygen and Heat Release Rate Monitoring in Hydrogen Tubular Flames
Evolving combustion applications require efficient devices whose complexity continues to grow, but they require accurate simulations in turbulent reacting environments that are difficult to validate. Thus, fundamental laminar configurations, such as tubular flames, are an attractive means to independently study turbulent characteristic (stretch and curvature) effects on reaction chemistry. The laminar nature of tubular flames makes them prime candidates for validating full and reduced chemical mechanisms in direct numeric simulations against measurements. This work seeks to provide further insight into tubular flame structure by imaging atomic oxygen profiles; testing the validity of the latest chemical mechanisms in a stretched and curved environment; and re-examining the utility of common heat release tracers in hydrogen-fueled combustion. Quantitative atomic oxygen measurements are conducted in H2-O2 tubular flames via femtosecond, Two-photon Absorption Laser Induced Fluorescence (fs-TALIF) where signal quenching is corrected through Raman scattering measurements of major species. The study finds that temperature-dependent quenching of the fluorescence signal must be considered for accurate quantitative fs-TALIF. Measured atomic oxygen profiles in non-cellular (one-dimensional) tubular flames (diluted with N2 or CO2) are shown to be well-predicted by the latest chemical mechanisms. However, no new mechanism can accurately describe the two-dimensional flame in the presence of cellular instability. Atomic oxygen profiles are reasonably predicted within the cells when N2 is the diluent, but dearth regions and curvature trends are not well-modeled, and temperature is over-predicted. When diluted with CO2 all of the mechanisms over-estimate O-atom number densities by 340% though temperature is more accurately predicted. In addition to these measurements, heat release markers for hydrogen combustion are also explored. Knowledge of heat release in combustion devices is necessary to avoid destructive thermo-acoustic instabilities; however, it cannot be directly measured. Thus, combinations of potential chemical tracers are numerically compared to calculated heat release in tubular flames. The legacy tracer, electronically excited OH, is shown to under-predict heat release in zones of low reactivity while hydrogen atom is determined to more accurately trace heat release through each reaction zone of the flames.