Quantitative atomic hydrogen measurements in premixed hydrogen tubular flames

Auto-ignition of near-ambient temperature H2/air mixtures during flame-vortex interaction

“…While auto-ignition was not observed, the tubular flame configuration involves a steady stretch rate (between 200-400 s −1 in Refs. [14,15]) and is not perturbed by vortices.…”

“…Curved H 2 /air flames are well known to cause spatial variations in radical concentration due to differential and thermal (Soret) diffusion, e.g. [7][8][9][10][11][12][13][14][15][16][17]. It generally is expected that relatively large H concentrations are confined to relatively high temperature regions of the flame due to rapid recombination at lower temperatures (via H + O 2 + M k3 → HO 2 + M), with differential and thermal diffusion leading to elevated (lowered) concentrations in regions of positive (negative) curvature.…”

“…It generally is expected that relatively large H concentrations are confined to relatively high temperature regions of the flame due to rapid recombination at lower temperatures (via H + O 2 + M k3 → HO 2 + M), with differential and thermal diffusion leading to elevated (lowered) concentrations in regions of positive (negative) curvature. However, quantitative measurements of H concentrations using two-photon femtosecond laser induced fluorescence in lean laminar tubular H 2 /air flames found that the H concentration could remain in the range of 2-5% of the peak value in regions with temperatures (measured using Raman scattering) less than about 400 K [14,15]. These regions of elevated H concentration occurred upstream of highly positively curved flames, and were not predicted by simulations employing detailed chemistry, multi-component diffusion, and the Soret effect.…”

A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics

“…In their scheme, H was excited by a 205 nm fs laser, and the subsequent H α fluorescence at 656 nm from the transition n = 3 → n = 2 was detected, as shown by the red line in Figure 2. This excitation scheme has also been adopted for quantitative measurements [44] and 2D imaging [45]. Another extinction scheme, shown as the blue line in Figure 2, was proposed by our group [51].…”

Collision‐independent detection of molecular two‐photon excitation by time‐resolved parametric four‐wave mixing

“…Such multiphoton approaches have been particularly important in providing optical access to species that cannot be excited via single‐photon transitions because the necessary far‐UV photons would be strongly absorbed by other components in the medium of interest . In the case of femtosecond‐ (fs‐) duration pulses, two‐photon‐excitation efficiencies are further aided by an increased breadth of photon frequency pairs that can contribute, resulting in efficient detection of both molecular and atomic species. This previous work has emphasized two further advantages associated with two‐photon fs‐laser excitation: (1) high peak intensities combined with low average pulse energies, as compared to ps‐ and ns‐pulse excitation schemes, result in negligible photolytic production of the target species; and (2) measurements can be made at high (1‐kHz) repetition rates associated with commercially available fs‐laser systems.…”