Lubomir A. Ribarov

Flame flow tagging velocimetry with 193-nm H 2 O photodissociation

In a new nonintrusive, instantaneous flow tagging method called hydroxyl tagging velocimetry (HTV), a molecular grid of hydroxyl (OH) radicals is written into a flame and the displaced grid is imaged at a later time to give the flames velocity profile. Single-photon photodissociation of vibrationally excited H(2)O, when a 193-nm ArF excimer laser is used, produces a tag line of superequilibrium OH and H photoproducts in a high-temperature flow field that itself may contain ambient OH. The tag line OH concentration is composed mostly of direct OH photoproducts, but OH is also indirectly produced through H photoproduct reactions with oxygen-bearing species. For lean and modestly rich flames the OH tag lifetime is of the order of 1 ms. For very rich H(2)-air flames (equivalence ratio of 4.4) the lifetime drops to 200 ns. After displacement the position of the OH tag line is revealed through fluorescence caused by OH (A-X) (3 <-- 0) excitation by using a 248-nm tunable KrF excimer laser. A HTV grid of multiple tag lines, providing multipoint velocity information, is experimentally demonstrated in a turbulent H(2)/N(2)-air diffusion flame.

Ozone Tagging Velocimetry Using Narrowband Excimer Lasers

Ozone tagging velocimetry (OTV), a nonintrusive, unseeded, time-of-flight velocity measurement technique, consists of a write step, where a 193-nm pulsed excimer laser creates an O 3 line via O 2 uv absorption, and a subsequent read step, where a 248-nm excimer laser photodissociates the O 3 and fluoresces the vibrationally exdted O 2 product, revealing the tag line displacement. For the flrst time, instantaneous OTV images and velocity measurements are reported in airflows at room temperature. The narrowband lasers are tuned to the O 2 Schumann-Runge transitions improving the OTV signal strength by a factor of six over that obtained using two broadband lasers. This improvement is less than expected from absorption ratio estimates, due in part to incomplete laser locking efficiency and possibly to laser bleaching. Diffusion of the O 3 tag line is shown to be important only for write-read delay times of the order of milliseconds or greater Modeling of O 3 concentration vs time shows O 3 is long lived at room temperature and relatively insensitive to water vapor, but O 3 peak concentration and lifetime greatly decrease at high temperature, though high pressure increases peak O 3 concentration

MULTILINE HYDROXYL TAGGING VELOCIMETRY IN REACTING AND NONREACTING EXPERIMENTAL FLOWS

A new compact micro-lens optical system has been developed to produce a 7 x 7 multiline optical grid for Hydroxyl Tagging Velocimetry (HTV). These lines provide additional spatial information and aid the software tracking of the flowfield by increasing its resolution. Single-photon photodissociation of groundstate H2O by a ~193-nm ArF excimer laser “writes” a 7 x 7 beam molecular grid with very long gridlines of superequilibrium OH and H photoproducts in either room air flowfields or in H2-Air flames due to the presence of H2O vapor. The displaced OH tag lines’ positions are revealed through fluorescence by A 2 Σ + (v′ = 0) ← X 2 Πi (v″ = 0) OH excitation using a ~308 nm pulsed frequency-doubled dye laser. A time-of-flight software (DaVis 6.0 Stereo PIV/PTV, LaVision, GmbH) determines the instantaneous velocity field. HTV tag lifetime comparisons between experimental results and theoretical predictions are discussed.