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 flame’s velocity profile. Single-photon photodissociation of vibrationally excited H<sub>2</sub>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<sub>2</sub>-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 (<i>A</i>–<i>X</i>) (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<sub>2</sub>/N<sub>2</sub>–air diffusion flame.
© 1999 Optical Society of America
(280.1740) Remote sensing and sensors : Combustion diagnostics
(280.2470) Remote sensing and sensors : Flames
(280.2490) Remote sensing and sensors : Flow diagnostics
(280.7250) Remote sensing and sensors : Velocimetry
(300.6450) Spectroscopy : Spectroscopy, Raman
Joseph A. Wehrmeyer, Lubomir A. Ribarov, Douglas A. Oguss, and Robert W. Pitz, "Flame Flow Tagging Velocimetry with 193-nm H2O Photodissociation," Appl. Opt. 38, 6912-6917 (1999)