Two complementary unseeded molecular flow tagging techniques for gas-flow velocity field measurement at low and high temperature are demonstrated. Ozone tagging velocimetry (OTV) is applicable to low-temperature air flows whereas hydroxyl tagging velocimetry (HTV) is amenable to use in high-temperature reacting flows containing water vapour. In OTV, a grid of ozone lines is created by photodissociation of O 2 by a narrowband 193 nm ArF excimer laser. After a fixed time delay, the ozone grid is imaged with a narrowband KrF laser sheet that photodissociates the ozone and produces vibrationally excited O 2 that is subsequently made to fluoresce by the same KrF laser light sheet via the O 2 transition B 3 − u (v = 0, 2) ← X 3 − g (v = 6, 7). In HTV, a molecular grid of hydroxyl (OH) radicals is written into a flame by single-photon photodissociation of vibrationally excited H 2 O by a 193 nm ArF excimer laser. After displacement, the OH tag line position is revealed through fluorescence caused by OH A 2 +-X 2 (3 ← 0) excitation using a 248 nm tunable KrF excimer laser. OTV and HTV use the same lasers and can simultaneously measure velocities in low and high temperature regions. Instantaneous flow-tagging grids are measured in air flows and a flame. The velocity field is extracted from OTV images in an air jet using the image correlation velocimetry (ICV) method.

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(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.

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