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.

This paper discusses the experimental setup of a 3D LIF imaging facility. The advected scalar concentration field of a LIF dye is digitized quasi-instantaneously. The x- and y-resolution are determined by the camera optic, the z-resolution by the scan process. In the absence of scan noise, spatial resolution can be designed to be equal in all three directions. The thus obtained 3D field can be used as input signal for an optical flow measurement technique. The scope of the imaging is mainly constrained by the local Reynolds number of the flow and the state of current high-speed digital cameras and laser scanners. The reciprocal relationship of the important parameters is discussed along with some implementational examples for the turbulent free jet.

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