Unseeded molecular flow tagging in cold and hot flows using ozone andhydroxyl tagging velocimetry

Abstract

Two complementary unseeded molecular flow tagging techniques forgas-flow velocity field measurement at low and high temperature aredemonstrated. Ozone tagging velocimetry (OTV) is applicable to low-temperatureair flows whereas hydroxyl tagging velocimetry (HTV) is amenable to use inhigh-temperature reacting flows containing water vapour. In OTV, a grid ofozone lines is created by photodissociation of O2 by a narrowband 193 nmArF excimer laser. After a fixed time delay, the ozone grid is imaged with anarrowband KrF laser sheet that photodissociates the ozone and producesvibrationally excited O2 that is subsequently made to fluoresce by the sameKrF laser light sheet via the O2 transition B 3Σu-(v' = 0, 2) ← X 3Σg-(v'' = 6, 7). In HTV, amolecular grid of hydroxyl (OH) radicals is written into a flame bysingle-photon photodissociation of vibrationally excited H2O by a 193 nmArF excimer laser. After displacement, the OH tag line position is revealedthrough fluorescence caused by OH A 2Σ+-X 2Π (3←0) excitation using a 248 nm tunable KrF excimer laser. OTV and HTV use thesame lasers and can simultaneously measure velocities in low and hightemperature regions. Instantaneous flow-tagging grids are measured in airflows and a flame. The velocity field is extracted from OTV images in an airjet using the image correlation velocimetry (ICV) method.