Hydroxyl tagging velocimetry

Hydroxyl tagging velocimetry (HTV) is a velocimetry method used in humid air flows. Themethod is often used in high-speed combusting flows because the high velocity and temperatureaccentuate its advantages over similar methods. HTV uses a laser (often an argon-fluorideexcimer laser operating at ~193 nm) to dissociate the water in the flow into H + OH. Before entering the flow optics areused to create a grid of laser beams. The water in the flow is dissociated only where beams ofsufficient energy pass through the flow, thus creating a grid in the flow where theconcentrations of hydroxyl (OH) are higher than in the surrounding flow. Another laser beam (at either ~248 nm or ~308 nm) inthe form of a sheet is also passed through the flow in the same plane as the grid. This laserbeam is tuned to a wavelength that causes the hydroxyl molecules to fluoresce in the UV spectrum. The fluorescence is then captured by a charge-coupled device (CCD) camera. Usingelectronic timing methods the picture of the grid can be captured at nearly the same instantthat the grid is created. Hydroxyl tagging velocimetry (HTV) is a velocimetry method used in humid air flows. Themethod is often used in high-speed combusting flows because the high velocity and temperatureaccentuate its advantages over similar methods. HTV uses a laser (often an argon-fluorideexcimer laser operating at ~193 nm) to dissociate the water in the flow into H + OH. Before entering the flow optics areused to create a grid of laser beams. The water in the flow is dissociated only where beams ofsufficient energy pass through the flow, thus creating a grid in the flow where theconcentrations of hydroxyl (OH) are higher than in the surrounding flow. Another laser beam (at either ~248 nm or ~308 nm) inthe form of a sheet is also passed through the flow in the same plane as the grid. This laserbeam is tuned to a wavelength that causes the hydroxyl molecules to fluoresce in the UV spectrum. The fluorescence is then captured by a charge-coupled device (CCD) camera. Usingelectronic timing methods the picture of the grid can be captured at nearly the same instantthat the grid is created.