Hydroxyl tagging velocimetry (HTV) is a

velocimetry Velocimetry is the measurement of the velocity of fluids. This is a task often taken for granted, and involves far more complex processes than one might expect. It is often used to solve fluid dynamics problems, study fluid networks, in industria ...

method used in humid air flows. The method is often used in high-speed combusting flows because the high velocity and temperature accentuate its advantages over similar methods. HTV uses a laser (often an argon-fluoride

excimer laser An excimer laser, sometimes more correctly called an exciplex laser, is a form of ultraviolet laser which is commonly used in the production of microelectronic devices, semiconductor based integrated circuits or "chips", eye surgery, and micro ...

operating at ~193 nm) to dissociate the water in the flow into H + OH. Before entering the flow

optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

are used to create a grid of laser beams. The water in the flow is dissociated only where beams of sufficient energy pass through the flow, thus creating a grid in the flow where the concentrations of

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

(OH) are higher than in the surrounding flow. Another laser beam (at either ~248 nm or ~308 nm) in the form of a sheet is also passed through the flow in the same plane as the grid. This laser beam is tuned to a

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

that causes the hydroxyl molecules to fluoresce in the UV spectrum. The

fluorescence Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with colore ...

is then captured by a

charge-coupled device A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...

(CCD) camera. Using electronic timing methods the picture of the grid can be captured at nearly the same instant that the grid is created. By delaying the pulse of the fluorescence laser and the camera shot, an image of the grid that has now displaced downstream can be captured. Computer programs are then used to compare the two images and determine the displacement of the grid. By dividing the displacement by the known time delay the two dimensional velocity field (in the plane of the grid) can be determined. Flow ratios, however, are shown to affect the impingement locations, where increased air flow ratios can reduce the required combustor size by isolating reaction products solely within the secondary cavity. Other

molecular tagging velocimetry Molecular tagging velocimetry (MTV) is a specific form of flow velocimetry, a technique for determining the velocity of currents in fluids such as air and water. In its simplest form, a single "write" laser beam is shot once through the sample spa ...

(MTV) methods have used

ozone Ozone () (or trioxygen) is an Inorganic compound, inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , break ...

(O₃), excited oxygen and nitric oxide as the tag instead of hydroxyl. In the case of

the method is known as ozone tagging velocimetry or OTV. OTV has been developed and tested in many room air temperature applications with very accurate test results. OTV consists of an initial "write" step, where a 193-nm pulsed

creates

grid lines via

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

(O₂) UV absorption, and a subsequent "read" step, where a 248-nm

photodissociates the formed O₃ and fluoresces the vibrationally excited O₂ product thus revealing the grid lines' displacement.

References

* * * * * * {{DEFAULTSORT:Hydroxyl Tagging Velocimetry Measuring instruments Laser applications Measurement Fluid dynamics Transport phenomena