Hydroxyl tagging velocimetry
Encyclopedia
Hydroxyl tagging velocimetry (HTV) is a velocimetry
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
operating at ~193 nm) to dissociate the water in the flow into H + OH. Before entering the flow optics
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
(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
that causes the hydroxyl molecules to fluoresce in the UV spectrum. The fluorescence
is then captured by a charge-coupled device
(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.
Other Molecular tagging velocimetry
(MTV) methods have used ozone
(O3), excited oxygen and nitric oxide as the tag instead of hydroxyl. In the case of ozone
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 excimer laser
creates ozone
grid lines via oxygen
(O2) UV absorption, and a subsequent "read" step, where a 248-nm excimer laser
photodissociates the formed O3 and fluoresces the vibrationally excited O2 product thus revealing the grid lines' displacement.
Velocimetry
Velocimetry is the measurement of the velocity of fluids, as often used to solve fluid dynamics problems, or to study fluid networks, as well as in industrial and process control applications, or in the creation of new kinds of fluid flow sensors...
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
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
operating at ~193 nm) to dissociate the water in the flow into H + OH. Before entering the flow optics
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
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
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
(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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
that causes the hydroxyl molecules to fluoresce in the UV spectrum. The fluorescence
Fluorescence
Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
is then captured by a charge-coupled device
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
(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.
Other Molecular tagging velocimetry
Molecular tagging velocimetry
Molecular Tagging Velocimetry 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 space...
(MTV) methods have used ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
(O3), excited oxygen and nitric oxide as the tag instead of hydroxyl. In the case of ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
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 excimer laser
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
creates ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
grid lines via oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(O2) UV absorption, and a subsequent "read" step, where a 248-nm excimer laser
Excimer laser
An excimer laser is a form of ultraviolet laser which is commonly used in the production of microelectronic devices , eye surgery, and micromachining....
photodissociates the formed O3 and fluoresces the vibrationally excited O2 product thus revealing the grid lines' displacement.