Acronym: PLIF

Title: Planar Laser-Induced Fluorescence

Brief Description: In a PLIF measurement, a laser source, usually pulsed and tunable in wavelength, is used to form a thin sheet of light which traverses the flow field of interest. If the laser wavelength is resonant with an optical transition of a species present in the flow, a fraction of the incident light will be absorbed at each point within the illumination plane. A fraction of the absorbed photons may subsequently be re-emitted with a modified spectral distribution, which changes for different molecules and varies with local flow field conditions. The emitted light, known as fluorescence, is collected and typically imaged onto a solid-state array camera, usually image-intensified or cooled to provide time-gating and improved sensitivity. The amount of light detected by a pixel of the camera depends on the concentration of the interrogated species within the corresponding measurement volume and the local flow field conditions, i.e., temperature, pressure and mixture composition.

Parameter Space Covered:
Flowfield imaging of:
1) species concentration/mole fraction (e.g., Na, OH, NO, O2, CH, CO, acetone)
2) temperature,
3) velocity,
4) pressure

1) flowfield must contain molecular species with an optical resonance wavelength that can be accessed by laser
2) temperature measurements typically require two laser sources
3) velocity measurements typically practical only for high Mach number flows (near sonic or supersonic)
4) signal-to-noise ratio often limited by detector shot-noise
5) fluorescence interferences from other species, especially from hydrocarbons in high pressure reacting flows
6) attenuation of laser sheet across flow field or reabsorption of fluorescence before it reaches detector can lead to systematic errors

1) concentration/mole fraction: depends on species, e.g., < 10% systematic error for OH measurements in turbulent non-premixed flames, > 3-5% random error for single-shot images
2) temperature: application dependent; as an example, for measurements based on ratios of OH images over a temperature range of 1000-3000 K systematic errors of ~1-2% and random errors of >5-10% should be expected for " single- shot & quot; measurements

Sample Applications:
1) OH imaging in flames and combustors
2) NO imaging for NOx production in burners
3) acetone imaging for mixing of fuel and air
4) temperature imaging in flames and supersonic/hypersonic flows over bodies
5) velocity imaging in supersonic jets

1) Hanson, R. K., " Combustion Diagnostics: Planar Flowfield Imaging & quot;, Twenty- First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, PA, 1987, pp. 1677-1691.
2) Eckbreth, A. C., " Laser Diagnostics for Combustion Temperature and Species," Abacus Press, Kent.
3) Seitzman, J. M. and Hanson, R. K., " Planar Fluorescence Imaging in Gases, & quot; Chapter 6 in Instrumentation for Flows with Combustion, ed. A. M. K. P. Taylor, Academic Press, London (1993).

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