PLIF Imaging of Fuel Fraction in Practical Devices and LII Imaging of Soot

Abstract

In this paper we discuss the development and application of 2D imaging methods for the study of fuel-air mixing and the in-situ measurement of soot in flames. Fuel imaging is based on planar laser-induced fluorescence (PLIF) with pulsed U V lasers. Fuel concentrations may be estimated from the fluorescence of common fuel components or through the addition of a fluorescent species or seed. PLIF of fuel is shown to be applicable to both premixed and non-premixed combustion. The choice of fluorescent seed and the role of interfering species, such as combustion derived polyaromatic hydrocarbons (PAH) and laser-induced incandescence from soot, are discussed. The application of the method is illustrated with an example of an isothermal mixing study in a gas turbine combustor sector rig and measurements in a simple flame. The results demonstrate that PLIF can achieve high spatial resolution, circa 0.4mm, in a combustor with dimensions of order of 400mm. The dynamic range of the measured intensities exceeds 1500 with typical signal-to-noise ratios of better than 100: I . A major source of interference in PLIF studies of non-premixed flames is laser-induced incandescence (LII) from soot particles. We discuss the role of LII both as an interference in PLIF imaging and as an imaging method for soot volume fraction. A detailed analysis of the physics of LII is presented on a theoretical model developed. Results from the model and supporting experimental data are presented. LII images from a highly turbulent sooting flame indicate that soot structures, probably in the form of thin sheets as small as 100 pm across, are formed through vortex mixing. E 8450The derivation of temperatures of gas phase media from spectra generated by DFWM, induced by frequency scanned narrow linewidth lasers, is briefly reviewed. The technique of multiplex or broadband DFWM is described, by which spectra, and hence temperatures, may be obtained by a single shot of a novel broadband "modeless" laser. Experimental measurements of broadband DFWM lineshapes from OH are briefly described and compared with theoretical predictions. Results of temperature measurements in premixed methanelair flames using broadband DFWM spectra are reported. Systematic errors arising from the effect of absorption are discussed, improvements to and applications of the technique to turbulent media are briefly considered.Optical methods for measuring temperatures in hostile environments are attractive since they allow remote sensing and are inherently non-invasive. These features are especial-ly useful for diagnostics of combustion processes where physical probes may perturb the medium. Laser based techniques have been developed using linear and nonlinear scat-

Laser-induced incandescence for soot particle size measurements in premixed flat flames

“…A delayed detection gate biases the LII measurements in favor of large particles. 2,7,27,29 Such is also true for long gate widths. 29 The detected LII signal has to be calibrated to give an absolute value of soot volume fraction.…”

“…2,7,27,29 Such is also true for long gate widths. 29 The detected LII signal has to be calibrated to give an absolute value of soot volume fraction. A well-characterized flame, for which the soot volume fractions have been calculated from, for example, extinction measurements, is often used for this purpose.…”

Effects of volatile coatings on the morphology and optical detection of combustion-generated black carbon particles.

“…Laser-induced incandescence (LII) is a technique that has been developed over decades to measure soot particle abundances and physical properties in combustors, such as engines [4][5][6][7][8][9][10] and flames [11][12][13][14][15][16][17], and in combustion exhaust streams [18][19][20][21]. More recently it has been applied to measurements of ambient atmospheric black-carbon particles [22][23][24][25][26][27][28][29].…”

Modeling laser-induced incandescence of soot: a summary and comparison of LII models

“…LII involves heating particles with a highpower pulsed laser (usually with a pulse duration of several nanoseconds) and measuring the radiative emission from the hot particles. The magnitude of the signal depends on the particle volume fraction, making it a useful technique for measuring soot spatial and temporal distributions [1][2][3][4][5][6][7][8][9][10][11][12][13]. The signal decay rate depends on the specific surface area of the particles.…”

“…Considerable effort has been devoted to developing models capable of predicting LII signals in response to pulsed laser heating over a range of fluences [4,10,. Current models solve the energy-and mass-balance equations to predict the temporal response of the particle to pulsed laser heating.…”