Laser-induced incandescence measurements of soot in turbulent pool fires

Abstract

We present what we believe to be the first application of the laser-induced incandescence (LII) technique to large-scale fire testing. The construction of an LII instrument for fire measurements is presented in detail. Soot volume fraction imaging from 2 m diameter pool fires burning blended toluene/methanol liquid fuels is demonstrated along with a detailed report of measurement uncertainty in the challenging pool fire environment. Our LII instrument relies upon remotely located laser, optical, and detection systems and the insertion of water-cooled, fiber-bundle-coupled collection optics into the fire plume. Calibration of the instrument was performed using an ethylene/air laminar diffusion flame produced by a Santoro-type burner, which allowed for the extraction of absolute soot volume fractions from the LII images. Single-laser-shot two-dimensional images of the soot layer structure are presented with very high volumetric spatial resolution of the order of 10(-5) cm3. Probability density functions of the soot volume fraction fluctuations are constructed from the large LII image ensembles. The results illustrate a highly intermittent soot fluctuation field with potentially large macroscale soot structures and clipped soot probability densities.

High spatial resolution laser cavity extinction and laser-induced incandescence in low-soot-producing flames

“…Shaddix et al [9,10] exploited these combined measurements to produce a popular database of quantitative soot measurements in laminar hydrocarbon diffusion flames. The same technique was also applied by other researchers in the investigations of various flames, such as laminar diffusion flames [11][12][13], laminar premixed flames [14][15][16] and turbulent diffusion flames [17][18][19][20].…”

Temperature, Oxygen, and Soot-Volume-Fraction Measurements in a Turbulent C₂H₄-Fueled Jet Flame

“…The Nd:YAG laser fluence high enough to operate in the so-called plateau-level [7,13] regime, where the LII signal is largely insensitive to absorption of the interrogating laser beam at it propagates to the measurement volume at the center of the flame. Soot LII images were calibrated against laser light extinction measurements in a laminar C 2 H 4 diffusion flame using the approach described by Frederickson et al [7].…”

“…The Nd:YAG laser fluence high enough to operate in the so-called plateau-level [7,13] regime, where the LII signal is largely insensitive to absorption of the interrogating laser beam at it propagates to the measurement volume at the center of the flame. Soot LII images were calibrated against laser light extinction measurements in a laminar C 2 H 4 diffusion flame using the approach described by Frederickson et al [7]. Using this procedure, the accuracy of the sootvolume-fraction data is estimated to be 23%, which primarily results from uncertainties in literature values of the soot refractive index and from the uncertainty in the light-extinction data used for calibration.…”

“…Laser and optical diagnostics can overcome these disadvantages, but are additionally challenged by optical background arising from soot luminosity, scattering and absorption, and by heat transfer to the optical setups. Nevertheless, optical techniques have been successfully applied to sooting turbulent flames for spatially and temporally resolved measurements [3,4], and have recently been applied to meterscale turbulent pool fires for temperature and soot field evaluation [5][6][7][8] at the Sandia FLAME facility.…”

Two-dimensional laser-induced incandescence for soot volume fraction measurements: issues in quantification due to laser beam focusing

“…One problem which occasionally gets overlooked in 2D LII measurements is the issue of varying laser sheet width in the imaged region along the beam propagation direction caused due to the laser sheet focusing optics. One way to mitigate this problem is to use a very long focal length lens [8] or a combination of lenses [9,10]. Another possibility is to collimate the laser sheet both in the height and thickness directions instead of using a cylindrical lens to focus the laser sheet [11], but at the expense of a lower spatial resolution orthogonal to the beam sheet.…”