Investigations of the long-term effects of LII on soot and bath gas

Abstract

A combination of high-repetition rate imaging, laser extinction measurements, two-color soot pyrometry imaging, and high-resolution transmission electron microscopy of thermophoretically sampled soot is used to investigate the long-term and permanent effects of rapid heating of inflame soot during laser-induced incandescence (LII). Experiments are carried out on a laminar nonpremixed co-annular ethylene/air flame with various laser fluences. The high-repetition rate images clearly show that the heated and the neighboring laser-border zones undergo a permanent transformation after the laser pulse, and advect vertically with the flow while the permanent marking is preserved. The soot volume fraction at the heated zone reduces due to the sublimation of soot and the subsequent enhanced oxidation. At the laser-border zones, however, optical thickness increases that may be due to thermophoretic forces drawing hot particles towards relatively cooler zones and the rapid compression of the bath gas induced by the pressure waves created by the expansion of the desorbed carbon clusters. Additionally sublimed carbon clusters can condense onto existing particles and contribute to increase of the optical thickness. Timeresolved two-color pyrometry imaging show that the increased temperature of soot both in the heated and neighboring laser-border zones persists for several milliseconds. This can be associated to the increase in the bath-gas temperature, and a change in the wavelength-dependent emissivity of soot particles induced by the thermal annealing of soot. Ex-situ analysis show that the lattice structure of the soot sampled at the laser-border zones tend to change and soot becomes more graphitic. This may be attributed to thermal annealing induced by elevated temperature.

Laser-induced incandescence versus photo-acoustics: implications for qualitative soot size diagnostics

“…This is expected behavior, since the intensity of the photo-acoustic signal depends on the increase in sensible heat (i.e., internal energy) of the soot particles, assumed that all laser-induced heat is transferred from particle to gas. The increase in internal energy in turn is directly proportional to laser fluence, as can be seen from (4) and (5).…”

“…Here, E(m) is the absorption function, and ex is the laser excitation wavelength. From (4) and (5), it follows that particles in the Rayleigh regime absorb laser energy at a rate proportional to their volume.…”

“…When the contents of the probe volume are not refreshed between consecutive excitation laser pulses, the same soot volume will be probed multiple times. Since it is well known that soot particles are considerably modified by the excitation in an LII experiment [4,5], it is unlikely that a single soot particle will respond identically upon multiple exposures [6]. However, excitation at high-repetition rates renders another, independent detection method feasible, viz.…”

A review on determining the refractive index function, thermal accommodation coefficient and evaporation temperature of light-absorbing nanoparticles suspended in the gas phase using the laser-induced incandescence

“…This model assumed that soot begins to anneal at temperatures as low as flame temperature, whereas experiments showed that annealing starts above 4000 K. The model suggests that annealing leads to a reduction in the absorption and emissivity characteristics of soot, whereas experiments indicated a noticeable enhancement of such optical properties once soot was annealed. One possible explanation of laser action on the optical properties of soot has been presented in [54]. Using high-speed images, they found that, at high fluence (>0.3 J/cm 2 ), a much larger region is affected than the directly heated volume.…”

Measurements of Pressure Effects on PAH Distribution and 2D Soot Volume Fraction Diagnostics in a Laminar Non-premixed Coflow Flame

“…One should keep in mind that the LII signal lifetime at 16 bar is much shorter than the 500 ns gate (with our LII model 40 , we calculated that the LII signal from a 30 nm particle decays within less than 150 ns after the laser. We ignored the effects of bath-gas heating 41 . Thus, a 500 ns camera gate convolutes the entire LII signal.…”