Influence of rapid laser heating on the optical properties of in-flame soot

To understand the effect of rapid heating on the optical properties of in-flame soot and its potential influence on the laser-induced incandescence (LII) signal, the time-resolved extinction coefficient of soot is measured in diffusion and premixed flames during laser heating. Heating is performed using a 1064-nm pulsed laser with fluences ranging from 0.2 to 6.2 mJ/mm2. Extinction measurements are carried out using continuous-wave lasers at four different wavelengths. A rapid enhancement of extinction, by up to 10 % in the diffusion flame and 18 % in the premixed flame, occurs during laser heating most likely as a result of temperature-dependent optical properties and laser-induced thermal annealing of soot. The thermal expansion of flame gases causes a gradual decline of soot concentration for about 2 \u3bcs after the laser pulse. Significant loss of soot material by sublimation is observed at fluences as low as 1.03 and 2.06 mJ/mm2 for the diffusion and premixed flames, respectively. A secondary rise in extinction coefficient is observed from about 50 to 800 ns after the laser pulse at low monitoring wavelengths, attributed to the formation of light-absorbing gaseous species from the sublimated soot material. These effects may impact the LII signal and should be accounted for in LII analysis.Peer reviewed: YesNRC publication: Ye

A numerical and experimental study of a laminar sooting coflow jet-A1 diffusion flame

As the drive towards a better understanding of airline fuel combustion, and its associated emission characteristics continues, there is a need for fundamental numerical and experimental jet fuel studies. In the present work, a numerical and experimental study is conducted for a complex blended liquid fuel, Jet- A1, in an atmospheric pressure, laminar sooting coflow diffusion flame. The numerical model uses a surrogate mixture, comprising 69% n-decane, 20% n-propylbenzene, and 11% n-propylcyclohexane (by mole). The combustion chemistry and soot formation are solved using a detailed chemical kinetic mechanism with 304 species and 2265 reactions, detailed transport, and a sectional soot model including soot nucleation, heterogeneous surface growth and oxidation, soot aggregate coagulation and fragmentation, and PAH surface condensation. The problem is intractable by serial processing; therefore, distributed-memory parallelization is used, employing 192 CPUs. Experimentally, soot volume fraction and gaseous species concentration profiles are determined by a Laser Extinction Measurement method and Gas Chromatography, respectively, in a coflow diffusion flame of vaporized Jet-A1. These data are used to validate the model. Centerline species concentrations are satisfactorily reproduced by the model. The order of magnitude of the peak soot volume fraction is well predicted without calibrating any of the model constants to the experimental data, but discrepancies remain between numerical and experimental results on the radial locations of the peaks and the centerline soot concentration levels.Peer reviewed: NoNRC publication: Ye

Effective density and volatility of particles sampled from a helicopter gas turbine engine

The effective density and size-resolved volatility of particles emitted from a Rolls-Royce Gnome helicopter turboshaft engine are measured at two engine speed settings (13,000 and 22,000 RPM). The effective density of denuded and undenuded particles was measured. The denuded effective densities are similar to the effective densities of particles from a gas turbine with a double annular combustor as well as a wide variety of internal combustion engines. The denuded effective density measurements were also used to estimate the size and number of primary particles in the soot aggregates. The primary particle size estimates show that the primary particle size was smaller at lower engine speed (in agreement with transmission electron microscopy analysis). As a demonstration, the size-resolved volatility of particles emitted from the engine is measured with a system consisting of a differential mobility analyzer, centrifugal particle mass analyzer, condensation particle counter, and catalytic stripper. This system determines the number distributions of particles that contain or do not contain non-volatile material, and the mass distributions of non-volatile material, volatile material condensed onto the surface of non-volatile particles, and volatile material forming independent particles (e.g., nucleated volatile material). It was found that the particulate at 13,000 RPM contained a measurable fraction of purely volatile material with diameters below ∼25 nm and had a higher mass fraction of volatile material condensed on the surface of the soot (6%–12%) compared to the 22,000 RPM condition (1%–5%). This study demonstrates the potential to quantify the distribution of volatile particulate matter and gives additional information to characterize sampling effects with regulatory measurement procedures