Combustion kinetics of laser irradiated porous graphite from imaging Fourier transform spectroscopy

Abstract

The combustion plumes arising from laser-irradiated graphite targets were investigated experimentally using hyperspectral, imaging Fourier transform spectroscopy (IFTS). Porous graphite targets were irradiated with a 1.07 µm, 20-kW ytterbium fiber laser at irradiances of 0.25–4 kW/cm2. Emissive plumes from the oxidation of graphite in air were monitored using a mid-wave infrared imaging Fourier-transform spectrometer with spatial resolution of 0.52 mm per pixel. Strong spectral emission of CO and CO2 were observed in the infrared between 1900 and 2400 cm−1 with an instrument spectral resolution of 2 cm−1. A homogeneous single-layer plume, line-by-line radiative transfer model (LBLRTM) and two band models (EM2C and RADCAL) were applied to estimate spatial maps of temperature and column densities of CO and CO2 with a temporal resolution of 0.47 s per hyperspectral image. Steady surface temperatures of 1800–2900 K are achieved after ∼1 min for irradiances of 0.25–1.0 kW/cm2. A stable, gas phase combustion layer extends from 4 to 12 mm from the surface, with buoyancy driving a gas flow of ∼8 m/s. Plume extent and intensity is greater for the larger porosity (6 mm particle size) samples. Steady-state gas temperatures exceed surface temperature by up to 400 K. Column densities for CO and CO2 of up to 1018 molec/cm2 were observed. The CO/CO2 concentration ratio peaks at 2500 K. The initial rise with temperature is consistent with effective activation energies of 149–111 kJ/mol at distances between 0.72 mm and 3 mm, respectively.