Determination of Planck Mean Absorption Coefficients for Hydrocarbon Fuels

Abstract

Infrared transmission of propane, n-heptane and propylene samples was measured using Fourier transform infrared spectroscopy (FTIR) for temperatures ≤1000 K to facilitate calculation of absorption coefficients of fuel molecules at temperatures representative of non-premixed flame interiors. Spectrally resolved fits of the absorption coefficient data using a semi-empirical quantum-based expression provide a basis for calculating infrared spectra at any temperature. Experimentally-derived Planck mean absorption coefficients (κp) of these fuels are compared with that of methane calculated from the HITRAN database since methane absorption has been used to model fuel absorption in fires. κP's for propane and heptane have similar characteristics over the entire temperature range. Methane and propylene with their higher proportion of absorption in low frequency bands have peak κP at lower temperatures where blackbody radiation intensity peaks near the spectral range of these bands. Propylene with low frequency absorption bands associated with the C=C bond has the highest κP at temperatures <600 K. N-heptane has the largest κP at temperatures ≥800 K where blackbody emission intensity peaks near or above the spectral range of the C–H stretching bands. Implications of these results on fuel absorption of radiative heat transfer in flames are discussed.