Improvement of precision and accuracy of temperature imaging in sooting flames using two-line atomic fluorescence (TLAF)

Severe interference to the fluorescence signal from seeded indium has previously limited the application of current non-linear excitation regime two-line atomic fluorescence (NTLAF) to low sooting flames. The present paper reports new details of these interferences, from spatially- and temporally-resolved spectral measurements in a laminar, non-premixed sooting C2H4 flame burning in air. Three types of interference to the indium LIF signal have been identified: laser-induced fluorescence (LIF) from polycyclic aromatic hydrocarbons (PAH), elastic laser scattering (ELS) and laser-induced incandescence (LII) from soot particles. This knowledge was used to propose new methods to suppress interferences and select customized narrow-band filters for the signal collection, with a narrow bandwidth of ∼1.2 nm (full-width at half-maximum, FWHM) and a high peak transmission of ∼95%. The efficacy of the narrow-band filters for interference suppression in planar NTLAF measurements was assessed by comparing these measurements with those obtained with a set of conventional band-pass filters (FWHM = 10 nm), in both a soot-free flame and a sooting flame. The application of the narrow-band filters was found to increase both the accuracy and the precision of the temperature measurement, so that measurement accuracy that is due to the presence of interferences has been improved from 198 K to 10 K at a location where the soot volume fraction (fv) is 1.2 ppm. An average reduction of 40% in the standard deviation of measured temperature in single-shot measurements has also been achieved with the use of high transmission filters. With the use of narrow-band filters, NTLAF can be reliably applied in laboratory target sooting flames with peak soot volume fraction of several ppm.