Flame chemiluminescence from CO2- and N2-diluted laminar CH4/air premixed flames

Abstract

An experimental and numerical investigation of the chemiluminescence signals from OH*, CH*, and CO2* is conducted for laminar premixed conical CH4/air flames diluted with CO2 or N2. Experiments are conducted either at fixed equivalence ratio or fixed adiabatic flame temperature. An ICCD camera, equipped with different narrow bandpass filters, is used to record flame images at 307 nm (OH*), 430 nm (CH*), and 455 nm (CO2*). A spectrometer is also used to correct the OH* and CH* emissions from the CO2* broadband background emission. Measured chemiluminescence intensities are then compared to one dimensional freely-propagating-flame direct simulations accounting for the chemistry of the excited radicals. Simulations predict accurately the OH* chemiluminescence intensity, independently of the diluent nature and concentration. Correction for the CO2* background has a weak influence on the recorded OH* signal. Predictions of CH* emissions are also in good agreement with experimental data if the CO2* background intensity is subtracted from intensity measurements. Measured and calculated CO2* emissions lead to acceptable results using a simplified chemistry mechanism for CO2* and an heuristic model for its emission intensity. Finally, it is shown that CO2 dilution modifies chemiluminescence intensity couples and particularly the OH*/CO2* intensity ratio. These ratios regularly decrease with CO2 dilution, a feature which is reproduced by the simulations. It is then shown that the ratio OH*/CO2* is well suited to infer the CO2 diluent concentration in diluted CH4/air flames, a method which appears not feasible for sensing N2 in N2-diluted CH4/air flames.