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

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 con- ducted 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∗ back- ground 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