Experimental Characterization and Modeling for Equivalence Ratio Sensing in Non-premixed Flames Using Chemiluminescence and Laser-Induced Breakdown Spectroscopy Techniques

Real-time monitoring of local equivalence ratio is of primary importance for combustion efficiency improvement. Chemiluminescence and laser induced breakdown spectroscopy (LIBS) measurements can be powerful tools for sensing equivalence ratio. Radicals such as OH*, CH*, and C2* are formed within the flame front and consequently the related chemiluminescence emission can be considered as a marker to follow the evolution of combustion processes. From LIBS measurements, the ratio of two characteristic spectral lines of fuel (H) and oxidizer (O) is related to the equivalence ratio. In this work, LIBS measurements have been carried out in premixed flames of known equivalence ratio for calibration purposes. Then, the application to non-premixed flames has been performed to evaluate the local equivalence ratio. Chemiluminescence emission spectra have been also collected and the values of the OH*, CH* and C2* have been properly evaluated taking into account the contribution of CO2* broad band emission. A methane co-flow diffusion flame has been investigated along the centerline as well as at three cross sections and the local equivalence ratio profiles obtained by LIBS have been compared with CH* chemiluminescence profiles. Finally, the experimental data obtained were compared to the results of a detailed chemical kinetic model.