Experimental and numerical study of chemiluminescence in methane/air high-pressure flames for active control applications

Chemiluminescence of excited OH*, CH*, and C-2* radicals was investigated as a tool for combustion control. A parametric study in premixed methane/air flames is presented regarding the effects of pressure (1 to 10 bar) and equivalence ratio (0.6 to 1.1). The experimental geometry corresponds to a Bunsen-type burner, with pilot flames to achieve steady combustion at very lean conditions. The burner was set in a pressurized vessel to control ambient pressure. The chemiluminescence was spatially measured using an intensified CCD camera with interference filters centered on the three radical emission bands. A monochromator and a low-resolution spectrometer were used to obtain spectrally resolved data. The three diagnostic techniques show good agreement. The experimental results show that the chemiluminescence of the radicals investigated has different dynamics for given pressure and equivalence ratio conditions. The OH* radical seems more suitable for lean flames, while CH* and C-2* have a more monotonic behavior and stronger dynamics for richer flames. A numerical simulation with complex chemistry and transport modeling based on the PREMIX code was performed for two different kinetic schemes including OH* and CH*. A comparison is presented for integrated chemiluminescence (both spectrally and spatially), as well as for local excited radical concentration trends within the range of experimental conditions. Good qualitative agreement is found with the experimental results except for rich flames, where disagreements due to kinetic schemes are observed. As a conclusion, a new strategy for flame sensing using chemiluminescence over several wavelengths is proposed.