Two-line (CH∗/CO2∗) chemiluminescence technique for equivalence ratio mapping in turbulent stratified flames

Equivalence ratio fluctuations play an important role in exploring and controlling flame dynamic instabilities such as thermoacoustic combustion instabilities associated with lean premixed gas turbine (GT) engines. However, it is difficult to reliably measure local equivalence ratio, and so we often make estimates based on the ratios of chemiluminescent emissions. Recent studies have reported that equivalence ratio varies with chemiluminescence intensity ratio of CH* and CO2* in the visible spectrum (431 and 410 nm, respectively). A methodology based on this relationship is adopted in the present study to construct equivalence ratio maps for a series of turbulent premixed and stratified methane/air flames under globally lean conditions (mean or global equivalence ratio, ϕ¯=0.75), over a range of stratification with ϕ spanning 0.375–1.125 for the highest level of stratification. In contrast with the previously used CH*/OH* method, the current technique allows for a single visible range camera to be used for chemiluminescence imaging. A comparison of the current estimated equivalence ratio with previously calculated local equivalence ratio from Raman-scattering measurements shows that the two-line chemiluminescence technique can be reliably used to determine equivalence ratio in the instantaneous reaction zone of turbulent flames even at higher turbulence levels and mixture stratification conditions.