An experimental study on the effect of pressure and strain rate on CH chemiluminescence of premixed fuel-lean methane/air flames

This article reports systematic measurements of CH chemiluminescence from a premixed, laminar flame and a premixed, strained, counter-flow flame at ambient pressures ranging from 0.5 to 2.5 MPa and equivalence ratios from 0.66 to 0.86. The objective of this study was to investigate the possibility of using chemiluminescence as an active feedback control parameter to deduce the equivalence ratio, and therefore NOx production and combustion stability, for premixed flames at high pressure. The chemiluminescence signal from the first electronically excited state of CH to ground (at 431.5 nm) was detected through a band-pass, optical filter in combination with a photo-multiplier tube. CH chemiluminescence at high pressure was found to have a power law dependence with equivalence ratio and pressure, and was proportional to mass flowrate. Using the premixed counter-flow flame, strain was shown not to influence chemiluminescence. Further, a comparison of these findings with previous results of OH chemiluminescence produced a relationship between OH and CH chemiluminescence that can be used to estimate the equivalence ratio of a flame, independent of the mass flowrate. This relationship has immediate value in the context of a feedback control system for reducing pollution (NOx) emission from a premixed gas turbine engine.