Effects of flame unsteadiness on chemiluminescence intensity in turbulent premixed combustors

The measurement of heat release rate is of great importance in the study of thermo-acoustic instability occurring in lean premixed combustion and the chemiluminescence emission has been used as an indicator of heat release in combustion instability studies primarily for its relative simplicity. Most of the cases, the relationships between the chemiluminescence intensity and flow velocity/ equivalence ratio are obtained under stable flames and the calibrations are used to estimate heat release fluctuation for either forced or selfexcited (unsteady) flames, assuming the relationship is not affected by unsteady flame stretch. This paper presents results of experimental study of flame chemiluminescence from an atmospheric, swirl-stabilized, turbulent lean premixed flame with a main emphasis on the effect of unsteady flame stretch on the relationship between chemiluminescence intensity and heat release. Results show that the effect of unsteady flame stretch on the relationship between chemiluminescence intensity and heat release is different from species to species, indicating the amount of heat release estimated from chemiluminescence measurement can significantly overestimate the actual heat release if it is used without correcting for the effect.