Flame Spectra of a Turbulent Liquid-Fueled Swirl-Stabilized Lean-Direct Injection Combustor

Flame spectra within the UV/visible-light range are measured for a turbulent, liquid-fueled, swirl-stabilized, lean-direct injection combustor. The flame spectra are quite similar to those of lean premixed combustion, which can be attributed to the small droplet size and fast fuel/air mixing. Broadband background emissions around 431 nm, which mainly consist of CO2* chemiluminescence, are found to be self-similar with wavelength for both stable and unstable combustion. Chemiluminescence from OH*, CH*, and CO2* is found to be a nonlinear function of the airflow rate, the equivalence ratio, preheat temperature, and pressure. Procedures for the determination of the instantaneous heat release rate and equivalence ratios along the flame front are developed. For both combustion instability and forced flame responses, the prediction errors are within 2.0% in the mean heat release rate, the mean air consumption rate, and the mean equivalence ratio. The assumption that chemiluminescence is proportional to the instantaneous heat releaser rate is generally invalid. Probably the proportionality is valid only in the weakly turbulent or wrinkled flamelet region in the absence of equivalence ratio variations and strong acoustic oscillations.