Spectral characterisation of non-premixed H2/CO2 jet flames

The impact of CO 2 addition on the appearance and spectral characteristics of H 2 -fuelled flames was examined in two types of non-premixed jet flames, namely constant heat release rate and constant fuel jet exit velocity. The CO 2 volumetric fraction in the fuel stream was varied between 0 and 60%. Flame images and spectra for wavelengths 270–850 nm were acquired using a digital colour camera and a spectrograph, respectively. For both flame types, CO 2 addition introduced broadband CO 2 * chemiluminescence with prominent intensities in the blue range, owing to the chemical participation of CO 2 in combustion. It also suppressed OH* chemiluminescence and emissions from water molecules because of the thermal and chemical impact of CO 2 . Consequently, CO 2 addition led to a flame colour change from slightly red-dominated for pure H 2 flames to strongly blue-dominated, enhancing flame visibility by up to 4 times. The intensity of CO 2 * chemiluminescence reached a maximum for 40–50% CO 2 volumetric fraction for both flame types. Spectral intensities were evaluated for three selected wavelength bands, namely OH (305–315 nm), blue (400–540 nm), and IR (720–850 nm). The intensity ratios of three band combinations, namely blue/OH, blue/IR and OH/IR, demonstrated independence of the flame type and monotonic correlations with the CO 2 volumetric fraction, highlighting their potential application for industrial real-time monitoring of H 2 /CO 2 blending ratio in non-premixed H 2 /CO 2 flames. Flame height, evaluated from flame images, increased with CO 2 addition for laminar jet flames but decreased with increasing CO 2 addition for turbulent jet flames, regardless of flame type.