Flame fluctuations in Oxy-CO2-methane mixtures in swirl assisted distributed combustion

Swirl assisted oxy-methane combustion investigated here has focused on the impact of carbon dioxide dilution. Carbon dioxide dilution is essential as it lowers the flame adiabatic temperature and flame speed of oxy-fuel combustion leading to a more stable operation. However, carbon dioxide dilution may result in flame fluctuations, under fuel lean conditions. Experiments using a swirl burner examined the flame fluctuations with different amounts of CO2 dilution, with the goal to achieve stable oxy-CO2-methane combustion via colorless distributed combustion (CDC). This novel CDC technique has shown significant performance gains in gaseous fuel-air flames on emission reduction, enhanced thermal field uniformity, noise reduction, and improved flame stability. Results have shown that for oxy-CO2-fuel combustion, most of the heat release fluctuations had frequencies below 60 Hz. The results obtained revealed that increase in CO2 dilution increased the magnitude of flame fluctuation till a maximum of 28% O2 concentration in the oxidizer. Further increase in dilution gases and reduction in O2 concentration led to a more stable flame, along with more favorable conditions for colorless distributed combustion. Data analysis of flame images revealed the source of oscillations under different conditions. At maximum fluctuations with 28% O2, the flame oscillated between two modes leading to unstable operation and large fluctuation in the heat release signal. Decrease in oxygen concentration to below 27%, promoted a more stable flame behavior, occupying a larger flame volume, which is characteristic of distributed combustion. Further reduction in O2 led to flame blow off at around 21% O2. This work outlines the importance CO2 dilution in oxy-fuel flames for achieving stable distributed combustion and mitigate the increased oscillations normally encountered in higher oxygen concentration flames.