Fostering distributed combustion in a swirl burner using prevaporized liquid fuels

Highlights: • Examined the role of entrainment on Distributed Combustion with JP-8 and ethanol. • N₂ and CO₂ are used to simulate entrained combustion gases from within the combustor. • Low O₂ concentration in the mixture prior to ignition fostered distributed reaction. • Reaction distribution fostered at 9.2% O₂ and lower, with almost invisible flame. • 95% NO_x reduction demonstrated at distributed combustion with minimal impact on CO. - Abstract: Colorless Distributed Combustion (CDC) has presented itself as an environmentally friendly combustion method with significant benefits on ultra-low emissions, uniform thermal field (pattern factor), reduced noise, mitigation of instability and enhanced flame stability. CDC requisites include controlled entrainment of hot reactive species from within the combustor and their subsequent mixing with fresh reactants to form a low-oxygen concentration high-temperature oxidizer prior to ignition. This mixture results in distributed reaction over the entire combustion volume. To date, most of the CDC research efforts have focused on gaseous fuels only. In this paper, conditions fostering distributed combustion using JP-8 and alternative ethanol fuels. Data revealed that transition to CDC can be achieved at oxygen concertation of approximately 9.5% for both the fuels. This oxygen concentration was determined based on reaction field uniformity as identified from OH∗ chemiluminescence signatures. Under distributed combustion, emissions were substantially reduced by some 95% to result in NO_x emissions of less than 2 PPM with minimal impact on CO emission from both JP-8 and ethanol fuels at an equivalence ratio of 0.9, with even lower emissions at lower equivalence ratios. Combining the data obtained with liquid fuels with those from gaseous fuels revealed that, regardless of the fuel used, the oxygen concentration at which CDC prevailed can be predicted based on mixture temperature within a range of 0.75%. This knowledge enables designing a combustor to achieve CDC regardless of the fuel used. The data are useful in the design and development of fuel flexible CDC for high combustion intensity gas turbine applications.