On the flame-flow interaction under distributed combustion conditions

Colorless Distributed Combustion (CDC) has shown simultaneous benefits of high combustion efficiency, ultra-low pollutants emission, low combustion noise, uniform thermal field, and enhanced stability. Distributed combustion is fostered by reduced oxygen concentration and high temperature oxidizer to result in distributed reaction over a larger volume of the combustor and uniform thermal field. In this paper, the interaction between the velocity field (characterized through Particle Image Velocimetry) and the reaction region (identified through hydroxyl Planar Laser Induced Fluorescence) is investigated with focus on swirl assisted distributed combustion. A mixture of nitrogen and carbon dioxide was mixed with normal air upstream of the burner to simulate the hot reactive gases. The flowfield was characterized under non-reacting conditions to outline the impact of the added dilution on the flowfield. The results showed dilution to enhance both the inner recirculation and outer recirculation zones. Reacting flowfield, characterized to determine the impact of the temperature rise and density changes, showed maximum velocity region to shift downstream. Comparing the PIV data for reacting conditions with OH-PLIF revealed significant difference between normal swirl and CDC flames. In swirl flame, the flame was located around the shear layer of the entry jet (with both the inner and outer recirculation zones) where the velocity fluctuations and OH-PLIF fluctuations coincided. Flame transitioning to CDC pushed the reaction zone further downstream to locate at a position of lower velocity than what was found for swirl flames. In addition, the reaction zone occupied a much larger volume with lower signal intensity to exhibit distributed reaction. Experiments performed at same flow rates and velocities but with no reduction in oxygen concentration confirmed that the change in reaction behavior is attributed to the lower oxygen concentration rather than the increased flowrates due to dilution.