Schlieren analysis of non-MILD distributed combustion in a mixture temperature-controlled burner

It was recently demonstrated that distributed combustion is accessible outside the MILD combustion regime without needing inner or outer flue gas recirculation. The Mixture-Temperature Controlled combustion concept, which made it possible, offers excellent flame stability besides ultra-low emission. This concept is investigated presently to reveal the qualitative characteristics of the cold discharging mixture jet from the burner and its ignition. The Schlieren technique with a high-speed camera is the most suitable approach for this purpose, revealing the line-of-sight density gradients. Nine cases were evaluated, utilizing natural gas and diesel fuel, various equivalence ratios, and atomizing pressures. V-shaped flames were used as a baseline for comparing distributed combustion to it via direct images and velocity field using the PIVlab Matlab application. The results confirm the previous hypothesis that distributed combustion features a cold fuel-air mixture at the burner discharge that ignites downstream. The excellent flame stability comes from the fishbone-tiled coherent structures with significant random features, resulting in no characteristic frequency related to the flame. All these results comply with the previous findings by chemiluminescence emission and acoustic signal of distributed combustion, which techniques cannot be used to investigate the flame structure, unlike Schlieren imaging.