Repetitive autoignition and extinction of near-limit non-premixed n-dodecane spray cool flames

Experimental and numerical studies are performed on autoignition and extinction of non-premixed n-dodecane spray cool flames. A novel phenomenon of repetitive autoignition-extinction instability of near-limit non-premixed spray cool flames is observed and examined. The repeated autoignition and extinction cycles are mostly the results of the competition between timescales of fuel spray vaporization and low-temperature oxidation, where the dynamics of large droplets in polydisperse spray play an important role. It is also found that with the increase of the oxygen mole fraction or the oxidizer temperature, the spray cool flame stabilization time increases, and the cycles of autoignition and extinction become less frequent. Over a critical oxidizer temperature and oxygen mole fraction, the repetitive autoignition and extinction instability disappears, and a stable spray non-premixed cool flame can be observed. A one-dimensional two-phase model with detailed chemistry is employed to reveal the spray flame structure and dynamics. It is shown that the large droplets can penetrate the flame front and result in the autoignition-extinction instability. The experimental evidence and numerical results provide the insights of this novel phenomenon of repetitive autoignition-extinction instability of near-limit non-premixed spray cool flames. The results will contribute to the development of advanced low-temperature combustion engines and spray combustion models.