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

Experimental and numerical studies are performed on the non-premixed n-dodecane spray cool flames in a counterflow burner. A novel phenomenon of repetitive autoignition-extinction instability of near-limit non-premixed spray cool flames is observed and examined. The spray cool flame is established by a polydisperse n-dodecane fuel spray generated from a twin-fluid atomizer, ranging from sub-micron sizes up to 400 μm with Sauter mean diameter of 109 μm. Digital Inline Holography is used to measure spray size distribution and quantify the fuel fraction in spray and gas phases at experimental conditions. The chemiluminescence of the excited formaldehyde molecule in spray cool flame is recorded by an Intensified-CCD (ICCD) camera to examine the repetitive cycles of autoignition and extinction, and to measure the spray cool flame stabilization time in each cycle. The repetitive autoignition and extinction cycles are found to be mostly caused by the competition between chemical heat release from low-temperature fuel oxidation, and heat loss in fuel spray vaporization in a counterflow, 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. A one-dimensional two-phase monodisperse spray combustion model with detailed chemistry is applied to reveal the spray cool flame structure and dynamics. It is shown that spray sizes determine the spray cool flame structure, where small droplets pose minimal impacts like gaseous flame while large droplets penetrating the flame front have double-edged effects on spray cool flames, leading to autoignition-extinction events. The experiments and simulations provide insights of this novel phenomenon of repetitive autoignition-extinction instability of near-limit non-premixed spray cool flames.