Autoignition behavior of a spherical cluster consisted of a center fine droplet and surrounding twelve fine droplets

Autoignition behavior of spherical clusters of pure droplets was experimentally examined near the ignition limit. The fuel with the melting points little above room temperatures was selected as the test fuel. The three-dimensional droplet clusters having hexagonal closest packing structure were selected as the test clusters, which consisted of a center droplet and surrounding twelve droplets (thirteen droplets in total). The initial diameters of the droplets were much reduced from the authors' previous studies. The results showed that the ignition delay of the three-dimensional droplet clusters also had a minimum at a certain droplet spacing, which has been predicted by Niioka and coworkers as a result of rate-controlling process transition between reaction and evaporation. Autoignition of the droplet clusters could be achieved for the center fine droplet, which is out of ignition limit when isolated, in combination with finer surrounding droplets.