To investigate the mutual interactions between droplets in the spray combustion,combustion of 2-dimensionally arranged quasi-droplet clusters is studied undermicrogravity. Quasi-droplet samples, which are solid in room temperature and changeinto liquid just after the ignition, consist of alcohol (propanol, butanol, pentanol, orhexanol) and polyethylene glycol with a volumetric ratio of 2:1. Seven samplessustained by glass rods form a 2-dimensional quasi-droplet cluster. Electrically heatednichrome wires ignite all samples in the cluster simultaneously. Single envelope flamesthat surround the clusters appeared. The results show that the sample spacing has astrong effect on the shape and movement of the flame. Sample clusters with largesample spacings come to the external group combustion through the scavengingcombustion mode, whereas the small spacing clusters start directly with the externalgroup combustion. At large sample spacings, the distance from the edge of the samplecluster to the flame (flame distance) increases to a maximum value and then decreaseswith time. The period of flame growth is prolonged with decreasing sample spacing andfinally, at a small enough sample spacing, the flame distance keeps increasing until theflame disappears. This flame movement is attributed to the fuel vapor accumulationeffect, which becomes more dominant with decreasing sample spacing. The burninglifetime decreases monotonically and approaches the value of the single flame withincreasing sample spacing. The flame distance decreases monotonically and approachesthe single flame radius with increasing sample spacing also. These results renderimportant confirmations of the external group combustion phenomena and prove theimportance of the two kinds of unsteadiness, i.e., the scavenging combustion with largedroplet interval and the fuel vapor accumulation effect with small droplet interval, ingroup combustion.
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