Interactive combustion of two-dimensionally arranged quasi-droplet clusters under microgravity

Abstract

To investigate the mutual interactions between droplets in the spray combustion, combustion of 2-dimensionally arranged quasi-droplet clusters is studied under microgravity. Quasi-droplet samples, which are solid in room temperature and change into liquid just after the ignition, consist of alcohol (propanol, butanol, pentanol, or hexanol) and polyethylene glycol with a volumetric ratio of 2:1. Seven samples sustained by glass rods form a 2-dimensional quasi-droplet cluster. Electrically heated nichrome wires ignite all samples in the cluster simultaneously. Single envelope flames that surround the clusters appeared. The results show that the sample spacing has a strong effect on the shape and movement of the flame. Sample clusters with large sample spacings come to the external group combustion through the scavenging combustion mode, whereas the small spacing clusters start directly with the external group combustion. At large sample spacings, the distance from the edge of the sample cluster to the flame (flame distance) increases to a maximum value and then decreases with time. The period of flame growth is prolonged with decreasing sample spacing and finally, at a small enough sample spacing, the flame distance keeps increasing until the flame disappears. This flame movement is attributed to the fuel vapor accumulation effect, which becomes more dominant with decreasing sample spacing. The burning lifetime decreases monotonically and approaches the value of the single flame with increasing sample spacing. The flame distance decreases monotonically and approaches the single flame radius with increasing sample spacing also. These results render important confirmations of the external group combustion phenomena and prove the importance of the two kinds of unsteadiness, i.e., the scavenging combustion with large droplet interval and the fuel vapor accumulation effect with small droplet interval, in group combustion.-3-

Correlation of Burning Rate of the Interacting Liquid Droplets with Internal Circulation

“…These flame shapes under the present conditions are classified into external or internal group combustion in previous study of Chiu et al (1) , whose study was reinforced by the experimental results by Nagata et al (4) The present results that the flame penetration depth with decreasing mean droplet spacing decreases, and that the burning rate of the second droplet decreases because of the reduction of flame penetration depth, demonstrate a good agreement with previous studies. Figure 9 shows the ratio of burning rate of interacting droplets to that of single droplet at D = 15, in which the second droplet is under weak interaction with the first droplet.…”

“…The burning behavior of interacting droplets, however, is quite different from that of the single droplet. To investigate basic mechanism of spray combustion, the combustion of two droplets and droplet arrays has been studied experimentally (2) - (4) and theoretically (5), (6) . In practical combustion process, the interaction between atomized droplet and surrounding gas flow affects droplet vaporization rate and flame configuration which are quite different from those in a stagnant environment.…”

Droplet Array Combustion Experiments on Effect of Initial Droplet Diameter on Flame Spread Characteristic Time

“…Therefore, studies of droplet array combustion have been conducted under various experimental conditions. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] In most past experiments of flame spread along a droplet array, the initial droplet diameter was about 1.0 mm. Improving the spatial and temporal resolution of the flame spread phenomenon would facilitate observation.…”

Effects of Droplet Spacing on Evaporation of a Cluster of 13 Fuel Droplets

“…On the basis of this knowledge, experimental studies have been conducted with arrays or matrices of fuel droplets. For experimental convenience, the droplet size is relatively large, and the droplet number is limited around ten 1,2) . Larger droplets are preferred for better optical observation.…”