 |
Spray combustion is used in many combustors. In order for the atomized liquid fuel to stably burn, it is necessary to continuously cause group combustion in which the entire spray burns during spraying while the flame spread occurs near the flame base. However, the detailed mechanism of group-combustion excitation has not been elucidated. Researches on the flame spread between droplets using droplet arrays in microgravity have been conducted. In the Japanese Experimental Module "Kibo" aboard the International Space Station, experiments were conducted to clarify the mechanism of group-combustion excitation through flame spread between droplets. This study analyzes results of flame spread experiments with 67 droplets near the group-combustion-excitation limit of randomly distributed droplet clouds. A large-scale ignition phenomenon in which multiple droplets are ignited at the same time was observed, which has not been seen in the past experiments. We have hypotheses for this phenomenon that unburned droplets existing outside the flame-spread limit were heated by a group flame and created a flammable mixture, or heated unburned droplets vaporized with cool flames. We carried out verification experiments using droplet-cloud elements which are basic elements of randomly distributed droplet cloud. A flame surrounding droplets heated a droplet existing outside the flame-spread limit and the droplet vaporization was observed by a digital video camera with a back illumination. The droplet diameter was measured, and the vaporization-rate constant was calculated. The experiments were conducted for different numbers and positions of burning droplets. The results showed that increasing the number of heating droplets increased the vaporization-rate of the observation droplets. The observation droplets were almost completely vaporized. Furthermore, the vaporization-rate increased as the mass center position of the heating droplet was closer to the observation droplet. We also conducted another experiment in which a droplet was heated by a flame and pre-vaporized, and then the formed premixed gas was ignited from another flame. The flame appearing after the pre-vaporization was larger than the ordinary spreading flame. These results suggest that the pre-vaporization of the droplets gradually progresses due to the cool flame reaction occurring outside the flame-spread limit, leading to a large flame at the time of ignition. The large-scale ignition phenomenon in the randomly distributed droplet cloud occurred probably in the same way.
