Study on the Combustion and Microexplosion of an Emulsified Fuel Droplet in High Pressure Gaseous Environments

用加热的镍铬丝点燃了安装在高压燃烧室中的细石英纤维尖端悬浮的乳化燃料液滴。提供了一台CCD摄像机来观察液滴火焰，并确定燃烧寿命和微爆炸开始的等待时间。用精密热电偶测量了液滴温度。采用平面激光散射技术对液滴火焰中的碳烟流场进行了可视化研究。利用图像分析系统对散射光强图像的测量结果进行量化处理，得到碳烟浓度的二维分布。还提供了一种高速摄像机，用于对微爆炸引起的液滴的相变、相分离和二次雾化进行摄影观察。对以正十二烷和正十六烷为基础燃料的油包水和水包油乳化燃料进行了试验。结果表明，在超临界气体环境中，随着压力的增加，燃烧寿命缩短，趋于恒定。压力的增加导致最终液滴温度升高，低于基础燃料的饱和温度。这在高压下变得非常明显。水包油乳化燃料液滴在高温下发生相变和水层与油层的分离。悬浮在水面上的球形水滴，包裹在基础燃料层内。但油包水乳化燃料液滴的情况并非如此。微爆炸仅在低于极限压力时发生，且随着含水率的增加而略有增加。微爆开始的等待时间随压力的增加而缩短，与威布尔分布函数相关。液滴火焰中的最大碳烟量随着压力的增加和水分的减少而增加。

An emulsified fuel droplet suspended at a tip of fine quartz fiber installed in a high pressure combustion chamber was ignited with a heated nichrome wire. A CCD camera was provided to observe a droplet flame and to determine the combustion life time and the waiting time for the onset of microexplosion. The droplet temperature was measured by using a fine thermocouple. A planar laser light scattering technique was employed for the visualization of soot field in the droplet flame. The measured results of the image of scattered light intensity were quantified by using an image analyzing system to obtain the two dimensional profile of soot concentration. Also provided was a high speed video camera for the photographic observation of the phase transition, phase separation in the liquid phase and the secondary atomization of droplet due to microexplosion. Tested were water-in-oil and oil-in water emulsified fuel with the base fuel of n-dodecane and n-hexadecane.The results showed that the combustion life time decreased with an increase in pressure, approaching a constant in supercritical gaseous environments. Increasing pressure caused an increase in the final droplet temperature which was lower than the saturated temperature of the base fuel. This became remarkable at high pressure. The phase transition and the separation of water and oil layrs was observed at high temperature of the oil-in-water emulsified fuel droplet. A spherical droplet of water suspended afloat and enclosed within the layr of base fuel. This was not the case for water-in-oil emulsified fuel droplet. The microexplosion occurred only at the pressure lower than the limit which increased slightly with increasing water content. The waiting time for the onset of microexplosion which decreased with increasing pressure was correlated with the Weibull distribution function. The maximum quantity of soot in the droplet flame increased with an increase in pressure and a decrease in water content.