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

在高压燃烧室中，用加热的镍铬丝点燃悬浮在细石英纤维顶端的乳化燃料液滴。利用CCD相机观察微滴火焰，测定微爆炸的燃烧寿命和等待时间。采用精密热电偶测量液滴温度。采用平面激光散射技术对液滴火焰中的烟尘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，得到烟尘浓度的二维分布。同时配备了高速摄像机，用于照相观察微爆炸引起的相变、液相分离和液滴二次雾化过程。以正十二烷和正十六烷为基础燃料，对油包水和油包水乳化燃料进行了试验。结果表明，燃烧寿命随压力的增加而减小，在超临界气体环境中接近常数。压力的增加导致最终液滴温度的升高，最终液滴温度低于基础燃料的饱和温度。这在高压下变得非常显著。观察了水包油乳化燃料液滴在高温下的相变和油水分离现象。水珠：悬浮在水面上的球形水珠，包裹在基础燃料层内这与油包水乳化燃料液滴的情况不同。微爆炸仅在压力低于极限时发生，压力随含水量的增加而略有增加。微爆炸发生的等待时间随压力的增加而减小，符合威布尔分布函数。液滴火焰中最大烟尘量随压力的增加和含水量的降低而增加。

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.