A Study of Mechanism of Fuel Droplet Combustion using Laser Diagnostics in Microgravity

激光诱导荧光法（EXCIPLEX）用于远程、非侵入式测量微重力下燃料液滴燃烧的温度。将掺有萘和TMPD的燃料液滴置于静止气体环境中燃烧。用图像增强光学多通道分析仪测量了氮气激光激发下液滴的荧光发射光谱。结果表明，该诊断系统可成功应用于微重力条件下的液滴温度测量。两种不同波长的荧光发射强度之比是原位测定液滴温度的合适判据。利用平面激光散射技术对微重力条件下悬浮液滴周围火焰中烟尘浓度分布进行了二维可视化研究。利用图像分析系统从散射光的强度近似估计出烟尘浓度和瞬时烟尘量。所使用的燃料是由基础燃料、水和表面活性剂组成的油包水乳液。基础燃料是正十二烷。体积含水量在0 ~ 0.3之间变化。利用本工作所研制的仪器，可以在液滴周围的球面上观察烟尘区域。结果表明，烟气的燃烧行为和火焰的燃烧行为都是不稳定的。烟尘浓度最大值位于烟尘区内缘附近，与之前观测到的烟尘壳相对应。瞬时烟尘量的时间历程与瞬时火焰半径的时间历程相似。煤烟浓度最大值不随含水量的变化而变化，而煤烟量随水乳化而显著降低。

Laser induced fluorescence method (EXCIPLEX) has been employed for remote, non-intrusive measurements of the temperature of a fuel droplet burning under microgravity. A fuel droplet doped with naphthalene and TMPD was allowed to burn in a quiescent gaseous environments. The fluorescence emission spectra from a droplet subjected to the nitrogen laser excitation were measured with an image intensified optical multichannel analyzer. The results showed that a newly developed diagnostic system was found to be applicable successfully for droplet thermometry under microgravity. The ratio of fluorescence emission intensities at two different wavelengths was an appropriate criterion for in-situ determination of droplet temperature. Two-dimensional visualization of the soot concentration profile in a flame formed around a suspended droplet was carried out under microgravity using the planar laser light scattering technique. The soot concentration and the instantaneous amount of soot were estimated approximately from the intensity of the scattered light using the image analysis system. The fuels employed were water-in-oil emulsions composed of base fuel, water and sufactant. The base fuel was n-dodecane. The water content was varied from 0 to 0.3 in volume. The sooting region can be observed spherically around the droplet by using the apparatus developed in the present work. The results showed the unsteadiness of the sooting behavior as well as the flame behavior. The maximum of the soot concentration is located in the vicinity of the inner edge of the sooting region, which corresponds to the soot shell observed previously. The time history of the instantaneous amount of soot is similar to that of the instantaneous flame radius. The maximum of the soot concentration does not vary with the water content, while the amount of soot decreases significantly with the water emulsification.