Research on Flame Propagation and Heat Transfer in Fine Fuel Particle Cloud

研究了细颗粒燃料云的火焰传播机理及其传热过程。首先，利用形成煤粉与空气均匀混合物所必需的微重力环境，对煤粉云的火焰传播现象进行了详细观察。通过观察，发现了两种不同的火焰锋面结构，即(1)类似于气体混合物的连续火焰锋面和(2)燃料颗粒在火焰锋面前面自燃的离散火焰锋面。采用排列成行的发泡聚苯乙烯对大颗粒燃料的火焰传播过程进行了模拟。实验结果表明，当颗粒间距相对于火焰锋面位置足够大时，火焰锋面出现离散，而当颗粒间距较小时，火焰锋面出现连续传播。因此，粒子间距是决定火焰锋面结构的最主要因素之一。在对火焰锋面结构进行观察的基础上，研究了O2浓度、颗粒间距和颗粒大小对火焰传播速度的影响。根据上述实验结果，传热过程的重要性是由未燃烧颗粒与火焰前缘的相对位置决定的。特别是在离散火焰传播的情况下，火焰的传播速度由火焰前缘向未燃烧燃料的热量传递决定。为此，对点火后的传热过程进行了数值模拟。结果表明，火焰的传播机制可能有三种：1)燃料颗粒释放的混合气体的火焰传播；2)未燃烧的燃料颗粒附近的挥发分自燃；3)燃料颗粒表面着火。(1)和(2)出现机理的范围较宽，与在煤粉颗粒云中观测到的火焰结构相对应。少

Flame propagation mechanism and relating heat transfer process of fine fuel particle cloud have been investigated. At first, flame propagation phenomenon of pulverized coal cloud was observed in detail by using microgravity environment necessary to form homogeneous mixture of coal particle and air. According to the observation, two different structures of flame front, (1) continuous flame front like a flame front of gaseous mixture and (2) discrete flame front with autoignition of fuel particle ahead of flame front, are found out.The simulation of the flame propagation process with larger fuel particles was tired by using arranged foamed polystirene in line. The experimental results show that the discrete flame front appears when the particle spacing is large enough relative to the position of flame front, while the continuous flame propagation appears when the spacing is narrow. Therefore, the particle spacing is one of the most dominant factors to detrmine flame front structure. In a … More ddition to the observation of flame front structure, effects of O2 concentration, particle apacing, and particle size on flame propagation velocity were examined.According to the above experimental results the importance of heat transfer process which is determined by the relative position of unburned particle to the flame front. Especially, the case of discrete flame propagation the propagation velocity is determined by the heat transfer from the flame front to the unburned fuel. Thus, the numerical simulation on heat transfer process followed by ignition have been carried out. The results showed that the three possible mechanism on flame propagation, (1) flame propagation of gaseous mixture released from fuel particles, (2) autoignition of volatile matter close to the unburned fuel particle, and (3) surfase ignition on the fuel particle, are suggested. The range to appear the mechanism of (1) and (2) are wide and those are though to be corresponding to the flame structures observed in the pulverized coal particle cloud, respectively. Less