柴油机等径颗粒平面碰撞过程凝并特征

Coalescence features of planar collision between particulate matters of same diameter from diesel engine

针对后燃期柴油机颗粒物的碰撞过程,结合气溶胶颗粒的受力分析和基于弹性形变力、范德华力的非完全弹性碰撞运动方程,探讨了等径颗粒平面非完全弹性碰撞过程的两个特征-压缩、恢复和影响压缩恢复的主要特征参数.进一步研究了入射角、碰撞频率、压缩距离、恢复系数和凝并效率等特征参数相互之间的关系及其在碰撞过程中的作用.研究结果表明,任意粒径下,随着压缩过程的进行,范德华力均增强;对于相同粒径颗粒,在压缩过程中,弹性形变力始终大于范德华力,且两者差距逐渐增大.颗粒粒径分散性越大,颗粒间碰撞频率越大.15,45 nm颗粒分别与50 nm颗粒碰撞频率的比值约为1.9倍.等粒径颗粒碰撞过程中,初始相对速度保持不变时,入射角增大,压缩距离增大,对心碰撞时,压缩距离最宽;相同入射角下,压缩距离随着初始相对速度的增加而逐渐增大.随着恢复系数的增大,临界速度逐渐减小,凝并效率呈逐渐减小趋势.当恢复系数等于0.026时,凝并效率约为50.05%,能够实现较高效的碰撞和较快地促进颗粒凝并,当恢复系数大于0.063时,凝并效率接近于零,极大部分颗粒碰撞之后发生分离,阻碍了颗粒凝并.

It is usually assumed that collision means simultaneous coalescence for particulate matters of diesel engine combustion process. However, the limitation of this assumption is that the recovery phenomenon in collision period cannot be taken into consideration, while compression and recovery are two inevitable periods for collision of incomplete elastic particulate matters, in which Van de Waals force and elastic deformation force influence the coalescence features. In present research, the planar collision between particulate matters of same diameter in post-combustion period of diesel engine was studied and coalescence features of it was investigated further according to compression and recovery phenomena. In addition, the force analyses of particulate matters was utilized to reveal force effect and incomplete elastic collision function based on Van de Waals force and elastic deformation force was solved. In this case, coalescence feature parameters, namely, diameter, collision rate, incidence angle, compression distance and coalescence efficiency, as well as the relation between each other were investigated in terms of recovery phenomenon. The mathematic analyses showed that, Van de Waals force and elastic deformation force both gradually increased for particulate matters under any diameters when particulate matters compressed; furthermore, elastic deformation force was always larger than Van de Waals force with a trend of greater difference considering invariant diameters; and the larger the diameter was, the weaker the elastic deformation force became under the same compression distance, but Van de Waals force followed the opposite pattern of elastic deformation force. On the other hand, collision frequency of 50 nm between 45 and 40 nm was the lowest because higher diameter dispersivity caused greater collision frequency, for instance, it was 1.2×10^16 and 6.2×10^15 m^-3 s^-1 for particulate matters of 15 and 45 nm respectively. Besides this, the compression distance increased with raising incidence angle under the same initial relative velocity, which indicated that central collision had the widest compression distance; increasing initial relative velocity resulted in wider compression distance under the same incidence angle. Recovery factor was an essential element both for critical velocity and coalescence efficiency considering the central collision of 50 nm particulate matter; to be specific, both of critical velocity and coalescence efficiency reduced with raising recovery factor; the coalescence efficiency was approximately 50.05% when recovery factor was about 0.026 suggesting highly efficient collision and rapid coalescence; however, the coalescence efficiency was almost close to zero as long as the recovery factor was larger than 0.063, suggesting immediate separation after collision and barely no coalescence; in summary, a little variation of recovery factor could lead to the huge change of coalescence efficiency.