【作者】 王宇；

【导师】 姚强；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2009， 博士

【摘要】 在对燃烧源可吸入颗粒物形成与控制的研究中,火焰内部颗粒物控制已经随着科学研究以及诊断技术的发展而受到更广泛的关注。明确机理并有效控制颗粒成核、生长、团聚及氧化四个重要阶段将对颗粒物的脱除以及定向生产起到重要作用。碳烟颗粒是化石燃料火焰中的首要颗粒物。经过数十载的研究,其形成各个阶段的机理尚未得到科学、全面的描述。本文将从碳烟颗粒荷电这一特性入手,通过人为外加一个均匀分布的静电场,放大荷电特性对碳烟颗粒运动、体积浓度及形貌的影响。采用近10年来兴起的碳烟颗粒特性在线诊断技术(LII,LE,TSPD-TEM)对静电场影响下的碳烟颗粒展开定量研究。首先确定并量化描述研究对象。选择国际通用的McKenna燃烧器作为产生碳烟均匀分布的火焰。运用激光诊断和热泳探针采样技术确定了碳烟均匀分布特性与流量、当量比之间的关系,并通过对出口速度和火焰传播速度的计算给予进一步解释。建立了一个参数u量化碳烟分布的均匀程度。利用Maxwell电磁场计算软件确定了能产生均匀分布静电场的电极尺寸。通过激光诊断技术间接测量了荷电碳烟在上极板的沉积速率。得出火焰中的碳烟颗粒绝大多数荷电且有至少超过50%荷正电的半定量结论。碳烟颗粒在火焰中的滞留时间将随电场的电性和电场强度而改变。在确定了颗粒荷电电性后,测量了静电场影响下火焰中碳烟颗粒的绝对体积浓度以及形貌尺寸的变化,并通过对颗粒受力的计算,得出电场力与曳力随燃气流速、当量比、颗粒形貌、尺寸变化的规律,进一步解释了实验结果。证明了颗粒在形成初期(粒径小于100 nm的一次颗粒和团聚体),主要受到曳力作用,电场力主要改变颗粒运动方向;在颗粒团聚、氧化阶段(粒径约在100 ~ 200 nm),将主要受到电场力作用,脱离原流体运动,滞留时间改变,最终导致颗粒形貌特性的改变。最后通过测量闪变点和放电点,定量描述了电场对碳烟颗粒影响的极限情况,并在实验中发现了正电场作用下的二次稳定现象。更多还原

【Abstract】 In last 20 years, the research on the formation and control of combustion generated inhalable particles was conducted in more details. The state-of-art diagnostic processes provided better ways to do the on-line research. Soot is the most initial and important particle generated by combustion. However, it is important to note that no previous study has been reported on which a one dimensional flame is aligned normal to a uniform electric field. This has inhibited capacity to isolate the separate effects of the field and contributed to the lack at a quantitative study of the electrophoretic effect (without ionic wind) on soot particles. To meet these needs, the present research aims to investigate the effects of a uniform electric field on a series of one dimensional laminar premixed ethylene/air flames by using the state-of-art on-line diagnostic technologies (LII, LE and TSPD-TEM).The first aim is to find the uniform flame and electric field we need. Laser diagnostic technologies and thermophoretic sampling system were used to describe the uniformity of the soot distribution in the flames; and electric field was calculated to find the right set up scale to make it uniform.The second aim is to assess whether positively or negatively charged soot particles dominate in a flame. This was assessed by laser extinction, used to measure the variation of soot volume fraction due to the deposition under an electric field. It is found that more than 50% of charged soot particle in the flame are positive. The presence of electric field can affect the residence time of charged soot particles.The third aim is to measure the variation of soot volume fractions and particle morphology with change in the residence time induced by an electric field. Laser induced incandescence and thermophoretic sampling particles diagnostics were used. Drag force, electric force and thermopherotic force were calculated to provide a quantitative explanation for the experiment results. It is found that for the particles which smaller than 100 nm, drag force is the dominant force and the electric force can only change the fly direction of the soot; while for particles bigger than 100 nm (ususlly smaller than 200 nm in the flame), the number of charge will remarkably increase and the electric force will be the dominant force instead of drag force.The forth aim is to assess the effect on flame flicker of an applied electric field as a function of equivalence ratio. This was examined by determining the flickering points and the discharge points. The re-stabilization was found in a positive electric field, indicated the remarkable effect of the electric force.更多还原