摘要
纳米材料的广泛用途使其制备方法的研究越来越受重视。化学气相沉积法(CVD)是制备纳米粉体的一种很有效的方法,例如日常用的碳黑、钛白粉颜料(TiO_2)等都可以用该方法制得。颗粒尺寸、尺寸分布状况以及形态等特性对颗粒产品的性能都产生极大的影响,这就要求对生产装置的结构和操作参数要有很好的了解和控制。
本文应用CFD商业软件FLUENT,对火焰CVD法合成二氧化钛纳米颗粒的过程进行了数值模拟。首先对CVD法中的湍流扩散火焰进行了详细的模拟,在采用单步化学反应假设的条件下,通过比较得出非平衡壁面函数法下的RNGκ-ε模型的模拟结果与实验中观测到的火焰形状和温度场最为接近;在此基础上,将二氧化钛作为一种准气体模拟计算了火焰的温度场和各组分的浓度场。
其次引入颗粒动力学模型(Schild等人,1999)及大连理工大学化工学院谢洪勇老师的Kn修正模型。并编制了一段关于颗粒碰撞生长的计算程序,在FLUENT预先计算得到的温度场内对火焰CVD法合成纳米颗粒过程中颗粒的凝结生长过程进行了模拟。这里假定所有的先驱物TiCl_4在反应后全部转化为自由的单分子,忽略了过程中晶体与晶格的转变;接着,在高温火焰中TiO_2分子单体之间不断碰撞而凝结,长大形成单个的大颗粒。这里将气体中的颗粒或者颗粒聚集块看成一种假定的气体组分,忽略颗粒对于流体的影响。在这些假设基础上对颗粒尺寸进行了预测,模拟结果显示该模型对颗粒尺寸的预测与实验数据相差不大。文中还分析了火焰温度,氧化剂流量等对生成颗粒尺寸的影响。结果表明,温度越高就越容易形成大直径的颗粒,颗粒在火焰中的停留时间越长,生成的颗粒或聚集块的尺寸就越大。
Considerable interest lies in the synthesis and the use of nanosized particles for a variety of applications. Commodities such as carbon blacks, pigmentary titania or optical fibers for telecommunications are typical products of industrial aerosol reactors. Particle characteristics like size and size distribution or the morphology mainly influence the final product quality. This emphasizes the need for a tool for optimization of the reactor geometry and the operating parameters.
Using the commercial CFD-code FLUENT, the simulation of the growth process of Titania nanoparticle synthesized in a flame CVD process for nanoparticles is performed. With the suppose that the combustion reaction occurs in a single step, the calculated results show that the RNG k-ε turbulent model produces reasonable predictions for the temperature profile and the shape of the flame with the Non-Equilibrium wall function in comparison with other method. On the base of that, the oxidation of TiCl_4 was also included in this turbulent model with the pseudo-component titania and the effects of particles to the fluid are ignored. By using the additional fluid-particle dynamics (Schild et al. 1999)and Kn modified fuction by HongyongXie, growth processes of Titania nanoparticle in the turbulent diffusion flame was simulated, where the process of all precursor molecules converting to free TiO2 "monomer" molecules firstly when the reactions occurred; after that these monomers turned into large particles or aggregates by coagulation caused by Brown collisions between particles in the high temperature flame. Based on those assumptions, the size of particles is simulated, and the effects of flame temperature and the flow rates of oxygen on the sizes of particles and aggregates have been calculated. The results indicate a flame of higher temperature more easily leads to big particles; the size of particles or aggregates become bigger with the longer residence time.
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