Fundamental Processes of Nanoparticle Synthesis in Spray Flames: Evaporation, Mixing, and Chemical Reactions

喷雾火焰中纳米颗粒的合成通过蒸发、湍流混合和化学反应来表征。与由纯燃料提供燃料的喷雾火焰不同，用于喷雾火焰合成纳米颗粒的前体溶液强烈影响整个系统。特别是，由于缺乏关于前体溶液的物理性质和可能在液体内发生的反应的信息，因此很难获得由前体溶液组成的多组分液滴的蒸发情况。颗粒可能直接在液滴内部形成，这是不需要的，并观察到微爆炸，这直接影响整个过程。这些影响将在本项目中通过数值模拟的方式进行研究。此外，解析气相的一维模型将提供液滴后面的反应条件的证据，以提供关于全球系统内反应条件的信息。蒸发的前体影响燃烧行为。在这个项目中，化学反应和蒸发的耦合是通过使用喷雾火焰库来实现的，除了通常用于纯气体燃烧的表格外，在该库中还考虑了蒸发。特别是，使用混合分数和化学反应进度变量来考虑部分预混化学反应，这些变量将被整合到前体溶液的湍流喷雾燃烧模型中。发现了层流单分散富燃料喷雾火焰的多种解决方案：典型的双反应区，其中一个位于喷雾一侧，另一个位于喷雾一侧，以及一种新的结构，在气体一侧显示单个反应区，导致分离的蒸发和燃烧区，这两个区域是强耦合的：蒸发区将气体燃料送入反应区，反应区为喷雾蒸发提供能量。后一种情况将在SpraySyn燃烧器内火焰脉动和微爆炸发生的背景下进一步研究。这些新结构将建立一个喷雾火焰库，用于湍流前体喷雾火焰的模拟。湍流混合、蒸发和化学反应的相互作用可以利用联合概率密度函数整合到湍流模拟中。用混合分数、化学反应进程变量、蒸发速率和热焓等参数表征了前驱体溶液蒸发积分的喷雾火焰。参数研究将提供在这些火焰中合成纳米颗粒的初始条件的影响的证据。本项目开发的子模型将进入SpraySyn燃烧器的全球模拟，并传输给SPP中的其他项目合作伙伴。

Nanoparticle synthesis in spray flames is characterized through evaporation, turbulent mixing, and chemical reactions. In contrast to a spray flame that is fed by a pure fuel, the precursor solution that is used in spray flame synthesis of nanoparticles strongly affects the entire system. In particular, the evaporation of the multicomponent droplets consisting of the precursor solution is difficult to access because of the lack of information of the physical properties of the precursor solution and the reactions that may occur inside the liquid. Particles may be formed directly inside the liquid droplet which is unwanted and microexplosion is observed which directly affect the entire process. These effects will be studied in this project by means of numerical simulations. Moreover, one-dimensional models resolving the gas phase will provide evidence of the reaction conditions in the wake of the droplet to provide information on the reaction conditions within the global system.The evaporated precursors influence the combustion behavior. In this project, the coupling of the chemical reactions and the evaporation is achieved through use of spray flamelet libraries in which the evaporation is accounted for in addition to the usual tabulation for pure gas combustion. In particular, partially premixed chemical reactions are considered using a mixture fraction and a chemical reaction progress variable which will be integrated into the turbulent spray combustion modeling of the precursor solution. Novel multiple solutions of laminar mono-disperse fuel-rich spray flames were found: the typical double reaction zone, where one of them resides on the spray side and the other one on the gas side of the configuration, and a new structure that shows a single reaction zone on the gas side, leading to separate evaporation and combustion zones that are strongly coupled: the evaporation zone feeds the gaseous fuel into the reaction zone which delivers the energy for spray evaporation. The latter scenario will be studied further in the context of the pulsation of the flame in the SpraySyn burner and the occurrence of microexplosion. These new structures will build a spray flamelet library for use in the simulation of the turbulent precursor spray flame.The interaction of the turbulent mixing, evaporation, and chemical reactions may be integrated into the turbulent simulation using joint probability density functions. The parameters mixture fraction, chemical reaction progress variable, evaporation rate, and enthalpy are used that characterize the spray flame where the evaporation of the precursor solution will be integrated. Parameter studies will give evidence of the influence of the initial conditions of nanoparticle synthesis in these flames. The sub-models which are developed in this project will enter the global simulation of the SpraySyn burner and transferred to the other project partners in the SPP.