Direct numerical simulation of multi-physics reactive mass transfer at single and multiple bubbles

上升气泡的反应传质是许多工业上重要的化学过程的基础。必要的工艺强化可使工艺步骤更快，系统高度集中。在这种具有复杂的传质、传输和化学反应的局部相互作用的两相流中，附加的多物理变得相关。这包括：溶解气泡的体积效应；由于污染或添加剂而使气泡表面部分固定；离子物种由于固有电场与扩散通量的强烈耦合，通常导致混合物远离界面的局部电中性；交叉扩散效应以及较高浓度体系中的非理想性。这些复杂性增加了反应传质的多尺度性质，由于对流为主的传输，具有极薄的浓度边界层。除了实验研究外，要彻底了解基本子过程的局部相互作用，需要基于严格的数学模型的数值模拟。我们的方法采用了基于质量、动量和物种质量两相平衡的连续介质物理。基于流体体积(VOF)方法，建立了气泡传质三维直接数值模拟的双标量方法。该方法采用了次网格规模的模型，该模型在第一个资助期得到了大力改进，以考虑到现实的施密特数。在第二个资助期，将该方法推广应用到多个气泡的组合中，在具有周期边界条件的计算盒中上升，从而模拟无限大的气泡群。此外，现有的考虑污染影响的方法将得到改进，包括采用可变的吉布斯弹性来模拟部分固定的气泡界面。详细的数值模拟将对类群体条件下气泡周围的反应传质过程和体积混合有更深的了解。在优先程序的网络中，这些技术允许用其他项目中开发的不同模型化学来模拟泡状流中的传质。特别是，铁络合物在甲醇中的氧化和铁络合物在水中的硝化将被考虑。除了局部Sherwood数和增效性外，还将研究系统参数对产率和选择性的影响。除了研究自由上升的气泡群/链外，该项目还将通过包括气泡收缩和共轭传质在内的模拟，为泰勒气泡/流动的指导措施做出贡献。最后，分析了固有电场对离子物种电迁移的影响。所有实验装置的模拟结果都将与合作的同事讨论。将在SPP内合作开发缩小规模模型的改进相关性。

Reactive mass transfer from rising gas bubbles is the basis for many chemical processes of industrial importance. The necessary process intensification leads to faster process steps and highly concentrated systems. In such two-phase flows with complex local interaction between mass transfer, transport and chemical reactions, additional multi-physics becomes relevant. This includes: volume effects for dissolving bubbles; partly immobilized bubble surfaces due to contamination or additives; ionic species with strong coupling of diffusive fluxes by the intrinsic electrical field, typically leading to local electro-neutrality of the mixture away from the interface; cross-diffusion effects as well as non-idealities in systems of higher concentrations. These complexities add to the multi-scale nature of reactive mass transfer with extremely thin concentration boundary layers due to convection-dominated transport. Besides experimental investigations, a thorough understanding of the local interplay of elementary sub-processes requires numerical simulations based on rigorous mathematical modeling. Our approach employs continuum physics based on the two-phase balances of mass, momentum and species mass. Based on the Volume of Fluid (VOF)-method, we built on our two-scalar approach for 3D Direct Numerical Simulations of mass transfer at gas bubbles. The approach employs a subgrid-scale model which has been strongly improved in the first funding period to allow for realistic Schmidt numbers. In the second funding period, the method will be extended and applied to groups of several bubbles, rising in a computational box with periodic boundary conditions, thus simulation an infinite bubble swarm. Moreover, the existing approach to account for contamination effects will be improved by including a variable Gibbs elasticity to model a partly immobilized bubble interface. Detailed numerical simulations will yield deep insights into reactive mass transfer processes and bulk mixing around bubbles under swarm-like conditions. Within the network of the priority program, these techniques allow to simulate the mass transfer in bubbly flows with different model chemistry, developed in other projects. In particular, the oxidation of iron complexes in methanol and the nitration of iron complexes in water will be considered. In addition to local Sherwood numbers and enhancement, the influence of system parameters on yield and selectivity will be studied. Besides the investigation of freely rising bubble groups/chains, the project will contribute to the guiding measure of Taylor bubbles/flow with simulations including bubble shrinkage and conjugate mass transfer. Finally, the influence of electro-migration of ionic species due to the inherent electrical field will be analyzed. All simulation results for experimental setups will be discussed with the cooperating colleagues. Improved correlations for a scale-reduced modeling will be developed in cooperation within the SPP.