Mixing structures in bubble wakes of single bubbles and bubble swarms and their influence on gas-liquid mass transfer and chemical reactions

该建议的主要目标是揭示如何在自由上升的单个气泡的尾迹和气泡群中的混合结构可以被纳入紧凑的过程工程的传质相关性。通过这种方式，可以更容易地估计尾流对化学反应的影响。关于详细传质的知识对于生产日常生活中使用的大部分散装化学品至关重要，这些化学品在氢化，氧化或氯化等气液反应中合成。此外，气泡尾流中包含的高气体浓度也可能对充气反应器中的微生物和酶有害，导致底物抑制。为了预测有多少气体从气泡转移到周围的液体中，必须确定传质系数（或无量纲化的舍伍德数）。今天的传质模型（舍伍德相关）建立在双膜理论，渗透理论，或表面更新理论与不同的参数依赖于气泡相对于周围液体的速度和层流或湍流周围的流动条件。存在适应模型来解释表面活性剂对传质的影响。然而，所有的理论都假设，一旦气体溶解在液相中，它就立即充分混合，并且一旦离开气泡表面，通过气泡界面的所有流体包裹在气泡尾流中经历相同的条件。这幅图忽略了非均匀浓度尾流的影响，该尾流可能会影响气泡界面附近和气泡尾流中发生的反应，如Levenspiel所述，对于小到中等的Hatta数。到目前为止，气泡尾流中混合结构的细节还没有被考虑用于传质关联。在这项研究建议中，拉格朗日相干结构（LCS）的气泡尾流的3D自由上升的气泡和气泡群在停滞液体中进行分析，通过评估4D-PTV测量，使用最近开发的数学工具，源于动力系统理论。此外，将使用时间分辨激光诱导荧光（TRS-LIF）对溶解气体浓度进行定量评价。LCS将与浓度尾流进行比较，这两种测量结果都为开发新的舍伍德相关性提供了输入，该相关性将气泡尾流中相干流结构的影响纳入其中。

The main objective of this proposal is to unveil how mixing structures in the wakes of freely rising single bubbles and in bubble swarms can be incorporated into compact mass transfer correlations for process engineering. In this way the impact of the wakes on chemical reactions can be estimated more easily. The knowledge about the detailed mass transfer is crucial to produce a vast proportion of bulk chemicals used in everyday life that are synthesized in gas-liquid reactions like hydrogenation, oxidation, or chlorination. Furthermore, contained high gas concentrations in bubble wakes can also be potentially harmful to microorganisms and enzymes in gassed reactors leading to a substrate inhibition. To predict how much gas is transferred from a bubble to a surrounding liquid the mass transfer coefficient (or the nondimensionalized Sherwood number )has to be determined. Todays mass transfer models (Sherwood correlations) either build on the two-film theory, on the penetration theory, or on the surface-renewal theory with varying parameters in dependence on bubble relative velocity to the surrounding liquid and laminar or turbulent surrounding flow conditions. Adapted models exist to account for the effect of surfactants onto the mass transfer. All theories assume, however, that once the gas is dissolved in the liquid phase it gets immediately well mixed and that all fluid parcels passing the bubble interface experience the same conditions in the bubble wake once they leave the bubble surface. This picture neglects effects of a heterogeneous concentration wake that will likely influence a reaction taking place close to the bubble interface and in the bubble wake for small to intermediate Hatta numbers as according to Levenspiel. So far, the details of the structure of the mixing in the bubble wake are not considered for mass transfer correlations. In this research proposal, the Lagrangian coherent structures (LCS) in the bubble wake of 3D freely rising bubbles and bubble swarms in stagnant liquid will be analyzed by evaluating 4D-PTV measurements using recently developed mathematical tools stemming from dynamical systems theory. Additionally, a quantitative evaluation of the concentration of the dissolved gas will be obtained using Time Resolved Laser Induced Fluorescence (TRS-LIF). The LCS will be compared to the concentration wakes and both measurements supply input for the development of new Sherwood correlations that incorporate the effect of coherent flow structures in bubble wakes.