X-ray tomography studies of Taylor bubbles with mass transfer and surfactants in small channels

深入了解毛细管多相流中的动量、热量和传质过程，无论是从基础科学的角度，还是对于新型化学反应装置的实际设计和操作，如整体式微型反应器、微型换热器、燃料电池等，都具有重要意义。在微观尺度上，流动拓扑结构和液膜厚度、气泡形状和气泡周围液体速度分布的精确数据是开发传热传质模型和界面解析CFD程序的决定性输入。目前，对微通道中泰勒气泡形状的研究还很少。最近的实验研究使用高速成像技术研究了弹状物的长度和速度，使用PIV/LIF技术研究了液体速度场，并使用共聚焦激光扫描显微镜和激光聚焦位移扫描技术研究了最近的膜厚。但广泛使用的光学测量技术在两相流条件下具有很强的局限性，特别是不能分辨气泡的完整三维形状。光学技术进一步受限于透明通道和流体，而衍射将可实现的空间分辨率限制在约1微米。在该项目的框架内，申请人致力于开发一种X射线层析成像方法，该方法可以揭示毛细管两相流中泰勒气泡的三维形状，目标是以大约3μm的空间分辨率为目标。新的成像技术将用于记录不同流动类型(气-液、液-液)的气泡形状、通道横截面、壁面材料和毛细管数，并以这种方式为开发此类流动的传质模型提供数据，该模型正被SPP的其他合作伙伴所追求。测量技术的核心是一个系统，它提供泰勒气泡X射线投影的数字采集，并与气泡通过X射线风扇的过程同步。通过包括定制的脉冲计数时间选通X射线探测器的微焦点X射线成像器重复获取X射线投影。使用自适应的图像重建算法将投影数据重建为横断面图像。除了使用微焦点X射线源外，还计划进行使用同步辐射的实验研究。新的测量方法是独一无二的，远远超出了动态微观流动成像的当前技术水平。该技术所获得的流动结构数据将为毛细管多相流的细观和微尺度流动数值模型的发展提供支持。

A profound understanding of momentum, heat and mass transfer in capillary multiphase flow is of primary importance from the viewpoint of fundamental science as well as for practical design and operation of new chemical reaction devices, such as monolithic microreactors, miniature heat exchangers, fuel cells and others. Knowledge of the flow topology and precise data of the liquid film thickness, bubble shape and liquid velocity profiles around bubbles on the microscopic scale is a decisive input for the development of heat and mass transfer models as well as interface-resolving CFD codes. Today there is only coarse knowlegde on Taylor bubble shape in microchannels available. Recent experimental investigations explored slug length and slug velocity using high-speed imaging, liquid velocity fields using PIV/LIF and most recently film thickness using confocal laser scanning microscopy and laser focus displacement scanning. But the widely used optical measurement techniques have strong limitations in two-phase flow conditions and are particularly not able to resolve the full 3D shape of the bubbles. Optical techniques are further constrained to transparent channels and fluids and diffraction limits the achievable spatial resolution to about one micrometer. Within the frame of this project the applicant aims at the development of an X-ray tomography method which can disclose the three-dimensional shape of Taylor bubbles in capillary two-phase flow, targeting for the moment at about 3 μm spatial resolution. The new imaging technique shall be applied to record the bubble shape for different flow types (gas-liquid, liquid-liquid), channel cross-sections, wall materials and capillary numbers and this way provide data for the development of mass transfer models for this type of flow, which is pursued by other partners of the SPP. Core of the measurement technique is a system that provides a digital acquisition of X-ray projections from Taylor bubbles which is synchronized with the passage of the bubbles through the X-ray fan. The X-ray projections are repeatedly acquired by means of a microfocus X-ray imager comprising a custom-made pulse-count time-gated X-ray detector. The projection data is reconstructed to cross-sectional images using adapted image reconstruction algorithms. Additional to using a microfocus X-ray source experimental studies using synchrotron radiation are planed. The new measurement approach is unique and goes far beyond the current state of the art in dynamic microscopic flow imaging. The flow structural data acquired by this technique will foster the meso- and microscalic numerical flow model development for capillary multiphase flow.