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

从基础科学的角度来看，对毛细管多相流动量、传热和传质的深刻理解对于新型化学反应装置（如单片微反应器、微型热交换器、燃料电池等）的实际设计和操作至关重要。流动拓扑的知识和微观尺度上的液膜厚度、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代码的决定性输入。目前，关于微通道中泰勒气泡形状的知识只有粗略的了解。最近的实验研究使用高速成像技术研究了段塞长度和段塞速度，使用PIV/LIF技术研究了液体速度场，最近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膜层厚度。但目前广泛使用的光学测量技术在两相流条件下有很强的局限性，特别是不能解析气泡的完整三维形状。光学技术进一步受到透明通道和流体的限制，而衍射将可实现的空间分辨率限制在1微米左右。在本项目框架内，申请人的目标是开发一种x射线断层扫描方法，该方法可以揭示毛细管两相流中泰勒气泡的三维形状，目前的目标是约3 μm的空间分辨率。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、通道截面、壁面材料和毛细数，为建立这类流动的传质模型提供数据。测量技术的核心是一个系统，该系统提供泰勒气泡的x射线投影的数字采集，该系统与气泡通过x射线风扇的过程同步。x射线投影通过微聚焦x射线成像仪反复获得，该成像仪包括定制的脉冲计数时间门控x射线探测器。使用自适应图像重建算法将投影数据重构为横截面图像。除了使用微聚焦x射线源外，还计划使用同步辐射进行实验研究。新的测量方法是独特的，远远超出了目前的动态微观流动成像技术。该技术所获得的流动结构数据将促进毛细管多相流中、微观数值流动模型的发展。

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.