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

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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.