Multiphase flows in chemical reactors with micro- and millimeter-size channel structures such as monolith froth reactors, compact heat exchangers and fuel cells have received great attention in the last years. They are considered as a promising alternative to conventional reactors, such as fixed bed reactors and bubble columns which are mainly used for gas absorption, catalytic hydrogenation and biochemical conversions. Slug or Taylor bubble flow is a desired operating state for this type of contactors due to the frequent change of efficient gas-liquid contacting in the film around the bubbles and the enhanced mixing in the liquid slugs behind the bubbles. Consequently, capillary Taylor flow is currently a target of intensive investigations. However, a full understanding of design parameters and optimum operating conditions are still lacking. For milli- and microreactors mass transfer between gas and liquid phases depends upon various parameters such as bubble shape, relative velocity between the two phases, degree of liquid contamination and many more. To further advance the fundamental understanding of micro- and milli-channel reactors with Taylor flow, main design parameters and operating conditions were investigated, which include (a) the effect of bubble size, channel diameter and cross sectional shape of channel on the mass transfer coefficient of dissolving bubbles, (b) the influence of the presence of surface active agents on the bubble shape, velocity and also on the mass transfer rate of bubbles and (c) the intensification effect of oscillation of channels on the mass transfer performance of Taylor bubbles. For the study of gas-liquid mass transfer high-resolution X-ray radiography and tomography were used as measurement techniques. The X-ray imaging methods were chosen as their accuracy is less affected by changes in the refractive index, as it is the case for conventional optical methods. The mass transfer was calculated by measuring the changes in the size of the bubbles at constant pressure. The utilization of X-ray visualization enabled the acquisition of a series of radiographic images of bubbles. The images gave the volume, interfacial area and length of the bubble with high accuracy as a function of time and were used to evaluate the mass transfer coefficient using the mass conservation equations. In case of circular channels, the results show that Sherwood numbers have a large dependency on the bubble length and also equivalent diameter which is in accordance with previous results for larger channel diameters. However, the values of measured Sherwood numbers could not be predicted by available correlations which are valid only for larger pipes. As a result, a new mass transfer correlation in the form of Sherwood number as a function of Peclet number as well as bubble size ratio was derived. The proposed correlation is applicable for a large range of bubble sizes with high accuracy. The comparison of the results for the square and circular channels showed that despite the fact that the rise velocity of bubbles in the square channel is about three times higher than in the circular channel, the mass transfer coefficient is about the same. Furthermore, the results show that in square channels the dissolution curves are relatively even, while the dissolution curves of circular channels exhibit some distinguishable change in the slope. In addition, the results show that the calculated mass transfer coefficient based on the measured data show good agreement with the data predicted by the penetration theory. Regarding the influence of surfactants on the mass transfer in small channels with Taylor flow, it was shown that a small amount of surfactant reduces the mass transfer and its impact is more pronounced on small bubbles. Furthermore, it was demonstrated that the presence of surfactants causes the change of the bubble shape and leads to a slight increase of the liquid film thickness around the bubble and as a result the elongation of contaminated bubbles. Intensification of mass transfer in small channels with Taylor bubbles was investigated by measuring the motion, shape and dissolution rate of individual elongated Taylor bubbles of air and CO2 in water. The comparison of the results for the stationary and oscillating channel showed that mechanical vibration of the channel is able to enhance the mass transfer coefficient from 80% to 186%. Moreover, the mass transfer rate positively correlates with frequency and amplitude of oscillation, which is more pronounced at higher amplitudes. In addition, it was shown that the intensification of mass transfer with increase of amplitude/frequency of vibration is mainly attributed to the increase of bubble surface wave oscillations that causes an enlargement of contact area between the phases and also a reduction of mass transfer resistance in the liquid-side boundary layer.

Mit der Bekanntgabe der Entdeckung Röntgens 1895 begann nicht nur eine massive Bildproduktion, sondern auch ein reges radiologisches Ausstellungwesen, das sich an ein medizinisch-wissenschaftliches Publikum richtete und parallel zu wissenschaftlichen Kongressen stattfand. Die vorliegende Arbeit geht dieser Praxis des Ausstellens von Röntgenbildern und -apparaten im ersten Drittel des 20. Jahrhunderts nach und bezieht die Ausstellungen auf das im Entstehen begriffene medizinisch-wissenschaftliche Feld der Radiologie. Ausstellungen waren, so die zentrale These, keine dem radiologischen Erkenntnisprozess nachfolgenden Veranstaltungen, sondern Orte, bei denen ein radiologisches Apparate- und Bildwissen kommuniziert und erzeugt wurde. In der Arbeit wird die soziale, technische und expositorische Entwicklung des radiologischen Feldes parallel erzählt und eng miteinander verflochten gedacht. So geraten die engen Kopplungen zwischen Ausstellungsraum und Röntgeninstitut und zwischen technischer Entwicklung und professionellem Selbstverständnis von Radiologen in den Blick. / With the discovery of X-rays in 1895 not only an increased image production can be observed, but also the new format of the X-ray exhibition was created. The exhibitions were mainly aimed at a medical and scientific audience and were organized parallel to scientific congresses. The work traces this practice of exhibiting X-ray images and apparatus in the first third of the 20th century and relates the exhibitions closely to the medical-scientific field of radiology that developed during this period. According to the central thesis, exhibitions were not events that took place after the genesis and development of radiological facts in the clinic or scientific institutes, but spaces where knowledge about the new apparatus and images was communicated and generated. In the work, the social, technical and exhibition-related development of the radiological field is told in parallel and thought of in close relation to each other. Using exhibitions as an example, the close links between exhibition spaces and X-ray institutes and between technical development and the professional self-image of radiologists are brought into focus.