In order to allow for more reliable modelling of microscale processes in turbulent bubbly flows, detailed experiments in a laboratory-scale double loop facility and a bubble column with a diameter of 140 mm were performed. The range of bubble mean diameters considered in the measurements was between 2 and 4 mm. The average gas volume fraction was in the range between 0.5 and 5% for different experiments. To allow simultaneous measurements of bubble size, bubble velocity and liquid velocity field, a combined system of planar shadow imaging, particle tracking velocimetry (PTV) and particle image velocimetry (PIV) for online measurements was developed and applied. The measurements of the liquid phase velocities were realized by seeding the flow with polyamide tracer particles having a mean diameter of 65 μm. A background illumination was established by using an array of 551 LEDs with a size of 160 mm by 100 mm. A double image CCD camera with macro optics was used to record the images of tracers and bubbles simultaneously. Because of the small depth of field of the macro camera optics (<4 mm for the bubbles), it was possible to discriminate between bubbles and tracer particles inside and outside the camera's focal plane using the gradient of grey values. A set of digital image filters was applied to perform phase discrimination between bubbles and tracer particles, i.e. to obtain separate double images for bubbles and tracer, respectively. The contour of in-focus bubbles was determined using an edge detecting Sobel filter and a spline interpolation technique. Thereby, the bubble size, shape and orientation could be derived. The bubble velocity was obtained by applying PTV and the continuous phase velocity field was determined by PIV, using a successive refinement of the interrogation area. By recording and evaluating at least 500 double images, it was possible to determine bubble size distributions and mean as well as fluctuating velocities for both phases. Thereby, detailed data on the hydrodynamics of bubble-driven flows are provided.

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A bubble coalescence model was developed on the basis of the Lagrangian approach, where bubbles are followed through the turbulent flow field along their trajectories. The collision process was described using a stochastic inter‐particle collision model. Coalescence occurred if the contact time became larger than the film drainage time. The film drainage time was determined by a simple formulation according to classical approaches, whereas the contact time resulted directly from the collision model. The performance of the model was analyzed for bubble rise in a prescribed homogeneous isotropic turbulence field. Moreover, numerical calculations by the coupled Euler/Lagrange approach were performed and compared with measurements in a bubble driven loop facility. The comparison showed reasonably good agreement for liquid and bubble velocities.

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The hydrodynamics and bubble behavior in a laboratory-scale bubble column (diameter 140 mm) was analyzed using planar shadow image velocimetry. Different air flow rates were considered by using capillary aerators with different capillary diameter. This implies that a gas hold-up between 1 and 5% and a bubble number mean diameter between about 2 and 4 mm was realized. The imaging system consists of a background illumination utilizing a LED-array and a single CCD-camera which records simultaneously bubble and tracer images. The demarcation of the thickness of the imaging plane was realized by using a macrolens adjusted to small depth of field. To discard out-of-focus images of bubbles and tracer particles and to discriminate between both phases different gradient filters were applied. A Sobel filter was used to evaluate the bubble contours in order to obtain the area equivalent diameter, the eccentricity, and the bubble orientation. The velocity fields of both phases and horizontal profiles along the bubble column were determined by applying PTV (particle tracking velocimetry) for the bubbles and PIV (particle imaging velocimetry) for the tracer particles. For both phases axial and radial mean velocities as well as their fluctuating components were determined by averaging a sufficient number of double images. From these results also cross-sectional averages and global averages of turbulent kinetic energy and fluctuation energy of the bubbles were determined. It was found that the bubble fluctuation in the radial direction was higher than in the axial one for bubble sizes in the range between about 2.0 and 3.8 mm which is a result of the zigzag or helical motion of the bubbles. Finally also the bubble behavior was further analyzed by determining bubble eccentricity and orientation of the bubbles in the flow. These data are especially useful for modeling bubble oscillation and tumbling motion.