Publikationen

Electromagnetic induction in a conducting liquid that moves in an external magnetic field can be used for contactless flow measurement. In Lorentz Force Velocimetry (LFV), the induced force on the magnet is determined to obtain velocity information. This measurement principle may also be applied to conducting flows with gas bubbles encountered in metallurgical processes. This provides the motivation for our work, in which we study a single bubble rising in a liquid metal column as a model problem for LFV in two-phase flows. By using a small permanent magnet, one can not only detect the presence of a bubble but also obtain information on its position and velocity. Our numerical investigation aims at reproducing experiments with Argon bubbles in GaInSn alloy and at studying the electromagnetic induction in the flow in more detail. For three-dimensional and phase-resolving simulations we use the Volume of Fluid method provided by ANSYS FLUENT. The induction equation in the quasistatic limit is an elliptic problem for the electric potential. It is implemented in FLUENT with a user-defined scalar. The electric conductivity varies between the phases, and the magnetic field is given by an analytical expression for a uniformly magnetized cube. The comparison with the experiments also helps to validate the numerical simulations.

This study investigates the effect of a magnetohydrodynamic (MHD) shear flow on the growth and detachment of single sub-millimeter-sized hydrogen gas bubbles. These bubbles were electrolytically generated at a horizontal Pt microelectrode (100 [my]m in diameter) in an acidic environment (1 M H2SO4). The inherent electric field was superimposed by a homogeneous electrode-parallel magnetic field of up to 700 mT to generate Lorentz forces in the electrolyte, which drive the MHD flow. The growth and motion of the hydrogen bubble was analyzed by microscopic high-speed imaging and measurements of the electric current, while particle tracking velocimetry ([my]PTV) and particle image velocimetry ([my]PIV) were applied to measure the surrounding electrolyte flow. In addition, numerical flow simulations were performed based on the experimental conditions. The results show a significant reduction of the bubble growth time and detachment diameter with increasing magnetic induction, which is known to improve the efficiency of water electrolysis. In order to gain further insight into the bubble detachment mechanism, an analysis of the forces acting on the bubble was performed. The strong MHD-induced drag force causes the bubble to slowly slide away from the center of the microelectrode before its detachment. This motion increases the active electrode area and enhances the bubble growth rate. The results further indicate that at large current densities the coalescence of tiny bubbles formed at the foot of the main bubble might play an important role for the bubble detachment. Moreover, the occurrence of Marangoni stresses at the gas-liquid interface is discussed.

Small scale flow phenomena play an important role across engineering, biological and chemical sciences. To gain deeper understanding of the influence of those flow phenomena involved, measurement techniques with high spatial resolution are often required, presuming a calibration of very low uncertainty. To enable such measurements, a method for the in situ calibration of an interferometric flow velocity profile sensor is presented. This sensor, with demonstrated spatial resolution better than 1 [my]m, allows for spatially-resolving measurements with low velocity uncertainty in flows with high velocity gradients, on condition that the spatial behavior of the interference fringe systems is well-known by calibration with low uncertainty, especially challenging to obtain at applications with geometries difficult to access. The calibration method described herein uses three interfering beams to form the interference fringe systems of the sensor, yielding Doppler burst signals exhibiting two peaks in the frequency domain whose amplitude ratio varies periodically along the measurement volume major z-axis, giving a further independent value of the axial tracer particle position that can be used to determine the calibration functions of the sensor during the flow measurement. A flow measurement in a microchannel experimentally validates that the presented approach allows for simultaneously estimating the calibration functions and the velocity profile, providing flow measurements with very low systematic measurement errors of the particle position of less than 400 nm (confidence interval 95%). In that way, the interferometric flow velocity profile sensor utilizing the in situ self-calibration method promises valuable insights on small scale flow phenomena, such as those given in shear and boundary layer flows, by featuring reliable flow measurements due to minimum systematic and statistical measurement errors.