Experimentelle Untersuchung einer Flüssigmetall-Tropfenströmung unter Wirkung eines magnetischen Wechselfelds :
Experimental investigation of liquid metal droplet flow affected by a time-dependent magnetic field. - In: Technisches Messen, ISSN 2196-7113, Bd. 90 (2023), 10, S. 625-638
The present study investigates experimentally the effects of a time-dependent and spatially inhomogeneous magnetic field on liquid metal droplet flow down an inclined substrate. The flow is solely excited by the electromagnetic interactions between the electrically conducting melt and the applied magnetic field. The metal droplet consists of the eutectic alloy GaInSn which is liquid at room temperature. The magnetic field is generated in the gap between two metallic disks that are equipped with a special geometric arrangement of permanent magnets and put into a measured rotation. During the experiments, a droplet of a measured volume is positioned on an electrically non-conducting substrate that is slightly inclined against the horizontal direction. Droplet and substrate are placed in between the two rotating magnetic disks. In our experiments, we record the electromagnetically excited flow of the droplet downwards onto the substrate using a high-speed camera system. Applying standard techniques of digital image processing, we measure both the displacement position and velocity of the droplet as a function of time. We observe that, depending on the rotation rate of the disks and angle of inclination, the magnetic field eventually triggers this spreading process. In more detail, by evaluating the recorded data, we find that the magnetic field excites capillary waves at the free surface of the droplet. These surface waves contribute to a redistribution of volume towards the contact line formed at the downward-facing end tip of the droplet. This mode of transport steepens the contact angle, allowing the droplet to move. Besides the fundamental aspect of this work, the present study may contribute to the electromagnetic control of both the production of metallic microfibers and metallurgic coating processes as well as to the non-contact electromagnetic flow measurement technique of Lorentz force velocimetry applied to liquid metal free-surface flows.
https://doi.org/10.1515/teme-2023-0046
Modeling study of EMBr effects on molten steel flow, heat transfer and solidification in a continuous casting mold. - In: Metallurgical research & technology, ISSN 2271-3654, Bd. 120 (2023), 2, 218, S. 1-12
During continuous casting process, the internal molten steel flow pattern of the mold is one of the important factors affecting the quality of slab products. The application of electromagnetic braking (EMBr) technology in the slab caster provides an effective solution to improve the molten steel flow pattern in the mold. In the current research, one of the commonly used EMBr technology is studied, namely the Ruler-EMBr technology. In detail, the effect of magnetic flux density on the behavior of the molten steel jet flow, heat transfer, and solidification in a 1450 mm × 230 mm slab mold is numerically simulated through a Reynolds-averaged Navier-Stokes (RANS) turbulence model together with an enthalpy-porosity approach. The simulation results indicate that the electromagnetic force generated by the Ruler-EMBr can significantly suppress the diffusion of the impinging jet to the narrow face of the mold with the increase of magnetic flux density. By that, the impact of the upward backflow on the meniscus region in the mold is suppressed. Correspondingly, the uniformity of the temperature distribution in the mold is effectively improved. The parametric studies suggest that the optimized magnetic flux density is 0.3 T to ensure the improvement of steel quality with a casting speed of 1.6 m/min. By applying the magnetic flux density of 0.3 T, the Ruler-EMBr has a better capability to reduce the maximum amplitude of the surface velocity by 24.5% and increase the average surface temperature of the molten steel by 0.25% when compared to the case of No-EMBr. With this electromagnetic parameter, the Ruler-EMBr technology can well prevent the mold flux entrapment and promote solidified shell uniform growth along the casting direction.
https://doi.org/10.1051/metal/2023016
MHD flow of submerged jets behind the inlet disturbance. - In: Proceedings in applied mathematics and mechanics, ISSN 1617-7061, Bd. 22 (2023), 1, e202200200, S. 1-6
In a broad variety of configurations in technology and industrial applications, the properties of liquid metal flows subjected to strong magnetic fields, are largely governed by the dynamics of coherent structures, known to settle several basic types, such as thin shear layers, forming near the walls or within the fluid domain, vortices extended along the field, or planar and round jets. In some cases, these structures are created by the design, like a submerged jet formed by a sudden expansion from the nozzle into a blanket channel, or jets formed behind some flow obstruction. In the other cases this may be due to instability and evolution of secondary structures, for example, descending and ascending jets appearing as a result of convective instability in blanket channels. In this study, we undertake an attempt to affect liquid metal flow via inlet disturbance formed by a simple rod placed along the magnetic induction lines. The disturbance can generate flat jets behind the rod and, furthermore, a sustainable flow of anisotropic vortical perturbations further downstream the flow. We seek to analyze the most important mechanisms of the flow dynamics and effects of magnetic field on the integral system properties of enhancing mixing, mass and heat transport for such flow. The most optimal regimes of vortex generation are found to be governed by the magnetic interaction parameter (Stuart number). The exact ratio of the optimal Stuart number is found to be in a range between 20 and 40, based on the channel double width as a characteristic size. The observed vortices attain quasi-2D shape and exist at a length of dozens of duct calibers, being the strongest at higher flow rates. The obtained flow regimes and their turbulent properties are also found to resemble significant similarity to the results on quasi-2D turbulence found in prior studies of channel and duct flows under spanwise magnetic field.
https://doi.org/10.1002/pamm.202200200
From rectangular to diamond shape : on the three-dimensional and size-dependent transformation of patterns formed by single particles trapped in microfluidic acoustic tweezers. - In: Lab on a chip, ISSN 1473-0189, Bd. 23 (2023), 9, S. 2154-2160
Generally, the pattern formed by individual particles trapped inside a microfluidic chamber by a two-dimensional standing acoustic wave field has been considered only the result of the acoustic radiation force. Previous studies showed that particles can be trapped at the local minima and maxima of the first-order pressure and velocity fields. Thus, either a rectangular or a diamond pattern can be formed solely depending on the particle size, when the acoustic field is unchanged, and the material properties of the particles and the fluid are fixed. In this paper, we report about the co-existence of different patterns with particles of the same size. The actual shape of the patterns depends mainly on the ratio between particle diameter and wavelength. In addition, particles were found to be trapped at locations that coincide with the position of antinodes, even though the particles have a positive acoustic contrast factor. These phenomena imply that the trapping of individual particles cannot be described by the acoustic radiation force solely. Hence, further research is required, taking the viscous drag force caused by the fluid flow induced by the acoustic streaming effect into account.
https://doi.org/10.1039/D3LC00120B
A microfluidic magnetohydrodynamic pump based on a thermally bonded composite of glass and dry film photoresist. - In: Micro and nano engineering, ISSN 2590-0072, Bd. 18 (2023), 100173, S. 1-8
Miniaturized on-chip micropumps with no moving parts are intriguing components for advanced lab-on-chip systems. Magnetohydrodynamic pumping is one possibility but requires further research with respect to microsystems design and fabrication. In this paper, the design and fabrication of a magnetohydrodynamic micropump is discussed using a composite of patterned glass and stacked dry film photoresist as demonstrator platform. The magnetohydrodynamic pumping effect is achieved by the superposition of an electric ion current generated by integrated electrodes and an external magnetic field provided by a permanent magnet. As test electrolytes, potassium chloride with potassium hexacyanoferrate (III) and potassium hexacyane iron (II) were used. Seamless fluid channel sidewalls were achieved from stacked dry film resists, which appear to be cast from a single mold. A liquid-tight sealing of the microchannels was realized by covering them with a thermally bonded laser-structured glass lid. Although, a complete characterization of the pump performance was not yet realized, the micropump in its current state serves as a technology demonstrator for further research of microfluidic on-chip micropumps that utilize the magnetohydrodynamic effect and also for other microfluidic systems.
https://doi.org/10.1016/j.mne.2023.100173
The SCALEX facility - an apparatus for scaled fluid dynamical experiments. - In: Technisches Messen, ISSN 2196-7113, Bd. 90 (2023), 5, S. 296-309
The working conditions of the Scaled Convective Airflow Laboratory Experiment (SCALEX) at Technische Universität Ilmenau and sample experiments are reported. The SCALEX facility is a pressure vessel which allows for downscaling of laboratory experiments up to a factor of 20 by compression of gaseous working fluids, air or sulfur hexafluoride, to change the material properties of the fluid. The requirements and conditions for downscaling of fluid dynamical problems are discussed in detail. Long-term high and low pressure tests are conducted to screen the stability of the experimental environment inside the vessel against pressure and temperature fluctuations. Finally, a Rayleigh-Bénard convection experiment at an aspect ratio 10 is performed inside the SCALEX facility as a proof of concept. The reference experiment was conducted under 4.5 bar pressure for Ra = 1.9 × 10^5. However, the Rayleigh number could be varied in a wide range of Ra = 10^4 … 10^8. The flow investigation was pursued with stereoscopic particle image velocimetry in horizontal mid-plane through the convection cell. To improve the image quality the cameras were placed inside the pressure cell and tested up to 6 bar. Thus the feasibility of optical flow measurements at elevated pressures is shown.
https://doi.org/10.1515/teme-2022-0121
Particle detection and size recognition based on defocused particle images: a comparison of a deterministic algorithm and a deep neural network. - In: Experiments in fluids, ISSN 1432-1114, Bd. 64 (2023), 2, 21, S. 1-16
The systematic manipulation of components of multimodal particle solutions is a key for the design of modern industrial products and pharmaceuticals with highly customized properties. In order to optimize innovative particle separation devices on microfluidic scales, a particle size recognition with simultaneous volumetric position determination is essential. In the present study, the astigmatism particle tracking velocimetry is extended by a deterministic algorithm and a deep neural network (DNN) to include size classification of particles of multimodal size distribution. Without any adaptation of the existing measurement setup, a reliable classification of bimodal particle solutions in the size range of 1.14 μm–5.03 μm is demonstrated with a precision of up to 99.9 %. Concurrently, the high detection rate of the particles, suspended in a laminar fluid flow, is quantified by a recall of 99.0 %. By extracting particle images from the experimentally acquired images and placing them on a synthetic background, semi-synthetic images with consistent ground truth are generated. These contain labeled overlapping particle images that are correctly detected and classified by the DNN. The study is complemented by employing the presented algorithms for simultaneous size recognition of up to four particle species with a particle diameter in between 1.14 μm and 5.03 μm. With the very high precision of up to 99.3 % at a recall of 94.8 %, the applicability to classify multimodal particle mixtures even in dense solutions is confirmed. The present contribution thus paves the way for quantitative evaluation of microfluidic separation and mixing processes.
https://doi.org/10.1007/s00348-023-03574-2
Analysis of an unsteady quasi-capillary channel flow with time-resolved PIV and RBF-based super-resolution. - In: Journal of coatings technology and research, ISSN 1935-3804, Bd. 20 (2023), 1, S. 27-40
We investigate the interface dynamics in an unsteady quasi-capillary channel flow. The configuration consists of a liquid column that moves along a vertical 2D channel, open to the atmosphere and driven by a controlled pressure head. Both advancing and receding contact lines were analyzed to test the validity of classic models for dynamic wetting and to study the flow field near the interface. The operating conditions are characterized by a large acceleration, thus dominated by inertia. The shape of the moving meniscus was retrieved using Laser-Induced Fluorescence-based image processing, while the flow field near was analyzed via Time-Resolved Particle Image Velocimetry (TR-PIV). The TR-PIV measurements were enhanced in the post-processing, using a combination of Proper Orthogonal Decomposition and Radial Basis Functions to achieve super-resolution of the velocity field. Large counter-rotating vortices were observed, and their evolution was monitored in terms of the maximum intensity of the Q-field. The results show that classic contact angle models based on interface velocity cannot describe the evolution of the contact angle at a macroscopic scale. Moreover, the impact of the interface dynamics on the flow field is considerable and extends to several capillary lengths below the interface.
https://doi.org/10.1007/s11998-022-00664-4
Fertigung magneto-mikrofluidischer Mikropumpen durch Kombination von Glas, Trockenfilm-Resist und 3D-Druck. - In: Mikrosystemtechnik, (2022), S. 69-72
Experimental study of submerged liquid metal jet in a rectangular duct in a transverse magnetic field. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 953 (2022), A10
A liquid metal flow in the form of a submerged round jet entering a square duct in the presence of a transverse magnetic field is studied experimentally. A range of high Reynolds and Hartmann numbers is considered. Flow velocity is measured using electric potential difference probes. A detailed study of the flow in the duct's cross-section about seven jet's diameters downstream of the inlet reveals the dynamics, which is unsteady and dominated by high-amplitude fluctuations resulting from the instability of the jet. The flow structure and fluctuation properties are largely determined by the value of the Stuart number N. At moderate N, the mean velocity profile retains a central jet with three-dimensional perturbations increasingly suppressed by the magnetic field as N grows. At higher values of N, the flow becomes quasi-two-dimensional and acquires the form of an asymmetric macrovortex, with high-amplitude velocity fluctuations reemerging.
https://doi.org/10.1017/jfm.2022.879