On the influence of gas-liquid two-phase flow on Lorentz force velocimetry. - In: Measurement science and technology, ISSN 1361-6501, Bd. 29 (2018), 8, S. 085301, insges. 11 S.
Lorentz force velocimetry (LFV) is a contactless and non-invasive flow measurement technique for electrical conducting fluids. When the fluid passes a magnetic field, a Lorentz force will be generated. The force is proportional to the electrical conductivity, to the velocity of the fluid, and to the square of the magnetic flux density. It is possible to apply this technique over a range of conductivities varying from high values as for liquid metals to lower values as typically for electrolytes (e.g. salt water). The main challenge is measuring the force, which is six orders of magnitude smaller for electrolytes in comparison to liquid metals. It was shown that LFV is insensitive to the shape of different velocity profiles and especially to strongly asymmetric profiles as long as the volume flow is constant. However, the influence of a second gaseous phase on the force signal remains an open question. This is of fundamental interest especially for chemical and food processing industries. To examine this influence in detail, electrolyte flow with different volume flow fractions was investigated for the current study. By using frits, it is possible to generate a homogeneous distribution of the second (gaseous) phase in the electrolyte. Small electrolyte velocities or flow rates (v < 1.5 m s^-1, V1 = 225 l min^-1) enable the formation of slug flow and stratified wavy flow, and higher velocities (v > 1.8 m s^-1, V1 = 270 l min^-1) the formation of bubbly two-phase flow in the used test section. By this means, it is possible to study a wide range of flow patterns in a horizontal duct. The experimental results are in good agreement with the analytical estimations for fully dispersed flow. However, for a nonhomogeneous distribution of air the force signal deviates much stronger. An explanation can be the superposition of Lorentz forces and magnetic forces, caused by differences of the magnetic susceptibilities of water and air, which will be discussed in greater detail.
https://doi.org/10.1088/1361-6501/aaca8a
Multicomponent local Lorentz force velocimetry. - Ilmenau : Universitätsbibliothek, 2018. - 1 Online-Ressource (117 Seiten)
Technische Universität Ilmenau, Dissertation 2018
Lorenztkraft-Anemometrie (LKA oder LFV) ist ein berührloses elektromagnetisches Strömungsmessverfahren für Flüssigmetalle. Durch eine Relativbewegung zwischen einem elektrischen leitfähigen Fluid und einem statisch angelegten Magnetfeld werden Wirbelströme und eine strömungsbremsende Lorentzkraft erzeugt. Diese Kraft ist proportional zu der elektrischen Leitfähigkeit des Fluides und zu der Durchflussrate oder zu der lokalen Geschwindigkeit, welche abhängig von dem Anteil des durch das Magnetfeld aufgespannten Volumens ist. Gemäß dem dritten Newtonschen Gesetz wirkt eine gleich starke, jedoch entgegengesetzt gerichtete Kraft auf die Quelle des angelegten Magnetfeldes, die in unserem Fall Permanentmagnete sind. Entsprechend dem Ohm'schen Gesetz induzieren bewegende elektrisch leitfähige Fluide bei niedrigen magnetischen Reynolds-Zahlen ein elektrisches Potential, das die Ladungserhaltung gewährleistet. Diese Arbeit beginnt mit der Untersuchung des Beitrags des induzierten elektrischen Potentials in der gesamten Lorentzkraft. Dieses Problem wird numerisch und experimentell analysiert, indem zwei verschiedene Szenarien betrachtet werden: elektrisch leitfähige Wände mit endlicher Dicke und Aspektverhältnisvariation des Strömungsquerschnitts. In beiden Fällen stand die durch das elektrische Potential erzeugte Kraftkomponente immer entgegen der gesamten Lorentzkraft. Diese Kraftkomponente war empfindlich gegenüber den elektrischen Randbedingungen der Strömung, von denen isolierte und perfekt leitende Wände die Grenzfälle sind. Es wird gezeigt, dass die messbare Lorentzkraft beträchtlich erhöht werden kann, wenn die aus dem elektrischen Potential stammende Kraftkomponente verringert wird, indem entweder die elektrische Leitfähigkeit der Wand oder das Querschnittsverhältnis der Strömung verändert wird. Daher kann die Empfindlichkeit der Messtechnik erheblich verbessert werden. Im Anschluss an dieser Analyse konzentriert sich die vorliegende Arbeit auf die lokale Multikomponenten-Lorentzkraft-Anenometrie. Diese Technik basiert auf der Messung aller auf den Magnetsystemen wirkenden Kraft- und Drehmomentkomponenten. In diesem Fall sind die Magnetsysteme deutlich kleiner als der Strömungsquerschnitt. Die kleinen Magnetsysteme induzieren eine lokalisierte Magnetfeldverteilung im flüssigen Metall, die eine lokale aufgelöste Geschwindigkeitsmessung von flüssigem Metall ermöglichen. Die lokale Multikomponenten-Lorentzkraft-Anenometrie wurde mittels einer stetigen dreidimensionalen turbulenten Strömung innerhalb der Kokille eines Stranggussmodells untersucht, wobei das Arbeitsfluid GaInSn in eutektischer Zusammensetzung war. Die aus kubischen und kreuzförmigen Permanentmagneten bestehenden Magnetsysteme wurden an einem Sensor befestigt, der speziell für die gleichzeitige Erfassung aller drei Kraft- und drei Drehmomentkomponenten entwickelt wurde. Mit diesem Sensor war es möglich, Informationen über die dreidimensionale Geschwindigkeitsverteilung des flüssigen Metalls innerhalb der Kokille in der Nähe der Magneten zu erhalten. Zusätzlich könnte unter Verwendung eines kreuzförmigen Magneten das Drehmoment in der Feldrichtung des Magneten gemessen werden. Gemäß einem numerischen Modell der Experimente korreliert dieses Drehmoment mit der Wirbelstärke des Geschwindigkeitsfeldes in dieser Richtung.
http://nbn-resolving.de/urn:nbn:de:gbv:ilm1-2018000137
Marangoni convection at electrogenerated hydrogen bubbles. - In: Physical chemistry, chemical physics, ISSN 1463-9084, Bd. 20 (2018), 17, S. 11542-11548
Electrolytic gas evolution is a fundamental phenomenon occurring in a large number of industrial applications. In these processes gas bubbles are formed at the electrode from a supersaturated solution. Since dissolved gases can change the surface tension, a gas concentration gradient may cause the surface tension to vary locally at the interface of the gas bubble. Surface tension gradients may also form due to temperature gradients generated by ohmic heating of the electrolyte. In both cases, the resulting shear stress imposes a convection in the electrolyte and the gas bubble (Marangoni eect). This phenomenon may influence the entire electrolytic gas evolution process, e.g., by an enhanced mass transfer. In this study, the first evidence of the Marangoni convection near growing hydrogen bubbles, generated by water electrolysis, is provided. Microscopic high speed imaging was applied to study the evolution of single hydrogen bubbles at a microelectrode. The convection near the interface of the growing bubble was measured by using a time-resolved Particle Tracking Velocimetry (PTV) technique. The results indicate a clear correlation between the magnitude of the Marangoni convection and the electric current.
https://doi.org/10.1039/C8CP01050A
Preliminary experimental study on applicability of Lorentz force velocimetry in an external magnetic field. - In: Nuclear science and techniques, ISSN 2210-3147, Bd. 29 (2018), 6, Article 80, insges. 8 S.
Lorentz force velocimetry (LFV) is a noncontact technique for measuring electrically conducting fluids based on the principle of electromagnetic induction. This work aims to answer the open and essential question of whether LFV can work properly under a surrounding external magnetic field (ExMF). Two types of ExMFs with different magnetic intensities were examined: a magnetic field with a typical order of 0.4 T generated by a permanent magnet (PM) and another generated by an electromagnet (EM) on the order of 2 T. Two forces, including the magnetostatic force between the ExMF and PM in the LFV, and the Lorentz force generated by the PM in LFV were measured and analyzed in the experiment. In addition, ExMFs of varying strengths were added to the LFV, and the location of the LFV device in the iron cores of the EM was considered. The experimental outcomes demonstrate that it is possible for a LFV device to operate normally under a moderate ExMF. However, the magnetostatic force will account for a high proportion of the measured force, thus inhibiting the normal LFV operation, if the ExMF is too high.
https://doi.org/10.1007/s41365-018-0426-9
Numerical simulation of turbulent Taylor-Couette flow between conducting cylinders in an axial magnetic field at low magnetic Reynolds number. - In: Physics of fluids, ISSN 1089-7666, Bd. 30 (2018), 1, 015107, insges. 17 S.
Correction: Bd. 30 (2018), 2, 029901, insges. 1 S.
The effect of an axial homogeneous magnetic field on the turbulence in the Taylor-Couette flow confined between two infinitely long conducting cylinders is studied by the direct numerical simulation using a periodic boundary condition in the axial direction. The inner cylinder is rotating, and the outer one is fixed. We consider the case when the magnetic Reynolds number Rem 1, i.e., the influence of the induced magnetic field on the flow is negligible that is typical for industry and laboratory study of liquid metals. Relevance of the present study is based on the similarity of flow characteristics at moderate and high magnetic field for the cases with periodic and end-wall conditions at the large flow aspect ratio, as proven in the earlier studies. Two sets of Reynolds numbers 4000 and 8000 with several Hartmann numbers varying from 0 to 120 are employed. The results show that the mean radial induced electrical current, resulting from the interaction of axial magnetic field with the mean flow, leads to the transformation of the mean flow and the modification of the turbulent structure. The effect of turbulence suppression is dominating at a strong magnetic field, but before reaching the complete laminarization, we capture the appearance of the hairpin-like structures in the flow.
https://doi.org/10.1063/1.5003173
Numerical and experimental study on vorticity measurement in liquid metal using local Lorentz force velocimetry. - In: Measurement science and technology, ISSN 1361-6501, Bd. 29 (2018), 3, S. 035301, insges. 13 S.
Local Lorentz force velocimetry (local LFV) is a contactless velocity measurement technique for liquid metals. Due to the relative movement between an electrically conductive fluid and a static applied magnetic field, eddy currents and a flow-braking Lorentz force are generated inside the metal melt. This force is proportional to the flow rate or to the local velocity, depending on the volume subset of the flow spanned by the magnetic field. By using small-size magnets, a localized magnetic field distribution is achieved allowing a local velocity assessment in the region adjacent to the wall. In the present study, we describe a numerical model of our experiments at a continuous caster model where the working fluid is GaInSn in eutectic composition. Our main goal is to demonstrate that this electromagnetic technique can be applied to measure vorticity distributions, i.e. to resolve velocity gradients as well. Our results show that by using a cross-shaped magnet system, the magnitude of the torque perpendicular to the surface of the mold significantly increases improving its measurement in a liquid metal flow. According to our numerical model, this torque correlates with the vorticity of the velocity in this direction. Before validating our numerical predictions, an electromagnetic dry calibration of the measurement system composed of a multicomponent force and torque sensor and a cross-shaped magnet was done using a rotating disk made of aluminum. The sensor is able to measure simultaneously all three components of force and torque, respectively. This calibration step cannot be avoided and it is used for an accurate definition of the center of the magnet with respect to the sensor's coordinate system for torque measurements. Finally, we present the results of the experiments at the mini-LIMMCAST facility showing a good agreement with the numerical model.
https://doi.org/10.1088/1361-6501/aa9f85
Numerical and experimental study of the effect of the induced electric potential in Lorentz force velocimetry. - In: Measurement science and technology, ISSN 1361-6501, Bd. 29 (2018), 1, S. 015301, insges. 15 S.
Lorentz force velocimetry (LFV) is a contactless velocity measurement technique for electrically conducting fluids. When a liquid metal or a molten glass flows through an externally applied magnetic field, eddy currents and a flow-braking force are generated inside the liquid. This force is proportional to the velocity or flow rate of the fluid and, due to Newton's third law, a force of the same magnitude but in opposite direction acts on the source of the applied magnetic field which in our case are permanent magnets. According to Ohm's law for moving conductors at low magnetic Reynolds numbers, an electric potential is induced which ensures charge conservation. In this paper, we analyze the contribution of the induced electric potential to the total Lorentz force by considering two different scenarios: conducting walls of finite thickness and aspect ratio variation of the cross-section of the flow. In both the cases, the force component generated by the electric potential is always in the opposite direction to the total Lorentz force. This force component is sensitive to the electric boundary conditions of the flow of which insulating and perfectly conducting walls are the two limiting cases. In the latter case, the overall electric resistance of the system is minimized, resulting in a considerable increase in the measured Lorentz force. Additionally, this force originating from the electric potential also decays when the aspect ratio of the cross-section of the flow is changed. Hence, the sensitivity of the measurement technique is enhanced by either increasing wall conductivity or optimizing the aspect ratio of the cross-section of the flow.
https://doi.org/10.1088/1361-6501/aa9095
Vergleichende Analyse eines Ein- und Mehrkamerasystems zur simultanen, volumetrischen Temperatur- und Geschwindigkeitsmessung für die Mikrofluidik. - In: Technisches Messen, ISSN 2196-7113, Bd. 85 (2018), 2, S. 97-103
Im folgenden Artikel wird eine optische Messtechnik vorgestellt und qualifizert, welche die simultane Bestimmung des dreidimensionalen (3D) Temperaturfeldes und der drei Komponenten des dreidimensionalen Geschwindigkeitsfeldes (3D3C) in mikrofluidischen Anwendungen ermöglicht. Dabei wird die Temperatur über das Verhältnis der Intensität der Lumineszenzsignale von zweifarbigen Polymerpartikeln ermittelt, während das Geschwindigkeitsfeld gleichzeitig anhand der Verschiebung individueller Partikelbilder berechnet werden kann. Um die Tiefeninformation zu erhalten, wird das etablierte Astigmatismus particle tracking velocimetry Verfahren verwendet. Mit der beschriebenen Methode werden simultane, volumetrische Messungen des Temperatur- und Geschwindigkeitsfeldes in einem temperierten Kanal durchgeführt und mit numerischen Simulationen verglichen. Dabei werden zwei unterschiedliche Kamerasysteme verwendet, um die Emission der lumineszierenden Farbstoffen zu trennen. Einerseits kommt ein Zweikameraaufbau mit einem dichroitischen Spiegel und andererseits eine einzelne Farbkamera zum Einsatz. Die Ergebnisse zeigen eine gute qualitative Übereinstimmung zwischen Experiment und Simulation, wobei die Qualität der Messergebnisse für beide Kamerasysteme vergleichbar ist. Durch die Verwendung von nur einer Farbkamera kann der experimentelle Aufwand jedoch stark verringert werden.
https://doi.org/10.1515/teme-2017-0094
Local Lorentz force and ultrasound Doppler velocimetry in a vertical convection liquid metal flow. - In: Experiments in fluids, ISSN 1432-1114, Bd. 59 (2018), 1, 3, S. 1-12
We report velocity measurements in a vertical turbulent convection flow cell that is filled with the eutectic liquid metal alloy gallium-indium-tin by the use of local Lorentz force velocimetry (LLFV) and ultrasound Doppler velocimetry. We demonstrate the applicability of LLFV for a thermal convection flow and reproduce a linear dependence of the measured force in the range of micronewtons on the local flow velocity magnitude. Furthermore, the presented experiment is used to explore scaling laws of the global turbulent transport of heat and momentum in this low-Prandtl-number convection flow. Our results are found to be consistent with theoretical predictions and recent direct numerical simulations.
https://doi.org/10.1007/s00348-017-2457-0
Convective heat transfer in engine coolers influenced by electromagnetic fields. - In: Heat and mass transfer, ISSN 1432-1181, Bd. 54 (2018), 8, S. 2599-2605
In engine coolers of off-highway vehicles, convec- tive heat transfer at the coolant side limits both efficiency and performance density of the apparatus. Here, due to restrictions in construction and design, backwater areas and stagnation regions cannot be avoided. Those unwanted changes in flow characteristics are mainly triggered by flow deflections and sudden cross-sectional expansions. In application, mixtures of water and glysantine are used as appropriate coolants. Such coolants typically show an electrical conductivity of a few S/m. Coolant flow and convective heat transfer can then be controlled using Lorentz forces. These body forces are generated within the conducting fluid by the interactions of an electrical current density and a localized magnetic field, both of which are externally superimposed. In future applica- tion, this could be achieved by inserting electrodes in the cooler wall and a corresponding arrangement of permanent magnets. In this paper we perform numerical simulations of such magnetohydrodynamic flow in three model geometries that frequently appear in engine cooling applications: Carnot- Borda diffusor, 90˚ bend, and 180˚ bend. The simulations are carried out using the software package ANSYS Fluent. The present study demonstrates that, depending on the electromag- netic interaction parameter and the specific geometric arrange- ment of electrodes and magnetic field, Lorentz forces are suit- able to break up eddy waters and separation zones and thus significantly increase convective heat transfer in these areas. Furthermore, the results show that hydraulic pressure losses can be reduced due to the pumping action of the Lorentz forces.
https://doi.org/10.1007/s00231-017-2130-4