Reverse flows and flattening of a submerged jet under the action of a transverse magnetic field

Significance 

Under magnetic fields, the formation and evolution of jets is usually a common phenomenon that presents an important problem in modern astrophysics. For example, it is speculated that cosmic gamma-ray bursts are a result of explosions characterized by the formation of thin jets. From the magnetohydrodynamics (MHD) perspective, jet flows are characterized by a high magnetic Reynolds number representing the ratio between the advection and diffusion flux of the magnetic field. As a result, the effects of MHD on fluid jets are also a key consideration in various fields like nuclear fusion, though on smaller scales.

Magnetic fields have considerable effects on jet flows. Jets spread over a large distance under the axial magnetic field than without magnetic fields. In addition, magnetic fields delay the transition to turbulence due to their strong stabilizing effect. The jet profiles become more unsteady in the presence of magnetic fields due to the generation of traveling waves by the interaction of magnetic fields and secondary radial flows. Besides axial fields, transverse fields have also drawn more research attention, especially in continuous casting (CC). Nevertheless, only a limited amount of theoretical and experimental studies of submerged jets have been reported despite their practical significance.

Although the impact of magnetic fields has been investigated, the role of the transverse magnetic field in reducing velocity fluctuation is yet to be confirmed. Industrial investigations have also shown that not all magnetic fields produce quality metals. Further investigations have also been carried out to clarify the underlying physical mechanisms of jet flows. Most importantly, the theoretical analysis of low magnetic Reynolds number has revealed the development of long thin sheetlike structures along the direction of the magnetic as well as the occurrence of reverse flows. The existence of reverse flows has also been confirmed through continuous casting and large eddy simulations. However, the flow characteristics of submerged jets, including the occurrence of the thin sheetlike structures, are yet to be fully clarified.

To this note, Austrian researchers from University of Leoben (Montanuniversität Leoben): Associate Professor Abdellah Kharicha, Dr. Alexander Vakhrushev, Dr. Ebrahim Karimi-Sibaki, Professor Menghuai Wu and Professor Andreas Ludwig studied the occurrence of reverse flow and flattening of submerged jet under transverse uniform magnetic field. The flow characteristics for Reynolds (Re < 4500) and moderate interactions (N < 0.1) were numerically simulated. The original research work is currently published in the journal, Physical Review Fluids.

The research team findings revealed more complex phenomena than the widely expected MHD damping effects, which was in agreement with the previous predictions. The inlet round jet underwent anisotropic flattening along the direction of the magnetic field, while reverse jet flow zones were formed adjacent to the main jet. Even at large distances of ~800d, the decay rates and jet dynamics remained very sensitive to external electrical boundary conditions. Whereas the small-scale turbulent fluctuations were suppressed, the magnetic field formed within the shear region aligned with the large coherent vortices at the boundary between the main and adjacent reverse jets.

These fundamental findings have direct industrial applications, for instance in the continuous casting of steel, where the applied electromagnetic braking (EMBr) is used as a common flow control technique. It was revealed by the authors, that a typical double-roll flow in the CC mold can be transformed to the opposite single roll pattern with the growth of the EMBr power and consequent Hartmann numbers. These reverse zones develop from the jets towards a free-surface and occupy the top region of the CC mold in a form of an undesired reverse meniscus flow.

In summary, the Austrian study numerically investigated the spatial revolution of submerged jet flow in transverse magnetic fields. It was clear from their results that a viscous stress increase or porous does not provide a full explanation of the electromagnetic field-induced momentum redistribution. The full explanation was only achieved by taking into account the loops created by the induced electric current. In a statement to Advances in Engineering, Dr. Abdellah Kharicha explained their findings advance our understanding of submerged jet flow frequently observed in metallurgical industries where magnetic fields are used to control the liquid jets. A complex MHD phenomenon, happening in the CC mold, is currently investigated, aiming to assist the EMBr systems development for the metallurgical applications.

Reverse flows and flattening of a submerged jet under the action of a transverse magnetic field - Advances in Engineering
Figure (a) presents a submerged jet colored by the velocity field magnitude, where vectors display the flow direction, and the streamlines show the induced electric current density. According to the presented studies the submerged jet becomes flattened along the applied DC magnetic field. Moreover, due to the closure of the e-current lines through the quiescent liquid bulk the melt is brought to the motion and entrained into the reverse flow zones, developed from both sides of the jet.[1]These fundamental findings have direct engineering applications, for instance in the continuous casting process (CC) with the applied electromagnetic braking (EMBr). Typically, the flow in the CC mold has a so called doble-roll pattern, as in Figure (b). However, with the growth of the EMBr power the reverse zones are initiated above and below the fresh melt jets, detected in Figure (c). These zones develop towards a free-surface and finally occupy the top region of the CC mold in a form of a reverse meniscus flow in Figure (d).[2]

About the author

Assoc. Prof. (Priv.Doz.) Abdellah Kharicha is the head of Christian-Doppler laboratory for the metallurgical applications of Magnetohydrodynamics in Leoben, Austria. Kharicha is also a group leader of the Chair for “Simulation and Modelling of Metallurgical Processes”. The laboratory specializes in the simulations of MHD phenomena occurring in industrial metallurgical processes. A. Kharicha works also on different projects concerning the numerical modeling of flow-solidification interactions and on macro-segregation pattern formation.

Linkedin, ResearchGate

About the author

Dr. Alexander Vakhrushev

Since 2009, A. Vakhrushev develops numerical models of the multi-phase phenomena during continuous casting (CC) of the steel employing open-source CFD software. As a senior researcher, he deals with the magnetohydrodynamics effects on the turbulent flow, solidification, and free surface behavior during the thin slab CC process.

About the author

Dr. Ebrahim Karimi-Sibaki

Ebrahim is a Research Fellow since 2011. His main activities involve modeling and simulation of multiphysics phenomena in electro-metallurgical processes.

His research interests comprise: Solidification, melting, Magnetohydrodynamics, Electrokinetics, Electrodeposition, and Vacuum arc plasma

About the author

Prof. Menghuai Wu did his Master degree in 1986 at Northwestern Polytechnical University in China, PhD degree in 2000 at the Foundry Institute, RWTH Aachen in Germany, and Habilitation (professorial certificate) in 2008 at Montanuniversitaet Leoben in Austria. His main research interests are the modeling and simulation of solidification and related phenomena during different casting processes.

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About the author

Univ. Prof. habil. Dr. rer.nat. Andreas Ludwig studied Physics (1978-1987) at the University of Düsseldorf (Germany), obtained his Dr. rer.nat. at RWTH Aachen (Germany) in 1992 and in 1999 the habilitation (professorial certificate) on Material Physics also at the RWTH Aachen (Germany). In 1993/1994 he was scholar of the German Science Foundation at EPFL Lausanne (Switzerland). In 2013 he became full Professor at the chair of “Simulation and Modelling of Metallurgical Processes” at Montanuniversitaet Leoben (Austria).

References

Kharicha, A., Vakhrushev, A., Karimi-Sibaki, E., Wu, M., & Ludwig, A. (2021). Reverse flows and flattening of a submerged jet under the action of a transverse magnetic fieldPhysical Review Fluids, 6(12), 123701-123715.

Go To Physical Review Fluids

Vakhrushev, A., Kharicha, A., Karimi-Sibaki, E., Wu, M., Ludwig, A., Nitzl, G., Tang, Y., Hackl, G., Watzinger, J., & Eckert, S. (2021). Generation of Reverse Meniscus Flow by Applying an Electromagnetic Brake. Metall Mater Trans B, 52, 3193–3207.

Go To Metall Mater Trans B

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