Unexpected flow behavior in liquid metals

Some metals are in liquid form, the main example being mercury. But there are also huge amounts of liquid metal in the Earth’s core, where temperatures are so high that some of the iron is molten and undergoing complex flows. A team from Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has now simulated a similar process in the laboratory and made a startling discovery: under certain circumstances, the flow of liquid metal is much more turbulent than expected – and this has a significant impact on heat transport, presents the group in the journal Physical Review Letters.

Temperatures deep inside the Earth are so high that part of its iron core is liquid. This liquid iron is in constant motion, continually stirring and circulating. It acts like a dynamo, causing our planet’s magnetic field to be generated. One of the driving forces behind this complex iron flow behavior is the Earth’s rotation, another is what is called “convection”, driven by temperature differences: similar to how the Hot air rises above a radiator, where it displaces colder, relatively warm air. iron in the Earth’s core flows to cooler areas, resulting in heat transfer. However, so far little is known about how these processes take place in detail. To better understand them, experts must rely on theoretical calculations and computer simulations, as well as experiments that simulate what happens – at least to some extent – ​​at the laboratory scale.

One such experiment was conducted recently at HZDR’s Institute of Fluid Dynamics. “We took two cylindrical vessels – one relatively small the size of a bucket and the other barrel-shaped with a volume of 60 litres,” explained project leader Dr Tobias Vogt. “We filled these vessels with a metal alloy of indium, gallium and tin, which is liquid at room temperature.” The experts heated the bottom of the containers while cooling the top, creating a temperature difference of up to 50 degrees Celsius between the top and bottom layers.

Ultrasound offers an in-depth view

This substantial temperature difference caused the churning of the liquid metal inside the vessels: driven by convection, locally hotter flow areas such as columns rose and mixed with the colder parts – as a lava lamp. The metal alloy used by the team being opaque, they nevertheless had to resort to a particular analysis technique: “It’s an ultrasound method used in medicine”, explains Dr Sven Eckert, head of the magnetohydrodynamics department at the HZDR. “We have installed about twenty ultrasonic sensors on the tanks, which allows us to detect the circulation of liquid metal inside.”

While analyzing the data, the research group made a startling discovery. During the experiments, experts expected to find the clustering of individual flow areas to form a larger and more extensive structure, known as the large-scale circulation. “It’s comparable to a thermal wind, which is able to transport heat very efficiently between the top and the bottom,” Vogt said. “We were indeed able to observe this thermal wind in the smaller vessel – but with the larger vessel, the barrel, large temperature differences led to almost complete breakdown of the wind.” This meant that the heat was not transported as efficiently as might have been expected. “We believe the cause is the formation of much smaller scale turbulence rather than a few large vortices, which makes heat transport less efficient,” Vogt said.

Implications for battery technology

These new discoveries could have implications for what is happening in the Earth’s core: “To understand what is happening, experts are trying to extrapolate the results of laboratory experiments to the scale of the Earth”, explains Sven Eckert . “But we have now shown that heat is transported less efficiently under certain conditions than previous experiments had suggested.” This means that predictions for Earth will likely produce different values ​​as well. “However, the actual processes in the Earth’s core are far more complex than in our laboratory experiments,” added Tobias Vogt. “For example, the flux of liquid iron is also influenced by the magnetic field and the rotation of the Earth – ultimately, we know very little about these flux processes.”

In fact, the new findings could also prove relevant for technology, especially in areas involving liquid metals. For example, liquid metals are used in certain types of batteries as well as for future solar power plants and cold fusion reactors. To be able to further investigate heat transport in liquid metals, the HZDR team is currently working on an advanced analytical technique. “Special inductive sensors should record streams in even greater detail than before and produce true 3D images,” Sven Eckert remarked. “Our first measurements are very promising.”

Reference:

  1. Felix Schindler, Sven Eckert, Till Zürner, Jörg Schumacher, Tobias Vogt. Collapse of large-scale coherent flow in strongly turbulent liquid metal convection. Physical Examination Letters, 2022; 128 (16) DOI: 10.1103/PhysRevLett.128.164501
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