Ultrasound Velocity Measurement in a Liquid Metal Electrode

The overall goal of the following experiment is to measure the flow of a liquid metal electrode of the sort used for energy storage in liquid metal batteries. This is achieved by bringing the liquid metal to a steady temperature and putting it in contact with an ultrasound probe. As a second step, sound speed is measured, which is necessary for subsequent velocity measurements.

Next ultrasound probes are used in order to measure flow while varying the electrical current flowing through the electrode. The results show that increased current drives flow, which would improve battery performance. The main advantage of ultrasound over other methods like hot wires or through MR pairs, is that those make a measurement at a point.

Whereas ultrasound makes a measurement along an entire line through a fluid that gives us much more information with which we can glean more insight about the mixing and mass transport in energy systems like liquid metal batteries. Though this method can provide insight to liquid metal batteries. It can also be applied to other technologies that depend on fluid flow, such as flow batteries and fuel cells.

First, load the ultrasound transducer, lead, bismuth, stir stick, and thermocouples into a glove box following the instructions of the glove box manufacturer to minimize ingress in oxygen and moisture. After tuning the PID controller, use the furnace to melt at least 400 grams of lead bismuth. Insert an ultrasound transducer into the sound speed measurement device and tighten the swage connection to prevent leaks.

After inserting a thermal couple used a workstation to monitor and log temperature to transfer the molten metal to the sound speed measurement device, place the device on the furnace base for two minutes to gradually increase the temperature and avoid thermal shock. After removing heat sensitive equipment and materials from the area, add small amounts of molten metal at a time because thermal shock can damage The ultrasound transducer. Add lead bismuth until the transducer face and micrometer head are both completely submerged.

Once the temperature has stabilized and is within one degree Celsius for at least five minutes, set the micrometer tip to an arbitrary but known location. Record ultrasound echo measurements following the instructions provided by the instrument manufacturer using the micrometer dial. Move the micrometer tip by a known distance, then record ultrasound echo measurements following this.

Remove the molten metal from the sound speed measurement device and store it in a heat tolerant container. To determine sound speed, plot echo amplitude as a function of echo time for each of the two measurements, locate the echoes by fitting a Gaussian curve to each echo peak. Calculate sound speed by dividing displacement distance by the difference in echo peak times.

After melting 840 grams of lead bismuth, insert an ultrasound transducer into the battery vessel and tighten the swage connection to prevent leaks, ensuring that the furnace base is level to transfer the molten metal to the battery vessel. Place the vessel on the furnace base for five minutes to gradually increase the temperature and avoid thermal shock. Once heat sensitive equipment and materials have been removed from the area, add small amounts of molten metal at a time.

Place the furnace insulation around the battery vessel if it is not already there, then place the lid atop the battery vessel along with the negative. Current collector and thermocouples make all electrical connections for both power and signals. Use an OME to verify that no unintended electrical paths are present.

Checking that the electrical resistance between the negative current collector and all signal leads is at least one mega ohm. After waiting until the temperature reaches 150 degrees Celsius, begin logging and monitoring temperature, heat, power, battery voltage, and battery current. Be sure to set the sound speed using the appropriate temperature according to an accepted model.

Adjust a pulse repetition frequency such that echo depths are closely spaced. Then adjust a gate count such that the strong echo from the far wall of the vessel appears in the last few gates. Using instructions provided by the manufacturer, set the instrument for hardware triggering.

Next, begin logging and monitoring velocity with the ultrasound instrument by initiating triggering from the workstation. Record four velocity profiles per second For 30 minutes, set the battery current to five amps After waiting five minutes for the flow to stabilize, record four velocity profiles per second for 30 minutes. After repeating the previous steps for 10, 15, 20, 25, and 30 amps, stop logging data and turn off the furnace.

Disconnect the electrical connections and remove the furnace lid. Remove the molten metal from the battery vessel using the same procedures that were used when filling the vessel and store it in a heat tolerant container. Finally, add extra argonne to the glove box.

A measurement of sound speed in lead bismuth is shown here. Each curve showing measured echo is an average over 98 profiles spanning 7.4 seconds, and the calculated sound speed is 1, 795 meters per second at 138 degrees Celsius. One ultrasound velocity trace recorded without current in the electrode.

It's shown here. The positive velocities signify flow away from the transducer and the negative velocities signify flow toward the transducer. Though ultrasound measurements along one diameter do not give knowledge of the flow everywhere.

The measurements are consistent with a collection of convection rolls as sketched here by representing positive velocities in shades of red and negative velocities in shades of blue time can be plotted to make space time plots, which can convey temporal variation of the flow. This flow is disordered and a periodic consistent with what is expected from turbulent convection. The main flow is plotted here and one standard deviation is indicated.

Ultrasound velocity measurements with current running through the electrode are displayed here. Convection cells tend to align with the magnetic field lines produced by electrical current Organizing the flow increased organization in the presence of a magnetic field is consistent with prior observations in liquid metal convection experiments and theoretical predictions. After watching this video, you should have a good understanding of how ultrasound is implemented to measure velocity in liquid metal electrodes.

So ultrasound provides high resolution measurements of velocity even in an opaque fluid, and that's powerful. That's something we are going to use next to develop empirical models for liquid metal electrodes so that we can make predictions about their mixing and transport at technologically relevant conditions.