Three Meter Liquid Sodium Dynamo Experiment


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The 3m sphere through the door of the support frame.

The three meter experiment is a major research instrumentation project at the University of Maryland in College Park, MD.  It was funded by the National Science Foundation Earth Sciences / MRI program.  I have been a critical member of the research team (including Dan Lathrop, Santiago Triana, and Don Martin). My work includes mechanical, electrical, and thermal design, fabrication, instrumentation, safety design, and scientific work in this massive device.  

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Experiment cross-section

The experiment consists of two concentric rotating spheres, independently driven by 250kW electric motors. The 3m outer sphere is supported by a massive stainless steel frame I designed.  The rotating outer tank and its contents is twenty tons of sodium metal and stainless steel and can rotate up to four revolutions per second; you can see the device spinning at 2Hz in the video at the bottom of the page. We haven't gone to full design speed during sodium experiments, but we did run a mechanical test to full rated speed when the experiment was full of water.

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Frame vibration analysis: Algor beam modeling.

Mechanical vibration is a major concern with a machine like this.  Unlike many pieces of industrial equipment, the scientific goals require finely adjustable motor speeds over a large range.  This makes it important to understand and avoid, damp, or otherwise mitigate mechanical resonances.  I used Algor finite element beam analysis software to design a stiff and easily buildable frame with troublesome resonances out of most of the operating range.  

I also did experimental testing with shakers and accelerometers to test the predictions and understand the role of boundary condition and bolted joint nonlinearities and other complicated real-world issues.  In addition to vibration design and testing, I performed static and dynamic stress analyses and developed the final frame as a balance of strength, stiffness, cost and easy construction.

 

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Inner sphere stress analysis.

For equipment with major life safety implications, like the rotating outer sphere that contains 13 tons of caustic metal, we worked with professional and licensed engineering consultants.  This was not as much of a concern for the inner sphere, because it is completely contained by the outer.  I performed a stress analysis to decide a reasonable speed limit for the inner sphere.  We could not inspect the welds at the equator and near the poles of the inner sphere, so my calculations included some assessment of the role of incomplete weld fusion on the maximum safe rotation speed.

 

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Frame Drawings and Fabrication

In addition to designing the framework that supports the outer sphere as it spins, I produced construction drawings and constructed much of it.  I am an expert in metal fabrication including MIG and TIG welding.  In addition, I have extensive lathe, milling, and other machining experience.

 

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Outer sphere drive system.

The inner sphere on the three meter apparatus is driven directly by a 250kW three phase induction motor through a wireless torque sensor of my design, which has its own page to the right.  The outer sphere required is driven through a huge timing-style belt drive that I designed.  A 48 tooth pulley on the outer motor drives a 400 tooth pulley custom manufactured by Motion Systems in Warren, MI via a wide Kevlar and rubber power transmission belt.  The synchronous drive provides the necessary speed control and this high tech belt provides long life and good performance despite the high tangential speed of the drive pulleys.

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Rotating instrumentation.

I designed and implemented much of the scientific and utility instrumentation for this apparatus.  The main acquistion system rotates with the experiment and at the heart is the rotating computer pictured above.  This computer has a custom case that includes voltage conversion equipment for other instrumentation and easy interconnects.  

inside_rotcomp_sm.jpgInside the rotating computer.

The system is powered from three rotating sealed lead acid batteries. The equipment in the rotating frame communicates to the lab via an 802.11n network bridge.  I've done several custom instrumentation electronics projects for this apparatus, including the small box to the right in the picture above which contains several channels of biasing and signal conditioning circuitry for fluid pressure sensors.  Other instrumentation projects are described on other pages on the site.  

The main measurement of interest for the experiments with liquid metal are the magnetic field fluctuations induced (or hopefully, eventually, generated) by the flowing sodium.

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Honeywell SS94A1F sensors and mounts.

An array of 31 hall effect sensors is used to measure the magnetic field just outside the outer sphere.  The sensors are mounted to adjustable nylon blocks on stainless steel posts that I welded to the outer sphere surface.  The photo above shows the sensors just prior to installation, and the photo below shows an installed sensor.  The sensors are powered by a 10V regulator and the sensors' output voltages are acquired by a commercially available 16 bit acquisition card in the rotating computer.  

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Installed Hall Sensor

Dynamic pressure signals from the turbulent sodium flow are also acquired as an interesting scientific measurement.  The rotating computer also handles measurements of the battery voltage and current, sodium and outer shell temperatures, and the pressure of the bubble of inert gas at the north pole of the outer sphere. 

A separate acquistion computer that's fixed in the laboratory monitors the position of the inner and outer spheres with optical shaft encoders, and also has sensors that monitor the structural and mechanical health of the experiment. At this time multiple acquisition computers are synchronized using NTP, which is sufficient for our needs. Three accelerometers are mounted on the support frame. Two of them are 3-axis MEMS accelerometers mounted near the motors and the third is a higher sensitivity quartz unit mounted lower on the frame. These are monitored by the computer and our safety interlock system will shut down the machine if excessive vibration is detected. 

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3-Axis Accelerometer
 
 We also monitor the height of the bottom of the sphere with an optical height sensor. This would give some early warning of problems with the bottom bearing that holds the weight of the experiment, any wobble with respect to the base, or other mechanical issues.   
 
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Height Sensor
 
When we're running the experiment, we hold the sodium at about 118°C (melting point is about 98°C).  Temperature control is crucial; too hot and the system will become too full and leak.  Too cold and the small volume of inert gas we leave in the experiment can get too large and start to effect the fluid dynamics and measurements.  I designed, bought, and built much of the thermal control system. Because temperature control is very important, the temperatures of the sodium and the outer shell are monitored by redundant systems during running tiem, RTD sensors logged to the main acquisition computer and a second independent wireless temperature transmitter system. 
 
Temperature control is done by a hot oil system under computer control. A 30HP  centrifugal pump develops about 90psi to circulate Shell heat transfer oil at a flow rate of nearly 500 gallons per minute through a rotating coupling and the half-pipe thermal jacket integral to the sphere.

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Thermal system and fume scrubber.

  The oil passes through a 120kW computer-controlled electric heater and optionally through the fan-cooled heat exchanger units in the photo above.  Cooling is modulated by a computer controlled actuated bypass valve when the mechanical power dissipation in the fluid exceeds the heat losses to the lab.  

The bigger cylindrical tank and stack pictured above is a wet scrubber system we installed in case of a sodium fire.  We bought the SLY fume scrubber from a surplus outfit and rehabilitated it with a few replacement parts.  In an emergency situation, the air evacuated from the experimental space by a massive 50HP centrifugal blower passes through a water-flooded perforated plate which dissolves the caustic smoke.  The effluent is drained into the sanitary sewer.

Rotating at two revolutions per second during experiments.