Sandia National Laboratories researchers Jim Martin and Kyle Solis have what Martin calls a devil of a problem. They have discovered how to harness magnetic fields to create vigorous, organized fluid flows in particle suspensions. The magnetically stimulated flows offer an alternative when heat transfer is difficult because they overcome natural convection limits. Martin and Solis have even demonstrated a heat transfer valve that could potentially control the temperature of computer processors.
The devil is in the details, of course: The problem is that they are not sure how and why the flow patterns occur. Clearly, they say, it is a complex scientific behavior stemming from fundamental phenomena. Just because an effect is easy to generate doesn t mean that it s going to be easy to understand, Martin says.
Also, it is a tough problem to simulate because of the large scale of the flow patterns compared to the tiny particle size, he says. Martin and Solis, a student intern program doctoral researcher, have been generating flow patterns in magnetic platelet suspensions for about three years. They have published papers and led a review of the phenomenon at an international event.
The pair's research, funded by the Department of Energy's Office of Science, is concentrating on extending the fundamental understanding of novel heat transport in liquids; evaluating the effectiveness of various flows; and exploring what happens when researchers modify experimental parameters. Martin and Solis found the patterns occur only for magnetic particles shaped like plates essentially, magnetic confetti. Spherical and rod-like particles do not produce the effects.
The goal is making fluid flow on its own as in thermal convection. Industry forces convection using pumps and fans with associated seals and valves in contact with the fluid but sooner or later, those moving parts corrode and break down.
Martin and Solis make fluids move by adding a small amount of magnetic platelets to a liquid and applying modest, uniform AC magnetic fields. The phenomenon, which they have named isothermal magnetic advection, has shown good results for noncontact heat transfer. It would be useful for cooling microsystems, cooling in microgravity or for transferring heat in circumstances that prevent convection, they say.
“We don t have a lot of understanding of why these things occur, but we can determine what the effects are and how well it works, Martin says.
Because a uniform magnetic field can be scaled to any size, the technology could be practical in problems ranging from reactor cooling to microfluidics, a multidisciplinary field used in designing systems that handle minute volumes of fluids such as blood samples.
Martin and Solis used the phenomenon to create a heat valve they can control to transfer or block heat. They made flow cells a few inches long with blocks on the outside walls through which water flows to keep the blocks cold. The water blocks flank a chamber divided by a razor-blade-size heater made of plastic embedded with wire.
To test thermal transfer properties, the researchers run current through the heater and measure how hot it gets. Because the temperature depends on the heat transfer properties of the chamber s magnetically structured fluid, they control the temperature by controlling the flow created by platelets in the magnetic field.
Some fields effectively solidify the fluid and cause the heater to become very hot while others create strong flows so efficient in extracting heat that the temperature of the heater rises only about 0.5ºF (0.3ºC) higher than the water block temperature, Martin says.
Thus, it acts like a valve because it can control the transfer of heat over a 0.39 (1-cm) gap by a hundredfold, he says. Think of a water valve that can control water flow by a factor of 100 perhaps a little leaky, but still better than no valve. There is still room for improvement, he says.
“Heat transfer can be controlled over any size volume, and the relative efficiency of heat transfer actually increases with scale, Martin says. It is easy to create heat transfer over a large volume because the coils that produce magnetic fields are equally efficient at any size.
Isothermal magnetic advection could help efficiently manage overheating in computers, according to Sandia. The chips in modern supercomputers run ever hotter and use more power, and drawing heat away from them is a technical challenge that is limiting development, Martin says.