The system, which could improve the efficiency of electric power generation and other processes, is described in a paper by Department of Mechanical Engineering Professor Evelyn Wang, graduate student Jeremy Cho, and graduate Jordan Mizerak.
Prof Wang says this degree of control over the boiling process, independent of temperature, has not previously been demonstrated despite the ubiquity of boiling in industrial processes. Other systems have been developed to control boiling using electric fields, but these have required special fluids rather than water, and a thousand fold higher voltages, making them economically impractical for most uses.
The team’s result was accomplished by adding surfactants to water - creating a soapy liquid. The surfactant molecules, which carry an electrical charge, can be attracted to, or repelled by, a metal surface by changing the polarity of the voltage applied to the metal. This is claimed to switch the metal surface between being hydrophilic and hydrophobic.
Adding the surfactant causes the surface to become more hydrophobic, which increases the rate of nucleation to form bubbles. But reversing the charge on the surface causes the surface to become hydrophilic, and inhibits the formation of bubbles. The researchers found that they could achieve a tenfold change in the rate of bubble formation by switching the charge.
Being able to actively control the rate of bubble formation, in turn, allows control over the rate of heat transfer between the metal and the liquid. That could make it possible to make more efficient boilers for power plants or other applications, since present designs require a substantial safety margin to avoid the possibility of hot spots that could damage the equipment. While most such power plants operate at a steady rate most of the time, being able to control the heat transfer rates dynamically could improve their efficiency when ramping up or down from full power, and thus make it easier to make real-time adjustments in their output without losing efficiency. Similarly, liquid cooling for high-performance electronics also could be made more efficient by being able to control the rate of bubbling to prevent overheating in hotspots, the team says.
Power plant operators are conservative about making changes, since people depend on their output, so even though this system requires only relatively minor changes, a demonstration plant would be needed to prove the concept at operational scale.