A team of MIT researchers has succeeded in carrying out a systematic investigation of the factors that control boiling heat transfer from a surface to a liquid, according to a report by the university. The process is crucial to the efficiency of power plants and the cooling of high-power electronics.
According to a report by David L. Chandler of MIT, the research deals with a key transition point known as the critical heat flux (CHF), a value of heat transfer, per unit time and area, where a surface’s heat-transfer characteristics suddenly change. For example, when the cooling panels of an electronics system become covered with a layer of vapor that blocks heat transfer, the resulting rise in temperature can damage or destroy the equipment. The findings could raise the value of critical heat flux, providing extra safety margins or operating ranges for such equipment.
The research was carried out by seven MIT researchers and published in the journal Applied Physics Letters. Co-author Jacopo Buongiorno, an associate professor of nuclear science and engineering, says it could lead to more efficient heat exchangers, better thermal management of high-power electronics and safer nuclear reactors.
According to Chandler, until now, there has been no agreement on the relative importance of three surface attributes that could affect the onset of CHF:
- Wettability, or the ability of water to spread across a surface.
After a detailed investigation, seven MIT researchers found that the presence of a porous layer on a material’s surface is the most important factor by far.
While other researchers have studied these surface effects, Buongiorno explains, earlier analyses often changed multiple surface parameters at the same time, making it difficult to identify which was most important. Buongiorno’s team independently changed each of the three parameters, and obtained “some surprising results,” he says.
The work grew out of the team’s earlier studies of nanofluids — nanoparticles suspended in water — for possible use in nuclear-plant cooling systems. They found that the nanoparticles, which tended to deposit on surfaces, raised the CHF, potentially boosting plant safety.
For most applications such as fuel rods in nuclear power plants or liquid cooling systems in high-power electronics, it is desirable for CHF to be as high as possible. But for some applications such as drag-reduction on the surface of objects moving underwater, a low CHF is desirable. The new analysis shows how to reduce the CHF by applying a hydrophobic, porous coating to the surface.
The new work builds on earlier research by Buongiorno and his colleagues that looked at the flip side of CHF, a process called quenching. This is what happens when a hot material is put in contact with a cold liquid, such as when water is injected into an overheated fuel assembly in a nuclear plant or a glowing-hot piece of metal is submerged in cold oil to engineer its microstructure.
In such cases, the liquid’s contact with the hot metal can create a vapor barrier that effectively insulates the surface. Buongiorno says the metal could be “so hot that when you put water on it, it wouldn’t touch it.” This problem can be overcome by coating the surface with a porous hydrophilic layer that accelerates rewetting of the surface, enhancing heat transfer. Conversely, if rewetting is undesirable, a porous hydrophobic layer would be applied.
In addition to Buongiorno, Rubner and McKrell, the MIT research team included Robert E. Cohen, the St. Laurent professor of chemical engineering; graduate student Carolyn Coyle; Harry O’Hanley SM ’12; and Lin-Wen Hu, associate director of MIT’s Nuclear Reactor Laboratory. Nuclear-reactor vendor Areva NP supported the work.