Efficient Heat Transfer
Device Research Laboratory scientists aim to control changes in phase of gases and liquids to reduce power consumption in heating and cooling systems.
Efficient heat and mass transfer for devices from a thermal battery to air condition electric vehicles to cooling for steam power plants is the focus of Evelyn Wang, Associate Professor of Mechanical Engineering, at MIT, and principal investigator of the Device Research Laboratory. “In general, our lab works a lot on using micro and nanostructures for various thermal-fluid applications,” Wang said. Among key recent projects in Wang’s group are:
Former graduate student and postdoctoral associate Nenad Miljkovic pioneered highly scalable nanostructured coatings for copper tubing that shed water efficiently, boost heat flux by 25 percent and raise the condensation heat transfer coefficient by 30 percent. The work has applications for power plants, thermal desalination, dehumidification, and other industrial uses. Wang is principal investigator in a multi-site, ARPA-E funded project to develop a thermal battery for heating and cooling in electric vehicles. Postdoctoral associate Shankar Narayanan is leading work on an adsorption bed for the battery and graduate student Ian McKay is exploring how to integrate Miljkovic’s discoveries regarding superhydrophobic and superhydrophilic surfaces into the evaporator-condenser design.
Graduate student Andrej Lenert and colleagues are developing solar thermophotovoltaic systems based on photonic crystal absorbers and emitters that promise to deliver 10 percent efficiency. See related article.
Miljkovic and colleagues demonstrated enhanced jumping behavior of water droplets on a copper pipe which he made superhydrophobic by growing a thin layer of silanized copper oxide as reported in a November 2012 Nano Letters paper, “Jumping-Droplet-Enhanced Condensation on Scalable Superhydrophobic Nanostructured Surfaces.”
In an earlier paper, “Effect of Droplet Morphology on Growth Dynamics and Heat Transfer during Condensation on Superhydrophobic Nanostructured Surfaces,” Miljkovic showed that the shape of droplets profoundly affected the heat and mass transfer performance, and that, in particular, round suspended droplets, were much less effective in removing heat than mushroom-shaped partially wetting droplets. Most recently, in a Nature Communications article, “Electrostatic charging of jumping droplets,” Miljkovic showed that jumping droplets carry a net positive charge, which allows use of an external electric field to control droplet jumping.
By removing water more efficiently, engineers could reduce the size of cooling systems in steam power plants, for example.
|Associate Professor of Mechanical Engineering Evelyn Wang (center), post-doctoral associate Nenad Miljkovic (left) and graduate student Andrej Lenert (right) and with solar thermal photovoltaic experimental system in Rohsenow Kendall Heat Transfer Lab at MIT.
Photo: Denis Paiste, Materials Processing Center
Water shedding coupled with heat transfer
A key finding in Miljkovic’s experiments was that simply removing droplets more easily doesn’t guarantee better heat transfer. “You have to still harness the right type of droplet morphology, or the shape of the droplets to get enhancements. There are a lot of considerations that need to be taken into account to get enhanced heat transfer that are distinct from if you just look at the fluids problem (i.e. droplet removal and jumping),” Wang said. “The work naturally led to this insight to be able to design surfaces that give you the right, not just fluid characteristics (jumping), but also the heat transfer behavior, and that’s really important.”
“When you have a droplet on an atomically rough surface with chemical heterogeneity (hydrophilic and hydrophobic areas), basically it prevents the movement of the droplet, the droplet stays kind of immobile on the surface and it doesn’t allow the liquid to further propagate because of the fact there is this roughness on the surface and an associated work of adhesion that needs to be overcome,” Wang said. The droplet stays pinned to the surface, a phenomenon known as contact line pinning. To promote droplet jumping, that adhesion energy has to be overcome, which can be achieved with superhydrophobic surfaces Miljkovic said.
On a superhydrophobic surface (with micron scale pillars acting as roughness), droplets can exist in two different states, a Wenzel state in which pillars in the microstructure are penetrated and a Cassie state in which the droplet doesn’t penetrate but is suspended on top of the pillar tips. “You really need to get these partially wetting type, localized Wenzel base morphologies, not the suspended Cassie morphologies, which is what actually a lot of people have studied. They’ve been thinking for a long time that droplets that are suspended on top of these surfaces, because they have very low contact line pinning, that directly translates to enhanced heat transfer, but that’s not true because what dominates is resistance at the base during droplet growth prior to removal,” Wang said.
“The reason we ended up going with copper oxide is it favors the formation of these partially wetting droplets, so it’ll have this small liquid bridge at the base. It’ll form within the structures and then it’ll balloon out,” Miljkovic said. By addition of fluorinated silane to the copper oxide, silicon bonds to the oxide. “It just orients itself so that the silicon is bonded to the surface and this non-polar chain is vertically upwards and they self-assemble, these molecules, and form a grass of non-polar carbon-hydrogen atoms or carbon-fluorine atoms that coat the entire surface. It’s the fact that these chains are non-polar that make them hydrophobic,” Miljkovic said. “Water is polar, so it will tend to like itself more than wanting to interact with a non-polar molecular chain, so it will bead up on the surface.” One issue longer term for applications is durability of the extremely thin silane coatings, which are on the order of 1 to 2 nanometers.
Miljkovic finished his doctorate at MIT in June 2013, and then was a postdoc with Wang until September. He joined Professor Gang Chen’s lab Sept. 1, also in mechanical engineering and will be starting next fall as an assistant professor of mechanical science and engineering at the University of Illinois Urbana-Champaign.
Thermal battery work
Wang’s lab also is spearheading an effort with the University of Texas-Austin, University of California-Berkeley and Ford Motor Co. to develop a thermal battery to replace existing HVAC systems in electric vehicles and prolong their driving range. “You don’t want to use your electrical battery to drive your HVAC system because that consumes a lot of your electric battery power and you want to use that to increase the actual driving range.” Wang said. “The idea is you would charge this thermal battery with your electric battery at the same time and you would get heating and cooling for the duration of your driving.” The work was featured in a recent article in Automotive Engineering International.
Wang’s lab is developing an adsorption system, which relies on adsorbent materials such as zeolites and metal organic frameworks. Postdoctoral associate Shankar Narayanan is leading the adsorption work at MIT and Omar Yaghi’s group at Berkeley is developing the metal organic frameworks. The $2.7 million ARPA-E project is in the second of three years.
“The design of the adsorption bed is very important to try to maximize your heat and mass transport to provide the delivery that you need at the rate that you want in the passenger cabin for your electric vehicle. Ideas go from development of materials at the fundamental level all the way to systems architecture in prototype,” Wang said.“Our focus has been on the adsorption bed which is the heart of the concept. Now, it is integrated with the evaporator-condenser, which makes it unique.. We’ve also been developing zeolites as adsorbents, but they have pretty poor thermal conductivities. So to enhance the thermal transport, we’ve been incorporating high thermal conductivity binders ,” Wang said.
There may be a role for Miljkovic’s nanostructured superhydrophobic surfaces in the project as well. “Actually the evaporator-condenser design can be based on some of this (Miljkovic’s) work,” Wang said. “This is kind of the far out idea, but certainly I think that’s where the opportunity is to enhance performance.” Graduate student Ian McKay is working on that aspect of the thermal battery and how to use superhydrophobic and superhydrophilic surfaces for the evaporator-condenser design of this adsorption bed.
“The connection here is it's also a phase-change based system. It relies on similar fundamental physics but uses different materials to harness the behavior that we want for this particular application,” Wang said.