University of Illinois at Urbana-Champaign

Energy Transport Research Lab


Wettability


Wetability work at the ETRL is funded by UIUC.

Interactions between liquids and solids are ubiquitous in our physical environment and are typically characterized by the wetting angle that a liquid droplet makes on the solid surface. While wettability on flat and homogenous surfaces has been researched quite extensively, recent advances in micro-/nano-fabrication and coating technologies have enabled the development of smart engineered surfaces. Fundamental understanding of the wetting and liquid propagation behavior on these surfaces is important for a range of applications such as microfluidics, thermal management, lab-on-a-chip, desalination, optical, and biological systems.

At the Energy Transport Research Lab, we are working towards developing a better understanding of the change in wettability due to surface engineering [1], chemical heterogeneities [2], and in the presence of liquid-vapor phase change phenomena [3]. These studies are critical for elucidating the underlying physical mechanisms behind the other research topics being investigated in our lab. For example, hydrophilic surfaces made superhydrophilic due to structuring have resulted in enhancement of boiling and thin-film evaporation heat transfer coefficients. Conversely, hydrophobic structured surfaces, i.e. superhydrophobic surfaces have recently shown a promise to push the limits of condensation heat transfer [4].


Thin Film Evaporation


Evaporation work at the ETRL is funded by UIUC.

With the increase in processing speed in compact electronic devices, passive heat transfer cooling technologies with the ability cool heat fluxes of up to 100 W/cm2 are highly desired in the microelectronics industry. Conventional air cooling strategies are insufficient at these large heat fluxes. As a result, novel thermal management solutions such as thin-film evaporation that utilize the latent heat of vaporization of a fluid are needed. The high latent heat of vaporization associated with typical liquid-vapor phase change phenomena allows significant heat transfer with little temperature rise [5].

At the Energy Transport Reseach Lab, we are useing a combination of state-of-the-art silicon fabrication techniques with highly scalable metal oxidation to fabricate micro/nanostructured surfaces to implement thin-film evaporation [6]. The structures improve liquid spreading by generating capillary pressure in addition to increasing the thin-film region where the majority of the evaporation occurs. Moreover, the liquid-film thickness and the associated thermal resistance is minimum in the thin-film region making thin-film evaporation an attractive choice for dissipating high heat flux. However, one limitation in using microstructured surfaces for thin-film evaporation is drying out which occurs when the generated capillary pressure cannot transport enough liquid to sustain the evaporation. To overcome this limitation on the capillary pressure budget, we use biporous wicks (two level of porosity) to enhance liquid spreading as well as to achieve high heat flux and heat transfer coefficient. Nanostructure (first level porosity) is used to generate higher capillary pressure that assists liquid spreading while larger microchannels (second level porosity) are used to reduce the overall viscous loss by providing a less-viscous bypass path for fluid flow.


Condensation


Condensation

Condensation is a phase change phenomenon often encountered in nature, as well as in industry for applications including power generation, thermal management, desalination, and environmental control [7]. For the past eight decades, researchers have focused on creating surfaces allowing condensed droplets to be easily removed by gravity for enhanced heat transfer performance [8]. Recent advancements in nanofabrication have enabled increased control of surface structuring for the development of superhydrophobic surfaces with even higher droplet mobility and, in some cases, coalescence-induced droplet jumping [9].

At the Energy Transport Research Lab, we are theoretically [10] and experimentally [11] studying superhydrophobic and oleophobic [12] surfaces for enhanced condensation heat transfer for water and refrigerant based condensation systems. We work on identifying challenges and new opportunities to advance these surfaces for broad implementation into thermo-fluidic systems [4,7].

In addition, we study the fabrication, characterization, wettability [3], and interfacial dynamics of superhydrophobic materials during condensation to examine the role of surface structure on emerging droplet morphology [13], nucleation density [14], droplet growth rate [15], and departure characteristics. Furthermore, we seek to develop scalable [11] fabrication techniques for creating superhydrophobic surfaces with experimentally demonstrated heat transfer enhancement.


Freezing


Freezing

Frost has long been a problem in the eyes of human society. The effects of frosting can have dramatic consequences: downed power lines, damaged crops, stalled aircraft, as well as decreased performance of ships, wind turbines, and HVAC systems [16-18]. The buildup of frost on HVAC components results in a continuous decrease in cooling capacity, increased pressure drop, and therefore increased pumping costs. Currently used active chemical, thermal, and mechanical methods of ice removal are costly. Therefore, the development of passive methods preventing frost and ice accumulation is desirable.

The current approach to fabricate frost reducing surfaces focuses on the development of rough hydrophobic surfaces to increase the energy barrier for ice nucleation [19, 20], and to further reduce both the contact angle hysteresis, and the ice adhesion strength [21, 22]. Using this approach, many recent studies have shown superhydrophobic surfaces to be successful in preventing ice buildup for individual droplets being deposited on the surface or impacting the surface at some prescribed velocity [23]. However the typical working conditions encountered in industrial HVAC applications are better represented by condensation frosting, where the surrounding moist air first condenses on the surface to form liquid microdroplets that grow via coalescence, nucleate ice and grow in the form of frost [24]. When condensation frosting occurs on a superhydrophobic surface, the formation of highly mobile ‘Cassie’ droplets cannot be ensured, and the nucleating ice can impale the structure and result in increased ice-adhesion [25]. Hence, the early and energy-efficient removal of condensed microdroplets from the supercooled surface is of high priority.

At the Energy Transport Research Lab, we are currently studying a new approach to creating frost-reducing surfaces consisting of suitably designed, and highly scalable superhydrophobic surfaces. The key innovation in the proposed concept is the incorporation of the novel superhydrophobic nanostructures on the frosting surfaces (i.e. evaporator fins). Accordingly, the new mode of jumping-droplet-condensation [9], which has been previously demonstrated [11], can be harnessed to achieve an order of magnitude increase in heat transfer and hence condensed water removal rate (prior to frosting), compared to that of smooth hydrophilic metal surfaces.

The freezing work at the ETRL is funded by the Air Conditioning and Refrigeration Center (ACRC) at the University of Illinois at Urbana-Champaign.


Solar


Solar irradiation is a promising source of renewable energy, as the hourly incident solar flux on the surface of the earth is greater than annual global energy consumption [26]. Efficient harvesting of solar energy for steam generation is a key factor for a broad range of applications, from large-scale power generation, absorption chillers and desalination systems to compact applications such as water purification for drinking, sterilization and hygiene systems in remote areas where the only abundant energy source is the sun. Current methods of generating steam using solar energy rely on a surface or cavity to absorb the solar radiation, and transferring heat to the bulk liquid directly or via an intermediate carrier fluid, which require high optical concentration and suffer from high optical loss and surface heat loss [27], or vacuum to reduce convective heat loss under moderate optical concentration. The steam thus generated is usually in thermal equilibrium with the bulk liquid. Nanofluids—fluids seeded with nanoparticles—as another alternative have been studied [28-30] as volumetric absorbers, potentially minimizing the surface energy loss by uniform temperature in the fluid. Local generation of steam in a cold bulk liquid can be achieved through high concentrations or illumination of nanofluids by electromagnetic waves (generally, lasers) with high power intensity [31]. However, the high costs and optical concentrations limit the utilization of these approaches in stand-alone compact solar systems.

At the Energy Transport Research Lab, we are working on a new approach and corresponding material structure that localizes the solar energy where evaporation occurs and minimizes the heat losses leading to enhanced solar thermal efficiency at low optical concentration in open air while generating steam [32]. Under solar illumination, the developed structure forms a hot spot internally where evaporation occurs. The fluid wicks to the hot spot, evaporates and forms vapour which leaves the structure. This structure needs to have four main characteristics: high absorption in the solar spectrum, low thermal conductivity to suppress thermal conduction away from the hot internal region, hydrophilic surfaces to leverage capillary forces and promote fluid flow to the hot region, and interconnected pores for fluid flow to and from the structure.


Energy Harvesting


Energy harvesting or “energy scavenging” is the conversion of ambient energy present in the environment into electrical energy [33]. Typically, energy harvesting involves the conversion of small amounts of ambient energy to power small (<1 cm), low-power (<1 µW) electronic devices. In addition to being pollution free, the harvested energy is usually derived from waste energy streams that are otherwise not harnessed for useful work. Energy harvesting has therefore attracted much interest because of its potential use as a power supply in applications such as low-power wireless sensor networks [34] and electronic systems [35].

At the Energy Transport Research Lab, we are interested in jumping-droplet- based energy harvesting with nanoengineered superhydrophobic surfaces. Recent studies have shown that when small water droplets (<10–100 µm) merge on superhydrophobic nanostructured surfaces, droplets can spontaneously eject and charge via the release of excess surface energy irrespective of gravity [9]. We are interested in taking advantage of this unique droplet-charging phenomenon to study jumping-droplet energy harvesting, where the charged droplets jump between superhydrophobic and hydrophilic surfaces to create an electrostatic potential and generate power by direct condensation of atmospheric moisture [36].


References

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