Tuning Superhydrophilic Nanostructured Surfaces to Maximize Water Droplet Evaporation Heat Transfer Performance

Abstract

Spraying water droplets on air fin surfaces is often used to augment the performance of air-cooled Rankine power plant condensers and wet cooling tower heat exchangers for building air-conditioning systems. To get the best performance in such processes, the water droplets delivered to the surface should spread rapidly into an extensive, thin film and evaporate with no liquid leaving the surface due to recoil or splashing. This paper presents predictions of theoretical/computational modeling and results of experimental studies of droplet spreading on thin-layer, nanostructured, superhydrophilic surfaces that exhibit very high wicking rates (wickability) in the porous layer. Analysis of the experimental data in the model framework illuminates the key aspects of the physics of the droplet-spreading process and evaporation heat transfer. This analysis also predicts the dependence of droplet-spreading characteristics on the nanoporous surface morphology and other system parameters. The combined results of this investigation indicate specific key strategies for design and fabrication of surface coatings that will maximize the heat transfer performance for droplet evaporation on heat exchanger surfaces. The implications regarding wickability effects on pool boiling processes are also discussed.

Droplet Evaporation on Porous Nanochannels for High Heat Flux Dissipation

“…A high-speed camera is mounted overhead the arrangement to visualize wicking from the top, while a side camera at an inclination of 12 o is used to monitor the temporal variation of the amount of liquid sitting on the top. As the nanochannels are mostly closed (buried) with openings only at micropores, water wicks to much larger distances compared to open structures ,,,, as we have reduced evaporation compared to capillary flow; this allows us to visualize and detect the wicking front with ease. The two cameras are synchronized based on the instant when the droplet touches the surface, and the maximum error in synchronization is estimated to be 0.053 s based on the frame rates.…”

“…Thin-film evaporation manifests itself in nearly all evaporation processes, − and surfaces are designed to amplify its occurrence to achieve high heat flux removal. ,, For example, over the past few years, micro/nanostructures have been fabricated on surfaces to passively wick the liquid and augment thin-film meniscus area, thus enhancing heat transfer. ,,,− The way in which liquid is supplied to such structured surfaces gives rise to two distinct scenarios, first where the surface is partially submerged in a pool of bulk liquid thus providing unlimited supply of liquid to the structures , and second where liquid supply to the structured surface is limited but recurs at regular intervals. An example of the latter is spray cooling ,, where micro/macrosized droplets are dispersed, at a desired frequency, onto a heated surface where the droplets wick into the structures creating thin-film regions.…”

Spray Cooling on Enhanced Surfaces: A Review of the Progress and Mechanisms

“…For a nanostructured surface, capillarity is a dominant factor [147], and these dimensionless numbers are all much less than 1, meaning that the gravitational, kinetic and bulk surface energies can be neglected compared to the surface energy of the structured layer. The strong capillary pressure in the structured layer provides the driving force for liquid advancement against viscous resistance, creating a socalled hemispreading or capillary wicking region beyond the upper droplet contact line [87,129,138,[147][148][149], as shown in Fig. 14(f).…”

A Dynamically Responsive Surface with Switchable Wettability for Efficient Evaporation and Self-Cleaning Abilities

“…Droplet evaporation is essential in many applications, including lab-on-a-chip technologies, inkjet printing, − biosensing, manufacturing, and cooling. − Evaporation is strongly coupled to the wettability of a surface, as observed in several studies. For example, biphilic patterns consisting of hydrophilic and hydrophobic spots are known to affect droplet evaporation , and the boiling characteristics of water .…”