Abstract
In this study, the optimal surface pattern of low and high wettability regions for enhanced boiling heat transfer is investigated using aluminum superbiphilic surfaces. Samples are fabricated by combining chemical vapor deposition of a fluorinated silane to turn them superhydrophobic and nanosecond laser texturing to render selected areas superhydrophilic. Triangular lattice pattern of superhydrophobic circular spots is utilized with spot diameters between 0.25 mm and 1.0 mm and pitch values of 0.5–2.5 mm. Pool boiling heat transfer performance of superbiphilic surfaces is evaluated using saturated water at atmospheric pressure. A strong wettability contrast is shown to be important in ensuring high heat transfer performance of wettability-patterned surfaces. Highest heat transfer performance is achieved using 0.5 mm diameter spots with a spot pitch of 1 mm and a corresponding superhydrophobic area fraction of approx. 23%. The optimal pitch value will provide a high density of potentially active nucleation sites but still allow for the development of the thermal boundary layer thus not inhibiting the activation of neighboring spots. The size of (super)hydrophobic spots appears not to have a major influence on the boiling performance when using the optimal spot pitch. The developed superbiphilic surfaces increase the CHF and provide greatly enhanced heat transfer coefficients especially at medium and high heat fluxes, making them suitable especially for high-heat-flux applications.
Highlights
Progress in research and development of electronic devices, aerospace and space flight systems, nuclear power systems and refrigeration applications brings along increased cooling requirements as the size of the systems and their components decreases while their power dissipation requirement often increases, outlining the need for new cooling solutions [1]
The laser texturing was performed in open air atmosphere using a set of parameters obtained through preliminary testing, which were chosen so that superhydrophilicity was achieved alongside minimal damage to the surface and minimal roughness increase making the superhydrophobic spots welldefined
Scanning electron microscopy (SEM) imaging was performed using JEOL JSM-6500F SEM at an accelerating voltage of 15 kV utilizing a secondary electron detector to analyze the morphology of modified surfaces
Summary
Progress in research and development of electronic devices, aerospace and space flight systems, nuclear power systems and refrigeration applications brings along increased cooling requirements as the size of the systems and their components decreases while their power dissipation requirement often increases, outlining the need for new cooling solutions [1]. Betz et al [13] combined a PTFE coating and reactive ion etching to produce superbiphilic surfaces, i.e., surfaces with superhydrophobic spots and a superhydrophilic surrounding area They achieved heat transfer coefficients of over. A study of the effect of the hydrophobic spot pattern was performed by Jo et al [37,38], who report better low heat flux behavior with a small pitch value and a large spot size, while increasing the number of spots and decreasing their diameter results in higher heat transfer coefficients at high heat fluxes independent of the pitch value. The effects of spot size, spot pitch, (super)hydrophobic area fraction, pattern’s size scale and wettability contrast are elucidated through the comparison of boiling curves, heat transfer coefficients and high-speed video analysis
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