Flow Boiling Enhancement Using Three-Dimensional Contact-Line Pinning on Hierarchical Superbiphilic Micro/Nanostructures

Abstract

Flow boiling is a promising method for the cooling of sensitive computational and industrial components, facilitating the transportation of large quantities of heat at near-constant temperature and in a small form factor. The prevention of vapor film formation is a fundamental challenge for the enhancement of boiling systems, and an impetus therefore exists for the discovery of new techniques to segregate nucleating bubbles during their formation. Herein, we utilize the strong capillary forces generated by nanostructures to pin the liquid/vapor interface in three dimensions and thereby control the coalescence and flow interactions of developing bubbles. We demonstrate this principle on both symmetrical and asymmetrical superbiphilic microstructures, showing enhancement of peak heat transfer coefficient by 81% and 113%, respectively, when compared to the best superhydrophilic and superhydrophobic analogues. Our approach shows a potential future direction for engineered boiling micro/nanostructures, wherein bubble dynamics are directly manipulated on bespoke, three-dimensional substrates.

Flow Boiling Heat Transfer Enhancement Using Tuned Geometrical Contact-Line Pinning

“…Representing the nanograss as an ordered array of nanopillars with height of 2.6 μm, diameter of 200 nm, and pitch of 400 nm (estimated from SEM imagery), we may apply the wicking equation of Dhillon et al to estimate the capillary wicking forces on the superhydrophobic and superhydrophilic surfaces. Analysis of the capillary pressure at the superbiphilic interface reveals a stabilizing pressure of around 40 kPa toward the edge of the hydrophobic dot, so we may assume that the contact line adopts the same shape as the hydrophobic patch. High capillary pressure stabilizes the bubble’s contact line at the biphilic interface, thereby causing constant-contact-line (CCL) growth .…”

“…Recognizing that homogeneous wettability necessitates an innate compromise between CHF and HTC, there have been a number of recent investigations into boiling enhancement on surfaces engineered with heterogeneous wettability. − These surfaces are designed to benefit from the increased ebullition rate on hydrophobic patches, while a surrounding hydrophilic network simultaneously aids the transport of liquid to hotspots and hinders bubble expansion. Substantial performance enhancements have been recognized using this method; for example, previous studies have shown that hydrophobic islands on a hydrophilic surface can impart respective HTC enhancement of 100% and 113% in pool boiling and flow boiling compared to homogeneous surfaces, while CHF can be simultaneously augmented beyond that of a homogeneous surface.…”

“…23−28 These surfaces are designed to benefit from the increased ebullition rate on hydrophobic patches, while a surrounding hydrophilic network simultaneously aids the transport of liquid to hotspots and hinders bubble expansion. Substantial performance enhancements have been recognized using this method; for example, previous studies have shown that hydrophobic islands on a hydrophilic surface can impart respective HTC enhancement of 100% and 113% in pool boiling 28 and flow boiling 29 compared to homogeneous surfaces, while CHF can be simultaneously augmented beyond that of a homogeneous surface. Thus far, the majority of literature surrounding boiling on biphilic substrates has focused on pool boiling, with investigations being carried out into the influence of various design parameters such as dot geometry, 30 diameter/pitch, 31,32 peripheral length, 23 and hydrophobic/hydrophilic area ratios.…”

“…Thus far, the majority of literature surrounding boiling on biphilic substrates has focused on pool boiling, with investigations being carried out into the influence of various design parameters such as dot geometry, diameter/pitch, , peripheral length, and hydrophobic/hydrophilic area ratios . Hydrodynamic effects preclude the transferal of pool-boiling theories into meaningful predictions of microchannel flow boiling performance; however, there have been a limited number of attempts to connect the theories through flow-boiling investigations using biphilic microchannels, ,,− some of which are discussed later in the current article.…”

Recent progress in films with nanoengineered surfaces via bubble-induced self-assembly for energy applications

“…Theoretically, the boiling heat transfer capacity is mainly related to the thermal physical properties of the working medium and the characteristics of boiling surfaces. [26][27][28] Due to the limitation in the choice of working medium in a boiling system, recent research has given more attention to building up lms with nanoengineered surfaces which modify the characteristics of the substrates and enhance the boiling heat transfer using micro/nanofabrication technologies. [29][30][31] Conventional fabrication technologies such as high-precision milling, 32 wire cutting, 33 high-temperature sintering, 34 and electrochemical deposition, 35 as well as the emerging fabrication technologies including chemical vapor deposition (CVD), 36 layer-by-layer deposition, 37 additive manufacturing, 38 microlithography, 39 and evaporation-induced self-assembly (EISA), 40,41 have all been used to generate different patterned micro/nanostructures.…”