Superbiphilic patterned nanowires with wicking for enhanced pool boiling heat transfer

Highlights

• Superbiphilic (SBPI) patterned nanowires are applied to enhance boiling performance.

• Boiling heat transfer characteristics and bubble behaviors are experimentally investigated.

• SBPI surfaces significantly improve the heat transfer coefficient (HTC) due to the promotion of nucleation.

• Critical heat flux (CHF) enhancement limits of SBPI surfaces accompanied by wicking are broken in the present work.

• The mechanisms of CHF enhancement are discussed by analyzing both micro and macro scale liquid supply.

Abstract

The boiling performance, represented by the heat transfer coefficient (HTC) and critical heat flux (CHF), must be enhanced because the energy demand of industrial processes that generate a lot of heat increases under extreme conditions. Surface manipulations have been used to improve boiling performance by controlling interfacial characteristics. Specifically, biphilic or superbiphilic patterned surfaces have been widely utilized to enhance HTC and CHF. However, it remains a challenging issue to improve CHF on superbiphilic surfaces with wicking phenomena due to the suppression of liquid supply in hydrophobic regions. In the present work, to investigate the mechanism and experimentally break through the limits of CHF enhancement, artificially patterned superbiphilic (SBPI) surfaces with different superhydrophobic (SHPO) area fractions were produced, and conducted pool boiling heat transfer. By artificially promoting nucleation, all SBPI surfaces demonstrated a higher HTC than homogeneous wettability surfaces. Considering dynamic wicking and bubble behaviors, the SBPI successfully broke through the CHF of homogeneous superhydrophilic surfaces. It is concluded that the non-dimensional liquid supply factor, which reflects both wicking and bubble behaviors, is essential to design structured surfaces during boiling. The results can contribute to a strategy for further improving boiling performance by controlling wettability on nanoscale interfaces.

Introduction

High-heat-generating energy systems must implement efficient thermal management to guarantee their performance and safety. Boiling heat transfer is one of the intensive cooling mechanisms for heat dissipation, removing thermal energy through phase change phenomena of working fluid with a stable surface temperature of a device [1], [2], [3], [4]. Consequently, it has been utilized for thermally sensitive systems with high thermal loads, such as power generating systems, and immersion cooling of integrated electronic devices [5], [6], [7]. Many studies have been widely conducted to improve boiling performance because various industrial fields require extremely high-efficiency heat transfer [8], [9], [10], [11], [12], [13], [14]. Increasing heat transfer efficiency and ensuring thermal stability under extreme conditions are the two primary approaches to enhancing boiling performance. The heat transfer coefficient (HTC), the ratio of input heat flux to wall superheat, was proposed to evaluate heat transfer efficiency. The critical heat flux (CHF) is another major factor in determining boiling performance. CHF is the limiting point of fully developed nucleate boiling before forming a vapor film. When the thermal load reaches the CHF, a vapor film blocks the coolant supply to the heated surface; the low thermal conductivity of the vapor layer may cause thermal damage. Hence, simultaneous enhancement of the HTC and CHF should be achieved for effective thermal management and safety of the system.

Surface manipulation is an effective method to improve boiling performance via manipulating interfacial characteristics and consequent nucleation behaviors [15,16]. Surface wettability is the representative interfacial property of engineered surfaces closely related to CHF. Particularly, hydrophilicity improves the supply of working fluid to the boiling surface and delays the formation of vapor film to increase the CHF. In addition, the wicking coefficient, the rate of liquid propagation between structured surfaces with capillary effects, was used to evaluate the dynamic wetting characteristics. High-wicking ability has further enhanced CHF through additional heat dissipation by immediately supplying liquid to the local dry areas. Recently, various manipulated surfaces accompanied by wicking have been applied for enhanced boiling, and new CHF enhancement models have been proposed. Ahn et al. [17] provided liquid spreading models to explain CHF enhancement on nanostructured surfaces. The volume of liquid spread was evaluated by the volume difference between the initial and remained droplet. Kim et al. [18] used aligned nanopillars with different diameters to analyze the mechanism of CHF enhancement. Wicking distance could be measured using high-speed wicking propagation images, and the wicking coefficient was calculated by the ratio of wicking distance to the square root of time. Rahman et al. [19] described that additional heat dissipation by wicked liquid based on calculated wicked volume flux. Song et al. [20] investigated the wicking speed of various sandblasted surfaces, and described CHF enhancement using a unified descriptor considering both surface roughness and wicking speed. In early research to predict CHF, the correlations were provided considering the hydrodynamic instability [21], [22], [23], wettability [24], and roughness [25,26]. In the case of the surface with wicking phenomena, generally, the CHF can be described as the sum of the conventionally derived hydraulic model and the additional heat dissipation capacity provided by wicking.

Bubble departure characteristics, closely related to the HTC, are also important factors in boiling performance [27]. Early onset of nucleate boiling (ONB) facilitates developing two-phase heat transfer to a broad area with enhanced micro scale convection, evaporation, and quenching mechanisms during boiling. Hence, research on surface manipulations was conducted to promote the ONB and control nucleate site density on various engineered surfaces such as nanowires [28], channels [29], micro-cavity [30], reentrant structures [31], and micro-nano hybrid structures [32,33]. The bubble behaviors also had a significant effect on the CHF because it was directly involved in vapor film formation. Several studies have achieved CHF improvement by effectively separating liquid-vapor pathway using porous structures [34,35], honeycomb plate [36], 3d-printed composite porous structure [37], 3d-printed polymer structures [38].

Recently, novel methods to obtain the early onset of nucleation and separating liquid-vapor pathway have been implemented using the biphilic surface (i.e., heterogenous wettability surface). The objective of biphilic surface was to accomplish the synergetic effect of the heterogeneous wettability surfaces to promote nucleation activation on the hydrophobic and ensure the liquid supply on the hydrophilic regions, respectively. Betz et al., [39] applied biphilic surfaces to improve boiling heat transfer, and successfully demonstrated that CHF and HTC could be enhanced using hydrophilic networks with hydrophobic islands. Jo et al., [40,41] demonstrated meaningful results that boiling heat transfer improvement on biphilic patterned surfaces by analyzing bubble motions. Shen et al. [42] explained the early ONB and enhanced HTC on biphilic surfaces by experiments and simulations of bubble behaviors. Hsu et al. [43] conducted flow boiling utilizing different shapes and parameters of biphilic patterns. Zhang et al. [44] fabricated the 3D heterogeneous wetting microchannels and evaluated the boiling performance. Ateş et al. [45] investigated the boiling performance on superbiphilic surfaces at atmospheric and sub-atmospheric pressures. Recently, efforts to maximize boiling performance have been actively reported using optimum parametric design or nanoscale textured superbiphilic characteristics [46], [47], [48], [49]. The boiling experiments using the biphilic surfaces demonstrated that the HTC is enhanced by the early ONB, the starting point of the boiling mechanism; hydrophobic patterns regulate nucleate sites and increase the density of activated nucleation for enhanced HTC. Moreover, the bubble departure diameter has changed due to a strong pinning effect at the hydrophilicity-hydrophobicity boundaries [50,51]. Compared to bare surfaces or homogeneous wettability surfaces, controlled nucleate sites on biphilic surfaces have increased the CHF by separating the liquid supply and vapor departure regions. Several studies have applied superbiphilic (SBPI) surfaces, which employ nanostructured surfaces accompanied by wicking phenomena, to improve boiling performance [52,53]. Table A.1 in Appendix A of supporting information (SI) briefly summarizes the materials and boiling heat transfer results in previous works. Remarkably, the HTC has increased significantly on the SBPI surfaces due to superhydrophobicity which has stimulated bubble nucleation and maximized nucleate density. However, the CHF did not exhibit the expected improvement; rather, it decreased compared to the homogeneous superhydrophilic (SHPI) surface. The superhydrophobic (SHPO) dots significantly impact the deterioration of liquid propagation capabilities on wicking surfaces. Therefore, due to the wicking area reduction effects, the CHF decreases as the hydrophobic area fraction increases. Therefore, it remained a challenging task to improve CHF as well as maximize HTC on superbiphilic surfaces with wicking.

This study proposes strategically controlled SBPI patterns on wicking surfaces to improve HTC and CHF simultaneously during pool boiling heat transfer. The 5 μm-high silicon nanowires (SiNWs) were fabricated to obtain the SHPI base surface, which has wicking phenomena. As the primary design parameter, the SHPO area fraction is controlled by three different diameters of low surface energy-coated dots. Fig. 1 depicts the anticipated boiling performance enhancement mechanism. The superhydrophobicity will aid in nucleation from low heat flux. Active bubble departure behavior with a high nucleation site density contributes to efficient heat dissipation. High heat flux dissipation requires the maintenance of superhydrophilicity and wicking properties to supply coolant effectively. We demonstrate through experimental evaluations that a SBPI surface with less than 1% -SHPO area fraction is adequate for retaining a high liquid supply via minimal liquid propagation obstruction. Pool boiling experiments and visualization of bubble departure characteristics were conducted to validate the SBPI surfaces' strategy for enhancing boiling performance.