Enhanced boiling heat transfer on micro-structured surfaces via ultrasonic actuation

Abstract

Emerging issues in boiling heat transfer include enhancing heat transfer uniformity/stability, and critical heat flux (CHF). Microcavity structures improve heat transfer uniformity/stability by departing bubbles with arranged formation due to pinning effects and regular pitch. Ultrasonic actuation induces an acoustic field on departed bubbles, and this enhances contact line instability between the bubble and microcavity structures, resulting in an increased dissipation rate of smaller bubbles. In this study, we demonstrate that synergetic effects from microcavity structures and ultrasonic actuation can enhance CHF and thermal stability while also improving temporal/spatial temperature uniformity. Applying microcavity structures with ultrasonic actuation, we observe smaller and faster bubbles' departure with the proposed formation. These bubble departure characteristics on the microcavity surface with ultrasonic actuation enhance CHF and thermal stability by delaying bubble coalescence and ensuring liquid paths between smaller and faster-departed bubbles. Thus, when ultrasonic actuation is applied to the microcavity structure, CHF increased by 20%, and temporal/spatial temperature variations near CHF were reduced to less than 1/2 and 1/3, respectively, compared to no actuation case. This research will help to understand the interaction of ultrasonic wave and bubbles, and to show the way to overcome CHF limitations of passive methods using microsized structures.

Introduction

Boiling heat transfer is an effective cooling method that can achieve high heat transfer performance by utilizing latent heat when the phase change occurs. The most crucial factor for boiling heat transfer is to measure the critical heat flux (CHF) caused by vapor film formation, which induces surface failure [1,2]. Recently, it has become essential to enhance not only CHF but also thermal stability, i.e., spatial and temporal temperature stability relieving thermal stress, for long term cooling applications [3,4]. Previous boiling heat transfer studies have focused on passive methods to improve boiling heat transfer using macro and nano/microsized structures, specifically, nanostructures in the early stage [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]]. Boiling heat transfer studies using nanowire structures has shown that CHF can be increased by superhydrophilic and capillary pumping effects, which improve liquid supply due to its structural characteristics [[5], [6], [7], [8], [9], [10], [11], [12], [13]]. Also, many research groups have considered boiling heat transfer using microscale structures [[14], [15], [16], [17], [18], [19], [20], [21]], e.g., CHF, which has been enhanced by delayed bubble coalescence using artificially arranged microcavities and micropillar structures [[13], [14], [15], [16], [17], [18], [19], [20]]. CHF is particularly enhanced by enhanced surface wetting and delayed bubble merging, simultaneously, on micro/nano hybrid structured surfaces [[22], [23], [24]]. Macroscale structures, such as microforms (carbon, copper, etc), and three-dimensional (3D) structures have been studied as commercial boiling heat transfer applications due to manufacturing simplicity [25]. Besides, the passive boiling researches have been actively conducted to enhance the boiling heat transfer performance using not only the nano/microstructure but also the nanofluids. The nanofluids researches have been focused on its two kinds of enhancement mechanisms. Primarily, the surface roughness can be increased by fouling of nanoparticle on the heating surface, results in improvement of the boiling heat transfer performance, i.e., CHF and heat transfer coefficient (HTC). In detail, many papers have been reported that surface fouling owing to nanoparticle increases the surface roughness and decreases the contact angle according to Wenzel's model [26]. Thus, surface fouling owing to nanoparticles enhances the CHF by increased the wettability of the surface as well as enhanced capillary wicking on the rough structures [27]. Although the fouling of the nanoparticles is beneficial to enhance the CHF, if fouling progresses too much, fouling resistance much increased on the surface, resulting in significant HTC decrease. [28,29] So, many researchers conducted the numerical analysis by establishing fouling model for predicting the fouling resistance in two-phase heat transfer field such as boiling heat transfer, thermosyphon, heat exchanger, etc. [[30], [31], [32]] Secondly, the nanofluid can improve the conductivity of the fluid, resulting in HTC enhancement. In order to improve the thermal conductivity of the nanofluids, many kinds of nanoparticles, which have excellent thermal conductivity and dispersion property, are used in the nanofluid research such as the graphene-based particles, TiO₂ particles, FeO₃ particles, etc. [[33], [34], [35]]. These researches were mainly conducted by not only experimental methods but also numerical simulation. Especially, Artificial Intelligence (AI) techniques based on the machine learning concept, are applied to predict HTC in pool boiling heat transfer under nanofluidic conditions [36].

However, there are many difficulties to use passive methods (nanofluids, 3D-macro, and micro/nanostructures) in long-term applications. For example, nanofluids cause thick fouling layers and 3D-macro, micro/nanostructures are damaged by surface erosion, then these result in severe heat transfer performance degradation. Therefore, active methods using acoustic/ultrasonic actuation have been widely studied since this can enhance boiling heat transfer without fouling and surface damage from erosion even when used for long term applications. Applying ultrasonic actuation to the heated surface during boiling heat transfer provides significant enhancement in the single-phase region due to cavitation, which increases heat transfer by reducing the thermal resistance layer produced in single-phase heat transfer [37,38]. The heat transfer mechanism during two-phase boiling is mainly vapor rather than cavitation bubbles. Ultrasonic actuation reduces vapor bubbles interfacial forces and increases the bubble departure rate, hence enhances the heat transfer coefficient [[39], [40], [41], [42], [43], [44], [45], [46], [47], [48]]. However, ultrasonic actuation cannot affect CHF enhancement on the plain surfaces at the low frequency ultrasound (20–100 kHz) range. Because, bulk vapor forms when ultrasonic actuation is applied on the plain surfaces at high heat flux due to vigorous bubble merging, hence vapor volume is too large compared to ultrasonic actuation intensity. Thus, ultrasonic actuation cannot affect bubble behavior near CHF [49]. To overcome this limitation, it is crucial that bubbles depart individually even in high heat flux regions (near CHF). Therefore, microcavity structures are applied because these generate bubbles with regular size and pitch (time between bubbles) due to pinning effects [22]. Therefore, if bubbles are generated with regular size and pitch even in high heat flux regions using microcavity structures, we can infer that CHF will be enhanced under ultrasonic actuation by generating smaller and faster bubbles with specific formations compared to the case without actuation. Thus, CHF will be enhanced on the microcavity surface under ultrasonic actuation by delaying bubble coalescences that form the vapor film. Although significant research has investigated enhancing the heat transfer coefficient using ultrasonic actuation, few studies have focused on enhancing CHF by the synergetic effect of ultrasonic actuation and micro-sized structures.

In contrast to previous studies, the present paper investigated boiling heat transfer to enhance CHF using microstructures and ultrasonic actuation simultaneously. We conducted pool boiling heat transfer experiments using a 4-wire resistance temperature detector (RTD) sensor with five temperature measurement points to evaluate CHF as well as thermal stability on the plain and microcavity surface with and without ultrasonic actuation. Pool boiling heat transfer experiments were conducted under 20 °C subcooled conditions due to the ultrasonic piezoelectric transducer operating temperature limits. Shadowgraph bubble visualization was employed to observe bubble behavior on the plain and microcavity surface with and without ultrasonic actuation. We confirmed that CHF and thermal stability were simultaneously enhanced by synergetic effects from microstructures and ultrasonic actuation.