Flow boiling phenomena in a single annular flow regime in microchannels (I): Characterization of flow boiling heat transfer Fanghao Yang a , Xianming Dai a , Yoav Peles b , Ping Cheng c , Jamil Khan a , Chen Li a,⇑ a Department of Mechanical Engineering, University of South Carolina, 300 Main St, Columbia, SC 29208, USA b Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th St, Troy, NY 12180, USA c School of Mechanical and Power Engineering, Shanghai Jiaotong University, 800 Dong Chuan Rd, Shanghai 200240, China article info Article history: Available online 17 October 2013 Keywords: Enhanced flow boiling Microchannel Superhydrophilic silicon nanowire Single annular flow abstract Flow boiling with deionized water in silicon (Si) microchannels was drastically enhanced in a single annular flow boiling regime enabled by superhydrophilic Si nanowire inner walls. Part I of this study focuses on characterizing enhanced flow boiling heat transfer. Part II focuses on revealing mechanisms in governing pressure drop and critical heat flux (CHF). Compared to flow boiling in plain-wall micro- channels without using inlet restrictors (IRs), the average heat transfer coefficient (HTC) and CHF were enhanced by up to 326% and 317% at a mass flux of 389 kg/m 2 s, respectively. Additionally, compared with flow boiling in microchannels with IRs, HTC of flow boiling in the single annular flow was enhanced by up to 248%; while CHF in the new flow boiling regime was 6.4–25.8% lower. The maximum HTC reached 125.4 kW/m 2 K at a mass flux of 404 kg/m 2 s near the exits of microchannels. The significantly promoted nucleate boiling, induced liquid film renewal, and enhanced thin-film evaporation in the self-stabilized and single flow boiling regime are the primary reasons behind the significant heat transfer enhancements during flow boiling. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Flow boiling in microchannels has been extensively studied in the last decade [1–4] as it advantages for a range of applications including cooling high power microelectronics [1,5,6], compact heat exchangers, and chemical reactors [7–10]. Significant progress has been made in understanding two-phase heat transfer mechanisms [11,12], two-phase flow instabilities [13–15], and critical heat flux (CHF) mechanisms [16–18]. Various techniques such as micro reentry cavities [19], microporous structures [12], nanostructures [20–22], inlet restrictors (IRs) [15,23], pin fins [24], microjets [25], and seed bubbles [26,27] were developed to promote flow boiling in microchannels. Specifically, micro cavities and micro/nanostructures can improve nucleate boiling by increasing active nucleation site density. IRs can improve CHF conditions by effectively suppressing reverse flows and hence flow boiling instabilities. Microjets were used to promote convections by disturbing flows in microchannels. However, these reported techniques have various drawbacks, such as the dramatically increased pressure drop resulting from IRs [15], reduced reliability induced by complex structures [25–27], and low HTC on IRs and finned boiling surfaces [4,15,23,24,28]. Most recently, thermally induced high frequency two-phase oscillations were induced to generate mixing and hence significantly enhance flow boiling in microchannels [29,30]. However, none of these techniques aimed to enhance flow boiling in microchannels through manipulating or even controlling two-phase flow structures, i.e., regimes. During flow boiling in microchannels, boiling surfaces play crit- ical roles in governing bubble nucleation, growth, separations, interactions, and two-phase flow regimes. Microchannels are usu- ally microfabricated on silicon substrates by wet-etching or deep reactive ion etching (DRIE) [6,8,31,32]. The peak-to-peak roughness of etched silicon wafers can be as low as 3 nm at the bottom wall [33] and less than 300 nm at scalloped sidewalls [34], which are not favored by nucleate boiling due to the lack of favorable nucle- ation cavities, consequently, result in explosive boiling and low heat transfer rate because of high onset of nucleate boiling (ONB) [35]. Various artificial nucleation cavities were developed to enhance nucleate boiling [12,36,37]. Recently, one dimensional (1D) nanosturctures such as nanowires (NWs) [38,39] and carbon nanotubes (CNTs) [40–42] were used to enhance nucleate pool boiling and convective boiling in microchannels [20–22,40,43,44]. Enhanced HTC and CHF were reported because of the higher nucle- ation site density and enhanced wettability. However, the role of 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.09.058 ⇑ Corresponding author. E-mail address: li01@mailbox.sc.edu (C. Li). International Journal of Heat and Mass Transfer 68 (2014) 703–715 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt