Frontiers in Heat and Mass Transfer (FHMT), 3, 013001 (2012) DOI: 10.5098/hmt.v3.1.3001 Global Digital Central ISSN: 2151-8629 1 ONSET OF NUCLEATE BOILING IN MINI AND MICROCHANNELS: A BRIEF REVIEW Tomio Okawa *,† Department of Mechanical Engineering, Osaka University, Suita-shi, Osaka 565-0871, Japan ABSTRACT The present article summarizes the studies on the thermalhydraulic condition under which the onset of nucleate boiling (ONB) is triggered in subcooled flow boiling. Available correlations and experimental data show that the ONB is tended to be delayed in mini and microchannels. It is known that the ONB condition is significantly dependent on the surface condition even in conventional-sized channels. Accumulation of ONB data accompanied by the information on the surface condition is therefore considered of importance to elucidate the mechanisms of boiling incipience in microchannels. Discussion is also made for the bubble dynamics observed in mini and microchannels. It is indicated that the bubble behavior at ONB in mini and microchannels may significantly be different from that in conventional-sized channels and have greater impact on the system performance. Further studies on the bubble dynamics following nucleation at ONB are also requested to improve the design of heat transfer devices using mini and microchannels. Keywords: Microchannel, Subcooled flow boiling, Onset of nucleate boiling, Bubble dynamics. * Currently in Department of Mechanical Engineering and Intelligent Systems, The University of Electro-Communications, Chofu-shi, Tokyo 182-8585, Japan † Corresponding author. Email: okawa.tomio@uec.ac.jp 1. INTRODUCTION The use of small diameter channels is a promising way to increase the heat transfer area per unit volume in heat exchangers. Furthermore, the coolant flow rate required for dissipating a given amount of heat can be reduced if the latent heat transfer is employed. Flow boiling in mini and microchannels is hence expected to be a promising way to achieve effective heat dissipation particularly from small areas (Kandlikar, 2006), and has received increasing attention mainly due to extremely high power density encountered in recent miniaturized electronic devices (Mudawar, 2001; Thome, 2004). Although there exist several important parameters in flow boiling including the heat transfer coefficient and critical heat flux (Thome, 2004; Bergles and Kandlikar, 2005; Roday and Jensen, 2009), the present article focuses on the onset of nucleate boiling (ONB) in mini and microchannels. In subcooled flow boiling in conventional-sized channels, the vapor void fraction near the location of ONB can be positive only in the region close to the heated wall. In consequence, the void fraction just downstream of the point of ONB is small, and a rapid increase in the void fraction or the onset of significant void (OSV) occurs further downstream of the ONB point (Collier and Thome, 1994); the void fraction between the locations of ONB and OSV is hence usually neglected. However, since the location of ONB corresponds to the boundary between the single- and two-phase regions, pressure loss and heat transfer characteristics are different between upstream and downstream of the point of ONB. For instance, Basu et al. (2002) indicated that single-phase heat transfer prevails upstream of the ONB point, but the presence of bubbles permits various heat transfer mechanisms downstream of it. It is also reported that the bubble behavior at ONB can significantly be different depending on the experimental setup (Bibeau and Salcudean, 1994; Thorncroft et al., 1998; Okawa et al., 2005; Ahmadi et al., 2012). In consequence, many correlations and models have been developed so far to predict the location of ONB in subcooled flow boiling; some of them are discussed in the following section. In mini and microchannels, several investigators reported that the wall superheats needed for boiling incipience can noticeably be higher than those predicted by the conventional correlations (Hapke et al., 2000; Kennedy et al., 2000; Martín-Callizo et al., 2007; Qi et al., 2007). Since the presence of bubbles alters the characteristics of the pressure loss and heat transfer as in the conventional-sized channels, accurate prediction of ONB is of importance in the design of the heat exchangers or the heat sinks using microchannels. Furthermore, once the ONB occurs in microchannels, a bubble can grow rapidly and most of the channel cross-section may be occupied by bubbles (Kandlikar and Balasubramanian, 2004; Hetsroni et al., 2005a; Barber et al., 2010). This phenomenon may cause the onset of flow instability leading to a decreased CHF value and significant oscillations of system pressure and heat transfer rate (Hetsroni et al., 2005a; Bergles and Kandlikar, 2005; Kuo and Peles, 2008). Absence of apparent partial boiling prior to fully- developed nucleate boiling (Peng and Wang, 1993) and a considerable wall temperature drop at ONB (Piasecka et al., 2004; Qi et al., 2007) were also reported. It is considered that the ONB may have greater impact on the system performance in the heat transfer devices using mini and microchannels. The main purpose of the present brief review is to summarize available studies regarding the thermalhydraulic conditions under which nucleate boiling commences in forced-convective subcooled flow boiling. Since the bubble behavior in mini and microchannels can significantly be different from that observed in conventional-sized channels, studies concerning the bubble dynamics at ONB are also addressed. 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