Theoretical and experimental study of two-phase flow in micro-channels grooved into horizontal pipes

In recent decades, the use of heat-transfer-enhancing pipes as a way of improving thermal system efficiency has intensified. Among others, pipes with micro-channels etched in the inner wall are being widely employed in cooling systems and heat pipes - for both air-conditioning and solar thermal collectors - (for example, Webb, 1994). This type of pipe enhances heat transfer by intensifying convective boiling and/or capillary-driven pumping of liquid in the micro-channels. Due to its great potential, studies on the thermal-hydraulic behaviour of such systems have also increased in recent years. Most of these studies focus on practical considerations, such as the theoretical and/or experimental exploration of heat transfer coefficients for the most frequently used coolants (Yoshida et al., 1987; Yun et al., 2002; Honda and Wang, 2004). Theoretical studies deal preferentially with closed micro-channels (Peles et al., 2001), while modelling of two-phase flow in open micro-channels is rather limited. Peng et al. have investigated the rewetting characteristics of capillary-induced flow in specific cases, such as heated flat horizontal surfaces (Peng and Peterson, 1991), heated vertical flat plates with a porous coating (Peng et al., 1992) and heated horizontal pipes with vertical semicircular microgrooves etched in their inner surface (Peng et al., 1993). In the study presented here, Peng et al.'s model (1993) is generalized to any geometry and configuration of micro-channels, being improved with the definitions of dynamic and static liquid front positions. These definitions are quite useful in designing micro-channels for specific applications. The theoretical results are validated with experimental data from pool boiling of water and two carbon steel samples under different saturation conditions. © 2006 Elsevier Ltd. All rights reserved.