Surface tension effects on adiabatic gas–liquid flow across micro pillars Santosh Krishnamurthy, Yoav Peles * Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110, 8th Street, Troy, NY 12180, USA article info Article history: Received 5 February 2008 Received in revised form 6 July 2008 Accepted 13 August 2008 Available online 26 August 2008 abstract The effect of surface tension on adiabatic two-phase flow across a bank of 100 lm diameter staggered circular micro pillars, 100 lm long with pitch-to-diameter ratio of 1.5, for Reynolds number between 5 and 50, was investigated. Experiments with ethanol were performed and compared to results with water. Flow maps revealed similar flow patterns, but the transition lines were different for the two liquids. Void fraction measurements of the two fluids were also compared, and no significant deviations were observed. The two-phase pressure drop characteristics were significantly affected by the reduction in sur- face tension. Interfacial friction was attributed to this deviation, and a two-fluid model was developed to account for surface tension force. In addition, a modified form of Chisholm correlation was developed that accounts for surface tension. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Improvement of microfabrication techniques in the last decade has facilitated the design and fabrication of myriad micro flow do- mains for numerous microfluidic and microthermal applications. As a part of this effort, microchannels encompassing an array of micro-pillars for a range of applications, such as micro-chemical reactors (Losey et al., 2002), micro-rockets (Hitt et al., 2001; Lon- don, 2000), micro-biological systems (Christel et al., 1999), and mi- cro-heat exchangers (Kos ßar and Peles, 2006, 2007; Siu-Ho et al., 2007), have been explored. Better knowledge of surface tension ef- fects is important not merely from the fundamental stand point, but also to improve the design of many micro systems. In conventional scale, the hydrodynamic characteristics of two-phase flow were extensively investigated and were found to primarily depend on inertial and gravitational forces (Grant and Chislom, 1979; Xu et al., 1998b; Dowlati et al., 1988; Dowlati et al., 1990; Dowlati et al., 1992b; Schrage et al., 1988; Xu et al., 1998a). In microscale, these flow characteristics significantly deviate from those observed in conventional scale, mainly be- cause of the gradual dominance of surface tension and viscous forces, over inertial and gravitational forces (Kawahara et al., 2002; Chung and Kawaji, 2004; Kawaji and Chung, 2004; Cubaud and Ho, 2004; Krishnamurthy and Peles, 2007; Krishnamurthy and Peles, 2008)). Several new two-phase flow phenomena have been revealed, such as ring-slug/slug-ring flow in microchannels (Kawahara et al., 2002) and bridge flow in micro-pillar systems (Krishnamurthy and Peles, 2007). Void fraction and two-phase pressure drop were also altered at the micro scale. For example, the void fraction has been found to strongly depend on viscous forces for microchannels (Kawahara et al., 2002)(Dh = 100 lm) as well as for flow across micro pillars (Krishnamurthy and Peles, 2007). In mini-scale systems, the two-phase pressure drop has been successfully correlated by modifying the Chisholm correla- tion (Chisholm and Laird, 1949) to account for surface tension and viscous forces (Lee and Lee, 2001; Field and Hrnjak, 2007). For crossflows in micro-scale, Krishnamurthy and Peles (2007) ac- counted for the viscous effects by incorporating the Reynolds number in the C-parameter of a Chisholm-type correlation. In micro domains, surface tension and viscous forces begin to dominate. However, much is unknown about the effect of fluid properties, such as surface tension and viscosity, on the two- phase flow characteristics in small length scales. Kawahara et al. (2005) in their study on gas–liquid flow in microchannel observed that the surface tension did not significantly affect the void fraction. Krishnamurthy and Peles (2007) investigated adiabatic flow across micro pillars and reported new surface ten- sion dependent flow pattern and some unique two-phase flow characteristics. It was argued that the surface tension played a very important role in governing the flow hydrodynamics in cross flow systems. In the current study, the effect of surface tension on two- phase flow across a staggered array of 100 lm diameter by 100 lm deep micro pillars entrenched inside microchannel was investigated. Section 2 provides a detailed overview of the de- vice geometry and presents the essential microfabrication pro- cesses used. Additionally, an overview of the experimental setup and procedure is presented. Section 3 presents the meth- odology used for data reduction. The results and discussion sec- tion (Section 4) includes discussions on: (a) flow patterns and their transition, (b) void fraction, and (c) two-phase pressure drop. The physical mechanism governing the flow was modeled and a correlation was developed/modified to account for the 0301-9322/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmultiphaseflow.2008.08.001 * Corresponding author. E-mail address: pelesy@rpi.edu (Y. Peles). International Journal of Multiphase Flow 35 (2009) 55–65 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow