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The Journal of the Royal Institute of Thailand Volume II - 2010 An Evaluation of Viscosity Models for the Prediction of the Two-phase Pressure Drop in Two-phase Flow through a Circular Micro-channel 70 An Evaluation of Viscosity Models for the Prediction of the Two-phase Pressure Drop in Two-phase Flow through a Circular Micro-channel Sira Saisorn 1, 2 and Somchai Wongwises 2, 3 1 Department of Mechanical Engineering, King Mongkut’s Institute of Technology Ladkrabang Chumphon Campus, Thailand 2 Fluid Mechanics, Thermal Engineering and Multiphase Flow Research Lab (FUTURE) Department of Mechanical Engineering, King Mongkut’s University of Technology Thonburi, Thailand 3 Fellow of the Royal Institute, Academy of Science Abstract Adiabatic two-phase air-water flow experiments are conducted in this study. A fused silica channel, 120 mm long, with a diameter of 0.22 mm is used as the test section. The void fraction data obtained by image analysis correspond with the Amand-type correlation. The experimental frictional pressure drop data are compared with the homogeneous flow predictions. Several well-known two- phase viscosity models are evaluated in order to address the appropriate methods among them for application to micro-channels. Key words: two-phase flow, micro-channel, void fraction, pressure drop 1. Introduction Two-phase flow in micro-channel flow passages has been studied over the years. The clarifications of micro-scale effects on two-phase flow and heat transfer characteristics have become more necessary due to the rapid development of microstructure devices used for several engineering applications including medical devices, high heat-flux compact heat exchangers, and cooling systems of various types of equipment such as high performance micro-electronics, supercomputers, and high-powered lasers. Several investigators have proposed criteria to address the definition of a micro-channel. The proposed channel classifications are often based on different