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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 72 test sections: 20 mm x 2 mm, 40 mm x 1 mm, 40 mm x 2 mm, 40 mm x 3 mm and 60 mm x 2 mm at an equal length of 1 m. The flow phenomena, which were classified as bubbly flow, cap-bubbly flow, slug flow, churn flow and annular flow, were observed and recorded by a high-speed camera. The effects of gap size, channel width and liquid viscosity on the flow pattern transitions were also discussed. Saisorn and Wongwises (2008) reported the influence of the working fluid on flow characteristics in a 0.53 mm diameter channel. Air, nitrogen gas, water and de-ionized water were used as working fluids. The results of the two-phase air-water system were found to agree with those of working fluids other than air-water mixture. A new correlation was also developed based on their experimental data. From the above review of the literature, the two-phase flow phenomena in micro-channels are not entirely consistent with those in ordinary sized channels. However, some experimental data for micro-channels showed fair agreement with predictions developed based on ordinary sized channels. In general, the prediction methods derived from the separated flow assumption were considered to compare with the data, whereas the homogeneous flow assumption seemed to be unattractive for previous researchers. This is possibly due to the fact that the observed flow patterns are much less homogeneous under different conditions. Interestingly, it should be noted from Chung and Kawaji (2002) and Saisorn and Wongwises (2008) that although their observed flow patterns were not perfectly homogeneous, the measured void fractions for channels with diameters larger than 0.1 mm were found to vary linearly with volumetric qualities. Such a linear relationship between the void fraction and volumetric quality indicates that two-phase flow behavior is not far from the homogeneous flow assumption. The current work is, therefore, aimed at examining the applicability of several widely used viscosity models to the pressure drop prediction of air-water flow through a 0.22 mm diameter channel. 2. Experimental apparatus and procedure The experiments dealing with pressure drop measurements were carried out using the apparatus along with the instruments shown in Fig. 1. Air and water are used as working fluids in the system. Instead of a conventional pump, which may contribute to pulsation and fluid contamination, air and the liquid-filled tank are combined and operated as a pneumatic pump to supply a constant flow rate of liquid through the test section. The mixing chamber is designed to introduce the air-water mixture smoothly along the channel. The mixture flows freely from the channel outlet where atmospheric pressure is realized. The gas flow rates were able to be