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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 74 3. Results and discussion 3.1. Void fraction Void fraction is the fraction of channel cross-sectional volume that is occupied by the gas phase. This void fraction is one of the most important parameters used to evaluate the pressure drop components existing in two-phase flow in various channels. In the case of a horizontal channel, for instance, the void fraction is required before calculating the accelerational pressure drop. For channels with hydraulic diameters of less than 1 mm, it may be convenient to estimate void fraction by image analysis. Regarding the flow patterns, observed from the presented channel, which are throat-annular flow, annular flow and annular- rivulet flow, the void fraction is the average value estimated from around 50-70 photographed images. The image analysis is considered by assuming symmetrical volumes, covering spherical and ellipsoidal segments as well as cylinders, formed by the gas-liquid interface. The volumetric void fraction was determined, based on the micrometer scale with an accuracy of ± 0.05 mm. The void fraction data were found to agree well with the Armand-type correlation of Eq. (1) recommended for small channels. α = 0.833β (1) where α represents volumetric void fraction and β stands for volumetric quality. The linear void fraction distribution was also reported by Saisorn and Wongwises (2008) but their data corresponding to a 0.53 mm diameter channel are very compatible with the homogeneous flow model (α = β). Their void fraction val ues are slightly larger than the ones presented. Such deviations are due to the fact that for a larger diameter, a highly deformed gas-liquid interface is observed and hence the gas volume is decelerated through the liquid, resulting in the increment of residence time for gas volume in the field of view. The appearance of the gas volume with higher residence time indicates the relative high void fraction. Although the data presented do not correspond well with the homogeneous flow model, the void fraction still varies linearly with volumetric quality. This implies that the slip ratio is not very large and that the pressure drop data may possibly be predicted by the methods developed from the homogeneous flow assumption. Several widely used viscosity models will be examined in the next section.