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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 76 For turbulent flow (Re TP < 100,000) in a smooth channel, (6) where μ TP presented in Eq. (4) is two-phase viscosity. Several two-phase viscosity models proposed by different researchers are given in Eqs. (7) – (11). McAdams et al. model (1942): (7) Lin et al. model (1991): (8) Cicchitti et al. model (1960): (9) Dukler et al. model (1964): (10) Beattie and Whalley model (1982): (11) In the above equations, β is volumetric quality, and μ G and μ L represent gas and liquid viscosities, respectively. The frictional pressure drop data are compared with the predictions with different viscosity models as presented in Figs. 2-6 which contain the mean absolute error (MAE) providing the predictive accuracy of the corresponding models. In general, the agreement between the predictions and the data presented is not good.