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The Journal of the Royal Institute of Thailand Volume II - 2010 Sira Saisorna, Somchai Wongwises 75 3.2 Frictional pressure drop Prior to obtaining data for the two-phase frictional pressure drop, measurements of the total pressure drop are taken under various sets of different conditions. In this work, the total pressure drop of two-phase flow in a horizontal channel is composed of three terms: frictional pressure drop, accelerational pressure drop and pressure drop caused by the abrupt flow area. The latter component can be evaluated from an empirical correlation proposed by Abdelall et al. (2005). By subtracting the accelerational term and sudden contraction component from the total pressure drop, the frictional pressure drop data were obtained. Further details regarding the calculation method are available in Saisorn and Wongwises (2009). The frictional pressure drop data obtained are subsequently compared with the homogeneous flow predictions, taking into account several existing viscosity models. The homogeneous flow model assumes that gas and liquid phases flow with equal velocity. The gas-liquid mixture is considered as a single-phase flowing with average fluid properties. For pressure drop calculations based on the homogeneous flow, it follows that (2) (3) In Eqs. (2) and (3), (-dP f /dz) is the two-phase frictional pressure gradient, G is mass flux, D is the channel diameter, x is mass quality, ρ H is the average density of the homogeneous fluid, and ρ L and ρ G are liquid and gas densities, respectively. f TP shown in Eq. (2) represents the two-phase Darcy friction factor which is a function of the two-phase Reynolds number: (4) For laminar flow (Re TP ≤ 2100), (5)