Several engineering plants and components, such as nuclear reactors, boilers, bubble columns, chemical reactors or oil/gas wells may be characterized by two-phase flows. Appropriate and safe operation of these systems may be supported by the accurate prediction of the multiphase flow pattern with consistent estimation of the void fraction and flooding phenomenon. A preliminary approach for modeling flow patterns in countercurrent two-phase flows in vertical systems, based on the volume-of-fluid (VOF) method, is here presented. The general objective of the study is to investigate the feasibility of large scale two-phase systems simulations using the VOF method. The specific objective is to provide a first set of qualitative information about the fluid dynamics structures in this flow at specific operating conditions. Three-dimensional simulations are performed using a finite volume commercial CFD code. Validation of the numerical approach is achieved with experimental data taken from literature for typical air-water flows in bubble columns. The interaction of the downward water recirculation with the bubbles of air is indeed representative of a bubbly countercurrent two-phase flow and therefore may be a valuable test case. The use of the VOF method is here privileged, since no further closure relations are needed for exchange coefficients between the continuous and dispersed phases, and for breakup and coalescence of bubbles/drops. The validated model is therefore most likely applicable in complex situations where the flow behavior is unknown. Finally a specific analysis of a countercurrent two-phase flow is presented with the objective of simulating the flow within an industrial pipe where two immiscible fluids, with different density are injected. A fluid “A” is injected at the bottom of the pipe and it is supposed to exit the pipe in the upper part. A fluid “B”, immiscible in “A”, is injected at the top of the pipe in countercurrent and it is supposed not to influence the path of fluid “A” from inlet to the exit. Fluid “B” may be used for reacting somehow with fluid “A”, but in this paper only the fluid dynamic condition is considered.

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