Fig. 6-3 Prediction of bubbly flow in a large vertical pipe (inner diameter 0.48 m) Multidimensional prediction of bubbly flow in a large vertical pipe has been succeeded by an adequate modeling for the interaction between a bubble and the surrounding fluid. This method can predict the flow circulation that an up-flow is realized in the center region of the pipe and a downward-flow is attained near the wall.

Fig. 6-4 Simulation of bubble formation through an orifice and of subsequent bubble coalescence A numerical simulation method has been newly developed for capturing the moving interface between gas and liquid phases and a direct simulation becomes possible for micro-process such as bubble generation, bubble coalescence and bubble break-up.

Bubbly flow can be expected at various locations in nuclear reactors, where extreme conditions occur, including high temperatures and high pressures, which make it very difficult to investigate the bubble behavior experimentally. Numerical simulations of bubbly flow are therefore very useful, but they also present difficulties, in the prediction of quantities such as the liquid velocity and the distribution of the void fraction (gas phase volume-fraction). Ten years ago, simulations of the behavior could only be performed using one-dimensional treatments, but with today's computing power more precise simulations are possible, in three dimensions, and with direct simulation of the individual bubbles. Recently, accurate predictions of bubbly flow in a large vertical pipe have been obtained by multidimensional modeling of the interaction between a bubble and the surrounding fluid. As shown in Fig. 6-3, the method can predict the experimentally observed flow circulation, where an upward flow occurs in the central region of the pipe and a downward flow occurs near the wall. Our numerical method includes a simulation of the moving interface between the gas and liquid phases, and, as shown in Fig. 6-4, direct simulations are possible for micro-processes such as bubble generation, bubble coalescence and bubble break-up. The numerical simulation method that we have developed has made studies of bubbly flow much more economically viable, by reducing the number of experiments which are needed, and by improving our ability to plan experiments and interpret their results.

Reference A. Ohnuki et al., Development of Multidimensional Two-Fluid Model Code ACE-3D for Evaluation of Constitutive Equations, JJAERI-Data/Code 96-033 (1996).

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