DNS of a turbulent steam/water bubbly flow in a vertical channel
Résumé
Two-phase turbulence has been studied using a DNS of an upward turbulent bubbly flow in a so-called plane channel. Fully deformable monodispersed bubbles are tracked by the Front-Tracking algorithm implemented in TrioCFD code on the TRUST platform. Realistic fluid properties are used to represent saturated steam and water in pressurised water reactor (PWR) conditions. The large number of bubbles creates a void fraction of 10%. The Reynolds friction number is 180. After the transitional regime, the flow is simulated until convergence of statistics is achieved. Time- and space-averaging is used to compute main variables at the average scale (e.g. void fraction, phase velocities). Budget of forces and Reynolds stresses are also computed from the local fields. They provide reference profiles to improve momentum transfer closures and turbulence modelling. The velocity profile and the flow-rate are compared to a similar single-phase flow simulation. Strong buoyancy forces create a large relative velocity. Averaged surface tension forces also play a significant role in the flow equilibrium. In the prospect of assessing a single-pressure Euler-Euler two-fluid model, the macroscopic momentum jump condition is deduced from averaging DNS fields. The resulting balance shows that the classical assumption of opposite forces acting on each phase should be revised. Indeed, neither surface tension, nor pressure difference is negligible.
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