Abstract
This study aims to investigate the pressure changes, bubble dynamics, and flow physics inside the U- and C-shaped pipes with four different gravitational directions. The simulation is performed using a 1D centerline-based mesh generation technique along with a two-fluid model in the open-source software, OpenFOAM v.6. The continuity and momentum equations of the two-fluid model are discretized using the pressure-implicit method for the pressure-linked equation algorithm. The static and hydrostatic pressures in the two-phase flow were consistent with those of single-phase flow. The dynamic pressure in the two-phase flow was strongly influenced by the effect of the buoyancy force. In particular, if the direction of buoyancy force is the same as the flow direction, the dynamic pressure of the air phase increases, and that of the water phase decreases to satisfy the law of conservation of mass. Dean flows are observed on the transverse plane of the curve regions in both C-shaped and U-shaped pipes. The turbulent kinetic energy is stronger in a two-phase flow than in a single-phase flow. Using the 1D centerline-based mesh generation technique, we demonstrate the changes in pressure and the turbulent kinetic energy of the single- and two-phase flows, which could be observed in curve pipes.
Highlights
Flows in pipes are most commonly observed in an industrial system; many studies on straight and curve pipelines for single- and twophase flows have been presented
In the case of a curve pipe with two-phase flows, the volume fraction of two-phase fluids could be determined by the balance between the centrifugal and buoyancy forces
Investigating the effects of buoyancy and centrifugal forces on the two-phase flow is critical for understanding the flow physics in curve regions and the redistribution of volume fraction
Summary
Flows in pipes are most commonly observed in an industrial system (e.g., energy plant and refrigeration); many studies on straight and curve pipelines for single- and twophase flows have been presented. Takemura et al [14] performed an experiment to investigate the flow behavior and pressure drop in air–water two-phase flows by imposing two opposite gravitational directions on a U-shaped pipe, i.e., a U-shaped pipe and an inverted-U-shaped pipe. Inverted-U-shaped pipes are preferable for industrial design Based on these backgrounds, it is essential to perform a comprehensive study to investigate the single- and two-phase flows in C- and U-shaped pipes simultaneously. We assessed the effects of four different gravitational directions (i.e., upward-C-shaped pipe, downward-Cshaped pipe, U-shaped pipe with upward injection, and U-shaped pipe with downward injection) on flow energy, pressure drop, volume fraction, flow behaviors, and turbulence physics This different pipe positioning is often observed at actual industrial sites. The post-processing of CFD after the CFD simulation could be expedited, because the domain had already been divided into subzones
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