For Vmix < Vcrit in the horizontal pipes, flow regimes with a stationary bed above which a compact layer of beads is flowing are observed (see bottom left picture in Fig. 3 that corresponds to Vmix = 1.2 m s1, i.e. Vmix = 2.5V0). The more the mixture velocity decreases, the more the solids tend to settle down. These regimes are thus such that sC (s ’ 32% for bottom left picture in Fig. 3) and are characterized by a large pressure drop that is caused by a decrease in the discharge section. In the ascending part of the flexible pipe, a layer of solids located at the bottom of the pipe and that
is flowing backwards is even observed for these low velocities, as already observed by Yamaguchi et al. [17] (see top left picture in Fig. 3 that is taken at Vmix = 0.9 m s1). The flow in this regime is very unstable and the transit time needed to reach a stationary state is very long, of the order of twenty minutes—the typical time for a solid to flow through the whole pipe being 30 s.
For Vmix < Vcrit in the horizontal pipes, flow regimes with a stationary bed above which a compact layer of beads is flowing are observed (see bottom left picture in Fig. 3 that corresponds to Vmix = 1.2 m s1, i.e. Vmix = 2.5V0). The more the mixture velocity decreases, the more the solids tend to settle down. These regimes are thus such that sC (s ’ 32% for bottom left picture in Fig. 3) and are characterized by a large pressure drop that is caused by a decrease in the discharge section. In the ascending part of the flexible pipe, a layer of solids located at the bottom of the pipe and that
is flowing backwards is even observed for these low velocities, as already observed by Yamaguchi et al. [17] (see top left picture in Fig. 3 that is taken at Vmix = 0.9 m s1). The flow in this regime is very unstable and the transit time needed to reach a stationary state is very long, of the order of twenty minutes—the typical time for a solid to flow through the whole pipe being 30 s.
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