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Simulating a forced external flow loop with 2 phases |
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August 22, 2018, 03:50 |
Simulating a forced external flow loop with 2 phases
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Senior Member
Cees Haringa
Join Date: May 2013
Location: Delft
Posts: 607
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Hi all,
I am trying to simulate a bubble column with external flow loop (leaving at the bottom, re-entering halfway). There is a pump in this external loop, and only liquid runs through - the inlet is below the sparger and no considerable amount of gas is being entrained. The average flowrate through the external loop is known. In my simulations, I need to take this flow-loop into account; Lagrangian particle tracking is being done for a component in the liquid phase, and these particles can and will enter the flow-loop. Hence, simply modeling it as an in/outlet is not an option. As general setup I use: euler-euler multiphase (fixed bub. size), k-e realizable, mixture formulation (dispersed diverged). Top of the column may be pressure outlet or degassing (for the latter, headspace section is removed). So far, I have great difficulties in modeling the flow through this loop and hope you have some suggestions. I have tried the options below. In all cases, a small cell-zone (about 1/50th of the tube length in the center) was designated as the "pump-zone". I have tried enforcing the flow by: - Setting a fixed velocity X = Qliq/A in the flow direction. This worked, but only if I (1) set the other velocities to 0 and (2) also fixed k and epsilon in the zone (based on the loop-average values). It only works when I use pressure-outlet; with degassing, gas starts to be generated out of nowhere inside this pump section (!) and the whole thing crashes. Even with pressure-outlet, things go wrong when I add particles: a considerable fraction of them gets stuck at the pump-zone; they stick at the face-centers for some reason, even though the particle velocity is finite and equal to the liquid velocity! I have no idea why this is, or how to fix it. - Setting a momentum source term counteracting the loop pressure drop: Setting a momentum source in the flow direction works fine in single phase. However, in multi-phase, the pressure drop over the pipe changes considerably and as soon as I add gas, the flow in the pipe reverses, gas gets entrained, and the whole thing goes to hell (sudden spikes in velocity to 200 m/s or so randomly occur). This happens both with degassing and pressure-outlet. I have tried using an UDF that adapts the source term size, but the random velocity jumps that occur made it unmanagable. - Using a fan-zone: not compatible with Eulerian multiphase, so no option. I am running out of ideas, so I hope there are some solutions to be found here. Hope to hear from you! Cees |
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