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Displacing visco plastic fluid with Newtonian fluid in a vertical circular pipe with buoyancy effects Jeon, Jaewoo


In this thesis, displacement flows in a vertical pipe are studied when Newtonian fluids displace visco-plastic fluids. The density combinations between displacing and displaced fluids are varied from density unstable through iso-density to density stable, and captured dimensionlessly using Atwood numbers. In density unstable cases, three flow regimes are classified: central, mixed/turbulent and asymmetric regimes. These regimes are partially classified by a buoyancy parameter. However, we found that the buoyancy parameter has a limit in classifying the flow regimes. Once the flow enters the turbulent regime, spread of the dispersive mixed region is characterized by fitting the mean concentration changes to the solution of an 1D linear advection diffusion equation, i.e., turbulent diffusivity (or dispersivity) dominates in this regime. In iso-density cases, all flows are classified in central regime but the shapes of static layers are classified as: smooth, wavy and corrugated. We found that Re, Newtonian Reynolds number, differentiates the static layer shapes. Transitional Reynolds numbers are identified as Re = 345 for corrugated to wavy and Re = 1000 for wavy to smooth. The transitional Re for turbulent regime is identified at around 4000. Lastly, we observed that viscous fingering is common in density stable cases. Viscous fingering is observed for large effective viscosity, ratio of a viscoplastic fluid to a Newtonian fluid, and a ratio of shear stress to a yield stress of a displaced fluid ratio is small, and starts from an elongated thin layer finger. In the regime, the wall shear stress is too small to yield the visco-plastic fluid from the wall and the mobility of the displacing fluid is relatively high, so it seeks a way to channel though the visco-plastic fluid. The transitional Re for mixed/turbulent regime was not found within our experimental range. The displacement efficiency, described in the ratio of a front velocity to a mean velocity in density stable cases increases by approximately 15%, compared to density unstable and iso-density. Density unstable experiments can have better efficiency than iso-density experiments due to entering mixing regime in lower Reynolds numbers. However, the differences in the efficiency are generally small.

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