Abstract
ABSTRACTIn this study, computational fluid dynamics (CFD) modeling is used to simulate Taylor bubbles rising in vertical pipes. Experiments indicate that in large diameter (0.29 m) pipes for an air–water system, the bubbles can rise in a oscillatory manner, depending on the method of air injection. The CFD models are able to capture this oscillatory behavior because the air phase is modeled as a compressible ideal gas. Insights into the flow field ahead and behind the bubble during contraction and expansion are shown. For a bubble with an initial pressure equal to the hydrostatic pressure at its nose, no oscillations are seen in the bubble as it rises. If the initial pressure in the bubble is set less than or greater than the hydrostatic pressure then the length of the bubble oscillates with an amplitude that depends on the magnitude of the initial bubble pressure relative to the hydrostatic pressure. The frequency of the oscillations is inversely proportional to the square root of the head of water above the bubble and so the frequency increases as the bubble approaches the water surface. The predicted frequency also depends inversely on the square root of the average bubble length, in agreement with experimental observations and an analytical model that is also presented. In this model, a viscous damping term due to the presence of a Stokes boundary layer for the oscillating cases is introduced for the first time and used to assess the effect on the oscillations of increasing the liquid viscosity by several orders of magnitude.
Highlights
Numerous experimental studies have investigated the rise of elongated, bullet-shaped gas bubbles in pipes charged with either stagnant fluids (e.g., Figure 1) or cocurrent flows
The solution from the computational fluid dynamics (CFD) model for this case replicates the expected behavior – a stable Taylor bubble is produced and the liquid surface rises at a constant rate until a rapid expansion of the bubble is observed as it approaches the surface, which can be seen in the middle line of Figure 6
It has been demonstrated that a great deal can be learnt about the behavior of oscillating Taylor bubbles rising in a quiescent liquid through the use of URANS CFD simulations
Summary
Numerous experimental studies have investigated the rise of elongated, bullet-shaped gas bubbles in pipes charged with either stagnant fluids (e.g., Figure 1) or cocurrent flows. The work is continued in James, Lane, and Corder (2008), who present solutions validated by experimental data, which show that the liquid surface rises due to bubble expansion as the bubble moves into regions of lower hydrostatic pressure They confirm that slug oscillations occur when an initial gas overpressure is used; no detail is provided. These studies use an approach in which the Taylor bubble is held at a constant position by having walls move vertically downwards around it This approach does not take into consideration the hydrostatic pressure experienced by the bubble, nor does it include the free surface at the top of the liquid column, so it cannot be used to model the expansion and oscillatory behavior reported in the experiments of Pringle et al (2015).
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