Abstract
Hydrodynamic behaviors of bubble stream flow in fractal tree-shaped microchannels is investigated numerically based on a two-dimensional volume of fluid (VOF) method. Bubble breakup is examined in each level of bifurcation and the transition of breakup regimes is discussed in particular. The pressure variations at the center of different levels of bifurcations are analyzed in an effort to gain further insight into the underlying mechanism of bubble breakup affected by multi-levels of bifurcations in tree-shaped microchannel. The results indicate that due to the structure of the fractal tree-shaped microchannel, both lengths of bubbles and local capillary numbers decrease along the microchannel under a constant inlet capillary number. Hence the transition from the obstructed breakup and obstructed-tunnel combined breakup to coalescence breakup is observed when the bubbles are flowing into a higher level of bifurcations. Compared with the breakup of the bubbles in the higher level of bifurcations, the behaviors of bubbles show stronger periodicity in the lower level of bifurcations. Perturbations grow and magnify along the flow direction and the flow field becomes more chaotic at higher level of bifurcations. Besides, the feedback from the unequal downstream pressure to the upstream lower level of bifurcations affects the bubble breakup and enhances the upstream asymmetrical behaviors.
Highlights
Segmented bubble in gas–liquid multiphase flow is ubiquitous in a variety of applications such as sample analysis [1,2,3], sorting [4], mixing [5] and heat transfer [6,7] in microfluidics
The dynamic behaviors of bubbles are strongly affected by the local geometry [15], which is significant to the efficiency of treatment, especially when bubbles are acting as drug delivery vessels
The fractal tree-shaped channel can be considered as a derivative from multi-leveled T-junctions and the bubble behaviors in fractal tree-shaped channels are similar to those flowing through a single T-junction [24]
Summary
Segmented bubble in gas–liquid multiphase flow is ubiquitous in a variety of applications such as sample analysis [1,2,3], sorting [4], mixing [5] and heat transfer [6,7] in microfluidics. The behavior of bubbles splitting in bifurcation channels is usually studied in the form of a single bubble flowing in through a T-junction, especially in numerical simulations [27]. The interaction of fluids between different levels, the variation of pressure and the dependence between bubble volume distribution and global structure of the network is still unrevealed. The deformation, splitting and coalescence characteristics of the bubbles at each level of the tree branches are compared and analyzed in an effort to gain further insight into the development and evolution mechanism of distinct flow patterns. As mentioned above, the fractal tree-shaped network is highly efficient in mass distributing and the numerical simulation results are beneficial for the design of innovative drug delivery system. Sci. 2019, 20, 5516 of the feedback from the fluid flow downstream on upstream bubble dynamics is significant in the configuration of tree-shaped delivery networks
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