Numerical investigation of the stability of bubble train flow in a square minichannel

Abstract

The stability of a train of equally sized and variably spaced gas bubbles that move within a continuous wetting liquid phase through a straight square minichannel is investigated numerically by a volume-of-fluid method. The flow is laminar and cocurrent upward and driven by a pressure gradient and buoyancy. The simulations start from fluid at rest with two identical bubbles placed on the axis of the computational domain, the size of the bubbles being comparable to that of the channel. In vertical direction, periodic boundary conditions are used. These result in two liquid slugs of variable length, depending on the initial bubble-to-bubble distance. The time evolution of the length of both liquid slugs during the simulation indicates if the bubble train flow is "stable" ͑equal terminal length of both liquid slugs͒ or "unstable" ͑contact of both bubbles͒. Several cases are considered, which differ with respect to bubble size, domain size, initial bubble shape, and separation. All cases lead to axisymmetric bubbles with the capillary number in the range of 0.11-0.23. The results show that a recirculation pattern develops in the liquid slug when its length exceeds a critical value that is about 10%-20% of the channel width. If a recirculation pattern exists in both liquid slugs, then the bubble train flow is stable. When there is a recirculation pattern in one liquid slug and a bypass flow in the other, the bubble train flow may be stable or not depending on the local flow field in the liquid slugs close to the channel centerline. These results suggest that a general criterion for the stability of bubble train flow cannot be formulated in terms of the capillary and Reynolds number only, but must take into account the length of the liquid slug.

Influence of the contact angle on two‐phase flow in microreactors for nitrobenzene–hydrogen–stainless steel/carbon

“…Nowadays, standard CFD codes are able to dynamically describe the interface in immiscible flows in mini channels by consideration of surface tension force and wall adhesion. [17,18,23,24] The implementation (Eqns (9) and (12)) provides so-called dynamic boundary conditions for the adjustment of the curvature of the interface near solid walls and therefore explains the deviation of apparent contact angles on the walls in relation to the SCA in our studies (Table 3). [23] In order to correctly model the physical motion of the contact line between three phases (here gas-liquid-solid) depending on the wall boundary conditions, additional user effort is needed to implement the required boundary condition into codes; e.g.…”

“…[17] The latter was verified by experimental observations of a bubble train flow. [18] There, it was found that, for the determination of the interface between gas and liquid phases, the recommended method is VOF in combination with the geometrical reconstruction method, both available in the FLUENT commercial code. This is why we used in this study again the ANSYS FLUENT system with VOF to evaluate the bubble formation in our microreactor.…”

Numerical modeling of multiphase flows in microfluidics and micro process engineering: a review of methods and applications

“…Liu and Wang (2008) extended this study to vertical square and equilateral triangular channels with 1 mm hydraulic diameter. Wörner and coworkers performed comprehensive numerical simulations of concurrent upward and downward Taylor flow in millimeter sized square vertical channels with an in-house PLIC-VOF code (Ghidersa et al 2004;Wörner et al 2005;Wörner et al 2007;Ö ztaskin et al 2009;Keskin et al 2010). The use of periodic boundary conditions in the axial direction allowed restriction of the computational domain to a single flow unit cell (which consists of one bubble and one liquid slug).…”

Numbered-up gas–liquid micro/milli channels reactor with modular flow distributor

“…Oztaskin et al [45] demonstrated that as the slug lengths is equal to or lower than the channel diameter, Taylor flow is not stable because the velocity profile in the liquid slug is not fully developed. The non-stable Taylor flow result in bubble coalescence.…”

“…As the liquid film thickness decreases and because there are sharp bends in the transport channels and reaction channels (see Figure 5), it is possible that partially dry walls could form. The partially dry walls can induce bubble coalescence especially at lower slug lengths (when the length is similar or lower than the channel diameter [45]). Bubble coalescence generate pressure fluctuating over the reaction channels (σ(q C ) increase) and produces larger flow non-uniformity.…”