Liquid Co-flow Research Articles

Prediction of self-organised morphology for biphasic liquid co-flow in mesodomain using energy minimisation approach

In this study, the energy minimisation principle is used to map the self-organisation pattern of biphasic liquid–liquid co-flow in the mesodomain. In particular, we focus on the specific role of gravity in tuning the patterns. The total energy of the system expressed in terms of flow and geometric parameters is minimised by a multivariate non-linear method and the flow morphology corresponding to the minimised total energy is inferred for up, down and horizontal flow of the two phases. The analysis predicts the formation of stable segmented flow by the mechanisms of dripping, squeezing and jetting and also the transition to parallel flow with change in operating conditions and flow orientation. The predicted morphology and flow parameters are validated against experimental observations of the present study and also against available data from literature for mesoscale flow.

Near-wall flow response to large air bubbles rising in inclined water channels

The two-strip electrodiffusion probe is employed to investigate the wall shear rate induced by large air bubbles rising in stagnant or coflowing water within inclined rectangular channels. Synchronized video recordings of bubble movements captured by a high-speed camera provide additional information on the bubble shapes and terminal velocities. The measurements are carried-out in a channel with easily adaptable geometry (three heights and various widths) over a wide range of operation parameters (air volumes and liquid velocities) and inclination angles (from horizontal to vertical arrangement). The main objective of this experimental study is to elucidate the influence of individual operating parameters on the bubble-induced wall shear rate. The typical profile of wall shear rate measured at the center of a flat channel can be characterized by a positive peak at the bubble front location, a negative plateau corresponding to the reverse flow in a liquid film around the bubble, and highly fluctuating values in a wake behind the bubble. Just as the large bubbles in flat channels exhibit a close similarity in their frontal shapes, so do the wall shear rate profiles induced by differently sized bubbles. The magnitude and profile of measured wall shear rate is found to be controlled primarily by the distance between two opposite walls squeezing the rising bubble, thus in flat channels by the channel height. By contrast, widening the channel has no significant effect, even though it contributes to a significant increase in the bubble rise velocity. When the channel is tilted, the rising bubbles are pushed towards the roof wall and thus two distinct wall shear rate profiles are measured at the opposite walls of the channel. The liquid coflow then contributes positively not only to the bubble velocity but also to the magnitude of wall shear rate in the near-wall flow region.

Between droplets and fluid thread—the role of gravity in meso-scale flow

How gravity affects immiscible liquid co-flow is best illustrated through experiments in inclined conduits. In the macro-domain, gravity leads to flow stratification while in the microscale, the phase distribution is practically insensitive to conduit tilt. The influence of flow orientation in the intermediate scale conventionally known as meso-domain or milli-channel, although noted, has not been discussed earlier. In the present study, flow morphology is experimentally investigated during up, down, and horizontal co-flow of a biphasic liquid mixture in a glass conduit of diameter 2.38 mm. In all orientations, the dispersed phase flows either as droplets/plugs or as a continuous thread. Gravity modulates the process of thread pinch off and regulates the domain of thread/droplet flow. Apart from flow orientation, we also note entry arrangement to influence droplet detachment in horizontal conduit. The experimental observations are explained from a simplified analysis based on momentum and energy considerations; the defining parameters are fluid properties and flow rates, conduit dimension, and flow orientation. The proposed analysis, albeit the approximations, has successfully predicted thread pinch off for the present experiments. Pinch off from the thread tip is noted to be cyclic and comprises several steps, of which inception of necking to its completion is only a part.

The effect of liquid co-flow on gas fractions, bubble velocities and chord lengths in bubbly flows. Part II: Asymmetric flow configurations

This paper describes the effects of uniform and non-uniform liquid co-flow on the bubbly flow in a rectangular column (with two inlets) deliberately aerated unevenly. The two vertical bubbly streams, comprising uniform bubbles, started interacting downstream of the trailing edge of a splitter plate. This study quantifies the emergence of buoyancy driven flow patterns as a function of the degree of a-symmetric gas sparging and (non-)uniform liquid co-flow by using Bubble Image Velocimetry (BIV) and dual-tip optical fibre probes. Without liquid co-flow, small differences in the gas fraction of the left and right inlet had a large effect on the mixing pattern, whereas a liquid co-flow stabilized a homogeneous flow regime and the flow pattern was less sensitive to gas fraction differences. Void fractions, bubble velocities and chord lengths were measured at two fixed position in the flow channel, whereas BIV provided a global overview of the flow structures. A correlation was developed to predict (a-symmetric) operating conditions for which the gas fraction of the left and right inlet are balanced, such that the bubble motion is governed by advection and no buoyancy driven flow structures arise. The data obtained is highly valuable for CFD validation and development purposes.

The effect of liquid co-flow on gas fractions, bubble velocities and chord lengths in bubbly flows. Part I: Uniform gas sparging and liquid co-flow

Unique experiments were performed in a homogeneously sparged rectangular 400×200×2630 mm (W×D×H) bubble column with and without liquid co-flow. Bubbles in the range 4–7 mm were produced by needle spargers, which resulted in a very uniform bubble size. Dual-tip optical fibre probes were used to measure horizontal profiles of gas fractions, bubble velocities and bubble chord lengths for superficial gas velocities Usg in the range 0.63–6.25 cm/s and superficial liquid velocities Usl up to 20 cm/s. Images of the bubble column were captured and a Bubble Image Velocimetry technique was adopted to calculate bubble (parcel) velocities. For low gas fractions, when a homogeneous flow regime occurred, both methods agreed very well and the optical fibre probes were found to be rather accurate for our bubbles. A liquid co-flow was found to have a calming effect and to stabilize a homogeneous bubbly flow regime, with less spatial variation in gas fractions and bubble velocities. Bubble chord lengths were almost normally distributed and do not exhibit the theoretical triangular probability density functions. The mean cord lengths were in the range 1.9–3.5 mm and found to increase with Usg and to decrease slightly with increasing Usl, while a liquid co-flow significantly reduced the standard deviation of the chord length distribution.

The Limerick bubbly flow rig: Design, performance, hold-up and mixing pattern

As Euler-Euler CFD simulations of bubbly flows suffer from uncertainties due to the many underpinning models, there is an obvious need of accurate experimental data for validation. With this in mind, a new bubbly flow test rig was built to be operated with and without liquid co-flow, with bubble size as uniform as possible in the range 4–7mm, and with a very even horizontal bubble distribution. We designed the gas sparging system such that we can also produce an essentially bi-modal bubble size distribution. The column consists of two square sections to allow for studying the mixing of two originally separated bubbly flows with either the same or a different bubble size. The bubbles are produced from 2×196 needles, bubble sizes are determined with high-speed imaging and with a simple acoustical method, overall volume fractions in the column by means of air chamber pressure measurements. Overall volume fractions are presented as a function of gas and liquid flow rates, with slip velocity mostly increasing with increasing void fraction. First results are obtained on (a) producing bi-model bubble size distributions and the pertinent volume fractions in the column, and (b) flow patterns in the case of unequal aeration.

Experimental investigation on the bubble formation from needles with and without liquid co-flow

We report experiments on bubble formation from needles with and without liquid co-flow, carried out with needles in the range of 0.79<dn<2.06 mm, for gas flow rates up to 4.5 cm3/s per needle, and with liquid co-flow velocities up to 0.4 m/s. Bubble sizes and frequencies were obtained by means measuring an acoustic signal in the pressurized chamber upstream, which is validated by high-speed imaging analysis. Bubble contours, bubble growth curves and time return plots were obtained to analyse the bubble formation process. Different bubbling regimes are distinguished and a novel dimensionless pressure ratio is proposed to forecast the emergence of weeping and the transition from constant flow rate bubbling to constant chamber pressure bubbling. A single correlation for the non-dimensional bubble size with and without liquid co-flow was developed and validated with the experimental data obtained in the present study.

Wall shear stress induced by a large bubble rising in an inclined rectangular channel

The rise of single air bubbles in inclined rectangular channels was experimentally investigated. Two-segment electrodiffusion probes were used to measure wall shear rate profiles along the passing bubbles. They provided information on reverse flow in a liquid film separating the bubble from the wall, capillary waves appearing at the bubble tail, and near-wall flow fluctuations in the bubble wake. The corresponding bubble shapes and rise velocities were obtained from simultaneous visual observations done by a high-speed camera. The experiments were carried out for three channel depths (1.5, 4, and 8mm), various channel inclinations (from 5° to 90°), bubble volumes (from 1 to 80ml), and liquid up-flow velocities (from 0 to 0.2m/s). In vertical channels, the wall shear rate trace of a bubble rise is primarily influenced by the channel depth. As the frontal shape of large bubbles does not change with the bubble size, also the wall shear rate measured under these bubbles evolves in the same manner. In inclined channels, the liquid film is unequally distributed above and under the bubble with the maximum reverse flow observed under the bubble at middle inclinations. Laminar liquid co-flow makes the liquid film around the bubble thicker and in inclined channels slightly pushes the bubble toward the center-line position. The bubble velocity scaling based on the channel perimeter is confirmed to be suitable for vertical channels with stagnant liquid. The linear relationship between the bubble rise and liquid mean velocity is identified under co-flowing conditions at all channel inclinations.

CFD predictions for flow‐regime transitions in bubble columns

AbstractThis work evaluates the ability of multiphase computational fluid dynamics (CFD) models to predict known flow regimes in air–water bubble columns. An initial grid‐resolution study shows that grid spacing of 0.25 cm or smaller must be used for adequate resolution. The ability of the two‐fluid model to predict homogeneous‐ and transitional‐flow behavior is analyzed next, and the flow predictions are found to be highly dependent on the model formulation (that is, bubble‐induced turbulence, drag, virtual mass, lift, rotation, and strain). At low gas velocities, homogeneous flow is observed for only a particular set of force models. At higher gas velocities, the same set of models yields reasonable predictions of transitional flow for small columns. Bubble size and liquid coflow also affect flow structures and flow stability at high gas flow rates. Scale‐up to larger column diameters is studied for both the homogeneous‐ and transitional‐flow regimes. In the homogeneous regime, the flow behavior is found to be independent of column diameter. However, because of neglect of coalescence, transition to churn‐turbulent flow is not observed at high gas velocities for large column diameters. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Transition from bubbling to jetting in a coaxial air–water jet

In this Brief Communication we study experimentally the flow regimes that appear in coaxial air–water jets discharging into a stagnant air atmosphere and we propose a simple explanation for their occurrence based on linear, local, spatiotemporal stability theory. In addition to the existence of a periodic bubbling regime for low enough values of the water-to-air velocity ratio, u=uw∕ua, our experiments revealed the presence of a jetting regime for velocity ratios higher than a critical one, uc. In the bubbling regime, bubbles form periodically from the tip of an air ligament whose length increases with u. However, when u&amp;gt;uc a long, slender gas jet is observed inside the core of the liquid coflow. Since in the jetting regime the downstream variation of the flow field is slow, we performed a local, linear spatiotemporal stability analysis with uniform velocity profiles to model the flow field of the air–water jet. Similar to the transition from dripping to jetting in capillary liquid jets, the analysis shows that the change from the bubbling to the jetting regime can be understood in terms of the transition from an absolute to a convective instability.

Temporal instability of swirling gas jets injected in liquids

A temporal linear stability analysis of an inviscid incompressible swirling gas jet injected into a co-flowing liquid was conducted. The ratio of the tangential velocity to the axial velocity (swirl number) played a significant role in the destabilization of the gas jet. Even at small gas Weber numbers, the presence of swirl caused the higher order azimuthal modes to become unstable; the growth rates, and the dominant and limiting wave numbers of the higher order modes were greater than those of the varicose and sinuous modes. The differences in growth rates, limiting and dominant wave numbers of the various azimuthal modes became significant at large gas Weber numbers. An increase in liquid viscosity resulted in a reduction in the growth rates and the dominant wave numbers. The liquid co-flow velocity controlled the phase velocity of the unstable modes.

Three-Dimensional Temporal Instability of Compressible Gas Jets Injected in Liquids

A temporal linear stability analysis was conducted to study the growth rates of three-dimensional disturbances on a compressible inviscid gas jet injected into an incompressible viscous liquid coflow in the absence of gravitational effects. The primary parameters that governed the growth rates of the disturbances were the gas Weber number, Mach number, Ohnsorge number, density ratio, and velocity ratio. As the Mach number was increased, the range of unstable wave numbers and the growth rates of the unstable modes were increased. Also, the maximum growth rate was shifted to larger wave numbers (smaller wavelengths). The disturbance velocities were small, on the order of the coflowing liquid velocity. The growth rates and range of unstable wave numbers were reduced as the liquid coflow was increased. With an increase in liquid viscosity, the growth rates were reduced and the maximum growth rates were shifted to longer wavelengths. A reduction in the ratio of the liquid density to the mean gas density resulted in an increase in the growth rates of the unstable disturbances and the disturbance velocities