Air Slug Research Articles

Bubble formation by shearing-to-squeezing transition in a T-junction

A shearing-to-squeezing transition process for bubble formation in a T-junction was reported using microscopy and digital image technology. The investigations were conducted in a polydimethylsiloxane (PDMS) T-junction. According to the aspect ratio of air slug and bubble generation frequency, the bubbles generated in the microchannel were identified to three types, namely dispersed bubble, short-slug bubble and long-slug bubble. The corresponding modes of bubble formation were identified as shearing, transition and squeezing modes, respectively. Additionally, the microbubble size and generation frequency with various gas pressures and velocity of liquid flows in each mode were investigated. The relevant dimensional arguments were performed to characterize the microbubble formation process in the T-junction. A new scaling law, based on the formation mechanisms, was proposed to predict the size of the microbubbles and showed a good agreement with the experimental results.

Research of the fluid flow regime during the milk pipeline washing

The aim of the research was to determine the change in the volume of fluid slug moving in the milk pipeline for calculating the initial length of the fluid slug and thus the air slug length. This allows to control the fluid and air inlet valve during circulatory washing of milking equipment with milk line and provides a stable plug flow during washing. The article deals with the problems of formation of a wave flow and plug flow regime in a milk line in dependance to the degree of filling and shear stress on the surface of the phases. As the result, the dependence curves have been obtained. They show that when the degree of filling the milk line with liquid is from 0.4d to 0.6d, the probability of slug formation decreases. To determine the loss of a fluid slug volume moving in a milk line, mathematical calculations with the use of the boundary layer theory have been carried out. As the result, the curves of the time of fluid and air intake into the system depending on the length and diameter of the milk pipeline have been obtained. These engineering calculation methods allow to set the operating parameters of the valves for fluid and air inlet into the system during washing. To calculate the parameters of pipelines with an internal diameter of 48, 63, 70 and 98 mm often used in milking equipment, a Delphi-based program was written. The article provides an example of calculating the time of opening and closing the valve for the inlet of fluid and air during washing for a milk line with 48 mm pipe diameter, 120 m length and pressure in the system of 48 kPa. Experimental studies have confirmed the reliability of the calculations, the loss of the liquid plug length for the milk pipeline Ø50. 8 mm is on average 5 cm per 1 meter of the fluid path.

Statistical Characterization of Flow Structure of Air–water Two-phase Flow in Airlift Pump–Bubble Generator System

The aim of this study is to identify the flow structure in an airlift pump–bubble generator system by using experimentally obtained differential pressure signals. Differential pressure measurements were applied at both the bottom and top test sections. The normalized time variation of the differential pressure data was analyzed using the probability density function (PDF), power spectral density function (PSDF), Kolmogorov entropy, and discrete wavelet transform (DWT). The results indicate that the water movement mechanism in the riser pipe could be divided into three regions, namely fixed liquid, locally moving liquid, and fully moving liquid, depending on the supplied superficial air velocity. Moreover, the chaotic in riser pipe increases with the increase of the submergence ratio at high supplied superficial air velocity. The chaotic level at the bottom of the test section was higher than that at the top of the test section at low supplied superficial air velocity. Finally, the observed flow patterns closely conformed to the previous definition and flow pattern maps. The bubbly flow was classified into clustered, homogeneous, and cap bubble regimes. The slug flow regime was classified into bubbly stable slug, bubbly unstable slug, and slug-churn, wherein the frequencies of both the liquid and air slugs increased with an increase in the air superficial velocity and submergence ratio.

Effect of Sand Fines Concentration on the Erosion-Corrosion Mechanism of Carbon Steel 90° Elbow Pipe in Slug Flow.

Erosion-corrosion of elbow configurations has recently been a momentous concern in hydrocarbon processing and transportation industries. The carbon steel 90° elbows are susceptible to the erosion-corrosion during the multiphase flow, peculiarly for erosive slug flows. This paper studies the erosion-corrosion performance of 90° elbows at slug flow conditions for impact with 2, 5, and 10 wt.% sand fines concentrations on AISI 1018 carbon steel exploiting quantitative and qualitative analyses. The worn surface analyses were effectuated by using laser confocal and scanning electron microscopy. The experiment was conducted under air and water slug flow containing sand fines of 50 µm average size circulated in the closed flow loop. The results manifest that with the increase of concentration level, the erosion-corrosion magnitude increases remarkably. Sand fines instigate the development of perforation sites in the form of circular, elongated, and coalescence pits at the elbow downstream and the corrosion attack is much more obvious with the increase of sand fines concentration. Another congruent finding is that cutting and pitting corrosion as the primitive causes of material degradation, the 10 wt.% sand fines concentration in carrier phase increases the erosion-corrosion rate of carbon steel up to 93% relative to the 2 wt.% sand fines concentration in slug flow.

Real-Time Detection of Slug Velocity in Microchannels.

Microfluidics processes play a central role in the design of portable devices for biological and chemical samples analysis. The bottleneck in this technological evolution is the lack of low cost detection systems and control strategies easily adaptable in different operative conditions, able to guarantee the processes reproducibility and reliability, and suitable for on-chip applications. In this work, a methodology for velocity detection of two-phase flow is presented in microchannels. The approach presented is based on a low-cost optical signals monitoring setup. The slug flow generated by the interaction of two immiscible fluids {air and water} in two microchannels was investigated. To verify the reliability of the detection systems, the flow nonlinearity was enhanced by using curved geometries and microchannel diameter greater than 100 m. The optical signals were analyzed by using an approach in a time domain, to extract the slug velocity, and one in the frequency domain, to compute the slug frequency. It was possible to distinguish the water and air slugs velocity and frequency. A relation between these two parameters was also numerically established. The results obtained represent an important step in the design of non-invasive, low-cost portable systems for micro-flow analysis, in order to prove that the developed methodology was implemented to realize a platform, easy to be integrated in a System-on-a-Chip, for the real-time slug flow velocity detection. The platform performances were successfully validated in different operative conditions.

Periodic Trends in Two-Phase Flow Through a Vertical Minichannel: Wavelet and Multiscale Entropy Analyses Based on Digital Camera Data

Abstract By changing the air and water flow relative rates in the two-phase (air-water) flow through a minichannel, we observe aggregation and partitioning of air bubbles and slugs of different sizes. An air bubble arrangement, which show non-periodic and periodic patterns. The spatiotemporal behaviour was recorded by a digital camera. Multiscale entropy analysis is a method of measuring the time series complexity. The main aim of the paper was testing the possibility of implementation of multiscale entropy for two-phase flow patterns classification. For better understanding, the dynamics of the two-phase flow patterns inside the minichannel histograms and wavelet methods were also used. In particular, we found a clear distinction between bubbles and slugs formations in terms of multiscale entropy. On the other hand, the intermediate region was effected by appearance of both forms in non-periodic and periodic sequences. The preliminary results were confirmed by using histograms and wavelets.

Experience coating bubbles with solvent in a downcomer

A downcomer, as employed in the Jameson Cell, was considered for producing solvent-coated bubbles for possible application in a technique referred to as “air-assisted solvent extraction” (AASX). It was found that the bubble bed in the downcomer started to collapse upon introducing solvent, evident from decreasing air holdup, decreasing aspirated air rate, and formation of air slugs. Neither changing solvent introduction method, solvent composition nor addition of frother or salt alleviated the collapse, which could become complete (air rate became zero) at about 900 ppm solvent. To explain, the bubble bed, which in the downcomer can approach 60% air holdup (Atkinson et al., 1993; Jameson and Manlapig, 1991), was likened to a foam that is known to collapse in the presence of hydrophobic liquid (oil) droplets due to a bridging/spreading mechanism that expels water between contacting bubbles. Tests with hydrophobic solids, graphite and talc, gave increased bubble bed stability showing it is the liquid nature of the solvent that is the key factor causing the collapse. One combination, talc and MIBC, did provide stability against solvent addition. Froth collapse/stability mechanisms are discussed. It is noted that the Jameson Cell is successfully employed to remove solvent droplets in SX plants. The solvent levels in those cases appear to be less than 200 ppm where collapse is only partial and the phenomenon may not be noticed.

Laser induced fluorescence measurement of liquid film thickness and variation in Taylor flow

In the present work, experiments are done to understand the distribution of liquid film in Taylor flows occurring in small circular mini channels using laser-induced fluorescence. Gas–liquid Taylor flow is generated in 3 different channels of inner diameter 1.5, 2.1 and 3.1 mm to understand the effect of flow parameters and nature of forces on the liquid film thickness. Rhodamine-B, a fluorescent dye is mixed with water, the primary phase in the two-phase mixture and is excited by a pointed laser beam of wavelength 531 nm. The liquid film thickness with the excited fluorescent dye emits light at 610 nm which is captured using a CMOS high-speed camera. A long pass filter is used to filter the reflected laser light before being captured by the CMOS sensor camera. Subsequently, the image obtained is processed with a set of image processing techniques to determine the liquid film thickness. Different slug shapes are obtained for various combinations of flow velocities and are found to depend on the Bond number. Further, it is also found from the experiments that asymmetry in the flow regime is found to depend on the capillary forces acting on the flow and it increases with the capillary number. The film thickness is also found to vary across the air slug length and its distribution along the flow length is also presented.

The loss coefficient for fluctuating flow through a dominant opening in a building

Wind-induced fluctuating internal pressures in a building with a dominant opening can be described by a second-order non-linear differential equation. However, the accuracy and efficiency of the governing equation in predicting internal pressure fluctuations depend upon two ill-defined parameters: inertial coefficient <TEX>$C_I$</TEX> and loss coefficient <TEX>$C_L$</TEX>, since <TEX>$C_I$</TEX> determines the un-damped oscillation frequency of an air slug at the opening, while <TEX>$C_L$</TEX> controls the decay ratio of the fluctuating internal pressure. This study particularly focused on the value of loss coefficient and its influence factors including: opening configuration and location, internal volumes, as well as wind speed and approaching flow turbulence. A simplified formula was presented to predict loss coefficient, therefore an approximate relationship between the standard deviation of internal and external pressures can be estimated using Vickery's approach. The study shows that the loss coefficient governs the peak response of the internal pressure spectrum which, in turn, will directly influence the standard deviation of the fluctuating internal pressure. The approaching flow characteristic and opening location have a remarkable effect on the parameter <TEX>$C_L$</TEX>.

Study of dynamics of two-phase flow through a minichannel by means of recurrences

By changing air and water flow rates in the two-phase (air–water) flow through a minichannel, we observed the evolution of air bubbles and slugs patterns. This spatiotemporal behaviour was identified qualitatively by using a digital camera. Simultaneously, we provided a detailed analysis of these phenomena by using the corresponding sequences of light transmission time series recorded with a laser-phototransistor sensor. To distinguish particular patterns, we used recurrence plots and recurrence quantification analysis. Finally, we showed that the maxima of various recurrence quantificators obtained from the laser time series could follow the bubble and slugs patterns in studied ranges of air and water flows.

Dynamics and Noise Level Estimation in Two-Phase Flow through a Mini Channel

By changing air to water flow rates of a two-phase flow through a minichannel we identified aggregation and partitioning of air bubbles and slugs of various sizes. It was found that for some flowconditions air bubbles formed periodic patterns. The identification of the spatio-temporal behaviourswas performed with a laser transitivity sensor and confirmed by a digital camera. We used the Hurstexponent to distinguish instabilities in air slugs, their breakups and aggregations. In addition, we performed noise level estimation.

Self-drainage of viscous liquids in vertical and inclined pipes

The rate of drainage of a viscous liquid from initially full cylindrical tubes inclined at various angles to the vertical (0°, 30°, 45° and 60°) was studied in glass and polymethylmethacrylate (Perspex™) tubes of various lengths and diameters using three food materials: honey (Newtonian) and two variants of Marmite™ spread (both exhibiting complex rheological behaviour, including shear-thinning and thixotropy). The behaviour was marked by an initially steady rate of drainage in which an air slug descended the tube, followed by slower drainage from an annular film remaining on the wall. Eventually the liquid stopped draining as a filament and entered a dripping regime. Drainage was insensitive to the tube material, whereas the stages of drainage were influenced by the geometry and angle of inclination. Quantitative models are presented for the rate and extent of the initial drainage stage, the rate in a second linear stage (where it existed), and the rate of drainage in the third, falling rate stage. The fourth and final stage, characterised by drop formation, was not modelled. The initial rate can be predicted with reasonable accuracy, allowing the time to remove approximately 50% of the material in a short waiting phase to be calculated, e.g. t=8νL/R2g for a Newtonian liquid with kinematic viscosity ν in a vertical pipe of radius R and length L. The agreement with the other models is less exact but they capture the general trends reasonably.

Visualization of Terrain-induced Slugging in W-shaped Pipeline

Gas-liquid two-phase flow in a circular pipeline is commonly encountered in an inclined pipeline of an offshore plant. To visualize gas-liquid flow phenomena in an inclined pipeline, the w-shaped transparent pipeline was fabricated with internal diameter of 2″ and slope angle of 25°. The terrain-induced slug flow in a steady-state was visualized at fixed water flow rate of 1 m3/hr and 80% GVF (Gas Volume Fraction). The air and water flow is initially maintained in stratified or wavy flow at t = 0 s. When the velocity difference between the air and water is high enough, the Kelvin-Helmholtz wave motion starts to occur just after at t = 0 s. As the wave reaches the top, the upward water flow is faced with the downward water flow in the main visualized region. When the airway is clogged, the air slug is formed at t = 0.02 s. When a huge tidal wave is observed at t = 0.1 s due to different velocity between the upward water and the downward water flow, the air slug travels at a greater velocity than the water flow. As the tidal wave enlarges its growth, the chaotic motion with scattered bubbles is exhibited at the gas-liquid interface. A series of the air slugging is periodically observed after near 0.5 s. At the second v-shaped pipeline, the slugging phenomena become even more severe due to an irregular water inflow from the first v-shaped pipeline.

Fluidic Active Transducer for Electricity Generation.

Flows in small size channels have been studied for a long time over multidisciplinary field such as chemistry, biology and medical through the various topics. Recently, the attempts of electricity generation from the small flows as a new area for energy harvesting in microfluidics have been reported. Here, we propose for the first time a new fluidic electricity generator (FEG) by modulating the electric double layer (EDL) with two phase flows of water and air without external power sources. We find that an electric current flowed by the forming/deforming of the EDL with a simple separated phase flow of water and air at the surface of the FEG. Electric signals between two electrodes of the FEG are checked from various water/air passing conditions. Moreover, we verify the possibility of a self-powered air slug sensor by applying the FEG in the detection of an air slug.

Dynamics of Two-Phase Flow through a Minichannel: Fourier and Multiscale Entropy Analyses

By changing a air flow rate of the two-phase (air-water) flow through a minichannel weidentified aggregation and partitioning of air bubbles and slugs of different sizes and air bubble arrangement into periodic patterns. The identification of these spatio-temporal behaviour was doneby digital camera. Simultaneously, we provide the detailed studies of these phenomena by using thecorresponding sequences of light transmission time series recorded by a laser-phototransistor sensor.To distinguish the instabilities in air slags and their breakups and aggregations we used the Fourierand multiscale entropy analysis.

Self-aggregation Phenomenon and Stable Flow Conditions in a Two-Phase Flow Through a Minichannel

Abstract By increasing a water flow rate of the two-phase (air–water) flow through a minichannel, both the partitioning of air slugs into air bubbles of different sizes and small air bubbles aggregation into larger air bubbles were identified. These phenomena were studied in detail by using the corresponding sequences of light transmission time series recorded with a laser-phototransistor sensor. To distinguish any instabilities in air slugs along with their break-ups and aggregations, the recurrence plots and recurrence quantification analysis were applied.

Channel aspect ratio effect on thermal performance of air–water slug flow through U-bend channels

This study investigates the effect of channel height-to-width ratio (Aspect Ratio, AR) on heat-transfer rates, pressure-drop coefficients (f) and thermal performances of air–water flows through horizontal and vertical U-bend rectangular channels at intermittent slug and slug-annular flow conditions. Interfacial two-phase flow structures, local and area-averaged Nusselt numbers (Nu), f coefficients, channel-wise averaged void fractions (α) and thermal performance factors (TPF) for three sets of horizontal and vertical U-bend channels of AR = 1, 0.83 and 0.33 are measured with liquid Reynolds numbers (ReL) and air-to-water mass flow ratios (AW) in the range of 1500 ≤ Re ≤ 10000 and 0 ≤ AW≤0.024. Early transitions from slug flow to slug-annual flow along with the shortened air slug and the extended period of the trailing-edge bubbly flow over each intermittent cycle are promoted by decreasing AR to elevate both heat transfer rates and pressure drops for the U-bend channels with small AR. A set of selective Nu, f and TPF data illustrates the interdependency between Nu, f, TPF and the air–water flow structures in present test channels with different AR. The area averaged endwall Nu for each U-bend test channel (Nu¯) and the corresponding f and TPF are cross-examined to generate a set of heat-transfer and pressure-drop correlations, which permit the evaluations of isolated and interdependent ReL, AW and AR effects on Nu¯ and f, to assist various design applications.

Scaling methods for wind tunnel modelling of building internal pressures induced through openings

Appropriate scaling methods for wind tunnel modelling of building internal pressures induced through a dominant opening were investigated. In particular, model cavity volume distortion and geometric scaling of the opening details were studied. It was found that while model volume distortion may be used to scale down buildings for wind tunnel studies on internal pressure, the implementation of the added volume must be done with care so as not to create two cavity resonance systems. Incorrect scaling of opening details was also found to generate incorrect internal pressure characteristics. Furthermore, the effective air slug or jet was found to be longer when the opening was near a floor or sidewall as evidenced by somewhat lower Helmholtz frequencies. It is also shown that tangential flow excitation of Helmholtz resonance for off-centre openings in normal flow is also possible.

Studies of Air Slug Distributions and Preliminary Membrane Fouling by Optical Monitoring in a Side-Stream Membrane Module

This article is aimed to propose an alternative method to characterize an air/water flow and new designs of rotating air distributors for membrane fouling control in a side-stream membrane module for a membrane bioreactor. A new optical sensor technique was experimentally demonstrated for the characterization of slug flows in a tubular ultrafiltration membrane module. The experimental results showed that the optical sensor system had high sensitivities for measurements of slug velocities, lengths, and frequencies in each membrane tube. The phenomena of slug coalescence/spilt were discussed. The rotating air distributor significantly improved the air slug distributions and had the potential to reduce the membrane fouling.

A debubbler for microfluidics utilizing air-liquid interfaces

We present a debubbler for the removal of air bubbles in aqueous liquids in microfluidics. The debubbler is realized through an array of cylinder-shaped air-liquid interfaces, or air-pillars, formed directly into the microfluidic channels by surface tension. The debubbler demonstrated effective trapping and removal of both chemically generated bubbles (several nanoliters to several microliters in volume) and air slugs with several microliters in volume, which were formed by high infusion rates of bubble series introduced into the microchannel.