Size Of Slugs Research Articles

Quantitative study for control of air–liquid segmented flow in a 3D-printed chip using a vacuum-driven system

The formation of droplets or bubbles in a microfluidic system is a significant topic requiring device miniaturization and a small volume of samples. Especially, a two-phase segmented flow can be applied to micro-mixing for chemical reactions and the treatment of heat and mass transfer. In this study, a flow of liquid slugs and bubbles was generated in a 3D-printed chip and controlled by a single pump creating a vacuum at the outlet. The pump and chip device were integrated to form a simple and portable system. The size and flow rate of liquid slugs, obtained through image processing techniques, were analyzed considering several parameters related to hydraulic resistance and pressure drop. In addition, the effect of segmentation on mixing was observed by measuring the intensity change using two different colored inks. The hydraulic resistance of air and liquid flows can be controlled by changing the tube length of air flow and the viscosity of liquid flow. Because the total pressure drop along the channel was produced using a single pump at the outlet of the channel, the size and flow rate of the liquid slugs showed a near linear relation depending on the hydraulic resistances. In contrast, as the total pressure varied with the flow rate of the pump, the size of the liquid slugs showed a nonlinear trend. This indicates that the frequency of the liquid slug formation induced by the squeezed bubble may be affected by several forces during the development of the liquid slugs and bubbles. In addition, each volume of liquid slug segmented by the air is within the range of 10–1 to 2 µL for this microfluidic system. The segmentation contributes to mixing efficiency based on the increased homogeneity factor of liquid. This study provides a new insight to better understand the liquid slug or droplet formation and predict the segmented flow based on the relationship between the resistance, flow rate, and pressure drop.

X-ray tomography analysis of bubbles and slugs in a fluidized bed with inter-particle force

Bubbles and slugs coexist in a fluidized bed with strong inter-particle forces. However, the effects bubbles and slugs impose over fluidizing stability, and chemical reaction efficiency are different. The present work distinguished bubbles and slugs with the aid of X-ray tomography, and the evolution of their properties with varying inter-particle force in a fluidized bed were studied. The results show that the gas-holding capacity of the emulsion phase improves as inter-particle forces initially increase. In this process, the bubble frequency decreases noticeably, and the bubble size slightly increases. Bubbles rise slower than theoretically predicted because inter-particle force reduces the particle flow. The frequency and the size of slugs increase with inter-particle forces. As forces initially increases, the slug size increases with measurement height. An opposite trend is observed when inter-particle forces continues to strengthen. Near the gas distributor, the slug rising velocity is independent of size. In contrast, large slugs have a higher velocity than small slugs near the bed surface. Finally, three slugging patterns are distinguished, and the influence they impose on particle flow is analyzed.

Experimental research on the mechanisms of condensation induced water hammer in a natural circulation system

Natural circulation systems (NCSs) are extensively applied in nuclear power plants because of their simplicity and inherent safety features. For some passive natural circulation systems in floating nuclear power plants (FNPPs), the ocean is commonly used as the heat sink. Condensation induced water hammer (CIWH) events may appear as the steam directly contacts the subcooled seawater, which seriously threatens the safe operation and integrity of the NCSs. Nevertheless, the research on the formation mechanisms of CIWH is insufficient, especially in NCSs. In this paper, the characteristics of flow rate and fluid temperature are emphatically analyzed. Then the formation types of CIWH are identified by visualization method. The experimental results reveal that due to the different size and formation periods of steam slugs, the flow rate presents continuous and irregular oscillation. The fluid in the horizontal hot pipe section near the water tank is always subcooled due to the reverse flow phenomenon. Moreover, the transition from stratified flow to slug flow can cause CIWH and enhance flow instability. Three types of formation mechanisms of CIWH, including the Kelvin-Helmholtz instability, the interaction of solitary wave and interface wave, and the pressure wave induced by CIWH, are obtained by identifying 67 CIWH events.

Curvature induced intensification of biodiesel synthesis in miniature geometry

In the present study, mini-serpentine reactors of different R/D ratios are tested for mass transfer limited transesterification reaction to produce biodiesel. Experiments are carried out in three test sections made of glass capillary of id 2 mm with different R/D ratios and results are compared with a straight test-section of similar dimension. Yield increases almost 1.26 times in the presence of bend (serpentine geometry), as it induces the secondary flow in the liquid film surrounding the slug. Reverse flow in bend generates eddies surrounding the slug, known as Dean Vortices. This counter-flow enhances the transport rate significantly. An optimum Fatty Acid Methyl Ester (FAME) yield of 98 % is observed in serpentine test-section with R/D ratio 3 in just 1.18 min of residence time. Percentage FAME yield relies upon the prevalence of shape and size of long slugs or slender short slugs in subsequent tubes. FAME yield diminishes for the first case and enhancement is observed in the latter case.

Effects of an immersed tube in deep and shallow slugging fluidized beds

The effects of an immersed tube on slugging fluidization and particle behaviors in slugging fluidized beds were investigated using computational fluid dynamics. Both shallow and deep slugging fluidized beds with and without an immersed tube were simulated using the two-fluid model. The presence of an immersed tube gave rise to regions of high voidage below the tube and regions of low voidage above the tube. Formation of the high voidage region occurred by particle deflection away from the immersed tube, while that of the low voidage region resulted from particle vortices above the tube. The presence of an immersed tube was observed to facilitate bubble formation and bubble coalescence in both shallow and deep slugging fluidized beds. In shallow beds, an immersed tube regularized pressure fluctuations but increased the amplitude of fluctuations. In deep beds, an immersed tube reduced the frequency of pressure fluctuations by facilitating wall slug formation. Based on random sampling and statistical testing (Z-test) of mean slug lengths, it was concluded that mean slug lengths increased when U was increased from 0.90 m/s to 1.0 m/s in the deep free column (p-value = 0.0422). Based on an F-test of the slug length variance, it was concluded that there was larger variability of slug lengths for U of 1.0 m/s than for U of 0.90 m/s in the deep free column (p-value = 0.0202). However, the results of these statistical tests on mean slug lengths and slug length variance for the deep single tube column showed that U had no significant effect on mean slug length or slug length variance at the 5% significance level. Thus, one of the effects of an immersed tube in a deep slugging fluidized bed was regulation of the size of slugs formed and prevention of any significant increase in mean slug size or slug size variability.

Formation of gas-liquid slugs in millimeter-scale T-junctions – Slug size estimation framework

Gas-liquid slug flow in micro/millimeter-scale channels is preferred in a broad range of applications because of its high mass and heat transfer rates. In most applications, the size of slugs needs to be precisely known, as the mass transfer characteristics are influenced by the slug length and its velocity. Here, a formation mechanism of gas-liquid slug is explored for the millimeter-scale T-junctions, which is different from that reported for micrometer-scale ones, and a framework is newly presented for the estimation of the slug size, which is based on a mass balance. As opposed to the gas slug size determined solely by channel geometry and liquid to gas flow ratio for micrometer-scale case, the size is strongly influenced by liquid and gas velocities.

Conformance Improvement in Oil Reservoirs by Use of Microemulsions

SummaryThe performance of many improved and enhanced-oil-recovery (EOR) techniques in conventional reservoirs is frequently degraded by conformance problems. The presence of high-permeability streaks or thief layers between injection and production wells typically results in premature water breakthrough, high water cut, and deficient volumetric sweep. As a result, significant oil volumes in the reservoir might not be contacted by the injection fluid.Several conformance-improvement techniques (e.g., foams, gels, resins) have been developed and practiced in improved-oil-recovery operations. Each technique has its own advantages and limitations related to deployment practicality, effectiveness, and durability. In this paper, we introduce a novel conformance-improvement method (CIM) that we consider practical, effective, and durable. The CIM process consists of cyclical injections of pulse slugs of surfactant alternating with brine. The slug compositions are selected on the basis of the rheological behavior of the microemulsion phase. The chemical slugs are configured such that the viscosity of the injected fluids is kept low to preserve injectivity and to ensure the invasion of the conformance agent toward the thief zones. The trailing brine slugs are designed to produce a high-viscosity microemulsion as they mix with the leading surfactant slugs in the reservoir. The proposed process leads to a reduction in the effective mobility of the fluids in the thief layers. As a result, the chase waterflood (WF) would divert into previously uncontacted layers to improve the sweep efficiency.The potential of the proposed CIM in improving oil recovery is demonstrated by various simulations of reservoir cases under waterflooding. We performed various sensitivities to investigate the effectiveness of the proposed process that include well spacing, permeability contrast, size of the thief layers, heterogeneity, and the size of the chemical pulse slugs. Simulations showed that this process is effective in addressing reservoir-conformance issues, and therefore it has the potential to improve the sweep efficiency and the recovery factor (RF) in reservoirs with distinct thief layers. The treatment surfactant volumes are relatively small, which enables this process to be cost-effective.

Agent-based models for first- and second-order emergent collective behaviours of social amoeba Dictyostelium discoideum aggregation and migration phases

Collective behaviour in nature provides a source of inspiration for engineering artificial systems (e.g. robotics, ecosystems of services), due to their inherent mechanisms favouring adaptation to environmental changes and enabling complex emergent behaviour to arise from a relatively simple behaviour of individual entities. The first-order emergence, also referred to as swarm intelligence, is well studied, while higher order levels of emergent behaviour have not received much attention yet. Second-order emergent behaviour arises from the interactions of individuals, which are themselves the result of first-order emergent behaviour. Dictyostelium discoideum provides a compelling case for studying both first- and second-order emergence. Individual cells move around on their own when there is plenty of food. When food is scarce, cells self-aggregate towards a leading center cell (first-order emergent behaviour) to build a super-organism, similar to a slug. The slug displays properties that none of the cells has on its own (e.g. sensitivity to light and heat). It moves as a whole (second-order emergent behaviour) looking for a suitable place to transform into a fruiting body (also known as sporocarp), where later the cells resume their individual behaviour. This paper focuses specifically on the aggregation and migration phases of Dictyostelium discoideum. We present two agent-based models, implemented in Matlab for first order and Python for second order. They display a series of emergent properties, among others homogeneous aggregation territories size (first order) and merging of slugs or new property as sensitivity to light (second order). Future works involve implementing and experimenting both first- and second-order emergence in swarm robotics, and identification of design patterns for engineering higher order emergent behaviour in artificial systems.

On-demand generation and mixing of liquid-in-gas slugs with digitally programmable composition and size

Microscopic droplets or slugs of mixed reagents provide a convenient platform for performing large numbers of isolated biochemical or chemical reactions for many screening and optimization applications. Myriad microfluidic approaches have emerged for creating droplets or slugs with controllable size and composition, generally using an immiscible carrier fluid to assist with the formation or merging processes. We report a novel device for generation of liquid slugs in air when the use of a carrier liquid is not compatible with the application. The slug generator contains two adjacent chambers, each of which has a volume that can be digitally adjusted by closing selected microvalves. Reagents are filled into the two chambers, merged together into a contiguous liquid slug, ejected at the desired time from the device using gas pressure, and mixed by flowing in a downstream channel. Programmable size and composition of slugs is achieved by dynamically adjusting the volume of each chamber prior to filling. Slug formation in this fashion is independent of fluid properties and can easily be scaled to mix larger numbers of reagents. This device has already been used to screen monomer ratios in supramolecular nanoparticle assembly and radiolabeling conditions of engineered antibodies, and here we provide a detailed description of the underlying device.

SLUG FLOW HEAT TRANSFER IN MICROCHANNELS: A NUMERICAL STUDY

Two-phase microchannel slug flow has attracted significant interest among researchers due to its wide range of applications. This paper will present some as yet overlooked features of slug flow heat transfer in microchannels, using numerical simulation. Numerical simulations are carried out using ANSYS FLUENT with the volume of fluid (VOF) method with a 2D axisymmetric geometry for a 100 µm inner diameter microchannel with constant wall temperature boundary conditions. Effects of slug flow parameters on heat transfer are discussed showing the heat transfer rate follows a similar trend to existing numerical results. Our results show a significant increase in Nusselt number with liquid−liquid two-phase flow compared to single-phase flow, by up to 200%. The study revealed that capillary number, size of the liquid slugs, as well as the liquid film thickness have an important impact on heat transfer.

Production of a high-velocity water slug using an impacting technique

A pulsed water jet consists of a series of discrete water slugs travelling at high velocity. Immediately after striking a target, these slugs apply high-intensity, short-duration transient stress pulses reaching an amplitude known as the water hammer pressure, followed by low-intensity, long-duration stationary stress at a lower stagnation pressure. The magnitude and duration of the water hammer and stagnation pressures are controlled by the size and quality of the water slugs. The use of water jets for rock cutting in mining operations is a centuries-old technology; however, practical methods for producing high-energy water slugs repeatedly have proven difficult. This can be partly due to the fact that the geometrical properties of a jet and so its effectiveness in creating damage is controlled and influenced by the method that is employed to generate the water slugs. This paper investigates the quality of a single water slug produced using an impacting technique where a hammer strikes a piston, resting on top of a water-filled chamber. The coherence and integrity of the jet core was of concern in this study. The impact impulse of the formed water jet was measured in a Kel-F target material using an embedded PVDF (Polyvinylidene fluoride) shock gauge. The recorded stress waveform was then used to determine the unity and endurance of the water slug stream once travelled through air.

Effects of Flow Rate on Formation Behavior of Two-Phase Slug Flow in a Microchannel T-junction

Droplets of slug flow made on micro channel T-junction is used for emulsion, solid beads and so on as the applications. It is necessary to control a size of slugs for these applications and also needed to understand the mechanism of formation process of slug flow. In the present study, relationship between the formation behavior of slug flow at the T-junction and length of droplet of two-phase slug flow has been studied experimentally and numerically. We observed formation behavior of two phase slug flow at the T-junction in detail by using silicon-glass micro channel which width and depth are 100 μm respectively and measured length of the droplet of slug flow by the observation. We also simulated two-phase slug flow in a micro channel to examine the mechanism of formation process of slug flow by using phase-field model lattice Boltzmann method. We confirmed that length of droplet which is formed by same flow rate of continuous and dispersed phase became shorter with increasing of Weber number by experimental results and numerical simulation. In addition, it is also confirmed that interface shape has influence on slug length. By numerical simulation, it is suggested that interface shape can change as a competition between shear-stress and interface-tension forces.

Hydrodynamics of oscillating slug flow inside mini channels: a state of art review

Improvements in the fabrication capability have given rise to mini/micro systems for heat and mass transfer operations. The motion of uniform/oscillating menisci or air-plug/liquid slug inside such systems (i.e., capillaries/channels) is a challenging hydrodynamic problem that is relevant in many interesting practical applications. Understanding the hydrodynamics of such flows will help us in manipulating the performance parameters, which will further improve the efficiency of these systems. The present paper is an attempt to review the experimental, theoretical/analytical, and modelling methodologies applied to capillaries to predict these properties. Out of the different flow patterns inside the capillaries, oscillating slug flow has been reviewed intensely. In general, flow properties are well understood and predicted for fully formed unidirectional Taylor bubbles in circular as well as rectangular channels. However, the effect of impurities on interfacial tension cannot be fully accounted so far. In addition, there is still uncertainty about the size of bubbles and slugs, while there is little literature available for oscillating slug flows/meniscus. In view of the state of art on such flows, further recommendations have been made for future research direction to explore various hydrodynamic characteristics.

Effect of gas pulsation on long slugs in horizontal gas–liquid pipe flow

Long liquid slugs reaching a length of several hundreds of pipe diameter may appear when transporting gas and liquid in horizontal or nearly horizontal pipelines. These long slugs may cause system vibration, separator flooding, and operational problems for the downstream processing facilities. Although mainly short hydrodynamic slugs have been observed in offshore gas and oil production fields over the past years, the appearance of the long slugs is becoming more common as many production fields are now more mature and reach end of field life, giving reduced production rates and reduced operational pressure. In this paper we investigate the effect of gas pulsation at the pipeline inlet on the length and frequency of long slugs for gas–liquid flow in horizontal pipelines. Experiments have been carried out in a 137 m long air–water horizontal flow loop with an internal diameter of 0.052 m. The results show a strong relation between the frequency of gas pulsation and the resulting slug flow. Particularly gas pulsation can decrease the size of the long liquid slugs significantly.

Observations of Transient Flooding in a Proton Exchange Membrane Fuel Cell Using Time-Resolved Neutron Radiography

The generation and transport of water in both liquid and gas phases during device operation are important areas to understand both steady state and transient performance of proton exchange membrane fuel cells. Localized concentrations of liquid water within the cell can cause flooding, which leads to decreases in cell performance, fuel starvation, degradation, or, in extreme cases, collapse of cell output. A variety of experimental and simulation techniques have been used to elucidate flooding events; yet, a comprehensive understanding of what leads to flooding and the specific details of how flooding affects fuel cell performance, especially during transient operation, have not been completely developed. The work reported here couples direct observations of liquid water flooding, primarily in the gas flow channel, with measurements of cell performance, outlet temperature, and outlet dew point during a step change in current density. Liquid water buildup and water slug dynamics were monitored with the temperature and electrical performance of the cell in real time. The size of the water slugs was connected to the cell performance to illustrate how the liquid water influences cell operation and how the conditions of the cathode gas flow control the liquid water content of the cell.

Hydrodynamics of Taylor flow in small channels: A Review

The improved mass transfer characteristics of Taylor flow, make it an attractive flow pattern for carrying out gas—liquid operations in microchannels. Mass transfer characteristics are affected by the hydrodynamic properties of the flow such as thickness of the liquid film that surrounds the bubbles, bubble velocity, bubble and slug lengths, mixing, and flow circulation in the liquid slugs, and pressure drop. Experimental, theoretical, and modelling attempts to predict these properties are reviewed and relevant correlations are given. Most of these refer to capillaries but there are number of studies on square channels. In general, flow properties are well understood and predicted for fully formed Taylor bubbles in a developed flow and in clean systems, particularly in circular channels. However, the presence of impurities and their effect on interfacial tension cannot be fully accounted for. In addition, there is still uncertainty on the size of bubbles and slugs that form under certain operating and inlet conditions, while there is little information for channels with non-circular cross-sections.

Channel processes following land use changes in a degrading steep headwater stream in North Island, New Zealand

In headwater streams in steep land settings, narrow and steep valley floors provide closely coupled relationships between geomorphic components including hillslopes, tributary fans, and channel reaches. These relationships together with small catchment sizes result in episodic changes to the amount of stored sediment in channels. Major sediment inputs follow high magnitude events. Subsequent exponential losses via removal of material can be represented by a relaxation curve. The influence of hillslope and tributary processes on relaxation curves, and that of altered coupling relations between components, were investigated along a 1.3 km reach of a degrading channel in the 4.8 km 2 Weraamaia Catchment, New Zealand. Extensive deforestation in the late 19th and early 20th centuries, followed by invasion of scrubs and reforestation, induced changes to major erosion types from gully complexes to shallow landslides. Changes in the size and pattern of sediment slugs from 1938 to 2002 were analysed from air photographs tied to detailed field measurement. The rate and calibre of sediment flux changed progressively following substantive hillslope input in a storm in 1938. Subsequently, the channel narrowed and incised, decoupling tributary fans from the main stem, thereby scaling down the size of sediment slugs. As a consequence, the dominant influence on the behaviour of sediment slugs and associated relaxation processes, changed from tributary fans to the type and distribution of bedrock outcrops along the reach.

DIETARY INDUCTION OF OPISTHOBRANCH MORPHOLOGY: PLACIDA DENDRITICA (ALDER & HANCOCK, 1843) ON DIFFERENT GREEN ALGAL HOSTS

The relationship of phenotypic morphological and behavioural variation was investigated for the ascoglossan (=sacoglossan) opisthobranch Placida dendritica. Morphological attributes were documented for individual slugs collected from three algal host species along the central Oregon coast, USA, the green algae Codium setchellii, C. fragile, and Bryopsis corticulans. Because individuals on B. corticulans generally were substantially larger than conspecifics on Codium spp., the species appeared polymorphic. Comparably sized slugs from different host genera also differed significantly in morphology: individuals from Codium spp. had more numerous, shorter cerata on wider bodies than did conspecifics from B. corticulans. Principal components analysis indicated that despite considerable overlap of morphological characters, ceratal shape features were diagnostic of algal host. When slugs collected from C. fragile were maintained on C. fragile and B. corticulans for 4 days in the laboratory, the size and shape of slugs fed different diets diverged, and changes were consistent with field data. Thus, phenotype was labile and changed as a result of modified diet. Although a variety of molluscs exhibit dietary induction of morphology, a general awareness of the phenomenon has only recently begun to emerge, despite its important ecological and taxonomic implications.

Inhibition of cAMP-Dependent Protein Kinase in Dictyostelium Prestalk Cells Impairs Slug Migration and Phototaxis

Dictyostelium slugs expressing a dominant inhibitor of the cAMP-dependent protein kinase (PKA) selectively in prestalk cells are insensitive to the environmental signals that normally induce culmination (Harwood et al., 1992a). When such slugs do, eventually, attempt to culminate they are unable to activate stalk-specific gene expression and they become arrested in their development. We show here that they are also about half the size of normal slugs and that they move at about half normal speed, even after their size difference has been taken into account. They are also less accurate in orienting their direction of movement towards the light. Thus, in addition to its role in cellular differentiation, PKA is required for several important aspects of slug behaviour.

Linear Model Studies of the Immiscible CO2 WAG Process for Heavy-Oil Recovery

Summary This paper presents experimental work that quantifies the effect of water-alternating-gas (WAG) variables on the immiscible CO2 flooding process for moderately viscous heavy oils. The results may be used to determine such parameters as total CO2-slug size, WAG ratio, and number of WAG slugs. Analysis of the relative efficiency of each WAG slug is discussed.