Slug Formation Research Articles

Rapid Coalescence of Bubbles Driven by Buoyancy Force: Implication for Slug Formation in Basaltic Eruptions

AbstractIn basaltic eruptions, bubbles move freely and collide within a volcanic conduit, leading to frequent bubble coalescence. Understanding the dynamics of buoyancy‐driven coalescence of bubbles is crucial for predicting the explosivity of basaltic eruptions. We examine the evolution of the bubble volume distribution while considering buoyancy‐driven coalescence and expansion due to decompression. We find that, at lower decompression rates, the bubble volume distribution rapidly evolves into a power‐law distribution with an exponent of approximately as . This suggests that, in basaltic magma, the repeated coalescence of bubbles rapidly forms large bubbles within 45 min to 3 days. We then examine the occurrence of eruption styles, specifically Strombolian or Hawaiian, under the assumption that the bursts of slugs, produced from bubble coalescence within the conduit, trigger Strombolian eruptions. Consequently, we identify a critical condition for the transition between eruption styles in terms of the ascent velocity of magma. This critical ascent velocity is consistent with the observed transitions between Strombolian and Hawaiian eruptions at Izu‐Oshima and Kilauea.

Influence of nanoparticle concentration on the flow regimes of crude oil – Nanosuspension in a microchannel

This paper presents the results of an investigation into the effect of silica nanoparticles on two-phase flow regimes in a Y-shaped microchannel. The following fluids were subjected to investigation: oil, water, and a water-based suspension containing SiO2 nanoparticles at a weight concentration of 1 ≤ φ ≤ 10 %. A systematic investigation of the flow regimes revealed the following: slug, parallel, and droplet regimes. The ranges of existence of the obtained flow regimes were determined and flow regime maps were constructed as a function of fluid flow rates and dimensionless numbers. The results of the study demonstrated that the slug length and frequency of slug formation are influenced by varying silica concentrations. Furthermore, the correlation between slug length and nanoparticle concentration remains consistent as the suspension flow rate increases. The dependence of the ratio of oil layer width to channel width for suspensions with varying nanoparticle concentrations was quantified. A mathematical simulation of two-phase flow of immiscible fluids was conducted. The results of the numerical simulation demonstrated the existence of the flow regimes that had been previously identified through experimentation. The aforementioned methodology may therefore be employed for the investigation of the two-phase immiscible flow of oil and nanosuspension.

Experimental and Numerical Study of Air Flow Reversal Induced by Fire in an Inclined Mine Working

Effective fire prevention in mine workings and tunnels requires a thorough theoretical analysis of the heat and mass transfer processes within these structures. This involves using established models to calculate non-isothermal air flow dynamics in long tunnels and mine workings. While the ventilation of tunnels has been extensively studied, significant challenges persist regarding mine ventilation systems, particularly due to their complex and branched topology. This study aimed to address these challenges and gaps in mine ventilation. We designed a laboratory bench to simulate an inclined mine working with a heat source (fire) and validated a mathematical model of heat and mass transfer in such settings. Using experimental measurements, we verified the model’s accuracy. It is important to note that our experimental and theoretical analyses focused solely on the thermal effects of a fire, without considering the release of harmful impurities. Using the validated model, we conducted multiparameter simulations to identify the conditions leading to the formation of a thermal slug in an inclined mine working and the subsequent reversal of air flow. The simulation data enabled us to determine the dependency of the critical heat release rate on the aerodynamic parameters of the mine working. Additionally, we evaluated the changes in average air density within a mine working at the critical heat release rate. These findings are crucial for the further development of a network-based method to analyze air flow stability in mine ventilation networks during fires.

Novel concept for 3D-printed, baffled micro-fluidized beds for the regulation of bubble behavior and the suppression of solid back-mixing: Experiments and simulations

In Micro-Fluidized Beds (MFBs), large bubbles (or slugs) and strong solid back-mixing might hinder good mixing, heat and mass transfer, while reducing conversion rates, and limiting selectivity in certain gas–solid reactions. In this study, a series of novel baffled MFBs were produced using 3D-printing. The influence of the number and position of baffles on the regulation of bubble performance and solid back-mixing in MFBs was investigated through fluidization experiments and Two-Fluid Model (TFM) simulations. The results show that the introduction of baffles effectively suppress slug formation and expand the bubbling regime. Baffles placed at mid-height disrupt fully developed slugs more effectively than those at the bottom. Increasing the number of baffles enhances bubble-breaking efficiency. Numerical simulations suggest that baffles positioned in the lower portion of the bed are more effective for the inhibition of global solid back-mixing. This study shed new light on the design and optimization of baffled MFBs.

Theoretical analysis of the vapor-liquid slug-train dynamic formation in an ultra-maximum hydraulic diameter oscillating heat pipe in horizontal orientation

The formation of liquid slug is critical to the pulsating flow of vapor-liquid slug-train (VLST) in an ultra-maximum hydraulic diameter oscillating heat pipe (UHDOHP) in horizontal orientation. The high-speed vapor causes the fluctuations in the vapor-liquid interface and forms local low pressure at the wave peak, which promotes the wave peak growth to create a liquid slug. Therefore, how to obtain a sufficient vapor velocity is important. The results show that the required heat to obtain sufficient vapor velocity cannot exceed the maximum load-bearing heat that maintains the effective evaporation of liquid and the unobstructed flow of vapor. And the effects of liquid reflux limit (RL), sonic limit (SL) and critical heat flux (CHF) appear sequentially as the diameter increases. In the effective range of RL, the optimal filling ratio decreases with diameter. And reducing the effective length can weaken the RL limitation and increase the effective range of SL. Moreover, increasing the diameter and filling ratio can weaken the limiting effect of CHF. This study reveals the start-up of UHDOHP in a horizontal orientation and provides a guideline for promoting the VLST formation.

An insight into the severe slugging characteristics in a catenary flexible riser

This paper reports the experimental results of the severe slugging (SS) flow characteristics in a downward inclined pipeline-catenary flexible riser system. The non-intrusive optical measurement with high-speed cameras was employed to capture the evolution of liquid slugs and the gas–liquid interface. Five SS patterns are identified in the considered flow velocity range. There are two kinds of severe slugging I (SSI-1 and SSI-2) with the essential difference in the occurrence of the fast blockage stage before the slug formation (SF) stage. The severe slugging II (SSII) is characterized with the longest liquid slug less than a riser length and the absence of slug production stage. The liquid slug is further shortened in severe slugging III (SSIII), exhibiting local liquid fallback and accumulation of multiple slugs in the riser. The severe slugging transition (SST) occurs due to the switching between the SSI-1 and SSII. The flow regime partition is plotted in the vsl (liquid superficial velocity)–vsg (gas superficial velocity) diagram, presenting alteration as the inclination angle of upstream pipeline varies. Generally, the SSI is easier to form at a higher inclination angle. The intermittent occurrence of hydrodynamic slug in the upstream pipeline in the SF stage contributes to the appearance of SSI-2 to SST. The SS cycle has approximately a negative exponent relation with vsg, while the riser base pressure changes exponentially with the increase in vsl.

Experimental study of flow boiling characteristics of open microchannels with elliptical cavities and elliptical ribs

Open microchannel heat sinks (O-MCHS) stand out among many thermal management technologies due to their extra open gaps that reduce pressure drop and mitigate flow instability. Recently, there has been a growing trend of incorporating microstructures onto microchannels in order to enhance the hydrothermal performance of heat sinks. However, the heat transfer characteristics and bubble behavior of O-MCHS with microstructures are still rarely studied. In this study, open copper-based microchannel heat sinks with elliptical cavities and elliptical central ribs (O-EC-ECR) and elliptical cavities and elliptical side ribs (O-EC-ESR) were designed and manufactured; the flow patterns, heat transfer performance and pressure drop were systematically studied and compared with those of rectangular open microchannel heat sink (R O-MCHS). The working fluid was degassed DI water and the experiments were performed with the inlet subcooling of 75 ℃ at efficient heat fluxes of 668.00–2416.16 kW/m2 under various mass fluxes ranging from 421 to 801 kg/m2·s. The experimental results demonstrated that the O-EC-ECR exhibited the best two-phase heat transfer performance among three structures at all mass fluxes. The average two-phase heat transfer coefficient of O-EC-ECR was increased by 12.2 % compared to R O-MCHS at the mass flux of 801 kg/m2·s. The average wall temperature of O-EC-ECR was found to be 10.1 ℃ lower than that of R-O-MCHS and 6.7 ℃ lower than that of O-EC-ESR at the mass flux of 801 kg/m2·s. This should be attributed to the dramatically increased convective heat transfer area as well as the separating and breaking effect of the central ribs on the bubbles, which was manifested by the increase in heat transfer coefficient and critical heat flux (CHF). In addition, the central ribs improved the flow boiling stability by effectively suppressing the generation of confined bubbles and the formation of slugs. The arrangement of the side ribs and cavities made bubbles easier to condense, which facilitated the mixing of hot and cold fluid. At low to medium mass fluxes, the O-EC-ESR only showed superior heat transfer performance during the bubbly flow dominated nucleate boiling regime. Whereas high mass flux can improve the boiling heat transfer performance; the average heat transfer coefficient of O-EC-ESR was increased by 6.1 % compared to R O-MCHS. The average pressure drop of O-EC-ESR was decreased by 21.8 % compared to R O-MCHS and 25.6 % compared to O-EC-ECR at the mass flux of 801 kg/m2·s.

Experimental study of boiling heat transfer of inclined down-ward facing heated curved wall under low flow and pressure conditions

This study aims to investigate the behavior of bubbles and the mechanisms of heat transfer in a downward-facing curved channel under boiling flow conditions. A curved wall, representing a portion of the external surface geometry of a nuclear light water reactor vessel, is used to examine the influence of channel curvature on boiling phenomena. Experiments are conducted across a range of heat fluxes to understand the behavior of bubbles and slugs at different thermal loads on the curved surface. High-speed videography is employed to visualize bubble dynamics and their impact on heat transfer. The study quantifies parameters such as slug bubble velocity, length, width, and frequency, and examines their dependence on heat flux. The findings reveal that under a heat flux range of 30 to 100 kW/m2, the bubble motion and dynamics exhibit varying patterns, with the generation of smaller and faster bubbles. At approximately 150–200 kW/m2, a transition phase occurs, alternating between isolated nucleates and the formation of large slugs. Beyond 200 kW/m2, in the slug boiling regime, bubbles undergo substantial transformations, characterized by the emergence of larger vapor structures and more intricate boiling patterns. The resulting heat transfer rate is, thus, affected by the dynamics and behavior of these vapor structures, as well as the streamwise evolving thermal boundary layer and the surface curvature of the heater.

Experimental investigation of the flooding phenomenon in a pulsating heat pipe unit cell

This study presents an experimental investigation on characterization and quantification of flooding phenomenon in a pulsating heat pipe (PHP) unit cell at different orientations, evaporator and condenser temperatures. A transparent capillary tube having a diameter of 2.5mm is used as a PHP unit cell, and n-pentane is used as the working fluid. The meniscus oscillations along with flooding events are recorded using a high-speed camera. Three different flooding mechanisms are recognized, viz., wave transport with film drainage delay, wave transport with slug formation, and droplet entrainment. The flooding frequency is determined for each flooding mechanism at different combinations of the operating boundary conditions. The adverse impact of flooding on the performance of PHP is realized apparently by considering its influence on the amplitude and frequency of meniscus oscillations. The approximate liquid deficit in evaporator caused by flooding is also estimated for one of the mechanisms, viz. wave transport with slug formation. Thus, role of flooding in accession of the operational limit of a PHP is signified. Further, the hydrodynamics of flooding is studied in detail. The superficial velocity of vapor at the onset of flooding and film-vapor interfacial wavelength are determined for a few cases and compared to the respective predictions based on some of the most widely accepted correlations. It is found that none of the considered correlations could predict the flooding velocity of vapor for the present experiments. The study signifies the need to study flooding more intensively for PHPs in near future.

Experimental study and prediction on the thermal management performance of SDS aqueous solution based microchannel flow boiling system

This study reports the flow boiling of SDS aqueous solution in straight and tree-shaped microchannel for thermal management. Under different volume flow rate and heat flux, the effect of SDS on the flow pattern, heat transfer and pressure drop in different channel structures is clarified. The results reflect that unique flow patterns like bubble stacking, bubble cluster and activated bubbles are observed in the flow boiling of SDS aqueous solution. 400 mg/kg and 200 mg/kg SDS aqueous solution has the best overall heat transfer enhancement performance in straight (25% improvement) and tree-shaped (22% improvement) microchannels respectively. The heat transfer enhancing mechanism of SDS can be summarized as suppressing the formation of large volume vapor slug and promote the rewetting of the bottom channel surface. Based on this mechanism, SDS can also advance the ONB and extend the stable nucleate boiling state, 100 mg/kg, and 400 mg/kg SDS aqueous solution show longest stable nucleate boiling state in straight and tree-shaped microchannel respectively. Increasing solute concentration of SDS leads to larger pressure drop and stronger pressure drop fluctuation in both microchannels, but increasing volume flow rate will suppress this extra resistance and instability caused by SDS. Targeted heat transfer and pressure drop correlations are also presented.

Liquid–Liquid Two-Phase Flow and Size Prediction of Slug Droplets in Microchannels

The liquid–liquid two-phase flow and size prediction of slug droplets in flow-focusing microchannels with different downstream orifice sizes were investigated experimentally. Aqueous solution of 50%-glycerol and mineral oil with 4 wt.% surfactant sorbitanlauric acid ester (Span 20) were used as the dispersed and continuous phases, respectively. Three characteristic flow patterns were identified: slug flow, dripping flow, and jetting flow. The slug flow region decreased but the jetting flow region increased with the decrease in the size of the channel orifice. Afterwards, the universal flow pattern maps of the liquid–liquid two-phase in three microchannels were obtained based on dimensionless analysis. Furthermore, two slug droplet formation regions were found: when φ−1Cac < 0.01, the droplet formation was mainly driven by the squeezing force Fp, while when φ−1Cac > 0.01, both the squeezing force Fp and shear force Fτ contributed to droplet formation. Additionally, the prediction correlations of the dimensionless sizes of the slug droplets in both regions were established based on the flow rate ratio of the two-phase, the dimensionless orifice size, and the Capillary number of the continuous phase. The predicted results are in good agreement with the experimental values.

Experimental study of lithium solvent extraction in a T-shaped microchannel

ABSTRACT The extraction of lithium from the waste of used batteries is going to be necessary due to the limited resources of lithium. One of the most common methods to extract is liquid–liquid extraction with an organic solvent. In this study, Bis(2-ethylhexyl) phosphoric acid (D2EHPA) solvent is used for liquid–liquid extraction of lithium-ion in a T-shaped microchannel. Lithium chloride solution has been used as a dispersed phase and, D2EHPA solvent with sunflower oil has been unitized as a continuous phase. The effect of some parameters, such as the flow rate of the continuous phase and the amount of the consumed solvent on the time of slug formation process, slug length and, extraction percentage has been investigated. Results showed that the time of the slug formation decreases by increasing the flow rate of the continuous phase. Also, it found that with increasing the pH of the aqueous phase by 36% resulted in a 15% reduction in the length of the slug. With an increase of 67% of the consumed solvent, the slug length increases by about 16%. By increasing the amount of the consumed solvent, the slug length increases. In continuation, with increasing the slug length, extraction percentage increases.

Study on heterogeneous gas-solid structure and interparticle collision behavior in a vibrated fluidized bed of Geldart D particles using CFD-DEM simulations

The spatiotemporal characteristics of the heterogeneous gas-solid structure and interparticle collisions in fluidized beds of Geldart D particles with and without vibration were comparatively studied by the coupled CFD-DEM simulations. The effects of vibration on the spatiotemporal distributions of particle configuration and particle dynamics were revealed utilizing data visualization and frequency domain analysis. The results show that the input vibration facilitates the formation of square-nosed slugs and improves the uniformity of gas-solid contacts. Moreover, vibration causes fluctuations in static air pressure on the entire bed simultaneously. It is revealed that the introduction of vibration can significantly increase the intensity and uniformity of interparticle collisions, the number of particles involved in collisions, and the total kinetic energy of the bed. The results also show that there exists a positive correlation between average collision number and solid volume fraction while an inverse correlation between granular temperature and solid volume fraction.

Pneumatic Conveying

Pneumatic conveying is the transportation of bulk solids in enclosed pipelines via a carrier gas, typically air. The local flow pattern in a pipeline is a function of the conditions, and slug flow can form under certain conditions. Slug flow is a naturally occurring, wave-like flow where the bulk material travels along the pipeline in distinct `slugs'. Establishing the environment for the formation of slugs within the conveying system is essential to maximise the overall system efficiency and minimise damage to the bulk material. MISG2021 considered a wide range of mathematical approaches to slug formation and travel. These two key problem areas have the most significant potential to impact the system design and efficiency. Critical interconnected facets of pneumatic conveying systems were investigated and an overview for future work was developed. Many of the avenues uncovered during the MISG2021 require more time for in-depth analysis. This analysis and framework will aid in optimising conveying system design and provide insight to construct more efficient pneumatic conveying systems.

Investigation of Mass-Transfer Performance for Biodiesel Reaction in Microchannel Reactor using Volume-of-Fluid with Species-Transport Model

A microchannel reactor improves the overall mass and heat transfer as compared with a conventional reactor. This is attributed to the creation of a high area-to-volume ratio and enhanced mixing due to the presence of the vortices inside the slug. In this paper, the mass-transfer performance was studied using a cross-junction microchannel. Subsequently, the computational fluid dynamic (CFD) method was used to observe the oil concentration contour inside a slug using volume-of-fluid (VOF) with the species-transport model. Based on the simulation results, the oil concentration was accumulated in both the slug’s rear and front regions. Hence, the creation of four vortices resulted in the creation of dead zones at the low-oil-concentration region. Furthermore, it has been observed that an optimum flow rate in a microchannel reactor is required to achieve a high mass transfer. A higher oil concentration was measured during the slug formation at a low flow regime due to the long residence time. In contrast, a high mass transfer has been reported during the slug-moving stage due to the higher vortices velocity, resulting in enhanced mixing and mass transfer. Hence, slug forming and the moving stage substantially influenced mass transfer at low and high flow rates, respectively.

Study of the Hydraulic Characteristics of Slug Flow in the Microfluidic Y-shaped Junctions of Three Different Cross Angles

The slug flow in microchannels has been applied for microdroplet/bubble production in recent years. The comparison investigation of hydrodynamic properties of slug flow in three different Y-shaped microchannels was conducted by numerical methods. The numerical simulation results show that the VOF model can describe the velocity and pressure field in the Y-shaped microchannel. It was found that the pressure in the microchannel from upstream to downstream showed an overall decreasing trend, and the liquid phase pressure between the two gas slugs was less than the internal pressure of the adjacent gas slug. The midline pressure of the bubble generation moment is less than the rest of the state. Among the three Y-shaped junctions, the absolute value of the pressure difference at the moment of slug bubble formation is the smallest when the angle of the Y-junction is 90°. Compared to 120° and 90° Y-junctions, the pressure and velocity field of the 60° Y-junction is the most unstable when the slug flow generates.

ANALISA KECEPATAN SLUG ALIRAN DUA FASE DI DOWNSTREAM T-JUNCTION MINICHANNEL HORIZONTAL DENGAN RADIUS BELOKAN (r/dh) 0.7

Uneven distribution of the phases between in the main channel when two-phase flow passes through the T-junction it can cause the formation of slug flow which affects performance in the downstream area. The slug is formed due to the acceleration of the gas phase moving towards an average or stable velocity. The bend radius of the T-junction affects the formation of slug because the radius can increase the velocity of the gas phase. Research on the slug velocity was carried out in the downstream region of the horizontal mini channel T-junction. The working fluid used is air as the gas phase and water. The flow pattern and velocity slug analysis were carried out by visualizing the flow at a distance of ±30 mm from the T-junction, that works as a mixer of the working fluid. High-speed camera is used to record video and processed in the form of image processing with the MATLAB program. Two-phase flow slug velocity analysis conducted in the downstream area of ​​the horizontal minichannel T-junction, it can be concluded that the slug velocity tends to increase linearly with increasing superficial velocity of air and water. Comparison of slug velocity experimentally with the results of calculations using the equations of Fukano and Kariyasaki (1993) and Sudarja et al (2018) shows conformity with less than 10% of error margin, while Nicklin et al (1962) is ±30%.

Experimental investigation and semi-empirical correlations for a pulsating heat pipe filling with hybrid nanofluids applied for low-grade energy recovery

Pulsating heat pipe (PHP) is a high-efficiency thermal transportation technology being utilized in the low-grade energy fields. In current work, the novel-hybrid working fluid - graphene oxide nanoplatelets (GO nanofluid) and surfactant solution were jointly utilized to reduce flow resistance and enhance overall thermal performance of PHPs. Sodium Dodecyl Sulfate (SDS) was employed as a surfactant to prepare the water-based surfactant solutions of 500 PPM. The charge ratio was maintained at 60%, vertical bottom heating position of PHP was applied with the power range of 10–100W. Our experimental results illustrated that the oscillation frequency of temperature fluctuation was higher than that with charged GO-SDS hybrid nanofluid in comparison with DI water and pure GO nanofluid. Additionally, maximum average improvement rate of thermal performance 8.55% was successfully achieved in 0.03 wt% GO-SDS hybrid nanofluid than that with pure GO-nanofluid. Furthermore, multiphase fluid flow inside PHP has been comprehensively analyzed, and its flow regimes such as formation of foam and slug, coalescence and breakup of bubbles, and annular flow were observed. On site visualization results illustrated that a fast nucleation of vapor bubbles could be promoted in the evaporator section due to lower surface tension. Specially, a series of bubbles were expected to form cluster for GO-SDS hybrid nanofluid, which further enhanced the oscillation circulation inside PHP. This experimental research and its results could contribute to the optimization and application of PHP.

Slugging in horizontal channel flow

Abstract The stratified flow of gas–liquid mixtures is important in many industries, in particular oil and gas, where the formation of liquid slugs is usually undesirable. In this paper, we analyse the hydraulic model for turbulent, two-layer, gas–liquid flow in a horizontal channel developed by Needham et al. (2008, The development of slugging in two-layer hydraulic flows, IMA J. Appl. Math., 73, 274–322) who considered only the thin layer limit, for liquid layers of arbitrary thickness. This allows us to consider the effect of the prescribed inlet velocity ratio and the interaction of the gas layer with the upper wall of the channel on the formation of slug formation. We show that there is a critical Froude number above which small-amplitude disturbances to the uniform flow are unstable and grow to form non-linear roll waves. We investigate the existence of periodic travelling wave solutions and find that these are strongly influenced by the existence of two equilibrium solutions that are not present when the liquid layer is thin. The system of ordinary differential equations that describe travelling wave solutions has a complicated phase portrait which we study in some typical cases. We find that periodic solutions that have the channel close to filled with liquid can exist, and we identify them as slugs. We also study an initial value problem driven by noise at the inlet of the channel and find that structures locally similar to these periodic travelling waves are generated. We provide numerical evidence for the formation of slugs in cases where the initial liquid to homogeneous velocity ratio is greater than 0.5.

Hydrodynamics and mass transfer studies of liquid-liquid two-phase flow in parallel microchannels

Parallel or multiple microchannels (PMC) are being tried for high throughput operations. Extremely fewer reports on liquid/liquid two-phase flow in PMC exists in the open literature compared to the gas/liquid flow especially utilizing the new designs. In this work, a bifurcation-based PMC was employed to study hydrodynamics and the mass transfer aspect of solvent extraction of propionic acid. Slug formation, slug volume, specific surface area, extraction efficiency and mass transfer coefficients were evaluated. PMC exhibited better results than the single microchannels (SMC), making them superior in performance than the SMC. The standard deviation for uniform flow distribution is within 2%. A maximum specific surface area was obtained in PMC than the single channels, which is 10513 m2/m3. Similarly, the maximum extraction efficiency of 98% is achieved in PMC at flow rates 16 times higher than the SMCs. The overall pressure drop in PMC is significantly less than in the SMCs. Thus PMC is a suitable choice for process intensification in process industries.