Slug Flow Research Articles

Vortex bifurcation and air entrainment mitigation using multi-point intakes

Air entrainment is a concern of paramount importance in the process industries, including nuclear power plant, hydraulic machines, water treatment plants, hydrocyclones, and power generation turbines. The ingression of the air into the equipment results in the reduction of the hydraulic efficiency. In the current paper, a combined experimental and numerical investigation has been performed using the commensurate high-speed photography and volume of fluid-based numerical simulations. The mitigation strategies for the reduction of the air ingression include the reduction of the critical height by increasing the number of intakes and changing the intake configuration. The viability of these solutions and the hydrodynamics behind the interfacial evolution leading to the air entrainment have been studied in detail. The air ingression progresses with the establishment of the flow patterns ranging from bubbly flow, slug flow, to the annular flow. The reduction of the critical height can be accomplished by increasing the number of intakes for the same cross-sectional area leading to the reduction in the Froude number Fr=Vdown/gHlocal<1. Further increasing the number of intakes beyond four leads to the minimal reduction in the critical height in the tank.

An experimental study on air-oil flow patterns in horizontal pipes using two synthetic oils

Investigations on the gas-oil mixture flow of highly viscous oils have become increasingly significant in various industries for the past couple of years. This study examines the characteristics of air-oil flow patterns to understand the effect of the oil phase's thermophysical properties and flow rates on flow patterns. An experimental study was conducted to visualize the air-oil mixture flow in horizontal pipes. The diameters of the horizontal pipes were selected as 20 and 40 mm, respectively. Two different oils having substantially different viscosities were employed as the liquid phase. Based on the visualization results, air-oil mixture flow is categorized into seven flow patterns: stratified smooth, stratified wavy, annular, plug, slug, bubbly, and dispersed bubble flows. Along with representative images, this study demonstrates the essential characteristics of individual flow patterns. Furthermore, it discusses the influence of each phase's flow rate, oil viscosity, and surface tension on flow patterns. The processes of liquid entrainment in stratified wavy flow, bubble tip breakup in plug flow, and liquid slug aeration in slug flow are also illustrated, respectively, with the corresponding sequential representative images and schematic diagrams.

Investigations of the Formation Mechanism and Pressure Pulsation Characteristics of Pipeline Gas-Liquid Slug Flows

The pipeline system is widely used in marine engineering, and the formation mechanism and flow patterns of two-phase slug flows are of great significance for the optimal design of and vibration prevention in a complex pipeline system. Aiming at the above problems, this paper proposes a modeling and solving method for gas-liquid slug flows. First, a VOF-PLIC-based coupling gas-liquid slug flow transport model is conducted. Second, to reduce the fuzzy boundary between the gas-liquid coupling interfaces, an artificial compression term is added to the transport equations, and the formation and evolution mechanism of severe slugging flow in piping systems is investigated. The pressure pulsation and gas content characteristics of the gas-liquid coupling process are explored. Research results found that the slugging phenomenon occurs at the gas-liquid interface, where liquid slugging frequency reaches its peak. The pipeline system has prominent periodic characteristics of the slugging phenomenon, and the period decreases when the gas-phase converted speed rises; pressure fluctuation amplitude increases, and the gas-phase velocity change is the inducing factor for the drastic change of pressure fluctuation. The research results can offer theoretical references for optimal designs of and vibration prevention in marine pipeline systems.

Enhancing Biodiesel Production: A Review of Microchannel Reactor Technologies

The depletion of fossil fuels, along with the environmental damages brought by their usage, calls for the development of a clean, sustainable and renewable source of energy. Biofuel, predominantly liquid biofuel such as biodiesel, is a promising alternative to fossil fuels, due to its compatible direct usage within the context of compression ignition engines. However, the industrial production of biodiesel is far from being energy and time efficient, which contributes to its high production cost. These inefficiencies are attributed to poor heat and mass transfer of the transesterification reaction. The utilisation of microchannel reactors is found to be excellent in escalating heat and mass transfer of the reactants, benefitting from their high surface area-to-volume ratio. The microchannel also intensifies the mixing of reactants via the reactor design, micromixers and the slug flow patterns within the reactor, thus enhancing the contact between reactants. Simulation studies have aided in the identification of mixing regimes within the microchannel reactors, induced by various reactor designs. In addition, microwave irradiation heating is found to enhance biodiesel production by localised superheating delivered directly to the reactants at a molecular level. This enables the reaction to begin much earlier, resulting in rapid biodiesel production. It is postulated that the synergy between microchannel reactors and microwave heating would catapult a pathway towards rapid and energy-efficient biodiesel production by enhancing heat and mass transfer between reactants.

Experimental Study on Flow Characteristics of Oil-Water Two-Phase Flow in Single-Node Cross Fracture

Single-node cross fracture is the basic unit of discrete fracture network. In this paper, the flow characteristics of oil-water two-phase flow in single-node cross fracture model are investigated by experimental method. The model was divided into sixteen groups according to different angles, the oil and water phase flow rates were changed sequentially in each group and the velocity of oil and water phase were controlled to be in the low velocity flow rate range all the time during the experiment. The fluid flow process was recorded during the experiment by using the high-speed camera and the results were analyzed in detail. Under the condition of low velocity flow, oil-water two phases were slug flow in fracture. Effects of fracture crossing angle and oil-water phase velocity on the length of water section and the ratio of water-oil section length were also considered. The peristaltic pump speed is used instead of flow rate for the discussion, and the results show that for the same cross-fracture angle model, when the water pump speed is unchanged, the length of the water section gradually decreases in the form. of a power function with the increase of the speed of the oil pump. When the oil pump speed is unchanged, as the water pump speed increases, the length of water section increases in the form. of a linear function and the length ratio of wateroil section increases in the form. of the power function. Under different speeds of oil pump and water pump, the influence of fracture crossing angle on the length of water section and the length ratio of water-oil section is different.

Effect of slug characteristics on the nonlinear dynamic response of a long flexible fluid-conveying cylinder

Hydrocarbon flows in a marine riser may appear in multiple phases with varying flow patterns, among which, slug flows are known to exhibit complex flow characteristics due to fluctuations in multiphase mass, velocities, and pressure changes. Bulk of the literature concerning internal single and two-phase flow induced vibrations have largely focused on the use of a linearized tension beam model. In this study, the fluid-structure interaction phenomena of a submerged long flexible cylinder conveying two-phase slug flows is investigated taking into account the geometric and hydrodynamic nonlinearities. A semi-empirical theoretical model which consists of nonlinear structural equations of coupled out-of-plane, in-plane and axial structural motions is presented for the analysis and prediction of two-phase slug flow induced vibrations (SIV). The model includes equations involving centrifugal and Coriolis forces to capture mass variations in the slug flow regime. A numerical space-time finite difference approach combined with a frequency domain analysis is used for analyzing the highly nonlinear three-dimensional responses. The model assumes constant geometric properties across the span of the cylinder and utilizes an idealized slug unit concept, wherein slug properties are considered to be fully developed and undisturbed by pipe oscillations. Model validations are performed through comparisons with published experimental internal flow-induced vibration results. Parametric study captures the effects of several key slug characteristics on the vibration responses of a fluid-conveying cylinder. The results demonstrate amplitude-modulation response, mean displacements, three-dimensional cylinder displacements due to geometric nonlinear couplings and the influence of internal flow velocities. Higher dominant modes and oscillation frequencies were observed when the cylinder experiences large amplitude motions at specific slug formations. A new dimensionless parameter is introduced to predict scenarios of high amplitude oscillations for long flexible cylinders carrying two-phase slug flows.

Research on void drift between rod bundle subchannels

Void drift between subchannels in a rod bundle is a crucial phenomenon affecting the calculation accuracy of thermal-hydraulic parameters in SMRs. It holds significant importance in enhancing the precision of safety analysis for SMRs. Existing research on experiment and model of void drift between rod bundle subchannels is relatively rare, and the accuracy of model calculations requires improvement. In this study, experiments on gas-liquid two-phase non-equilibrium flow were conducted to measure the redistribution of two-phase flow induced by void drift in a 1 × 2 rod bundle. The experiment results indicated that in bubby flow regime with void fraction less than 0.3, the void diffusion coefficient showed little variation with changes in void fraction. However, in slug flow and annular flow regimes with void fraction exceeding 0.3, the void diffusion coefficient significantly increased with an increase in void fraction. Furthermore, a new void drift model was developed and validated based on a subchannel code. The overall predicted uncertainty for the outlet void fraction in the rod bundle benchmark was less than 13%.

Experiments and predictions for severe slugging of gas-liquid two-phase flows in a long-distance pipeline-riser system

Experiments concerning the flow regimes observed in gas-liquid two-phase flows in a long-distance pipeline-riser system were carried out and analyzed in this paper. The test loop with 0.075 m inner diameter consisted of a 314 m horizontal pipeline, followed by a 25 m downward-inclined pipe, and finally a 21.5 m riser pipe. An industrial-scale global flow regime map including 5 flow regimes was derived from differential pressure measurements in the riser and was used as a reference for improvements. Electrical capacitance volume tomography installed in the horizontal section was used to reconstruct the phase interface and calculate the average gas void fraction (GVF). An appropriate model for this system was selected by comparing the experimental GVF with different, widely used models. The occurrence of severe slugging flow (SS) can be predicted accurately using an improved model involving additional corrections that consider pipeline length, accurate GVF calculation, and gas density. This study supports the development of a suitable SS prediction model enabling safe design, operation, and control of practical systems.

Pore-scale visualization of hydrate dissociation and mass transfer during depressurization using microfluidic experiments

Natural gas hydrate is a potential low-carbon energy resource. Extracting this resource efficiently remains a challenge, due to the intricacies involved in multiphase reactive transport during hydrate dissociation. This study employs microfluidic experimental techniques to observe pore-scale interactions during depressurization-induced hydrate dissociation, utilizing high-resolution spatial and temporal imagery. Supervised machine learning algorithms segmented these microscopic images, enabling the analysis of phase saturation changes and hydrate dissociation rates. The study identifies two distinct hydrate forms in microfluidic chips: hydrate films and crystals, each exhibiting unique dissociation characteristics. Hydrate films decomposed rapidly in response to pressure reductions below equilibrium with the dissociation rate of O(10−1 %/s) under stationary gas–water conditions. When considering gas–water migration, the gas encapsulated in the hydrate film will be displaced by water, leading to a reduced dissociation rate. The hydrate crystal dissociation is much slower with the dissociation rate of O(10−3 %/s) under stationary gas–water conditions compared to the hydrate film, hindered by mass transfer limitations. Gas-water migration can enhance crystal dissociation. The formation and expansion of gas microbubbles under depressurization accelerated hydrate dissociation within a confined temporal and spatial range, which was known as the self-promotion mechanisms. Significantly, depressurization-induced gas slug flow increases the dissociation rate by more than one order of magnitude, compared to that in stationary gas–water conditions. The depressurization process played a significant role, not only by facilitating thermodynamic feasibility for hydrate dissociation but also by inducing the critical gas slug flow. These results of the present study provide theoretical guidance for improving natural gas hydrate production efficiency.

Research on the Mechanism of Flow-Induced Vibration in the Cooling System of a Double Crystal Monochromator

To investigate the mechanism of flow-induced vibrations in the cooling system of a double crystal monochromator (DCM), this paper utilizes a multi-physics numerical simulation approach, employing ANSYS and FLUENT platforms to simulate the flow state of liquid nitrogen in the cooling system and explore the amplitude response of the DCM. Initially, simulations were conducted to examine the flow state of liquid nitrogen with varying frequency and amplitude pulsations. Subsequently, modal analysis was employed to investigate the amplitude response of the DCM in the pitch direction vibrations under pulsating excitation. Finally, this research investigated the influence of high heat load-induced liquid nitrogen boiling on a DCM. The results indicate that pipe resistance is the fundamental cause of vibration induced by pulsating excitation. Low-frequency excitation enhances the amplification factor of DCM vibration. In contrast, due to the rapid conversion of fluid kinetic energy to pressure potential energy, high-frequency excitation increases the pulsation amplitude in the pipe. Additionally, there is a linear relationship between the amplitude of liquid nitrogen velocity fluctuations and the response amplitude of a DCM. The slug flow formed after liquid nitrogen boiling generates low-frequency pulse signals, and intermittent fluid impacts cause significant vibrations in the DCM. These research findings provide a reference for the analysis and design of ultra-high-stability DCM cooling systems.

Influence of slug flow on sand dune transport in a 3.6° upward inclined pipeline

AbstractThis study presents an experimental investigation of sand conveying from a stationary flatbed through two‐phase liquid–gas flows as a function of the fluids flow rates and pipeline orientation (α = 0°, and α = +3.6°). The characteristics of sand particle transportation by saltation, sand dune formation process and morphologies are visualized using a transparent cylindrical acrylic pipeline and digital photography. It was observed that slug flow regime was the dominant mechanism to lift the sand particles for both horizontal and upward inclinations. It was also found that sand dunes deconstructed more quickly at an upward inclination than horizontal position. For the upward inclination, the conveying phenomenon is characterized by sand bed lifting, suspension, and backward entrainment below the air bubble in the water film. Due to the increased liquid flow rates, higher dune velocity is recorded for the inclined condition. The dune pitch length grew for the horizontal configuration after the transient phase, as a result of the gravitational force effect, while it remained constant for the inclined orientation. In both conditions the slip face angle decreased with time; however, for the inclined configuration, this angle was lower because of the higher upstream liquid flow rate. These results provide important insights into the effect of pipe inclination on bed‐load mode solid transport by two‐phase flow inside a closed circular conduit. The obtained experimental data can be used to validate future numerical investigations.

Experimental investigation of the two-phase flow distribution and pressure drop characteristic within the cathode bending type channel of fuel cell

The accurate prediction of the two-phase flow distribution within the channel is of great significance for enhancing fuel cell water management and improving output efficiency. The current study constructs a novel transparent fuel cell visualization test system and experimentally investigates the characteristics of two-phase flow pattern distribution, pressure drop, and output voltage within the bending type channel. Furthermore, a focused analysis of slug flow which may induce fuel cell flooding is conducted. The results show that the two-phase flow pattern maps of both the bent and straight channels are highly consistent. The force balance analysis is performed to explain the internal mechanism that the bending structure has less influence on the two-phase flow pattern. Additionally, the bending structure has a significant impact on the channel pressure drop magnitude and time-domain characteristics. The two-phase pressure drop can be used as an empirical tool to judge the flow pattern and occurrence position. The fuel cell output voltage jump phenomenon occurs under slug flow pattern, and there is a strong positive correlation between the voltage jump amplitude and current density, with a Pearson correlation coefficient of 0.924. The research results provide theoretical guidance for two-phase flow prediction within the channel and flood prevention of the fuel cell.

Pattern identification of kerosene-water two-phase flow in Y-shaped microchannels using terahertz spectroscopy

In this paper, the terahertz time-domain spectroscopy (THz-TDS) is proposed to investigate the behavior underlying kerosene-water two-phase flow in Y-shaped rectangular microchannels. The microchannel is initially saturated with water and water forms an external continuous phase at low kerosene flow velocities (vk), while a dispersed phase is formed at high vk. The flow process of kerosene-water two-phase flow in microchannel was in situ monitored using a THz-TDS peak intensity, and four flow patterns such as droplet flow, plug flow, slug flow and deformed interface flow are identified over a wide range of vk from 2.08 to 43.13 cm/s at selected water flow velocity (vw) of 13.96 cm/s and 27.71 cm/s. Due to the strong dependence of flow patterns on the vw, only droplet, plug and slug flows are observed at high vw. In addition, with the vw increasing from13.96 cm/s to 27.71 cm/s the ranges of vk for droplet flow and plug flow patterns are expended by ∼100%. A parameter β = k/vw, where k is the slop of the peak intensity of THz-TDS against vk, was proposed to characterize the kerosene-water two-phase flow patterns, and each β only corresponded to one phase for various vk at different vw. These results render THz-TDS a feasible technology for investigating the behavior of oil-water two-phase flow inside microfluidic devices and lay the foundation for experimental and numerical investigations on multiphase flow in microchannels for further study.

Automatic measurement of slug flow processes from in‐line videos

AbstractSlug flow processes draw much attention in chemical and pharmaceutical manufacturing thanks to their ability to eliminate downtime costs and batch‐to‐batch variation. Using in‐line video can monitor the current process status and improve the stability of the slug flow. However, there are no effective image processing methods aiming at such videos from chemical experiments due to limited training sample size and the variety of experimental settings. The paper proposes a training‐free method to automatically detect and measure the slugs from in‐line videos. Multiple image features are fused to identify the slug shapes under various lighting conditions, a six‐point model is fitted to achieve better volume estimation, and a consistency score is estimated to quantify the detection uncertainty. The proposed algorithm achieves similar results to manual labeling for various types of slug flow processes, improving the accuracy of slug column estimation by a large margin compared to the existing automatic methods. Finally, using it to monitor the slug flow process's stability and the change of the slug volume in an injection point are demonstrated. The method not only shows that handcrafted image features have the capability for detection and segmentation from chemical experiment images but also paves the road for a training‐based algorithm for the task.

Flow boiling of liquid nitrogen in a horizontal macro-tube at low pressure: Part Ⅱ – Heat transfer characteristics

Nitrogen is the most important cryogen in superconductivity. This study investigates the heat transfer characteristics of flow boiling in a horizontal macro-tube with an inner diameter of 10 mm, particularly focusing on flow boiling at negative gauge pressure. The experiments cover the following ranges: inlet pressure from −31.0 to 2.5 kPa, mass flux from 27.0 to 71.3 kg/(m2·s), and heat flux from 0 to 28.68 kW/m2. The effects of operating parameters on the heat transfer coefficient (HTC) are examined, and the HTC data are compared with the predicted value from four correlations. At the bottom wall, the HTC distribution exhibits a “two-peak” shape. The primary peak is dominated by nucleate boiling in bubbly, plug, and slug flow with low vapor quality, while the secondary peak is dominated by convective evaporation with higher vapor quality. These two regions are roughly divided by the vapor quality of 0.3 to 0.4. In stratified flow, the HTC at the top wall is generally lower than that at the bottom. The HTC increases as the pressure decreases, particularly in the convective evaporation region. This is attributed to the high liquid–vapor density ratio, extensive thermal conductivity, and substantial surface tension at lower pressure. The effect of heat flux or mass flux on the HTC at the bottom wall differs between the nucleate boiling and convective evaporation regions. Increasing the heat flux improves the HTC where nucleate boiling dominates, while increasing the mass flux significantly improves convective evaporation, especially at higher heat flux. All selected correlations overestimate the HTC at the top wall. The Shah correlation demonstrates the best prediction accuracy for the HTC at the bottom wall, both at near atmospheric pressure (Mean Relative Error, MRE = 21 %) and at negative pressure (MRE = 19 %).

Gas-liquid swirling flow behaviors and liquid-carrying mechanism inside a horizontal pipe

The behaviors of gas-liquid swirling flow under various swirler turns, neglected in previous studies, are essential for swirling flow applications. The role of swirler turns (1, 3, 5) in swirling flow characteristics was systematically investigated in a horizontal pipeline with an inner diameter of 32 mm. The temporal stability and circumferential symmetry of swirling liquid film become better with increasing swirler turns. The periodicity of the swirling liquid film interfacial wave induced by slug flow increases as the swirler turns decrease, while the main wave frequency decreases. Interestingly, for the original slug flow, the downstream void fraction probability density function curves are bimodal with 3 and 5 turns, while it is single-peaked at the swirler with 1 turn. The effectiveness of swirler with 1 turn, 3 turns, and 5 turns in promoting carrying liquid was 100 %, 82.08 %, and 89.22 %, respectively. The mechanism of liquid-carrying was explained in terms of perturbation waves and momentum transfer in the gas-liquid interface. Based on the homogeneous flow model and separated flow model, two predictive models for swirler pressure loss were proposed with maximum errors of 14.67 % and 17.86 %, respectively.

Liquid-liquid flow pattern and mass transfer in a rotating millimeter channel reactor

Abstract Currently, microchannels are widely used in liquid-liquid heterogeneous mass transfer systems due to its excellent mass transfer performance. However, because of the passive mixing principle of traditional microchannels, the improvement of mass transfer performance has a bottleneck. This work proposes a novel rotating millimeter channel reactor (RMCR), capable of achieving liquid-liquid heterogeneous mass transfer enhance by centrifugal force. Three typical flow patterns of slug flow, parallel-droplet flow, and parallel flow in the RMCR were observed by high-speed photography technology. The volumetric mass transfer coefficient (K O a) of the RMCR increased with the increase of the total volumetric flow rate and rotational speed (N) increased. Compared with N = 0 r/min, the K O a of the RMCR increases by 61.5 % at 200 r/min, ranging from 0.013 to 0.021 s−1. The RMCR proposed in this work is expected to be applied to the liquid-liquid heterogeneous mass transfer system with high processing capacity and easy plugging.

Flow pattern identification for internally threaded tubes based on pressure drop fluctuations

Internally threaded tubes, renowned for their exceptional heat transfer capabilities, are extensively used in heat exchangers. However, directly identifying two-phase flow patterns within internally threaded tubes poses a formidable challenge. To address this, this study conducted two-phase flow experiments with R600a within an internally threaded tube, encompassing a range of mass fluxes spanning from 40 to 130 kg m−2 s−1. The experimental observations unveiled three distinct flow patterns: plug, slug, and annular flow. To validate the accuracy of these observed flow patterns, they were compared with an empirical flow pattern map. Meanwhile, pressure drop signals corresponding to various flow patterns were recorded under different experimental conditions using a high-frequency data acquisition card. The experimental findings indicated that the pressure drop signals fluctuate because of the difference in gas-liquid phase compressibility, while the fluctuation period and flow pattern change simultaneously with the flow rate. Specifically, plug flow exhibited the longest pressure drop fluctuation periods and the sharpest probability density function (PDF) curve. In contrast, annular flow exhibited the shortest fluctuation periods and the least sharp PDF curve. Based on the distinctive characteristics of the PDF curve, a novel parameter, K, was introduced primarily derived from the sharpness of the curve. This led to the formulation of innovative transition criteria, which achieved a prediction accuracy of 86.99% across all datasets in this study. This work furnishes a straightforward methodology for distinguishing flow patterns. The advantages of the proposed criteria include the requirement of measuring only pressure drop fluctuations to identify flow patterns without direct observation.

Mass transfer phenomena of partially miscible liquids under liquid-liquid slug flow in a circular microchannel

Microreactors have been demonstrated to become an effective tool for increasing mass and heat transfer in heterogeneous chemical processes that are unachievable in batch or continuous stirred tank reactors. The key to superior performance is the availability of a large specific surface area (A/V) in the microsystem for mass and heat transfer between phases. Among the flow patterns generated in the two-phase liquid-liquid flow, slug is an ideal flow with a high stability characteristic, regular velocity, and uniform dimension throughout a microchannel system. The flow behavior gives slugs great potential to increase mass and heat transfer between phases, so optimizing the utilization of microreactors in many chemical process applications needs an in-depth understanding of the liquid-liquid hydrodynamic. Therefore, the current work aims to determine the mass transfer coefficient under liquid-liquid slug flow and the influence of slug dimensions, channel material, volumetric flow rate, and the flow rate ratio of the organic-aqueous phase on the mass transfer coefficient. The experiments were performed in 1 mm (ID) circular PTFE and silicone tubes, with a 30–40 ml/hour flow rate, and sodium hydroxide concentrations of 0.1, 0.13, and 0.15 M were used. The results exhibit that the most prominent overall mass transfer coefficient was 0.121/s, attained at 0.15 M sodium hydroxide concentration, with 40 ml/hour total discharge within the silicone channel. Microchannels generated smaller slug sizes at a higher aqueous phase flow rate than the organic phase. The smaller size provided a large specific surface area (A/V) for enhancing the mass transfer coefficient.

Formation of viscoelastic droplets in symmetrical parallelized microchannels

The dynamics of viscoelastic droplets and the distribution of viscoelastic fluids in symmetrical parallelized microchannels are studied by using a high-speed camera. Typical parallel and slug flows are observed. The droplet formation process is divided into four stages, and the pinch-off point of the droplet moves downstream under high dispersed phase flow rates. When the flow rate ratio of the continuous phase to the dispersed phase is very low, the expanding stage disappears and the remaining three stages gradually lose clear boundaries, and the slug flow transforms into parallel flow. The influence of fluid elasticity is manifested in the appearance of unbreakable filaments when the droplet is pinched off, and the duration and maximum length of filaments are positively correlated with the fluid elasticity. Increasing the velocity of the dispersed phase appropriately, the homogeneity of droplet generation can be effectively enhanced. Finally, a prediction model of droplet size is established.