Two-phase Slug Flow Research Articles

Flow and heat transfer for a two-phase slug flow in horizontal pipes: A mechanistic model

In view of the practical importance of gas–liquid two-phase flow in many applications, such as chemical engineering, petroleum engineering, nuclear engineering, etc., a reliable model of flow and heat transfer for two-phase flow is of practical importance in the two-phase flow analysis. Among various two-phase flow regimes, slug flow is most complicated due to the intrinsic randomness and intermittency. This paper aims at developing a novel mechanistic model of flow and heat transfer for two-phase slug flow in horizontal pipes. First, a hydrodynamic model of two-phase slug flow is developed using the concept of slug unit cell. Then, a heat transfer model is deduced based on the hydrodynamic model. The overall heat transfer coefficient is integrated by the local heat transfer coefficients of liquid slug, liquid film, and elongated bubble. The newly developed mechanistic model is well validated by the experimental results. Finally, the dependence of the heat transfer performance on the overall flow parameters, such as superficial liquid velocity and superficial gas velocity, and the local flow parameters, such as slug frequency, pressure drop, void fraction, and ratio of slug length to unit cell length, is comprehensively investigated. The heat transfer enhancement of two-phase slug flow compared with single-phase flow is mainly attributed to the turbulence increase in liquid by the injection of air and the decrease in thermal boundary layer by the frequent alternation between the liquid slug and the elongated bubble.

Entropy Generation for Negative Frictional Pressure Drop in Vertical Slug and Churn Flows.

It is widely accepted that the frictional pressure drop is impossible to be negative for pipe flow. However, the negative frictional pressure drops were observed for some cases of two-phase slug and churn flows in pipes, challenging the general sense of thermodynamic irreversibility. In order to solve this puzzling problem, theoretical investigations were performed for the entropy generation in slug and churn flows. It is found that the frictional pressure drop along with a buoyancy-like term contributes to the entropy generation due to mechanical energy loss for steady, incompressible slug and churn flows in vertical and inclined pipes. Experiments were conducted in a vertical pipe with diameter as 0.04 m for slug and churn flows. Most of the experimental data obtained for frictional pressure drop are negative at high gas–liquid ratios from 100 to 10,000. Entropy generation rates were calculated from experimental data. The results show that the buoyancy-like term is positive and responsible for a major part of entropy generation rate while the frictional pressure drop is responsible for a little part of entropy generation rate, because of which the overall entropy generation due to mechanical energy loss is still positive even if the frictional pressure drop is negative in vertical slug and churn flows. It is clear that the negative frictional pressure drops observed in slug and churn flows are not against the thermodynamics irreversibility.

Research on the Dynamic Responses of Simply Supported Horizontal Pipes Conveying Gas-Liquid Two-Phase Slug Flow

The dynamic responses of simply supported horizontal pipes conveying gas-liquid two-phase slug flow are explored. The intermittent characteristics of slug flow parameters are mainly considered to analyze the dynamic model of the piping system. The results show that the variations of the midpoint transverse displacement could vary from periodic-like motion to a kind of motion whose amplitude increases as time goes on if increasing the superficial gas velocity. Meanwhile, the dynamic responses have certain relations with the vibration acceleration. By analyzing the parameters in the power spectrum densities of vibration acceleration such as the number of predominant frequencies and the amplitude of each peak frequency, the dynamic behaviors of the piping system like periodicity could be calculated expediently.

MEASUREMENT OF VOID FRACTION DISTRIBUTION WITH SMALL WIRE MESH SENSOR IN WATER-AIR TWO-PHASE BUBBLE AND SLUG FLOWS IN VERTICAL SMALL DIAMETER PIPE

MEASUREMENT OF VOID FRACTION DISTRIBUTION WITH SMALL WIRE MESH SENSOR IN WATER-AIR TWO-PHASE BUBBLE AND SLUG FLOWS IN VERTICAL SMALL DIAMETER PIPE

Combined APSO-ANN and APSO-ANFIS models for prediction of pressure loss in air-water two-phase slug flow in a horizontal pipeline

Abstract Prediction of air-water two-phase flow frictional pressure loss in pressurized tunnels and pipelines is essentially in the design of proper hydraulic structures and pump systems. In the present study artificial neural networks (ANN) and adaptive neuro-fuzzy inference system (ANFIS) are employed to predict pressure loss in air-water two-phase slug flow. Adaptive particle swarm optimization (APSO) is also applied to optimize the results of the ANN and ANFIS models. To predict the pressure loss in two-phase flow, the frictional pressure loss coefficient needs to be determined with respect to the effective dimensionless parameters including two-phase flow Froude and Weber numbers and the air concentration. Laboratory test results are used to determine and validate the findings of this study. The performances of the ANN-APSO and ANFIS-APSO models are compared with those of the ANN and ANFIS models. Different comparison criteria are used to evaluate the performances of developed models, suggesting that all the models successfully determine the air-water two-phase slug flow pressure loss coefficient. However, the ANFIS-APSO performs better than other models. Good agreement is obtained between estimated and measured values, indicating that the APSO with a conjugated ANFIS model successfully estimates the air-water two-phase slug flow pressure loss coefficient as a complex hydraulic problem. Results suggest that the proposed models are more accurate compared to former empirical correlations in the literature.

Theoretical and experimental study on liquid film thicknesses of unsteady slug flows in a capillary tube

Liquid film thickness is an important parameter for predicting evaporation and condensation heat transfer performance of two-phase slug flow in a capillary tube. Under flow boiling condition, the bubble of two-phase slug flow is elongated and accelerated. In the present study, the effect of acceleration on the liquid film thickness of unsteady two-phase slug flow is theoretically and experimentally investigated. It is first theoretically found that the accelerated slug flow is affected by the dimensionless numbers of capillary number (Ca), Weber number (We), and Bond number (Bo). The relation of the dimensionless liquid film thickness with the capillary number, Weber number, and Bond number for unsteady accelerated slug flows was obtained. The liquid film thicknesses of two-phase slug flows under acceleration condition were directly measured using laser focus displacement metering technique. Circular capillary tube with inner diameter of 1 mm was used for the test tube, and water, ethanol and FC-40 were used as working fluids. The novel prediction correlation of liquid film thickness for accelerated slug flows was proposed and it well predicts all the experimental data for all conditions. As the Bond number increases, the liquid film thicknesses are gradually shifted in the direction of decreasing film thickness due to acceleration effect. At a fixed capillary number, initial liquid film thickness becomes thinner with increasing Bond number.

One-way coupled fluid–structure interaction of gas–liquid slug flow in a horizontal pipe: Experiments and simulations

Pipelines conveying a multiphase mixture must withstand the cyclic induced stresses that occur due to the alternating motion of gas pockets and liquid slugs. Few previous studies have considered gas–liquid slug flow and the associated fluid–structure interaction problems. In this study, experimental and numerical techniques were adopted to simulate and analyze the two-phase slug flow and the associated stresses in the pipe structure. In the numerical simulation, a one-way coupled fluid–structure framework was developed to explore the slug flow interaction with a horizontal pipe assembly under various superficial gas and liquid velocities. A modified Volume of Fluid and finite element methods were utilized to model the fluid and structure domains. The file-based coupling technique was adopted to execute the coupling mechanism. By contrast, slug characteristics were measured experimentally, while Bi-axial strain gauges were used to capture time-varying strain signals. Excellent agreements between the predicted and measured stress results were achieved with a maximum error of 10.2 %. It was found that at constant superficial liquid velocity, the maximum induced stresses on the pipe wall increased with increasing the slug length and slug velocity. While for the slug frequency, the maximum principal stresses decreased with increasing the slug frequency.

Numerical Simulation of Two-Phase Slug Flow Liquid-Carryover in a Converging T-junction

Numerical Simulation of Two-Phase Slug Flow Liquid-Carryover in a Converging T-junction

Comparison of Dynamic Mode Decomposition and Deep Learning Techniques for Two-Phase Flows Analysis

Dynamic mode decomposition (DMD) and deep learning are data-driven approaches that allow a description of the target phenomena in new representation spaces. This fact motivates their comparison in the analysis of flow data, generated through experimental setups and numerical simulations. The focused application is the processing of high-speed videos of horizontal two-phase stratified and slug flows regimes. Henceforth, in this work, we consider the traditional DMD, the sparsity-promoting DMD (SPDMD) and, in the deep learning context, we select an unsupervised convolutional autoencoder (CAE). In this avenue, it becomes imperative to compare DMD and deep learning with respect to: computational complexity of target techniques; reduced order modeling versus data representation; data set necessary to compute the dynamic modes and deep learning training; the preservation of the phase interface in the DMD and CAE space; data synthesis. In general, the results favor DMD in the considered applications.

Investigation of the Effect of Fluids Superficial Velocity on Initiation and Development of Slug Flow in a Horizontal Hydraulics Pipe

The aim of this study is to investigate the effect of fluids superficial velocity on the characteristics of two-phase slug flow in a horizontal hydraulics pipe covering slug flow initiation and development. Air and water were employed as the working fluids. Slug flow characteristics were visually observed at 25D and 200D in a 19 mm internal diameter of acrylic pipe for the initiation and fully developed slug flow conditions, respectively. Flow visualization was taken using high speed video cameras at 400 fps and then analyzed using image processing method. Based on the flow pattern characteristics, several slug flow initiation mechanisms were obtained. Under the investigated conditions, at low to medium air superficial velocity (JG) with low to high water superficial velocity (JL), slug flow initiation mechanisms consisted of wave growth, hydraulic jump, blockage, and slug decaying. While wave initiation, wave formation, disturbance, wave coalescence, and incompletely developed slug mechanisms were observed at medium to high JG with low to high JL. After slug flow was fully developed, it was found that the higher the water superficial velocity resulted in lower void fraction and higher slug frequency. In the meantime, pressure drop and liquid slug velocity increased considerably with the increase of both JG and JL. Maximum pressure drop of 8.35 kPa/m occurred at JL of 2.00 m/s and JG of 3.76 m/s. The highest slug frequency of 6.67 1/s was observed at JL of 2.00 m/s and JG of 2.12 m/s. This information is important for designing fluid flow in horizontal hydraulics pipes and for consideration in predicting the development of the slug flow in order to avoid the negative impact of slug flow in industrial applications.

Enzyme synthesis of cephalexin in continuous-flow microfluidic device in ATPS environment

We present an experimental study dealing with the transfer of the enzyme synthesis of cephalexin from a batch arrangement to a continuous-flow microfluidic system with integrated reaction product separation and enzyme recovery. Cephalexin is synthesized by penicillin acylase in a kinetic regime that is characterized by the appearance of a concentration maximum during the enzyme reaction. We determine proper reaction conditions providing a high cephalexin yield and relatively short reaction time by performing a set of batch experiments. Once the residence/reaction time was found, the reaction process is transferred into a continuous mode. We employ an aqueous two-phase system (ATPS) forming two-phase slug flow in a microfluidic capillary as the reaction-separation environment. Such a flow arrangement guarantees a uniform residence time of the reaction mixture in the reaction microcapillary, and it provides in situ extraction of cephalexin. ATPS also offers an easy way for enzyme recycling and addition of fresh reactants as a benefit. We optimized the composition of the ATPS based on phosphates, polyethylene glycol and water so that cephalexin showed a high affinity to one phase and a soluble enzyme to the other phase. Due to the low level of utilization of a soluble catalyst in the continuous flow arrangement, we recycled the reaction phase containing the dissolved enzyme. The recycling loop also contained microdialysis unit for removal of phenylglycine that tends to cause system clogging. We show that the final design of the microfluidic system with recycle can operate continuously for at least 5 h. Our integrated microfluidic platform represents a general solution for enzymatic reactions performed with simultaneous reaction product separation and enzyme recycle.

Slug flow's intermittent feature affects VIV responses of flexible marine risers

Slug flow is characterized by the variations of internal flow parameters with time and position. This study aims to investigate the slug flow effect (SFE) on the cross-flow Vortex-Induced Vibration (VIV) of a flexible riser. A steady slug-flow model is adopted using several assumptions and empirical correlations. A distribution of van der Pol oscillators is employed to create the VIV effect. A novel approach has been proposed to capture the intermittent feature which is validated by comparisons with experiments. The SFE is explored by comparisons of the dynamic behaviors of a flexible riser subject to two uniform currents, assuming the pipe is filled with still liquid, transporting a single-phase liquid flow and a two-phase slug flow. It has demonstrated that the SFE is much more profound than the single-phase internal flow effect. A flexible riser conveying a slug flow can exhibit quite distinguished dynamics owing to the intermittent feature. Apart from triggering new modes, the combination of slug flow frequency and VIV frequency can generate an amplitude-modulation phenomenon of excited modes. Most distinctively, beating behaviours which corresponds to the resonant synchronization have been captured. This study can contribute to fatigue analysis of a flexible riser conveying a two-phase slug flow.

Transition simulation of two-phase intermittent slug flow characteristics in oil and gas pipelines

Abstract This is a numerical study of transition simulation of two-phase intermittent slug flow characteristics in oil and gas pipelines using CFD analysis. The two-fluid free pressure model is employed to solve problem. Moreover, Riemann algorithm has been used to discretize the governing equations. The time history of slug flow initiation, intermittent slug flow repetition and slug flow development have been investigated in this study. The obtained results revealed that as time goes by the slug regime begins and grows and as the slug flow regime grows, it moves forward. Furthermore, it is concluded that when the first slug flow regime begins to grow and develop the second slug flow regime has been formed. Ultimately, it is found that as the first slug grows, the second slug is on the verge of growth.

Numerical simulation, validation, and analysis of two-phase slug flow in large horizontal pipes

Multiphase flow, especially two-phase gas-liquid flow, is of great importance for a variety of applications and industrial processes, for example in the nuclear, chemical, or oil and gas industries. In this contribution, we present simulation results for gas-liquid slug flow in large horizontal pipes. Six test cases with different oil, water, and gas flow rates are considered, which cover a wide range of different slug flows. The numerical predictions are validated by comparison with experimental data obtained from video observations. The relative error of the mean liquid level between experiment and simulation is less than 12.3% for all but one test cases. Furthermore, a frequency analysis is performed. The single-sided amplitude spectrum as well as the smoothed power spectral density are calculated. For both, experimental and simulation data, one observes an increase of the dominant frequencies if the ratio of liquid and gas superficial velocity is increased.

Acoustic Radiation, Cavitation Collapses, and Dryout in a Capillary Oscillating Heat Pipe

Abstract In an oscillation heat pipe (OHP), when two-phase flow oscillates to the condensation region, saturated vapor bubbles/slugs are subjected to a sudden temperature reduction or immediate subcooling. Rapid condensation ruptures vapor bubbles and generates cavitation erosions on the tube interior surface. In this article, a thorough study is performed to understand discrepancy of variation tendency between acoustic radiation and OHP temperature difference that both are operating temperature-dependent. On this basis, three temperature zones were identified: (1) low operating temperature zone with strong cavitation collapse and acoustic radiations, (2) optimal temperature zone with the minimum temperature difference and weakening cavitation collapses, and, (3) high-temperature zone where dryout and oscillation failures develop. At the optimal operating temperature, high-frequent oscillations reduce subcooling of two-phase flow, alleviating the impact of cavitation collapses and ceasing acoustic radiations. At high operating temperature, liquid surface tension dramatically reduces and dynamic contact angle significantly increases. Both the factors tend to lower the critical diameter necessary to maintain pressure difference and oscillating two-phase slug flow. When the critical diameter reduces to be less than the OHP tube diameter, liquid slugs are not able to seal the OHP capillary tubes, leading to dryout or insufficient heat and mass transfer.

Optimization and Extended Applicability of Simplified Slug Flow Model for Liquid-Gas Flow in Horizontal and Near Horizontal Pipes

The accurate prediction of pressure loss for two-phase slug flow in pipes with a simple and powerful methodology has been desired. The calculation of pressure loss has generally been performed by complicated mechanistic models, most of which require the iteration of many variables. The objective of this study is to optimize the previously proposed simplified slug flow model for horizontal pipes, extending the applicability to turbulent flow conditions, i.e., high mixture Reynolds number and near horizontal pipes. The velocity field previously measured by particle image velocimetry further supports the suggested slug flow model which neglects the pressure loss in the liquid film region. A suitable prediction of slug characteristics such as slug liquid holdup and translational velocity (or flow coefficient) is required to advance the accuracy of calculated pressure loss. Therefore, the proper correlations of slug liquid holdup, flow coefficient, and friction factor are identified and utilized to calculate the pressure gradient for horizontal and near horizontal pipes. The optimized model presents a fair agreement with 2191 existing experimental data (0.001 ≤ μL ≤ 0.995 Pa∙s, 7 ≤ ReM ≤ 227,007 and −9 ≤ θ ≤ 9), showing −3% and 0.991 as values of the average relative error and the coefficient of determination, respectively.

Experimental and numerical studies on the corrosion properties of AISI 316L stainless steel in two-phase upward slug flows

The corrosion properties of AISI 316L stainless steel in upward slug flow were initially investigated using the electrochemical measurements and computational fluid dynamics (CFD) simulations. The results demonstrate that the slug flow may destroy the passivation film on the specimen surface due to its intermittency and volatility, accelerating the pitting corrosion process on the electrode surface. It results in a higher corrosion rate in the two-phase slug flow than in the single-phase flow. The variations of superficial velocity lead to changes in the slug flow configuration. The variation has a greater impact on the Taylor bubble than the liquid slug. The changes in slug flow configuration result in differences in shear stress and mass transfer coefficient, leading to corresponding differences in electrochemical properties of AISI 316L stainless steel.

Analogue experiments on the rise of large bubbles through a solids-rich suspension: A “weak plug” model for Strombolian eruptions

Physical interactions between bubbles and crystals affect gas migration and may play a major role in eruption dynamics of crystal-rich magmas. Strombolian eruptions represent an end member for bubble-crystal interactions, in which large bubbles (significantly larger than the crystal size) rise through a crystal-rich near-surface magma. Indeed, volcanoes that produce Strombolian eruptions often generate ejecta with > 30 vol% (often > 45 vol%) average crystallinity. At Stromboli Volcano, Italy, average crystallinity can reach 55 vol%, which is approaching the eruptibility limit for magmas. At such high crystallinities the solids interact mechanically with each other and with bubbles. This complex rheology complicates the two-phase (liquid-gas) slug flow model often applied to Strombolian eruptions. To examine the effect of crystals on bubble rise, we performed analogue experiments in which large bubbles rise in a vertical tube filled with silicone oil and polypropylene particles. The particles have a slightly lower density than the oil, and therefore form a layer of oil + particles at the upper surface. We varied surface pressure, particle volume fraction, length of the particle-bearing cap, and bubble size to examine the ways in which these parameters influence Strombolian-type eruptions. We show that in experiments, suspended solids begin to affect bubble rise dynamics at particle volume fractions as low as 30 vol% (or, when divided by the random close packing value, a normalized particle fraction φ=0.64). Bubbles in experiments with higher particle contents deform as they rise and burst through a small aperture, generating surface fountains that begin abruptly and decay slowly, and longer-lasting acoustic signals of lower amplitude than in particle-poor experiments. Particle fractions > 38 vol% (φ>0.80) generated strong deformations on fast-expanding bubbles that applied a high stress on the cap, but they trapped bubbles that were less overpressured. Qualitatively, the gas release behavior observed in particle-rich experiments is consistent with observations of Strombolian eruptions. Moreover, we estimate that the observed crystallinity of pyroclasts at Stromboli volcano represents φ>0.8. From this we suggest a “weak plug” model for Strombolian eruptions that evolves towards a low-viscosity equivalent of Vulcanian-style plug failure with a more crystalline, stronger, and less permeable plug. Importantly, this model allows the rise of several bubbles in the conduit at the same time and suggests that longer-lasting, more pulsatory and complex eruptions may reveal an increase in near-surface crystallinity, shedding some light on changing conduit conditions that could help determine the different gas rise regimes involved in passive degassing, puffing, and different expressions of Strombolian explosions.

Investigation of slug flow characteristics in hilly terrain pipeline using ultrasonic Doppler method

An ultrasonic Doppler method is proposed for investigating the two-phase slug flow in a hilly terrain pipeline. To obtain the distribution of the velocity and gas–liquid interface, the velocity profile and echo profile of the slug flow were measured via ultrasonic Doppler velocimetry (UDV) with and without the addition of tracer particles in the liquid phase, respectively. An individual 8 MHz transducer with a diameter of 3 mm and an individual 2 MHz transducer with a diameter of 5 mm were employed for the liquid film and liquid slug measurement respectively. Using MATLAB programs based on the measured data, the investigation of the flow characteristics of the slug flow was divided into three parts: the liquid film, the front of the liquid slug, and the center of the liquid slug. The statistical analysis of the average length of every part indirectly reflected the formation and development behavior of the slug flow. The characteristic parameters of the gas–liquid interface, such as the thickness of the liquid film and the bubble distribution in the liquid slug, were extracted from the echo signals. The boundary layer of the liquid film was extracted from the velocity profiles. The average value and variation coefficient of the velocity profile were calculated for the liquid film and the center of the liquid slug. The effects of the superficial velocities and measured positions on the aforementioned flow parameters were examined. The UDV measurement and relevant data processing offer an effective method for the non-intrusive investigation of the slug flow in a hilly terrain pipeline.

The effect of two-parameter of Pasternak foundations on the oscillations of composite pipelines conveying gas-containing fluids

The effect of two-parameter of Pasternak foundations on the oscillations of composite pipelines conveying a two-phase slug flow is studied in the paper. To analyze the oscillations of the pipelines conveying gas-containing fluid, a viscoelastic model of the theory of beams and a model of Pasternak foundation are used. The Boltzmann-Volterra hereditary theory of the viscoelasticity is used to describe the viscoelastic properties of the pipeline material and earth foundation. A computational algorithm has been developed for solving problems of calculating the oscillatory processes in the pipelines conveying a two-phase gas-containing flow. On the basis of the computational algorithm developed, a set of applied computer programs has been created, which makes it possible to carry out numerical studies of pipeline oscillations. The effect of the flow rate of gas and liquid phases, the effect of tensile forces in the longitudinal direction of the pipeline, Pasternak foundations parameters, the parameters of singularity in the heredity kernels on the oscillations of the pipeline made of composite materials are numerically investigated. It is found that an increase in the length of the gas bubble zone leads to a decrease in the critical velocity of a two-phase slug flow.