Mechanism Of Two-phase Flow Research Articles

Numerical Simulation of the Droplet Formation in a T-Junction Microchannel by a Level-Set Method

To satisfy the increasingly high demands in many applications of microfluidics, the size of the droplet needs accurate control. In this paper, a level-set method provides a useful method for studying the physical mechanism and potential mechanism of two-phase flow. A detailed three-dimensional numerical simulation of microfluidics was carried out to systematically study the generation of micro-droplets and the effective diameter of droplets with different control parameters such as the flow rate ratio, the continuous phase viscosity, the interfacial tension, and the contact angle. The effect of altering the pressure at the x coordinate of the main channel during the droplet formation was analysed. As the simulation results show, the above control parameters have a great influence on the formation of droplets and the size of the droplet. The effective droplet diameter increases when the flow rate ratio and the interfacial tension increase. It decreases when the continuous phase viscosity and the contact angle increase.

Direct Measurement of Static and Dynamic Contact Angles Using a Random Micromodel Considering Geological CO2 Sequestration

The pore-level two-phase fluids flow mechanism needs to be understood for geological CO2 sequestration as a solution to mitigate anthropogenic emission of carbon dioxide. Capillary pressure at the interface of water–CO2 influences CO2 injectability, capacity, and safety of the storage system. Wettability usually measured by contact angle is always a major uncertainty source among important parameters affecting capillary pressure. The contact angle is mostly determined on a flat surface as a representative of the rock surface. However, a simple and precise method for determining in situ contact angle at pore-scale is needed to simulate fluids flow in porous media. Recent progresses in X-ray tomography technique has provided a robust way to measure in situ contact angle of rocks. However, slow imaging and complicated image processing make it impossible to measure dynamic contact angle. In the present paper, a series of static and dynamic contact angles as well as contact angles on flat surface were measured inside a micromodel with random pattern of channels under high pressure condition. Our results showed a wide range of pore-scale contact angles, implying complexity of the pore-scale contact angle even in a highly smooth and chemically homogenous glass micromodel. Receding contact angle (RCA) showed more reproducibility compared to advancing contact angle (ACA) and static contact angle (SCA) for repeating tests and during both drainage and imbibition. With decreasing pore size, RCA was increased. The hysteresis of the dynamic contact angle (ACA–RCA) was higher at pressure of one megapascal in comparison with that at eight megapascals. The CO2 bubble had higher mobility at higher depths due to lower hysteresis which is unfavorable. CO2 bubbles resting on the flat surface of the micromodel channel showed a wide range of contact angles. They were much higher than reported contact angle values observed with sessile drop or captive bubble tests on a flat plate of glass in previous reports. This implies that more precaution is required when estimating capillary pressure and leakage risk.

Impact of draft plate on the inter-chamber interaction in a two-chamber spout-fluid bed

The widespread application of the spout-fluid bed in the processing of thermoplastic composites, coal gasification, municipal solid waste gasification, and biomass gasification necessitates a deeply understanding on the flow mechanism of dense two-phase flow in the system in order to improve its design optimization and operation. By tracking the gas and solid phases respectively in the Eulerian and Lagrangian framework, the dense gas-solid flow in the two-chamber spout-fluid bed with and without draft plate was carried out using the computational fluid dynamics coupled with discrete element method (CFD-DEM) to evaluate the impact of the presence of draft plates and the plate length on inter-chamber interaction. The results demonstrate that the interaction of rising bubbles with the spouting channel gives rise to the unfavorable dancing spout phenomenon, which can be alleviated by draft plates. Insertion of the plates lowers the pressure drop magnitudes of both the spouting and background inlets, and the correlation coefficient of the two signals. Furthermore, the draft plates enhance the turbulent intensity of the solid phase in the fountain region and the vertical dispersion in the spout region. Increasing the draft plate length has a significant influence on gas-solid hydrodynamics and chamber interaction.

Multi-Physics Pore-Network Modeling of Two-Phase Shale Matrix Flows

We construct a three-dimensional pore-network model with mixed wettability to study the two-phase flow mechanisms in dry gas producing shales. Previous pore-scale modeling studies on shale have been focused on single-phase gas flow through the nano-pores. However, at most field sites, the majority of the injected fracking fluid does not return to the surface during the flow-back period. It is believed that a large portion of the fracking fluid imbibes into the shale matrix during the fracking process, and thus two-phase flow occurs. In addition, while the inorganic shale matrix is generally water-wet, the organic material embedded within the matrix is hydrophobic. As such, the system displays spatial heterogeneity of wettability. Other important physics are also coupled in the model. Pressure-dependent gas sorption effects are included in the organic pores, with pore size reduction accounted for in those pores. Compressibility and slip flow effects of the gas phase are included throughout the pore-network, with the latter underscoring the fact that the sizes of the nano-pores are comparable to the mean free path of the methane molecule. The coupled effects of these various physical processes are studied to determine the importance of each effect. Continuum-scale properties are computed, including relative permeability curves, as a function of fraction and structure of organic regions and type and magnitude of boundary conditions.

The detrimental effects of recruitment maneuvers on mucus clearance in an animal model of primary ards

The recruitment maneuver (RM) is a transient increase in trans-pulmonary pressure to reopen collapsed alveoli. During mechanical ventilation, mucus could be displaced toward the lungs, driven by the inspiratory flow, via a two-phase gas-liquid flow mechanism [1].

Research on the two-phase flow and separation mechanism in the oil-gas cyclone separator

The cyclone separator has attracted increasing attention due to its small size, rapid construction and high separation efficiency. This study investigated its gas-liquid two-phase flow and separation characteristics experimentally and numerically. A numerical model of two- phase flow in the cyclone separator was proposed using the Euler-Lagrange method. The distribution of pressure, tangential and axial velocity in the gas-phase flow field was obtained, and the oil droplet movement was traced. Separation efficiency was also studied experimentally, and the diameter distributions of oil droplets at the inlet and the outlet of the separator were measured by a Malvern laser particle size analyser to verify the simulation model. Based on high-speed photography technology, the oil film distribution and flow pattern on the wall of the cyclone separator were visualised. The variation of oil-gas two-phase flow in the cyclone separator was compared under various inlet flow rates. Based on the results, an improved structure was proposed, and the performance of the improved separator was investigated experimentally.

Causes of Secretion Retention: Patient Factors, Ventilation, Devices, Drugs

Retention of airway secretions is highly common in critically ill patients, on mechanical ventilation (MV). The endotracheal tube (ETT) plays a critical role in this context; indeed, upon inflation of the ETT cuff, mucociliary transport drastically impairs. Additionally, patients with neurological impairments or underlying diseases, i.e. asthma, chronic obstructive pulmonary disease, cystic fibrosis and non-cystic fibrosis bronchiectasis are at the greatest risk. Indeed, in these patients, MV rapidly disrupts the balance between overproduction of mucus and impaired clearance capabilities. Importantly, during MV, mechanically ventilated patients are positioned in the semi recumbent position and several laboratory studies suggested that in this position retained mucus might move toward the distal airways, driven by gravity. Additionally, airflow promotes clearance or retention of retained mucus, via a two-phase gas-liquid flow mechanism. In sedated, invasively ventilated patients, the inspiratory flow can be modulated through the ventilatory settings, and theoretically, mucus clearance could be promoted or hindered through adjustments of the ventilatory settings. Yet, these assumptions should be corroborated in large translational clinical trials. Importantly, humidification of respiratory gases plays an essential role in maintaining mucus clearance rate within the physiologic range. Thus, the most appropriate humidifier should be chosen on a case-by-case basis, and given the reported poor performance of heat-moisture exchanger during ventilation at high minute volumes, heated humidifiers should be a primary choice for patients requiring high ventilatory support. Finally, numerous drugs, commonly used in ventilated patients, i.e. oxygen, inhaled anesthetics, narcotics profoundly affect mucociliary clearance and increase mucus retention. Keywords: Asthma, bronchiectasis, chronic obstructive pulmonary disease, cystic fibrosis, endotracheal tube, mechanical ventilation, mucus clearance.

Efficient numerical methods for simulating surface tension of multi-component mixtures with the gradient theory of fluid interfaces

Abstract Surface tension significantly impacts subsurface flow and transport, and it is the main cause of capillary effect, a major immiscible two-phase flow mechanism for systems with a strong wettability preference. In this paper, we consider the numerical simulation of the surface tension of multi-component mixtures with the gradient theory of fluid interfaces. Major numerical challenges include that the system of the Euler–Lagrange equations is solved on the infinite interval and the coefficient matrix is not positive definite. We construct a linear transformation to reduce the Euler–Lagrange equations, and naturally introduce a path function, which is proven to be a monotonic function of the spatial coordinate variable. By using the linear transformation and the path function, we overcome the above difficulties and develop the efficient methods for calculating the interface and its interior compositions. Moreover, the computation of the surface tension is also simplified. The proposed methods do not need to solve the differential equation system, and they are easy to be implemented in practical applications. Numerical examples are tested to verify the efficiency of the proposed methods.

Void fraction correlations analysis and their influence on heat transfer of helical double-pipe vertical evaporator

An analysis of 50 void fraction correlations available in the literature was performed to describe two-phase flow mechanism inside two helical double-pipe vertical evaporators. The evaporators considered water as working fluid connected in countercurrent so the change of phase was carried out into the internal tube. The discretized equations of continuity, momentum and energy in each flow were coupled using an implicit step by step method. The selection of the void fraction correlations for the mathematical model was based on inclusion of some theoretical limits. The results of this analysis were compared with the experimental data in steady state for two different evaporators, obtaining good agreement in the evaporation process for only 7 void fraction correlations. The Armand and Massena correlation had a mean percentage error (MPE) of 3.08%, followed by Rouhanni and Axelsson I adquired MPE=3.16%, Chisholm and Armand obtained MPE=3.18%, Steiner as well as Rouhanni and Axelsson II with MPE=3.19%, Bestion reached MPE=3.20% and Flanigan presented MPE=3.21%. Furthermore, the experimental and simulated heat flux were acceptable (R2=0.939). Finally, the results showed that the drift flux parameter was important to evaluate the void fraction.

Optic Imaging of Two-Phase-Flow Behavior in 1D Nanoscale Channels

Summary Gas in tight sand and shale exists in underground reservoirs with microdarcy (µd) or even nanodarcy (nd) permeability ranges; these reservoirs are characterized by small pore throats and crack-like interconnections between pores. The size of the pore throats in shale may differ from the size of the saturating-fluid molecules by only slightly more than one order of magnitude. The physics of fluid flow in these rocks, with measured permeability in the nanodarcy range, is poorly understood. Knowing the fluid-flow behavior in the nanorange channels is of major importance for stimulation design, gas-production optimization, and calculations of the relative permeability of gas in tight shale-gas systems. In this work, a laboratory-on-chip approach for direct visualization of the fluid-flow behavior in nanochannels was developed with an advanced epi-fluorescence microscopy method combined with a nanofluidic chip. Displacements of two-phase flow in 100-nm-depth slit-like channels were reported. Specifically, the two-phase gas-slip effect was investigated. Under experimental conditions, the gas-slippage factor increased as the water saturation increased. The two-phase flow mechanism in 1D nanoscale slit-like channels was proposed and proved by the flow-pattern images. The results are crucial for permeability measurement and understanding fluid-flow behavior for unconventional shale-gas systems with nanoscale pores.

Evaluation of inhomogeneous model and the LCS based investigation in multiphase flows

In this paper, an evaluation of inhomogeneous model for computations of gas-liquid two-phase flow is presented, and the mechanism of gas-liquid two-phase flow in a bubble column is studied based on Finite-Time Lyapunov Exponents (FTLE) and Lagrangian Coherent Structures (LCS). The simulation is conducted with the homogeneous and inhomogeneous models respectively, and the numerical results are compared with the experimental data. It is shown that the inhomogeneous model can calculate the force of the gas more accurately and simulates the details of transient flows well due to the consideration of the interaction between the two phases. With inhomogeneous model, the periodic fluctuation of the bubble hose is captured and the velocity distribution coincides exactly with the experimental data. For the gas-liquid two-phase flow in the bubble column, the process of gaseous flow injected into water can be divided into two stages: the gas rising and gas fluctuation. The Lagrangian Coherent Structures (LCS) which consist of the ridges of the FTLE field can capture the boundary of vortex and the interface between the forward and backward flows in the liquid region, and the LCS have unique value for representing the divergence extent of neighboring particles in regions with different dynamics characteristics.

High Speed Photography and 3-D CFD Simulation of Pulsed Anti-Riots Water Cannon Launch Process

In order to study the atomization mechanism of gas-liquid two-phase flow, high speed camera was used to photo the water-jet, the jet images at different time were gained, CFD technology was used to simulate the launch process of water cannon in 3-D Model, Large Eddy Simulation and VOF model were used to describe the turbulent flow and track the gas-liquid interface in and out of launch pipe. The gas-liquid distribution image of experiment and simulation, water-jet velocity of experiment and simulation both matched well with other. The simulation results show that the flow pattern in launch pipe of water cannon changed from atomization pattern to annular pattern and finally slug pattern, the conclusion has significant meaning to the study of second spray and first atomization of water-jet.

Characteristic Variables Extracting of the Gas-Liquid Two-Phase Flow through V-Cone Flowmeter Based on Adaptive Optimal-Kernel Theory

In order to study the mechanism of gas-liquid two-phase flow, a method of characteristic variables extracting based on Adaptive Optimal-Kernel (AOK) theory was represented in the paper. First, to collect dynamic differential pressure signal of gas-liquid two-phase flow through a horizontal V-cone flow meter, and then the AOK theory was used to analyze the dynamic differential pressure signal. The movement law of two-phase flow was discussed through the time-frequency spectrum. Finally, four characteristic variables were defined by using the time-frequency spectrum and the ridge of AOK. After the characteristic variables were visual analyzed, the relationship between the different combination of characteristic variables and the flow pattern was obtained. The results show that, characteristic variables defined by this method can get a clear description of the flow information. This method provides a new way for the flow patterns identification.

The Analysis of Gas-liquid Two-phase Flow Patterns Based on Variation Coefficient of Image Connected Regions and Line-Correlation Algorithm

Five typical flow patterns are shot in the horizontal pipe test section by high-speed photography technology. Then the connected regions of flow pattern images are obtained. The connected region is different from the gray texture. Connectivity characteristics are morphological characteristics of the images, no influence by light and shade of the images, can explore deeply the image structural information. To analysis result, the influence of the standard deviation and the average of five typical flow patterns is eliminated through the combination of the images, the characteristics of four kinds of coefficient of variation and the statistical characteristics of the two connected regions. And the flowing mechanism of gas-liquid two-phase flow is more deeply characterized. From simple points and complex point, the gas-liquid two-phase flow patterns are comprehensively analyzed by the combination of two characteristic parameters of the straightness (straight line similarity and intermittent degrees).

CFD Modeling of the Fast Pyrolysis of Coal in Cold Flow Fluidized Bed

Abstract. Gas-solid fluidized beds are widely applied in many industries as reactors or heat/mass transferring units because of their good heterogeneous mixing behaviors and large transferring area between the gas and solid phases. In this study, based on the Eulerian-Eulerian approach, 2D model of gas-solid flow field in fluidized bed is simulated, and the drag force models of Gidaspow and Syamlal-O’Brien have been used to simulate and analyze the two-phase flow for exploring mechanism and interaction laws of two-phase flow.

Α new void fraction correlation inferred from artificial neural networks for modeling two-phase flows in geothermal wells

A new empirical void fraction correlation was developed using artificial neural network (ANN) techniques. The artificial networks were trained using the backpropagation algorithm and production data obtained from a worldwide database of geothermal wells. Wellhead pressure, steam quality, wellbore diameter, the fluid density and viscosity, and the dimensionless numbers Reynolds, Weber, and Froude were used as main input parameters. The target ANN output was defined by the optimized void fraction values (αopt), which were calculated from the numerical modeling of two-phase flow using GEOWELLS (a wellbore simulator). The Levenberg–Marquardt algorithm, the hyperbolic tangent sigmoid, and the linear activation functions were used for the development of the ANN model. The best ANN learning was achieved with an architecture of six neurons in the hidden layer, which made it possible to obtain a set of void fractions (αANN) with a good accuracy (R2=0.9722). These void fraction estimates were used to obtain the new correlation, which was later coupled into the simulator GEOWELLS for the prediction of pressure gradients in two-phase geothermal wells. The accuracy of the new correlation (αANN) was evaluated by a statistical comparison between simulated pressure gradients and measured field data. These simulation results were also compared with those data calculated by using Duns-Ros and Dix correlations, which were also programmed into GEOWELLS. Pressure gradients predicted with the new αANN correlation showed a better agreement with measured field data, which was also confirmed by the lower values of some statistical parameters (MPE, RMSE, and Theil's U). The statistical evaluation demonstrated the efficiency of the new correlation to predict void fractions and pressure gradients with a better accuracy, in comparison to the other existing correlations. These successful results suggest the use of the new correlation (αANN) for the analysis of two-phase flow mechanisms of geothermal wells.

Time-Frequency Signal Processing for Gas-Liquid Two Phase Flow Through a Horizontal Venturi Based on Adaptive Optimal-Kernel Theory

A time-frequency signal processing method for two-phase flow through a horizontal Venturi based on adaptive optimal-kernel (AOK) was presented in this paper. First, the collected dynamic differential pressure signal of gas-liquid two-phase flow was preprocessed, and then the AOK theory was used to analyze the dynamic differential pressure signal. The mechanism of two-phase flow was discussed through the time-frequency spectrum. On the condition of steady water flow rate, with the increasing of gas flow rate, the flow pattern changes from bubbly flow to slug flow, then to plug flow, meanwhile, the energy distribution of signal fluctuations show significant change that energy transfer from 15–35 Hz band to 0–8 Hz band; moreover, when the flow pattern is slug flow, there are two wave peaks showed in the time-frequency spectrum. Finally, a number of characteristic variables were defined by using the time-frequency spectrum and the ridge of AOK. When the characteristic variables were visually analyzed, the relationship between different combination of characteristic variables and flow patterns would be gotten. The results show that, this method can explain the law of flow in different flow patterns. And characteristic variables, defined by this method, can get a clear description of the flow information. This method provides a new way for the flow pattern identification, and the percentage of correct prediction is up to 91.11%.

Time-frequency spectral analysis of gas-liquid two-phase flow’s fluctuations

In order to study different flow patterns’ dynamic characteristics of gas-liquid two-phase flow, three time-frequency analysis methods are introduced to process the dynamic differential pressure signal of gas-liquid two-phase flow, such as the wavelet transform, Hilbert-Huang transform and adaptive optimal-kernel method. The results show that the main part of energy is transferred from frequency band 15—35 Hz to 0—8 Hz when the flow pattern changes from bubbly flow to slug flow and plug flow, and two spectrum peaks are observed at slug flow. The experimental results show that the time-frequency resolution of Hilbert-Huang transform and adaptive optimal-kernel is higher than that of wavelet transform. The extractions of ridge information based on adaptive optimal-kernel overcome the influence of fuzzy plane windowing effect, and enhance the readability of time-frequency plane information. The time-frequency analysis clearly shows the dynamic characteristics of different flow patterns, and describes the variation rules with time. It is helpful to further study the mechanism of gas-liquid two-phase flow.

Threshold velocities of particle-fluid flows in horizontal pipes and ducts: literature review

Numerous industrial processes such as conveying, fluidizing and handling involve two-phase (fluid-solids) flows. Investigation of the two-phase flow mechanisms have importance in both industrial manufacturing processes and natural phenomenon such as dust storms, erosion, air pollution and soil deposition in rivers and ocean flows. One of the most significant parameters for designing two-phase (fluid-solids) flow conveying systems is the fluid velocity magnitude. Inaccurate determination of the fluid velocity can lead to high energy consumption, particle attrition, pipe erosion and in some cases pipe blockage. This work reviews numerous studies presented in the literature concerning various threshold velocities in horizontal two-phase (fluid-solids) flow systems. The threshold velocities include: incipient motion, pickup from a layer of particles, pickup from hill-shape particle deposits, boundary saltation and minimum pressure velocity. Most of the studies were conducted in pneumatic and hydraulic conveying systems and large-scale wind tunnels systems. As each threshold velocity characterizes a transition between two or more flow regimes and mode of particle movement, all possible flow regimes of the particle-fluid horizontal conveying systems are present first. Then the threshold velocity models and correlations are analyzed and compared. The theoretical models are compared for the forces and torque vectors taken into account for various threshold conditions and particle package geometry. In addition, the main parameters influencing the pickup and saltation mechanisms and the dimensionless numbers are reviewed. Finally, four threshold velocities are used to characterize a generalized flow regime diagram for horizontal conveying systems. This diagram makes a significant contribution to researching and designing two-phase flow systems. For example, the diagram may assist in the transition conditions between dilute and stratified flow regimes. However, more theoretical and empirical research is required to discover all the possible states of horizontal systems.

Simulation on nucleate boiling in micro-channel

Using the VOF multiphase flow model, numerical simulations are conducted to investigate the nucleate boiling of water in micro-channels. The Marangoni heat transfer through the bubble surface is analyzed, and is compared with the incipient heat flux at the onset of nucleate boiling in micro-channels. The bubble growth in the channel is divided into two stages. At the initial stage, bubble growth is controlled by surface tension, while at the second stage the incipient heat transfer dominated the boiling process. In the results, the full process of bubble generating, growing, departing, combining, and shrinking in the channel is displayed. The simulated results with similar condition are agreed well with some experimental results in references. The method and discussion in the paper are helpful to the investigation of the mechanism of micro-scale two-phase flow and heat transfer.