Gas-Liquid Two-Phase Research Articles

Influence of Two-Phase Liquid-Gas Flow on Permeate Flux through Ceramic Membranes

The impact of two-phase flow onto the flux of the permeate in microfiltration of two sorbents-Bentonite and Lewatit S1468-with and without adsorbed zinc, was examined. Two different types of membrane, single-and multi-channel, based on I±-Al2O3, were used. In single-channel membrane application of aeration increased the permeate flux by 13 and 29% without adsorbed zinc and by 22 and 25% with adsorbed zinc for Bentonite suspension and by 4 and 5.5% both with and without adsorbed zinc for Lewatit S1468 compared to single-phase (without aeration) flow at 1.4 and 2.2 m s-1 of the gaseous phase rate, respectively. In multi-channel membrane the permeate flux increased not significantly without adsorbed zinc and by 22 and 25% with adsorbed zinc for Bentonite and by 12 and 13% without adsorbed zinc and not significantly with adsorbed zinc for Lewatit S1468 at 0.4 and 1.1 m s-1 of the gaseous phase rate, respectively.

Void fraction predictive method based on the minimum kinetic energy

This paper presents a new void fraction predictive method for two-phase gas–liquid flows inside tubes for horizontal and vertical upward flows based on the principle of minimum kinetic energy. Non-uniformities of the velocity profiles are taken into account in the development of the method through empirical factors defined by the momentum coefficients ratio. A regression analysis to correlate the momentum coefficients ratio based on a broad databank gathered from the literature covering horizontal and vertical upward flows is performed. The proposed method has proven more accurate than the current methods available in the literature, as it predicts 92 and 85 % of the experimental data for horizontal and vertical saturated flows, respectively, within a ±10 % error band. The experimental database includes results for saturated flow and covers mass velocities ranging from 70 to 800 kg/m2 s and from 37 to 4500 kg/m2 s and tube internal diameters from 0.5 to 13.8 mm and 6 to 89 mm for horizontal and vertical upward flows, respectively.

Development of Numerical Simulation Method for Compressible Gas-Liquid Two-Phase Flows

A numerical simulation method of compressible gas-liquid two-phase flow is developed for analyses of a cavitation bubble. Thermodynamic state of both phases is described with stiffened gas equation of state. Interface of two phases is captured by Level-Set method. As internal energy jump between two phases is critical for the stability of computation, total energy equation is modified so that inviscid flux of energy is smoothly connected across the interface. Detail of governing equations as well as their discretization is described followed by the result of one-dimensional simple example computation.

Numerical simulation of two-phase gas–liquid flow through gradual expansions/contractions

In the present work, turbulent two-phase flows of air and water are numerically simulated through a smooth area expansion/restriction with a constant opening angle. To model the multiphase flow, a two-fluid approach is adopted in which two sets of governing equations are solved simultaneously for continuous and dispersed phases and the coupling between these phases is forced by proper source terms. An extended version of two-equation k–ε turbulence model is used and the numerical result was validated in comparison to previously published experimental data. The effect of volumetric void fraction, Reynolds number and the opening angle is examined on the pressure distribution across the flow domain.

Numerical simulation of underwater explosion near air–water free surface using a five-equation reduced model

Underwater explosion phenomena is a complicated problem, however, it has different and important applications. This type of explosion includes strong shock waves with generation of low pressure cavitation׳s zone and deformable interfaces. The main objective of this article is simulation of interaction of shock wave with interface of two-phase gas–liquid flow and capturing the complicated interface generated from explosion. For this reason, a five equations reduced model is considered with using a new cavitation model including gravity force effect. From the numerical point of view, a Godunov method was applied using HLLC solver. By using MUSCL-Hancock strategy, second order accuracy is achieved. To verify the developed computer code, a one dimensional shock tube test case and two test cases including one dimensional cavitation in open tube and a two dimensional underwater explosion where its experimental results can be found in the related literature are used for comparison to justify the obtained results. The comparison of the results confirms an excellent accuracy of the numerical results. The proposed new cavitation model, on the other hand, is capable of calculating low pressures and simulating the dynamic creation and evolution of the bulk cavitation below the free surface.

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].

Adiabatic two-phase gas–liquid flow behaviors during upward flow in a vertical circular micro-channel

A two-phase air–water flow experiment was carried out to obtain flow pattern, void fraction, and pressure drop data from a vertical micro-channel, which was a fused silica tube with a diameter of 0.53mm and a length of 320mm. The adiabatic two-phase flow behaviors during vertical upward direction were experimentally investigated, and consequently, a buoyancy effect can be detected in this work. A flow visualization study was performed, leading to observation of slug flow, throat–annular flow, churn flow, annular flow, and annular–rivulet flow. The shape of the interfacial surface in the slug flow pattern was deformed during vertical upward flow, which was different from the bullet-shaped gas slug observed in the horizontal channel. The flow visualization results also indicate that the flow pattern map for vertical upward flow is not completely compatible with that for horizontal flow. Image analysis was performed to determine the void fraction, which increases linearly with increasing volumetric quality. The vertical upward flow gave a lower void fraction than the horizontal flow. The frictional pressure drop can be increased when the churn flow is formed in the channel. In addition, vertical upward flow can result in higher pressure drop when compared with the horizontal channel.

Advanced image processing of wire-mesh sensor data for two-phase flow investigation

Two-phase gas-liquid flows, the simultaneous flow of a gas and liquid in a pipe, are commonly found in several industrial activities, among them during the extraction and processing of crude oil. Wire-mesh sensors are flow imaging devices which are able to generate images of phase distribution of two-phase flows within a pipe cross section at high spatial and temporal resolutions. Appropriated image processing algorithms are however necessary in order to extract important flow parameter from raw data. In this paper, we describe a bubble identification algorithm based on three-dimensional image segmentation. Results of bubbles velocity and flow rate measurement based on the development algorithm are validated against reference models showing good agreement.

Numerical Study of Bubbly Flow Using the Second-Order Moment Turbulence Model and the Population Balance Method

A comprehensive computational fluid dynamics (CFD) turbulence model, coupled with a population balance method (PBM), is presented to simulate the two-phase gas–liquid bubbly flow. The simulation results are in good agreement with the experimental data. Furthermore, considering the bubble size distribution including the drag force and lift force, the results showed that the calculated results are in good agreement with the experimental measurements. The studies reveal the liquid recirculation and bubble uprising flow patterns, and anisotropic liquid and bubble normal Reynolds stresses. Moreover, both the liquid velocity gradient and the bubble–liquid interaction are important for the generation of liquid turbulence.

Refined reconstruction of liquid–gas interface structures for stratified two-phase flow using wire-mesh sensor

Wire-mesh sensors (WMS), developed at HZDR [13,4], are widely used to visualize two-phase flows and measure flow parameters, such as phase fraction distributions or gas phase velocities quantitatively and with a very high temporal resolution. They have been extensively applied to a wide range of two-phase gas–liquid flow problems with conducting and non-conducting liquids. However, for very low liquid loadings, the state of the art data analysis algorithms for WMS data suffer from the comparably low spatial resolution of measurements and from boundary effects, caused by e.g. flange rings – especially in the case of capacitance type WMS. In the recent past, diverse studies have been performed on two-phase liquid–gas stratified flow with low liquid loading conditions in horizontal pipes at the University of Tulsa. These tests cover oil–air flow in a 6-inch ID pipe and water–air flow in a 3-inch ID pipe employing dual WMS with 32×32 and 16×16 wires, respectively. For oil–air flow experiments, the superficial liquid and gas velocities vary between 9.2m/s≤νSG≤15m/s and 0.01m/s≤νSL≤0.02m/s, respectively [2]. In water–air experiments, the superficial liquid and gas velocities vary between 9.1m/s≤νSG≤33.5m/s and 0.03m/s≤νSL≤0.2m/s, respectively [17,18]. In order to understand the stratified wavy structure of the flow, the reconstruction of the liquid–gas interface is essential. Due to the relatively low spatial resolution in the WMS measurements of approximately 5mm, the liquid–gas interface recognition has always an unknown uncertainty level. In this work, a novel algorithm for refined liquid–gas interface reconstruction is introduced for flow conditions where entrainment is negligible.

Interrogating the effect of an orifice on the upward two-phase gas–liquid flow behavior

Experiments are reported on an air–water mixture flowing through an orifice in a vertical pipe. Time series of cross-sectionally averaged void fractions have been measured at nine axial positions by using a conductance probe technique. A series of six orifices with different thicknesses and apertures were employed. The Probability Density Function, the Power Spectral Density of the time series of cross sectionally averaged void fractions and the cross-correlation of time series from adjacent probes have been obtained to determine the effect of the orifice on the flow characteristics. The diameter area ratio and the thickness of the orifice have a higher influence on bubbly than on slug and churn flows. The recovery length is about 20, 10 and 7 pipe diameter downstream the orifice for these three flow patterns respectively. Homogenization effect needs a minimum liquid superficial velocity. Its position occurs depends on the value of this velocity and on the orifice fractional open area.Just downstream the orifice, the structure velocity increases for the bubbly and slug flows and decreases for churn flow. For bubble and slug flows, there is persistency of the frequency when passing through the orifice from the upstream to the downstream pipe.

Analysis of shock-wave propagation in aqueous foams using shock tube experiments

This paper reports experimental results of planar shock waves interacting with aqueous foams in a horizontal conventional shock tube. Four incident shock wave Mach numbers are considered, ranging from 1.07 to 1.8, with two different foam columns of one meter thickness and expansion ratios of 30 and 80. High-speed flow visualizations are used along with pressure measurements to analyse the main physical mechanisms that govern shock wave mitigation in foams. During the shock/foam interaction, a precursor leading pressure jump was identified as the trace of the liquid film destruction stage in the foam fragmentation process. The corresponding pressure threshold is found to be invariant for a given foam. Regarding the mitigation effect, the results show that the speed of the shock is drastically reduced and that wetter is the foam, slower are the transmitted waves. The presence of the foam barrier attenuates the induced pressure impulse behind the transmitted shock, while the driest foam appears to be more effective, as it limits the pressure induced by the reflected shock off the foam front. Finally, it was found that the pressure histories in the two-phase gas-liquid mixture are different from those previously obtained within a cloud of droplets. The observed behavior is attributed to the process of foam fragmentation and to the modification of the flow topology past the shock. These physical phenomena occurring during the shock/foam interaction should be properly accounted for when elaborating new physical models.

Two-phase flow phenomena assessment in minichannels for compact heat exchangers using image analysis methods

The paper describes a method of two-phase flow structure evaluation for minichannels. The two-phase flow structure appears in gas–liquid mixture. The research is based on innovative approach, with the use of stereology methods. Evaluation of the flow structure is made by image analysis. The images are obtained with high-speed visualization technique. The applied stereological analysis is based on the linear methods – the random secants method and directed secants method. Development of mini heat exchangers requires knowledge of the two-phase flow phenomena. The major result of conducted research is that for each flow structure there is a set of stereological parameters, enabling the quantitative estimation of the two-phase flow. It has been found that the interrelation of stereological parameters, during the change of the flow structure, can be used for controlling the operating conditions. The basic conclusion is that the knowledge about the character of the changes taking place in the flow structure may be used for constant process adjustment for various two-phase gas–liquid or gas–solid systems.

Numerical simulation of hydrodynamic wave loading by a compressible two-phase flow method

Hydrodynamic wave loading on and in offshore structures is studied by carrying out numerical simulations. Particular attention is paid to complex hydrodynamic phenomena such as wave breaking and air entrapment. The applied CFD method, ComFLOW, solves the Navier–Stokes equations with an improved Volume-of-Fluid method to track the movement of the free surface. A local height function keeps the surface sharp (no ‘flotsam and jetsam’). Application of two different fluid models, single-phase (only liquid) and two-phase (liquid and compressible gas) is presented, the latter model being capable of simulating bubbles of entrapped gas.Treatment of the density around the free surface is found highly critical for obtaining an accurate fluid distribution and velocity field. A newly-developed gravity-consistent density averaging method is applied to prevent spurious velocities around the free surface. The convective terms are approximated by a compressible, symmetry-preserving second-order upwind discretization. Time integration, using second-order Adams–Bashforth, is carried out with a generalization of the familiar pressure-correction method, in which the full acoustical part of the flow equations is treated implicitly.Numerical results are validated against experimental data for two test cases. As an example of internal wave loading, liquid sloshing dynamics are validated with experimental results for a 1:10 scale LNG tank section. In particular, the experimental pressure signal during a moment of air entrapment is compared with one-phase and two-phase flow simulations. The simulation of external wave loading is validated with data from an experiment with wave run-up against a 1:50 scale semi-submersible offshore structure. The test cases show that modeling of two-phase effects can be beneficial for simulating hydrodynamic wave loading.

Development of a Steady-State Point Model for Prediction of Gas/Oil and Water/Oil Pipe Flow

We report the development and testing of steady-state point models for two-phase gas–liquid and oil–water flow in pipes. A new point model for gas–viscous oil compared well with flow loop data using dense gas (SF6) and viscous oil. However, the flow regime prediction and the modeling of the transition from laminar to turbulent flow regime were found to be in need of further improvement. Data from stratified viscous oil–water experiments were analyzed using the point model for liquid–liquid flow. Traversing gamma densitometry was used to characterize the oil-in-water emulsion layer. Analysis of the measurements confirms the assumption of a flat interface between oil-continuous and water-continuous layers.

An Empirical Model to Predict the Transition Between Stratified and Nonstratified Gas–Liquid Two-Phase Flow in Horizontal and Downward Inclined Pipes

The specific objective of this work is to develop an empirical model to predict the existence of stratified flow in horizontal and downward inclined gas-liquid two-phase flow. The proposed model is in nondimensional form and attempts to emulate the qualitative trend of the Taitel and Dukler mechanistic model. The key advantage of the proposed method is that it is explicit in nature and, unlike Taitel and Dukler, it does not require use of a graphical or iterative solution. The empirical parameters used in the proposed model account for the effect of pipe diameter, pipe orientation, and the liquid density on the transition line between stratified and nonstratified flow. The accuracy of the proposed model is verified against the flow visualization data collected from more than 16 data sources consisting of 8 fluid combinations, pipe diameter in a range of 8.9 to 300 mm, liquid density in a range of 780 to 1420 kg/m3, and all downward pipe inclinations including horizontal.

Void Fraction Detection in a Gas-Liquid Two-Phase Flow Using a Propagation Attenuation Method

Based on the principle of the attenuation of an acoustic wave propagating in a gas-liquid two-phase flow, an experiment has been designed to measure the void fraction. The experiment system is consisted of three segments which arepressure stabilizing control system, ultrasonic measurement system and bubble injection system. The measurement process is three steps. First, a reference experiment is performed to acquire the received signal amplitude in pure liquid. Then, measurement of the received signal amplitude in different void fraction is accomplished. Last, the relationship between void fraction and signal amplitude attenuation is given on which the void fraction of two-phase flow can be detected. It is confirmed that void fraction detection using the present method achieves successfully.

Experimental Study of Two-Phase Air–Water Flow in Large-Diameter Vertical Pipes

Recently, due to an increase in production demand in nuclear and oil and gas industries, the requirement to migrate toward larger pipe sizes for future developments has become essential. However, it is interesting to note that almost all the research on two-phase gas–liquid flow in vertical pipe upflow is based on small-diameter pipes (D ≤ 100 mm), and the experimental work on the two-phase gas–liquid flow in large-diameter (D > 100 mm) vertical pipes is scarce. Under the above circumstances, the application of modeling tools/correlations based on small-diameter pipes in predicting flow behavior (flow pattern, void fraction, and pressure gradient) poses severe challenges in terms of accuracy. The results presented in this article are motivated by the need to introduce the research work done to the industries where the data pertaining to large-diameter vertical pipes are scarce and there is a lack of understanding of two-phase gas-liquid flow behavior in large-diameter (D > 100 mm) vertical pipes.The unique aspect of the results presented here is that the experimental data have been generated for a 254-mm inner diameter vertical pipe that forms an excellent basis for the assessment of modeling tools/correlations. This article (i) presents the results of a systematic investigation of the flow patterns in large-diameter vertical pipes and identifies the transition between subsequent flow patterns, (ii) compares it directly with the existing large- (150 mm) and small-diameter data (28 mm and 32 mm) in the same air–water superficial velocity range, (iii) exemplifies that the existing available empirical correlations/models/codes are significantly in error when applied to large-diameter vertical pipes for predictions, and last (iv) assesses the predictive capability of a well-known commercial multiphase flow simulator.

다단 원심펌프의 공동현상 유동해석

The purpose of this study is to investigate cavitating flow of the multistage centrifugal pump. Cavitation is observed in the impeller leading edge and trailing edge of the suction area. Head coefficients are measured under different flow operating conditions. The Rayleigh-Plesset cavitation model is adapted to predict the occurrence of cavitation in the pump. The two-phase gas-liquid homogeneous CFD method is used to analyze the centrifugal pump performances with two equation transport turbulence model. The simulations are carried out with three different flow coefficients such as 0.103, 0.128 and 0.154. The occurrence of cavitation described according to water vapor volume fraction. The head versus NPSH (Net Positive Suction Head) also measured using different flow coefficients. Development of cavitation in the centrifugal pump impellerI is discussed. It is showed that the simulation represents the head drop about 3%.

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.