Stratified Wavy Flow Research Articles

A bench-scale flow loop study on hydrate deposition under multiphase flow conditions

Gas hydrates are crystalline compounds containing water and small guest molecules. Hydrates, if not properly managed, can cause capital losses and raise safety concerns. In recent years, hydrate control strategies have changed from complete prevention to proper management. Hydrate management requires satisfactory understanding in all hydrate challenges, including agglomeration, deposition, and bedding. The importance of deposition motivates the investigation of its mechanisms. This work aims to provide further insights into hydrate deposition under multiphase flow conditions using a bench-scale single-pass flow loop. The flow loop was equipped with a horizontal test section, transparent windows, temperature sensors, and differential pressure transducers. The experiments were performed with a methane/ethane gas mixture at constant 38.7 bara. The mutual effects between the flow pattern and the deposit growth were observed. Most experiments in this study started with stratified flow. Non-uniform hydrate deposition was observed with relatively slow growth of the top deposit exposed to the gas phase and relatively fast growth of the bottom deposit with direct contact to the liquid phase. The hydrate buildup gradually changed the flow pattern from stratified flow to stratified wavy flow and eventually to slug/bubble flow. The deposit growth and the flow pattern change were reflected on the pressure drop measurement across the test section. The wall temperature showed a big effect on the growth rate and the steady-state thickness. The hydrate slurry presence and the water cut did not noticeably affect the steady-state deposit thickness, while the MEG addition seemed to reduce the thickness.

Experimental investigation of two-phase separation in T-Junction with combined diameter ratio

T-junctions are often used as partial phase separator to separate two phases gas/liquid feedstock in industrial applications. T-junctions are typically positioned in such a way that two-phase flow entering the T-junction split unevenly between branch and run arm, leaving the gas-rich flow in branch arm and liquid-rich flow in run arm. The T-junctions used in industry are either regular (D3 = D1) or reduced (D3 < D1) T-junction. There is no design standard available for the design of T-junction. Often, excessive liquid carry-over occurs in the branch arm, resulting in the breakdown of downstream equipment which is not designed to handle excessive liquid. In this study, converging T-junction was designed, developed, and experimentally tested. Converging T-junction was designed with a combination of two diameters D3 and D4 in the feedstock. The diameter of the lower pipe D3 of the branch arm was kept larger than the diameter of the upper pipe D4. The motivation behind this converging T-junction was based on the analysis of previous experimental data on regular and reduced T-junction, where a large diameter ratio T-junction resulted in high gas threshold while small diameter ratio T-junction resulted in low peak liquid carry-over. Thus, a converging T-junction will likely possess the best characteristics of both large and small diameter ratio T-junction. Converging T-junction with combined diameter ratios (D3/D1, D4/D1) of (01, 0.52), (01, 0.20), (0.67, 0.52), (0.67, 0.27) and (0.5, 0.27) were experimentally tested in slug flow regime. The inlet conditions were: Pressure - atmospheric, temperature - 25 °C, superficial gas velocities 0.388 ≤ JG1 ≤ 0.620 m/s and superficial liquid velocities 0.252 ≤ JL1 ≤ 0.582 m/s. It was found that the proposed converging T-junction's geometry performed very well. The separation efficiency of 93% was achieved in case of stratified-wavy flow with not more than 20% of the max. liquid carry-over and above 88% in case of slug flow with less than 35% max. liquid carry-over. This new geometry increased separation efficiency of 5–25% in stratified-wavy and 20–35% in case of slug flow as compared with conventional T-junction.

Experimental Study of the Two-Phase Flow Patterns of Air-Water Mixture at Vertical Bend Inlet and Outlet

Air-water two-phase flow in pipes introduces a noticeable challenge due to the complexity of the fluids. Thus, to estimate the best design and reasonable financing cost of the transportation pipelines where the bends are presenting a part of their accessories, the investigators should have been able to estimate the flow regime occurring at different directions. An experiment was carried out by using a 90o bend fixed with two pipes where the flow was upstream from a vertical to a horizontal pipe which were representing the bend inlet and outlet respectively. Two wire-mesh sensors were used for obtaining the data of the void fractions (α) at water superficial velocities (Usl) which changed from 0.052 to 0.419m/s, and air superficial velocities (Usg) from 0.05 to 4.7m/s. Furthermore, the characterization of flow regimes of the air-water flow at both bend inlet and outlet were competed accurately by using void fraction analysis of the time series, Power Spectral Density (PSD), tomographic images observed by the sensor program, and the Probability Density Function (PDF) method. The flow regimes of vertical flow lines at the bend inlet were observed as bubbly, cap-bubble, slug, and churn flow, whereas the flow regimes of the horizontal flow line at the bend outlet were characterized as having stratified, stratified wavy, bubbly, plug, slug, wavy annular, and semi-annular flow due to the gravity and bend effects.

Experimental investigation of intermittent airflow separation and microscale wave breaking in wavy two-phase pipe flow

We perform an experimental analysis of co-current, stratified wavy pipe flow, with the aim of investigating the effect of small scale wave breaking (microscale breaking) on the airflow. Particle image velocimetry is applied simultaneously in the gas and liquid phases. Active wave breaking is identified by high levels of vorticity on the leeward side of individual waves, and the statistics of the airflow above breaking and non-breaking waves are extracted from the gas-phase velocity fields. Keeping the liquid superficial velocity constant ($U_{sl}=0.1~\text{m}~\text{s}^{-1}$), we consider two experimental cases of different gas flow rates. The lowest flow rate ($U_{sg}=1.85~\text{m}~\text{s}^{-1}$) is slightly higher than the onset of microscale breaking, while the higher flow rate ($U_{sg}=2.20~\text{m}~\text{s}^{-1}$) is within the regime where wave breaking is observed to be frequent, and the root-mean-square interface elevation $\unicode[STIX]{x1D702}_{rms}$ is independent of gas flow rate. Results show that for the lowest gas flow rate considered, active wave breaking has a stabilizing effect on the airflow above the waves, reducing the sheltered region on the leeward side of the wave and the turbulence above the wave crest compared with non-breaking waves at similar steepness. At the higher gas flow rate the effect of active wave breaking is found to be small, and the main geometrical properties of the waves are found to dominate the evolution of the separated flow region.

Effect of helical micro-fins on two-phase flow behavior of R410A evaporating in horizontal round tubes obtained through visualization

This paper presents a new approach in visualization of the evaporation mechanism and the two phase flow. The visualization test section is a transparent micro-fin tube made by 3D printing. The 3D printing technology helps reproduce the real geometry in commercial metal micro-fin tubes. Even though the wall material is different from the real metal, this new method provides a more complete understanding of the mechanisms of flow boiling and the heat transfer enhancement due to micro-fin geometry. To investigate the effect of micro-fin geometries on two-phase flow behavior, R410A flow boiling experiments are conducted at 10 °C saturation temperature and visualized through a clear smooth tube and micro-fin tubes of 0°, 10° and 18° helix angles. The results showed that the annular flow pattern in the helical micro-fin tube occurred at a lower vapor quality than the smooth and axial micro-fin tubes. Helix angle affects the transition boundary of stratified wavy flow to wavy annular flow, which shifts downward (to lower mass flux and vapor quality conditions) as the helix angle increases. Other transitions are not significantly affected by the helix angle. Helical micro-fin tubes with different groove directions were made to understand the flow behavior on the upward side and downward side of the tube.

New turbulence modeling for air/water stratified flow

The prediction of interfacial turbulence characteristics is one of the still challenging of two-phase stratified flow. The evaluation of some important parameters such as interfacial heat transfer coefficient based on turbulence kinetic energy and turbulence dissipation rate in some models, intensifies the importance of turbulence flow correct simulation. High gradient of velocity and turbulence kinetic energy at the interface of two-phase stratified flow leads to a major overestimation or underestimation of flow characteristics without any special treatment. Consideration of a source function of turbulence eddy frequency at the interface is one of the common solution employed in past researches. Although this solution remedies some shortcomings of traditional methods in smooth stratified flow, its application in wavy stratified flow needs the other modifications. The examination of turbulence characteristics near the free surface reveals that, in addition to turbulence eddy frequency, the other source function of turbulence kinetic energy should be considered near the free interface. So, a new source function of turbulence kinetic energy is proposed at the interface based on flow condition. This new method has been employed for Fabre et al. (1987) experiment designed for air/water stratified flow. The results of simulation have a good agreement with experimental data and turbulence characteristic can be captured near the free surface.

A MULTIPHASE FLOW LOOP DESIGN FOR INVESTIGATING THE PARTIAL PHASE SEPARATION IN A PIPE TEE

This article presents a design and commissioning of a multiphase flow loop, which was developed for scrutinizing the partial phase separation characteristics of pipe Tees. Its length is 9m and its primary diameter is 0.078m (3 inches). For the ease of modification, its design was kept modular, so that it could be used for testing various other pipe profiles. To validate this flow loop, the separation of a stratified-wavy flow was tested in a regular diameter ratio pipe Tee, and the gathered results were compared with previously published data. A good agreement was observed between the two data sources, which suggests that this flow loop is suitable for a further experimentation.

Experimental investigation of droplet entrainment and deposition in horizontal stratified wavy flow

An experimental study of the local droplet parameters for an atmospheric horizontal stratified wavy flow was conducted in a horizontal circular pipe with an inner diameter of 40 mm and a length of 5.5 m. The local distributions of the droplet fraction, droplet velocity, droplet diameter, and droplet mass flux in the cross section of the pipe were measured using single optical fiber probes at four locations with length-to-diameter ratios of 17.5, 55.0, 80.0, and 105.0 from the inlet of the test section. The average droplet mass flow rate along the axial direction of the flow channel was also obtained from the droplet parameters. The combination of the droplet entrainment and deposition rate models of Schimpf et al. was evaluated for droplet mass flow rates obtained from five different experiments including the present experiment in the horizontal stratified flow. However, its prediction accuracy was not good for the present and REGARD data. Finally, new droplet entrainment and deposition rate models for the horizontal stratified flow were proposed based on those five experiments. The applicable ranges of the proposed models are 65,500–571,400 and 170–11,000 for the gas (Reg) and liquid (Rel) Reynolds numbers, respectively.

Experimental study of gas-liquid two-phase wavy stratified flow in horizontal pipe at high pressure

The regimes of gas-liquid two-phase flow in horizontal pipe at high pressure were experimentally investigated, and characteristics of wave stratified flow, such as wave length, wave height and propagation speed were calculated. The pressure drop was analyzed and friction factors, such as gas-wall, liquid-wall and interfacial friction factors, were calculated and new correlations were deduced. Influence of interface wave on interfacial friction factor was studied, and characteristics of interface wave were included to deduce new correlation for it, and good accuracy was proved. For the gas-water two-phase flow at 2 MPa, transition from wave stratified to annular flow appears with smaller gas superficial velocity compared with that at atmosphere, and intermittent flow appears with greater liquid superficial velocity. Regime transition models are tested, for the gas-liquid two-phase flow at atmosphere the sheltering coefficient with value of 0.01 is appropriate. However, it is promoted to be 0.06 at 2 MPa, and good performance of regime transition model was reached.

Experimental investigation of multiphase separation in different flow regimes through T-junction with an expander section

The experimental data for phase separation of the air-water mixture in a T-Junction with the expander section after the branch arm is presented in this work. The main and run arms of the T-junction are directed along the horizontal plane with the branch arm positioned in the vertical plane. The diameter of the main arm is 74 mm, with diameter ratio(s) of, 0.67, and 0.33 in relation to branch arm. At the inlet section of the T-junction, the flow regimes generated were stratified, stratified wavy and slug flow. At the inlet, the air and water superficial velocities are in the range of 0.25 - 0.140 m/s and 0.14-0.78 m/s respectively. The effect of the expander section after the branch arm, the air superficial velocity USA and water superficial velocity USw on liquid carryover (WL3/WL1)max in branch arm have been studied. Based on the experimental data obtained for T-junction with expander section, complete phase separation of air and water was observed in stratified and stratified wavy flow for all superficial velocities and improved phase separation for slug flow. In slug flow, increasing the liquid superficial velocity improves the phase separation but increasing the gas velocity decreases the phase separation. Finally, the volume weighted phase in this new T-junction design is compared with the phase separation data of a simple T-junction.

Improvement of horizontal stratification criteria in the SPACE code

The flow regime for two-phase flow plays an important role in nuclear safety analysis, because different physical models for two-phase flow are used depending on the flow regime. This paper deals with the flow regime criteria for horizontal stratification, which frequently occurs in the hot or cold leg during a loss-of-coolant accident (LOCA). Although there are several horizontal stratification criteria based on the Kelvin-Helmholtz theory, the condition for the onset of waves leading to a wavy-stratified flow or a non-stratified flow is still unclear. This study approached the horizontal stratification criteria from the perspective of viscous/inviscid flow instability theories. The criterion between smoothly stratified flow and transition region was adopted with viscous Kelvin-Helmholtz (VKH) analysis. The onset of ill-posedness for the governing equation set, which is related to the inviscid Kelvin-Helmholtz (IKH) analysis, was used to determine the transition criterion between the transition region and non-stratified flow. These criteria were incorporated into the SPACE thermal-hydraulic system code. Simulation results show good agreements with experimental data, and the predictability of horizontal stratification is thereby further improved.

Capacitance-Based Monitoring of a Three-Phase Crude-Oil Flow

A simple solution is presented for identifying the phase fraction and flow rate of a petroleum carrying pipeline with three-phase stratified smooth and wavy flow. The proposed method mounts a set of semicylindrical electrodes on the internal surface of the Plexiglas pipe to monitor the total capacitance and resistance of the three-phase contents. The phase fractions are determined by relating these electrical properties with the phase angles associated with the interface between water, oil, and gas phases of the mixture. To calculate the flow rate, measured capacitances from two sets of electrodes with known separation distance are cross-correlated to determine time delay. Calculated and measured results demonstrated close agreement. The percentage error exhibited by the proposed inclined capacitive plate technique is calculated and compared with published values.

Horizontal Air-Water Two-Phase Flow Measurement Using an Electrical Impedance Probe and a Venturi Flow Meter

Two-phase flow rate measurement is extremely important in many industrial sectors (nuclear, chemical, oil and gas, etc.). Nevertheless, the instrumentation required to perform these measurements is still very limited despite the huge research efforts during the last years. In the frame of the development of instrumentation for the two-phase flow rate measurement, the main aim of this study is the characterization of a spool piece consisting of a Venturi Flow Meter, and two Electric Capacitance Probes mounted in a horizontal test section, where an air-water two phase flow occurs. The probes response is analyzed versus superficial velocities of liquid and gas, flow patterns and void fraction. The test section consists of a nominal ID 80 mm horizontal transparent pipe equipped with a Venturi Flow Meter, a new capacitance probe with concave electrodes and a SIET designed electrical capacitance probe with ten long linear electrodes. In this experimental campaign, the air flow rate is in the range 0.048-0.119 kg/s (superficial velocity from 8 to 19 m/s) and the range of water flow rate is 0.0040-0.0535 kg/s (superficial velocity from 0.0006 m/s to 0.0103 m/s). These ranges of superficial velocities correspond to wavy stratified and annular-dispersed flow patterns along the test section.

Experimental study of horizontal two- and three-phase flow characteristics at low to medium liquid loading conditions

An experimental study is conducted using a 0.075-m ID pipe to investigate characteristics of two- and three-phase stratified flow in a horizontal pipeline. Experiments are conducted under low to medium liquid loading conditions which is common in wet-gas and long transportation pipelines. The flow characteristics investigated include flow pattern, liquid holdup and pressure drop. The experimental range covers superficial gas Reynolds numbers from 6314 to 200,734, superficial liquid Reynolds numbers from 160 to 4391 and water-cut values from 0 to 90%. Differential pressure transducers, quick closing valves and a high-speed camera are utilized to obtain the relevant data and the trends investigated. The observed flow patterns are stratified smooth, stratified wavy and stratified-annular flow. The transitions between flow patterns vary as a function of water-cut. The effect of water-cut on liquid holdup and pressure drop were relatively negligible especially at low water-cut conditions and the fine mixing of the oil-water mixture may be partially responsible for this. As a result, with the exception of flow pattern transitions, the performances of classical two-phase flow models (for the prediction of liquid holdup and pressure drop) appear unaffected when applied to air–oil–water 3-phase flows especially at high water-cuts.

Microscale wave breaking in stratified air-water pipe flow

We perform an experimental analysis of two-phase stratified wavy pipe flow, with the aim to detect and quantify the effect of small scale wave breaking. Particle image velocimetry is employed to analyze the velocity fields below individual waves, and a threshold for the vorticity on the leeward side of the crest is used to assess active wave breaking. Keeping the liquid flow rate constant, we analyze five experimental cases with increasing gas flow rates. The cases span the flow map from when first interfacial waves are observed, to the “amplitude saturation” regime, where the rms interface elevation is independent of the gas flow rate. While some wave breaking events are observed also in the wave-growth regime, wave breaking is found to be much more frequent when the gas flow rate is increased into the amplitude saturation regime, and 35%-40% of the waves passing the measurement section are assessed to be in a state of active breaking in this regime. A conditional averaging of the flow field is performed, and the turbulent dissipation rate below breaking and non-breaking waves is estimated. The effect of microscale breaking is observed down to a depth of 10 mm below the water surface. Below the crest of microscale breaking waves, the turbulent dissipation rate is increased by a factor 2.5 to 4 compared with non-breaking waves. This fraction increases with Usg, implying that the breaking events become more energetic as the gas flow rate is increased.

An Experimental Investigation of Flow Patterns During Condensation of HFC Refrigerants in Horizontal Micro-Fin Tubes

Condensation heat transfer coefficients and flow regimes in two different horizontal micro-fin tubes are examined during the condensation of refrigerants R-134a and R-410A. The present investigation has focused on determination and prediction of condensation heat transfer coefficients and finding the interrelation between heat transfer coefficients and the prevailing flow regimes. During flow visualization, flow regimes have been captured using borosilicate glass tube at inlet and outlet of the test condenser using high speed digital camera. Stratified, stratified wavy, wavy annular, annular, slug and plug flows have been observed at different mass fluxes and vapor qualities of the refrigerants. The observed flow regimes are compared with the existing flow regime maps proposed by Breber et al. [Prediction of horizontal tube side condensation of pure components using flow regime criteria, J. Heat Transfer 102 (1980) 471–476], Tandon et al. [A new flow regime map for condensation inside horizontal tubes, J. Heat Transfer 104 (1982) 763–768.] and Thome et al. [Condensation in horizontal tubes, part 2: New heat transfer model based on flow regimes, Int. J. Heat Mass Transfer 46 (2003) 3365–3387.] Thome et al. [Condensation in horizontal tubes, part 2: New heat transfer model based on flow regimes, Int. J. Heat Mass Transfer 46 (2003) 3365–3387.] flow regime map shows good agreement with experimental data.

Chaos in wavy-stratified fluid-fluid flow.

We perform a nonlinear analysis of a fluid-fluid wavy-stratified flow using a simplified two-fluid model (TFM), i.e., the fixed-flux model (FFM), which is an adaptation of the shallow water theory for the two-layer problem. Linear analysis using the perturbation method illustrates the short-wave physics leading to the Kelvin-Helmholtz instability (KHI). The interface dynamics are chaotic, and analysis beyond the onset of instability is required to understand the nonlinear evolution of waves. The two-equation FFM solver based on a higher-order spatiotemporal finite difference scheme is used in the current simulations. The solution methodology is verified, and the results are compared with the measurements from a laboratory-scale experiment. The finite-time Lyapunov exponent (FTLE) based on simulations is comparable and slightly higher than the autocorrelation function decay rate, consistent with previous findings. Furthermore, the FTLE is observed to be a strong function of the angle of inclination, while the root mean square of the interface height exhibits a square-root dependence. It is demonstrated that this simple 1-D FFM captures the essential chaotic features of the interface dynamics. This study also adds to a growing body of work indicating that a TFM with appropriate short wavelength physics is well-behaved and chaotic beyond the KHI.

Theoretical and experimental research on interfacial shear stress and interfacial friction factor of gas-liquid two-phase wavy stratified flow in horizontal pipe

The external flow field over wave interface is reached by the aid of conformal transformation thought, and velocity boundary layer of inner flow field is deduced also, which gives fundamental description of shear stress over wave interface. The viscous drag coefficient is induced with considering the characteristics of interface wave, such as wave height, wave length and ratio between wave lengths of both ascending and descending semi periods, which significantly impact the velocity gradient over the wave interface that accounts for the differences in distribution of local shear stress. The influence of fluid flow is studied also, which gives support that the earlier separating point of the fluid flow over wave interface the stronger depression to the turbulent perturbation is made, and a reduced drag force arrived. There are otherness and commonness between fluid flow over wave interface and interface appearing in air-water wave stratified flow, and a new model is proposed to intrinsically construct the interfacial friction factor encountered in wave stratified flow. The models for viscous drag coefficient and interfacial friction factor are tested, different models are studied, predicted values are compared with experimental data and that computed by solving there-dimension unsteady Navier-Stokes equations, which gives proof that the values predicted by newly proposed models well fit the real values.

Numerical Study on Diesel Oil Carrying Water Behaviors in Inclined Pipeline Based on Large Eddy Simulation

Water accumulation in low-lying areas of product oil pipelines always leads to corrosion damage of pipelines. Predicting the law of product oil carrying water can provide reference for the optimization of pipeline operation parameters and corrosion protection. A Volume of Fluid (VOF) model of oil-water two-phase flow based on Large Eddy Simulation (LES) was established to investigate the diesel oil carrying water behaviors in inclined uphill pipes. Through comparative study of experimental and simulation results based on Reynolds-Averaged Navier-Stokes (RANS) method, it is proved that the two-phase flow model based on LES-VOF can capture flow pattern characteristics such as wavy stratified flow, stratified flow and dispersed flow under different flow conditions. The model was used to simulate the oil carrying water process at different inlet oil flow velocities and inclination angles of uphill pipe on experimental scale. It was found that the shear rate of the oil-water interface increases with the increase of the inlet oil velocity, which leads to the increase of oil carrying water capacity. With the increase of slope angle, the two-phase flow pattern gradually transits from wavy stratified flow to dispersed flow. The model established in this paper provides a new method for the study of oil carrying water in product oil pipeline.

Simulation of High Viscosity Gas-Liquid Two-Phase Flow in a Horizontal Mini Pipe

Two-phase flow is used in many industries such as nuclear reactors, boilers, condensers, liquefactions of natural gas, etc. Two-phase flow is a flow in a pipe which has two states of fluid such as solid-liquid, liquid-gas, gas-solid. In a two-phase flow, there are three channels, namely vertical, horizontal and inclined channels. In the horizontal channel, the most widely found flow is the flow patterns of stratified flow, bubble flow, plug flow, stratified wavy flow, annular flow, and slug flow. Refer to the previous research above, the flow patterns were mostly obtained by using an experimental study. The advantage of using the simulation is the ability to predict the flow pattern and pressure gradient before doing the experimental study so it can be known earlier if it will have an insecure flow pattern, i.e. slug flow. This research was conducted to find the flow pattern and pressure gradient by using a Computational Fluid Dynamics (CFD) software, the Ansys Fluent 19.0 Student. The model which was used is the Volume of Fluid (VOF) with the fluid of air-water and glycerin (40%-70%). The length of the pipe was 200 mm, the inner diameter was 1.6 mm, and the length of the test section was 100 mm. Liquid superficial speeds (J L ) of 0.033 m/s; 0.149 m/s; 0.232 m/s; 0.539 m/s; 0.7 m/s; 2.297 m/s and 4.935 m/s were used, while the air superficial speed (J G ) was 9.62 m/s. The result of the simulation showed slug annular and churn flow patterns. Slug annular was formed at J L = 0.033 m/s; 0.149 m/s and 0.232 m/s with the glycerin content of 40% and 50%. Slug annular pattern was formed when the glycerin content was 60% and 70% with J L = 0.539 m/s. Viscosity affects the flow pattern, the higher the glycerin content, the higher the viscosity and the more fluid than air. The higher the J L and glycerin content, the higher the pressure gradient.