Horizontal Air Water Research Articles

Experimental Study on Two-Phase Countercurrent Flow Limitation in Horizontal Circular Pipes

The two-phase countercurrent flow limitation (CCFL) in horizontal channels is important in relation to nuclear reactor safety. In this study, we aim to investigate the CCFL characteristics and the flow behaviors in horizontal circular pipes with small diameters. The effects of pipe diameter and the water head in the upper plenum on CCFL characteristics are experimentally studied. An image-processing technique and statistical treatments are implemented to analyze the horizontal countercurrent flow. The results show that the CCFL characteristics for the horizontal circular pipes with small diameters can be well correlated using the dimensionless parameters, which are based on adding fluid viscosity to the Wallis parameters. The CCFL characteristics are significantly affected by the pipe diameter and are slightly affected by the water head above the horizontal pipe. The gas–liquid interface fluctuates with certain periods, and flow pattern transitions happen in the horizontal air–water countercurrent flow. As the air flow rate increases, the occurrence location of the liquid slug appears to shift towards the water entrance. In addition, the further away from the water entrance, the lower the average of liquid holdup.

Improvement of the two-fluid momentum equation for turbulent bubbly flows

A two-fluid model is widely employed to predict gas–liquid two-phase flows. Conventional two-fluid equations, derived by assuming that all phases are interpenetrating continua, occasionally yield impractical results. For example, the bubble velocity is predicted to be higher than the water velocity for a horizontal fully developed bubbly flow. One solution for this irrational result is two-fluid momentum equations based on the motion of fluid particles. This study revisits the particle-motion-based two-fluid equations. Previously, it was validated only with respect to the difference between the bubble and liquid velocities. Recently, we observed an erroneous prediction of the void fraction distribution by the existing particle-motion-based momentum equations. This study, through rigorous analysis, discovered that this error could be resolved by neglecting the turbulent kinetic energy term in the particle-motion-based equation for the gas phase. Subsequently, the revised momentum equations were applied to the vertical and horizontal air–water bubbly flows. The conventional and revised momentum equations produced similar results for upward bubbly flows because the relative velocity between the bubble and water phases was mainly governed by buoyancy. For horizontal bubbly flows, the revised momentum equations led to a downward shift in the liquid velocity distributions, which was attributed to the modified momentum diffusion term. At a higher modification factor of turbulent dispersion, the velocity distributions exhibited an upward shift. We conclude that the optimization of physical models should be based on the revised two-fluid momentum equations.

Uncertainty analysis for multiphase flow: A case study for horizontal air-water flow experiments

This work describes the procedure used to define the measurement uncertainties of horizontal two-phase air-water flow experiments conducted to determine influences due to pipe diameter on pressure gradient on such flows. These experiments were performed with 4 different pipe diameters, always using the same test section length, therefore varying the length-to-diameter (L/D) ratio. Several parameters were measured, such as volumetric/mass flow rate, pressures, temperatures and pressure drop; other parameters were calculated, such as the superficial velocities of each fluid, as well as their corresponding properties. The main parameters studied were the flow patterns for different velocity configurations and the two-phase pressure drop to be used for model improvement, thus the importance of uncertainties analysis. The sources of uncertainty were defined, detailed, systematically studied and quantified. Also, the reproducibility capacity of the experimental setups were analysed through the uncertainty analysis and proving them to be able for future similar studies. The flow maps with their uncertainties could help understand the thresholds for each defined flow pattern region, and the plots of two-phase pressure drop variation with diameter confirmed the homogeneous model as a possible approach to calculate pressure drop if the uncertainties are considered.

Measurement of void fraction in pipes by nuclear transmission based techniques

In this study, the application of nuclear transmission based techniques to the measurement of two-phase flow was evaluated and the experiment setup was described. Because neutrons are appreciably affected by interaction with hydrogen atoms, techniques based on transmission of thermal and fast neutron are particularly suitable for use with water or any other hydrogenous fluids. Based on the effective of water for gamma-rays attenuation on mass basis, transmission technique based on measuring of total gamma-rays has been examined. The beams of fast neutrons, thermal neutrons and gamma-rays emitted from 252Cf neutron source of strength 1.3 × 107 n/s were used. Void fraction is measured for steady-flow regimes in a horizontal air–water flow. Results are evaluated and the strength and limitations for each technique are given and discussed.

Numerical Study on Impulsive Pressure Fluctuations of Horizontal Air-Water Two Phase Slug Flow under Submerged Outlet Condition

Numerical Study on Impulsive Pressure Fluctuations of Horizontal Air-Water Two Phase Slug Flow under Submerged Outlet Condition

Pressure Drop of Horizontal Air–Water Slug Flow in Different Configurations of Corrugated Pipes

Abstract Corrugated pipes (CP) have regularly shaped and spaced cavities on their internal walls that can induce dynamic changes in the flow, such as increased pressure drops. Offshore petroleum production pipelines are an example of an industrial application of CPs, known as flexible lines. Slug flow is the most challenging flow pattern in those lines due to its complex hydrodynamics. A number of previous studies proposed correlations to predict the two-phase flow pressure drops in smooth pipes (SPs). However, limited researches have evaluated the pressure drops associated with liquid–gas slug flow in CPs. In this work, experiments to analyze the pressure drops in horizontal air–water slug flow under different configurations of CPs were carried out. The tests were performed in three different CP internal diameters (IDs) (26, 40, and 50 mm) with different cavity widths (1.2, 1.6, and 2.0 mm). The effects of the internal diameters and the cavity widths on the pressure drops associated with slug flow were analyzed. Results demonstrated that the pressure drops increase with increasing cavity widths. The experimental data were fitted and a pressure drop correlation using the concept of multiplier factor was proposed. Comparisons between predictions and the experimental data proved to be within ±10% accuracy.

Pressure Drop and Void Fraction in Horizontal Air–Water Stratified Flows with Smooth Interface at Atmospheric Pressure

This work presents and analyses the results of an experimental activity aimed at the characterization of stratified air–water flow conditions, which have been poorly analyzed in previous studies although they are significant for industrial applications. Tests were performed in a 24 m long, 60 mm inner diameter PMMA pipe; the superficial velocities ranged between 0.03 m/s and 0.06 m/s for the water and between 0.41 m/s and 2.31 m/s for air. The pressure gradient along the pipeline was determined and compared to the one obtained implementing two-fluid models available in the literature. Fair agreement with the models was found only at high values of the superficial gas velocities, i.e., above 1.31 m/s. Moreover, the void fraction was measured through a resistive probe and compared with the values predicted by available models. Since none of them was able to satisfactorily predict the void fraction in the whole range of superficial velocities, a drift flux model was successfully implemented. Eventually, with both the measured pressure gradient and the void fraction, a two-fluid model was implemented in order to determine the interfacial shear stress and to compare the outcome with the literature, emphasizing the influence of the operating conditions on the prediction performance.

Statistical features of the flow evolution in horizontal liquid-gas slug flow

The occurrence of the slug flow pattern is frequently detected in the petroleum industry. Theoretical and experimental works on the complex behaviour of this flow pattern are abundantly found, and their aim is to advance the state-of-the-technique on the behaviour of this kind of flow. The goal of the present work is to provide experimental data collected in five probing stations with resistive sensors mounted on a horizontal air–water slug flow in a 25.8-mm ID pipe. A signal processing technique provides the translational velocities, frequencies and void fractions that arise from different gas and liquid volumetric flow rates. Based on the obtained results, the effects of the liquid and gas superficial velocities were correlated with the probability density functions (PDFs) of the parameters variations observed in the slug flow development along the pipe. A quantitative criteria based on maximum likelihood estimates is used to determine the Weibull distribution for the translational velocity and the lognormal distribution type for the slug frequency distribution type.

Multi-Fluid VoF model assessment to simulate the horizontal air–water intermittent flow

Numerical simulation of the plug and the slug flow regimes have been performed using the Multi-Fluid VoF and Shear Stress Transport (SST) k–ω models in a horizontal pipe with 44mm diameter. The superficial velocities during the current study were set at 0.16, 1.64 & 3m/s for the gas phase and 1m/s for the liquid one. The pressure and the velocity equations were solved together, utilizing the PIMPLE algorithm in all cases of the present study. The VoF model, as well as the experimental outcomes, were applied to assess the results of Multi-Fluid VoF model. The qualitative comparison of numerical results with the experimental visualization revealed that the Multi-Fluid VoF model simulates precisely the interfacial structure of slug and plug flows, as well as the chronological formation of the slug. The length ratio of the gas slug to the liquid slug goes up with the gas superficial velocity increment; this issue increases the probability at low liquid hold-ups and decreases the higher ones. The Multi-Fluid VoF provided much more matching with the mentioned probability changes in comparison to the VoF model. The pressure drop calculated using Multi-Fluid VoF in the Lockhart–Martinelli framework illustrated a 21.8% improvement averagely in comparison to the VoF model. The obtained transitional slug velocities showed that the Multi-Fluid VoF model presented a maximum error of 8.17% versus the experimental values, while the mentioned error for the VoF model was estimated at 22.01%. Notwithstanding the mentioned advantages, the Multi-Fluid VoF model significantly increased, both the execution time of simulation as well as the associated costs.

Gas–Liquid Flow Rate Measurement Using a Twin-Plane Capacitive Sensor and a Venturi Meter

We introduce a flow meter to monitor the flow rate of individual phases in gas-liquid flows. The meter combines pressure data from a Venturi tube and void fraction data from a twin-plane capacitive sensor. By the twin geometry of the capacitive sensor it is possible to cross-correlate the two signals and estimate gas flow rate. A simple algebraic approach, which considers no-slip condition between phases and disregards influence of gas phase, combines pressure fluctuations of the Venturi meter with the measured phase fraction to estimate liquid flow rate. The proposed meter has been tested in horizontal air-water flow loops, 1-inch and 2-inches internal pipe diameter, in permanent and transient regimes. Calculated values were compared to reference measurements. The proposed meter performs well determining liquid flow rate, independent of the flow pattern, in the whole measured range with some more discrepancies at lower values of liquid flow rate. Gas flow rate presented good agreement with reference measures whereas here mainly slug flow pattern has been investigated. RMSD% in steady-state condition was 6.5% for liquid and 20.6% for gas, being satisfactory for many applications, considering the fact the introduced flow rate meter operates without the need for any adjustment based on reference measurements or previous knowledge/assumption of the gas/vapor quality parameter, as has been usual for other similar recent developments.

A robust image analysis technique for the study of horizontal air-water plug flow

The current work develops a robust image analysis technique for quantitative measurements in horizontal plug flow. To do this, experiments are performed to capture high-speed videos for horizontal air-water plug flow for two test conditions in a 38.1 mm ID pipe. A visualization block-mirror system is designed and fabricated to allow the simultaneous visualization of two-phase flow from both top and side views using a single camera. To obtain images of high quality for analysis, the angle and position of the camera relative to the visualization block are verified. With the obtained images, an algorithm is developed to detect and track the interface of plug bubbles. Due to the existence of contacting or overlapping small bubbles, additional algorithms are developed to segment the interfaces of plug bubble from those of small bubbles, including distance checking, interface breakpoints detection, and convex curvature segmentation. As a result, the nose position, local time-averaged axial velocity, and area-averaged time-averaged void fraction for plug bubbles can be obtained with this newly developed image analysis technique. To benchmark the newly developed image analysis code, local four-sensor conductivity probe measurements are performed. The axial velocity for plug bubbles generally agrees within ±10% between image analysis technique and the four-sensor conductivity probe measurement technique, while the plug bubble reconstruction approach can generate an area-averaged void fraction within ±10% of the probe measured value.

Experimental study of horizontal air-water plug-to-slug transition flow in different pipe sizes

The current work investigates the plug-to-slug transition in horizontal air–water two-phase flow in small (38.1 mm) and large (101.6 mm) diameter pipes. An extensive database is established to study the local interfacial structure in plug-to-slug transition flow. Detailed measurements across the flow area are performed for nine and six test conditions in small and large pipes, respectively, at three different axial locations downstream of the inlet using the local four-sensor conductivity probe. The effects of jg, jf, development length and pipe size are investigated. It is found that the number of small bubbles in the liquid plug/slug increases significantly in plug-to-slug transition with increasing jg, which are generated by the strong shear between the gas slug and liquid film. Due to the relative motion, these small bubbles either coalesce with the nose of the following plug/slug bubble, or slide between the plug/slug bubble and the wall, and then travel around the pipe circumference to reside beneath the large bubbles. This explains the large number of small bubbles observed at the top of the liquid film for the conditions at high gas flow rates. In the process of traveling downwards, some of the small bubbles coalesce with the slug bubbles. It is also found that increasing jg or jf decreases the size of the small bubbles. While shearing-off is believed to dominate as jg increases, turbulent-impact is enhanced as jf increases due to the increasing turbulence level in the liquid phase. Increasing jg, development length, or decreasing jf slightly increases the depths of the plug/slug bubbles; however, significant growth of plug/slug bubbles is observed in the axial direction. For the same condition, the contribution from large bubbles to total void fraction increases as pipe size increases, while the distribution of total void fraction is similar. The size of both small and large bubbles is found to be larger in the large diameter pipe. Due to the current bubble injection mechanism, small bubbles are generated at the inlet; they coalesce into large bubbles as the flow develops. The large bubble is found to accelerate as it grows along the axial direction, which can lead to a decreasing void fraction although pressure keeps decreasing.

Effects of pipe size on horizontal two-phase flow: Flow regimes, pressure drop, two-phase flow parameters, and drift-flux analysis

This paper performs experimental studies to investigate the effects of pipe size on horizontal air–water two-phase flow in a wide range of flow configurations. Some of the major characteristic two-phase flow phenomena of interest include flow regime transitions, two-phase frictional pressure drop, local interfacial structures, and drift-flux analysis. Based on the experimental results obtained in two pipe sizes, the flow regime maps and the correlations for two-phase frictional pressure drop employed in conventional nuclear reactor system analysis codes (TRACE and RELAP) are evaluated. It is found that the flow regime maps in the codes incorrectly predict plug and slug flows as bubbly flow. Effects of pipe size on flow regime transitions are observed, while these effects are not reflected on the maps employed in the codes. The frictional pressure drop analysis suggests that the closure relations used in both TRACE and RELAP can be improved for horizontal two-phase flow. The local time-averaged two-phase flow parameters acquired by a four-sensor conductivity probe including void fraction, interfacial area concentration, bubble velocity, and bubble Sauter-mean diameter provide quantitative descriptions for the interfacial structures in different pipe sizes. It is found that increasing the pipe size tends to cause bubbles to be more concentrated near the top wall in bubbly flow. As a result, the bubble layer occupies a smaller portion of the flow area as pipe diameter increases. Based on the experimental database, drift-flux analysis using both the 〈〈vg〉〉 vs. 〈 j〉 and 〈α〉 vs. 〈β〉 methods is performed. The effect of pipe size on drift-flux analysis is found to be negligible. The global slip is found to be less than one, indicating that the gas phase moves slower than the liquid phase in horizontal bubbly flow. This analysis also provides a predictive method to estimate the void-weighted bubble velocity and void fraction, which can be predicted with an average percent difference of ±3.3% and ±3.1%, respectively.

Flow disturbances induced by an orifice plate in a horizontal air-water flow in the slug regime

An experimental study of the flow disturbances induced by an orifice plate in an air-water slug flow is carried on a horizontal test section with 0.026 m and 1009 pipe diameters long. Three orifices with distinct area contraction ratios are tested at three axial positions along the test section. The instantaneous orifice pressure difference as well as the absolute pressure and void fraction along the test section are measured. The data is further processed to get the bubble nose velocities and the slug frequency. The pressure difference at the orifice and the absolute pressure along the test section reveal a pulsatile pressure behavior due the intermittent passage liquid slugs and elongated bubble through the orifice. The spectral analyses disclose a match between the frequency of passage of slugs through the orifice and the frequency of the orifice pressure fluctuations. The orifice disturbances on the void fraction, bubble nose velocity and frequency of passage exhibits a dependency on the orifice size and to the distance from the mixer.

Experimental study of interfacial structure of horizontal air-water two-phase flow in a 101.6 mm ID pipe

The current work seeks to investigate the interfacial structure and establish an extensive experimental database in horizontal air–water two-phase flow in a 101.6 mm inner diameter pipe. A wide range of flow configurations are studied including bubbly, plug and slug flows. A flow visualization study using the high-speed video camera enables qualitative description of bubbly-to-plug and bubbly-to-slug transitions, while the database of local time-averaged two-phase flow parameters obtained by the four-sensor conductivity probe enables quantitative study on the evolution of the flow. Detailed measurements across the flow area are performed for nine test conditions at three different axial locations downstream of the inlet. Using this database, the effects of superficial liquid and gas velocities, and development length on the evolution of interfacial structure are investigated. Similar characteristics are observed as in a counterpart 38 mm ID horizontal two-phase flow facility, which include (1) the bubbles are found to be more concentrated near the top wall in bubbly flow as superficial liquid velocity decreases at a constant superficial gas velocity, while increasing superficial gas velocity promotes the growth of bubble layer thickness; (2) in bubbly-to-plug transition, the void fraction of small bubbles decreases and the size of small bubbles increases, while in bubbly-to-slug transition, opposite trends are observed. Meanwhile, different characteristics on the evolution of the interfacial structure are also observed, which indicates the effect of increasing pipe diameter. The bubbly-to-plug/slug transition is found to shift to higher superficial liquid velocities as pipe diameter increases. It is observed that the gas phase is more concentrated near the top wall in the large diameter pipe. As a result, the distance between bubbles is smaller and there is a higher chance for bubbles to coalesce into large bubbles. The critical void fraction where the bubbly-to-plug/slug transition initiates decreases as pipe size increases.

Cellulose/PET humidifying elements having horizontal air and vertical water channels

New material and shape of a humidifying element were developed, which outperformed the widely- used criss-cross glasswool element. The new material consists of 50% cellulose and 50% PET. A parallel channel configuration was devised to reduce excessive pressure loss caused by the decreased height (5.0mm) from that (7.0mm) of the criss-cross configuration. For the same criss-cross configuration, the humidification efficiency of the cellulose/PET element was 26% higher than that of the glasswool element. For the same cellulose/PET material, humidification efficiency of the parallel channel configuration was 14% higher than that of the criss-cross configuration. As for the pressure drops, those of the glasswool element were 2–52% higher than those of the cellulose/PET element. For the same cellulose/PET material, the pressure drop of the parallel channel configuration was 14% higher than that of the criss-cross configuration. Data were compared with the predictions by existing correlations and those by the proposed model.

Characteristics of an interfacial wave in a horizontal air-water stratified flow

Interfacial wave parameters, in this case the frequency, height, velocity, and slope, were investigated experimentally in a horizontal air-water stratified flow. Experiments were conducted with a parallel wire conductance sensor and PIV visualization in a rectangular channel, of which the width and height are 40 mm and 50 mm, respectively. In the experiments, the flow condition covered the liquid Reynolds number Rel range of 450 to 3540 and the gas Reynolds number Reg range of 14,000 to 70,000. The results revealed that the observed wave types according to the flow conditions in the rectangular channel are similar to those in a horizontal pipe. The frequency, height, and slope of the interfacial wave show complicated tendencies according to the combination of Reg and Rel, which affects the coalescence and breakup of the wave. Specifically, the wave height and wave slope have opposite tendencies regarding the criterion of Reg = 34,000. For cases in which Reg ≥ 34,000, the interfacial drag force significantly affects the height and slope of the disturbance wave. In contrast, for Reg < 34,000, the growth of the wave has an important effect on the wave parameters. Finally, new empirical correlations for the frequency, height, and slope of the interfacial wave were proposed for application to the development of a droplet entrainment model in a horizontal stratified flow.

Do huge waves exist in horizontal gas-liquid pipe flow?

Huge waves are periodic interfacial structures which are observed in vertical co-current gas-liquid two-phase flow under churn and the transition between churn and annular flows. Published data examining vertical gas-liquid flow indicate that a huge wave has either a continuous gas core surrounded by a large-scale interfacial wave or a core with a highly-agitated mixture of gas and liquid.Employing a Wire-Mesh Sensor (WMS), the spatio/temporal investigation of high flow rate horizontal air-water flow divulged some recurrent liquid structures (one may call pseudo-slugs) analogous to huge waves of (vertical) churn flow. In both cases, the blow-through (penetration of gas into the liquid structure) was the most manifest feature.Different qualitative and quantitative methods were employed to compare the behavior of pseudo-slug to churn flow. The quantitative measures included Probability Density Function analysis (PDF), distribution coefficient in drift flux model, structural velocity, core average velocity, interfacial friction factor, and slippage number. Both flow regimes demonstrated similar behavior.

Experimental investigation of horizontal air–water bubbly-to-plug and bubbly-to-slug transition flows in a 3.81 cm ID pipe

The present study seeks to investigate horizontal bubbly-to-plug and bubbly-to-slug transition flows. The two-phase flow structures and transition mechanisms in these transition flows are studied based on experimental database established using the local four-sensor conductivity probe in a 3.81cm inner diameter pipe. While slug flow needs to be distinguished from plug flow due to the presence of large number of small bubbles (and thus, large interfacial area concentration), both differences and similarities are observed in the evolution of interfacial structures in bubbly-to-plug and bubbly-to-slug transitions. The bubbly-to-plug transition is studied by decreasing the liquid flow rate at a fixed gas flow rate. It is found that as the liquid flow rate is lowered, bubbles pack near the top wall of the pipe due to the diminished role of turbulent mixing. As the flow rate is lowered further, bubbles begin to coalesce and form the large bubbles characteristic of plug flow. Bubble size increases while bubble velocity decreases as liquid flow rate decreases, and the profile of the bubble velocity changes its shape due to the changing interfacial structure. The bubbly-to-slug transition is investigated by increasing the gas flow rate at a fixed liquid flow rate. In this transition, gas phase becomes more uniformly distributed throughout the cross-section due to the formation of large bubbles and the increasing bubble-induced turbulence. The size of small bubbles decreases while bubble velocity increases as gas flow rate increases. The distributions of bubble size and bubble velocity become more symmetric in this transition. While differences are observed in these two transitions, similarities are also noticed. As bubbly-to-plug or bubbly-to-slug transition occurs, the formation of large elongated bubbles is observed not in the uppermost region of bubble layer, but in a lower region. At the beginning of transitions, relative differences in phase velocities near the top of the pipe cross-section to those near the pipe center become larger for both gas and liquid phases, because more densely packed bubbles introduce more resistance to both phases.

Local characteristic of horizontal air–water two-phase flow by wire-mesh sensor

Two-phase flow widely exists in many industries. Understanding local characteristics of two-phase flow under different flow conditions in piping systems is important to design and optimize the industrial process for higher productivity and lower cost. Air–water two-phase flow experiments were conducted with a 16×16 conductivity wire-mesh sensor (WMS) in a horizontal pipe of a multiphase flow facility. The cross-sectional void fraction time series was analysed by the probability density function (PDF), which described the void fraction fluctuation at different flow conditions. The changes and causes of PDFs during a flow regime transition were analysed. The local structure and flow behaviour were characterized by the local flow spectrum energy analysis and the local void fraction distribution (horizontal, vertical and radial direction) analysis. Finally, three-dimensional transient flow fluctuation energy evolution and characteristic scale distribution based on wavelet analysis of air–water two-phase flow were presented, which revealed the structural features of each phase in two-phase flow.