Air-water Flow Research Articles

Detection and tracking of bubbles in two‐phase air water flow for non‐convergent sinusoidal channel

Bubble detection and tracking, which is essential for the enhancement of gas–liquid two-phase flow applications, is difficult due to optical noise surrounding bubble boundaries. Two-phase flow applications involving unconventional wavy channels further complicate these tasks by introducing non-linearity. In this work, the authors apply a path-based approach to transform a sinusoidal channel into a straightened horizontal channel for bubble detection and tracking. Segmented morphological operations are applied to the linear channel to identify each bubble as a single entity and to eliminate noise caused due to illumination conditions. The bubbles are detected through blob analysis and are associated across different frames based on motion, estimated by Kalman filter in both sinusoidal and path-based approach to the sinusoidal channel. The results show that the proposed path-based straightened channel gives more accurate results for the benchmark parameters of bubble count and bubble velocities than the sinusoidal channel with a percentage error reduction of 31.61% for bubble count and 27.36% for velocity.

Visualization Study of Air-Water Two-Phase Flow Pattern in Upward Vertical Flow of Square Channel

An important two-phase flow experiment study is carried out as a consideration in the application of piping system design that depends on two-phase flow. The purpose of this study was to determine the air-water flow pattern formed in a small square channel with vertical flow direction and to know the flow configuration. Channel 5 mm hydraulic diameter was used in this study. Bubble, slug, churn, and annular flow patterns are formed in the air superficial velocity range 0.33 - 7.31 m/s and superficial velocity of water is 0.13 - 1.2 m/s. A flow pattern map produced has not similarities with the mini-channel flow pattern map in previous studies. The velocity of bubbles and void fractions increases with the increase of superficial air velocity. The void fraction of present research has a good correlation to the results of predictions proposed by previous studies.

Two-phase flow measurements of an unsteady breaking bore

A key feature of breaking bores, jumps, and spilling breakers is the roller region, characterised by intense shear and recirculation, associated with air bubble entrainment and splashing. Detailed unsteady air–water flow measurements were conducted in a breaking bore propagating in a large-size channel, using an array of dual-tip phase-detection probes and an ultra-high-speed video camera. The results showed a steep roller front, with a very-dynamic air–water bubbly region, albeit with a relatively limited air–water roller region. In this study, a major challenge was the inconsistency in light intensity linked to the travelling nature of the bore. A simple flow visualisation technique was applied to retrieve the two-dimensional side-looking profile of the roller edge and average void fraction. The results were validated independently with a phase-detection probe. While the probe data lacked spatial variability, the study reinforces the needs of high-quality validation data set, including in unsteady transient flows.

Study of gas-liquid upward annular flow through a contraction

Transient response of two-phase flow is important in many analyses of nuclear reactor accident scenarios. In the present work, we study the response of vertical annular flow to a sudden or gradual pipe contraction. The three component fields of gas-liquid annular flow, namely, the liquid film, the entrained liquid droplets and the gas core, show different behaviour as they pass through an area change section because of their very different inertias. Pressure variation through the contraction is a result of the integrated effect of these responses. In order to throw light on these, pressure profiles along the upstream and downstream sections of the contraction have been measured in air-water flow through a vertical converging pipe section for diameter ratios of 1.5 and 2.0 and for half-cone angles of 8°, 15° and 90°. The gas-droplet field interaction has been studied using computational fluid dynamics simulations while the gas-film flow interaction has been studied using an analytical model for film flow. These studies highlight the role of droplet inertia as the principal contributor to the pressure change in the contraction. A semi-empirical model has been developed for the pressure loss coefficient in the contraction.

The liquid wave characteristics during the transportation of air-water stratified co-current two-phase flow in a horizontal pipe

The understanding of the interfacial wave structure and its effect to the interfacial friction factor is important to reveal the transition from stratified to the slug flows. However, the available theoretical of models, as well as the correlations to predict the wave interfacial characteristics, is quite rare. The discrepancy among researchers is often found. In the present work, the wave characteristics of air-water stratified flow were investigated experimentally. The inner diameter was 26 mm. The liquid superficial velocity (JL) raised from between 0.02 m/s and 0.075 m/s while the gas superficial velocity (JG) ranged from 4 m/s to 16 m/s. A high speed camera was used to capture high-quality visual data which was later processed with the image processing technique.As a result, the quantitative parameters of the stratified flow were successfully determined and analyzed. Next, the dimensional analysis was carried out to develop correlation to predict the flow parameters of the wave such as the wave frequency, the wave velocity, the wave amplitude, and the wavelength. Furthermore, it is found that the ratio of gas and liquid Reynolds number and Martinelli Parameter plays important role in almost all of the proposed correlation.

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.

Sand minimum transport conditions in gas–solid–liquid three-phase stratified flow in a horizontal pipe at low particle concentrations

Sand production in the life of oil and gas reservoirs is inevitable, as it is co-produced from reservoirs. Its deposition in petroleum pipelines poses considerable risk to production and can lead to pipe corrosion and flow assurance challenges. Therefore, it is important that pipe flow conditions are maintained to ensure sand particles are not deposited but in continuous motion with the flow. The combination of minimum gas and liquid velocities that ensure continuous sand motion is known as the minimum transport condition (MTC). This study investigates the effect both of sand particle diameter and concentration on MTC in gas/liquid stratified flow in a horizontal pipeline. We used non-intrusive conductivity sensors for sand detection. These sensors, used for film thickness measurement in gas/liquid flows, were used for the first time here for sand detection. We found that MTC increases with increase in particle diameter for the same concentration and also increases as the concentration increases for the same particle diameter. A correlation is proposed for the prediction of sand transport at MTC in air–water flows in horizontal pipes, by including the effect of sand concentration in Thomas’s lower model. The correlation accounts for low sand concentrations and gave excellent predictions when compared with the experimental results at MTC.

Experimental investigation of single- and two-phase pressure drop in slender rectangular 180° return bends

The pressure drop of single-phase water flows and two-phase air-water flows through slender 180° return bends with rectangular cross-section are investigated. The commonly used approaches consider return bends with relatively small curvature ratios whereas in this work the curvature ratios are 20, 40 and 58. The rectangular cross-section has a hydraulic diameter around 3.9 mm and an aspect ratio of 1.067 (4 mm × 3.75 mm). The measurements for the pressure drop are compared with other correlations. From the results, a relation is found to approximate the effects of curvature on the pressure drop with the principle of equivalent lengths. This relationship is also the base of the second proposed correlation of this work, which gives reasonable pressure drop predictions for air-water flows through slender 180° curved ducts. It has a maximum deviation below 35% for all considered values.

On void fraction and flow fragmentation in two-phase gas-liquid free-surface flows

In free-surface flows, the interactions between high-velocity liquid and atmosphere may lead to strong gas-liquid mixing and complex multiphase flow interactions. In this study, the void fraction power spectrum density (PSD) is used to provide an alternative view of the air-water flow composition. The high frequency range of the PSD curve reflects contributions of small chord times, while the low frequency range contains contributions of both small and large chord times. Likewise, the interactions between the smallest bubbles contribute approximately uniformly to the entire frequency range of the spectrum, while any interaction involving large bubbles/drops will be modulated by 1/f2. It is shown that the void fraction spectra are a powerful tool in providing an alternative view of the air-water flow composition.

Atomization parameters and fire suppression characteristics of water mist in simulated roadway

Water mist is considered to be one of the most effective ways to suppress fire in coal mines. The structure of an atomization device for water mist was determined based on preliminary research. By regulating the working conditions, including air pressure and water flow volume, we obtained water mist with varying particle size distributions, which proved to be the most important factor for fire suppression. Particle size distribution, characterized by a V99 and V50, changed from 50 to 1600 µm during the experiment at a given air pressure (0.6–0.16 MPa) and water flow volume (0.6–1.0 m3/h). The particle size distribution along an axial line can be divided into a wavy flow zone and a fully atomizing zone. Based on the experiments, three types of particle size distributions, 98, 498 and 1013 µm, were used to test the fire-suppression characteristics of water mist in a simulated roadway with a self-built platform. The result showed that water mist could suppress fire rapidly and the highest temperature dropped from 205 to 10 °C within 30 s.

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 Numerical Model for Bubble Column Reactors: Prediction of the Fractional Gas Holdup by the Implementation of the Drift-Flux Model

Comprehensive knowledge of hydrodynamics inside a reactor is crucial for the design and scale-up of bubble columns. Fractional gas holdup (αg) is an important parameter that should be obtained for the design of bubble column reactors. The estimation of this parameter depends mainly on experimental procedures. Drift-flux theory is one of the most practical and accurate models for calculating the gas holdup. Although many researchers have studied bubble column reactors, because of the limits of the experimental setting, there are few studies that have operated over a wide range of superficial gas velocities. In this work, a transient 3-D numerical simulation of upward air-water flow in the bubble column was performed over a wide range of superficial gas velocities (0.025-0.4 m/s) using the Eulerian-Eulerian model. The effect of the superficial gas velocity on the flow pattern was simulated, and two-phase flow regimes were classified into homogeneous, transition and heterogeneous regimes. Considering the importance of the drift-flux model, the values of the distribution parameter and the drift velocity were computed according to their definitions using the cross-sectional gas holdup and velocity profiles obtained via computational flow dynamic simulation. The results were verified against the experimental data, and a correlation is proposed for predicting the gas holdup.

Signals features extraction in radioisotope liquid-gas flow measurements using wavelet analysis

Knowledge of the structure of a flow is significant for the proper conduct of a number of industrial processes. In this case, a description of a two-phase flow regimes is possible by use of the time-series analysis in time, frequency and state-space domain. In this article the Discrete Wavelet Transform (DWT) is applied for analysis of signals obtained for water-air flow using gamma ray absorption. The presented method was illustrated by use data collected in experiments carried out on the laboratory hydraulic installation with a horizontal pipe, equipped with two Am-241 radioactive sources and scintillation probes with NaI(Tl) crystals. Signals obtained from detectors for slug, plug, bubble, and transitional plug – bubble flows were considered in this work. The recorded raw signals were analyzed and wavelet energy was extracted using multiresolution analysis. It was found that energies of wavelet approximation at 1-5 levels are useful to recognize the structure of the flow.

LABORATORY OBSERVATIONS OF DISSOLVED CARBON DIOXIDE TRANSPORT UNDER REGULAR BREAKING WAVES

Air bubbles and strong turbulence that form in water from breaking waves play important roles in gas transfer across the air-sea interface (Melville, 1996). The entrained bubbles increase the total area of air-water interface per unit volume and enhance local gas dissolution into water. The dissolved gases mix in the water mass diffuse by the strong turbulence. These gas transfer-enhancing factors have been parameterized by only wind speed in models of gas transfer velocity in the deep ocean. Bulk parameters based on wind speed cannot be used for a surf zone, where waves break due to shoaling. In a surf zone, the cross-shore distributions of entrained bubbles and the turbulent intensity vary as waves propagate. The physical process of gas transfer under the complex air-water turbulent flows in breaking waves has not been clarified. Thus, breaking-wave factors that enhance gas transfer in a surf zone cannot be parameterized. In this study, we observed the transport process of dissolved carbon dioxide (DCO2) under air-water turbulent flows in a laboratory surf zone using image measurement systems.

Two-phase flow structure assessment based on dynamic image analysis

Two-phase flow regime is an important industrial process phenomenon. Pressure drops, volume fraction and flow pattern are the most significant parameters describing the flow hydrodynamics of the two-phase mixtures. Authors propose a qualitative and quantitative description of the two-phase fluid flow structure, provided by the dynamic image analysis method. Method is based on high-speed visualization, stereological techniques and videogrammetry with grey level analysis and cross-correlation of signals. The study was conducted on a rectangular, vertical channel, with air-water co-current upwards flow. Dimensions of the channel are 5 × 50 × 1200 mm. The gas phase flux is 0.01 dm3/h and 0.16 dm3/h. The liquid phase flux is set to 0.01 dm3/h. The measurements and calculations were based on the stereology techniques, the Zuber-Finley and Mishima-Hibiki drift-flux model, and the grey level of the image. Two-phase flow parameters such as the void fraction, interfacial surface, free distance between objects, number of objects, average object size, drift and distribution coefficient was determined. Parameters were obtained as a function of time. Comparison of the image analysis results with the drift-flux models was made. To summarize, developed dynamic image analysis, provides information suitable for the assessment and reconstruction of the two-phase flow structure, which can be used for automatic process control.

Effect of length-to-diameter ratio on axial velocity and hydrodynamic entrance length in air-water twophase flow in vertical pipes

Effect of length-to-diameter ratio on axial velocity and hydrodynamic entrance length in air-water twophase flow in vertical pipes

A Study on Natural Circulation Flow Rate for Various Shaped Channels

The severe accident in Fukushima daiichi nuclear power plant revealed the importance of molten core cooling system without the electric power, then cooling system by natural circulation flow has attracted attention. The heat removal performance of this system depends on the natural circulation flow rate, so it is essential to predict the flow rate accurately for the safety design. The purpose of this study is to develop the evaluation method for the natural circulation, and to examine its accuracy dependence on the channel shape. Experiments were carried out at atmospheric pressure, using room temperature air-water flow. We used seven kinds of channels with various shape and diameter, and measured natural circulation flow rate and pressure drop for each channel. Predictive analysis was conducted by our method based on balance between driving force and pressure drop in the loop channel. The major results are as follows: (1) It is possible to predict the natural circulation flow rate with an error of less than 20 percent for various shaped channels. The average and standard deviation for the ratio between calculated and experimental flow rate is 0.98 and 0.047. (2) Two phase prediction method for pressure drop has maximum 15 % error. To improve the prediction accuracy, it is important to improve the prediction of the void fraction and the two-phase multiplier.

Experimental study on distribution parameter characteristics in vertical rod bundles

The drift-flux model is important for rod bundle two-phase flow analysis. A 5 × 5 rod bundles experiment has been performed under air-water two-phase flow conditions. The superficial liquid velocity ranges from 0.00 to 1.50 m/s while the superficial gas velocity ranges from 0.02 to 6.00 m/s. The void fraction has been measured by the impedance meter. Four typical drift-flux models developed for rod bundles have been compared with present data. By analyzing the experimental data and visualization, the distribution parameter may be influenced by the flow recirculation and turbulence significantly. In order to consider these two effects, a critical liquid velocity has been defined to determine which effect is dominant as the increase of liquid velocity, and a new critical void fraction correlation related to the flow-regime transition from cap-bubbly to cap-turbulent flow is incorporated to determine the distribution parameter. The new drift-flux model developed from existing models shows a good prediction capability.

An Evaluation of Correlations for Predicting Pressure Drop of Air-Water Flow in Narrow Rectangular Duct

Aiming at developing a more common method for predicting two-phase flow pressure drop for small channels, experiments on frictional pressure drop of air-water flow in a vertical narrow rectangular duct with a cross-section of 40 mm by 1.6 mm were conducted at atmospheric pressure. The mass flow rates of air and water covered the ranges from 0.03 to 12.5 kg/h and from 19 to 903 kg/h, respectively. It was found that the two-phase flow can be divided into three regions according to the liquid only Reynolds number, by which a modified Chisholm two-phase multiplier was proposed for predicting frictional pressure drop. Some leading correlations for predicting two-phase flow pressure drop were compared with the new correlation against current experimental data, the latter had and a mean deviation of 7.2%, showing a better agreement with the experimental results.

Optical flow estimations in aerated spillway flows: Filtering and discussion on sampling parameters

Image-based velocimetry captured a great interest in physical modelling of highly-aerated flows, for example in stepped spillways. The present study investigated the performance of a novel filtering technique based upon a detailed systematic comparison with intrusive phase-detection probe data. All measurements were conducted in a large-size stepped spillway. A sensitivity analysis provided recommendations in terms of optimum sampling and processing parameters for optical flow measurements in high-velocity air-water flows.