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• We perform simultaneous LIF film-thickness measurements in downward annular flows. • BBLIF is reliable in smooth/flat film regions, but corrections are needed for the mirror effect in PLIF. • Local features within the raw PLIF signals can be used to correctly identify the true interface. • Where films are rough/thick the mirror effect is suppressed, but BBLIF experiences larger errors. • Corrections improve both techniques, but simultaneous application is recommended in complex flows. This paper is devoted to the simultaneous application of two spatiotemporally resolved optical techniques capable of liquid film thickness measurements, namely Planar Laser-Induced Fluorescence (PLIF) and Brightness-Based Laser-Induced Fluorescence (BBLIF), to co-current downward annular gas-liquid flows. A single laser sheet is used to excite the liquid film, which has been seeded with a fluorescent dye, along a longitudinal/vertical plane normal to the pipe wall. Two cameras, one for each technique, are placed at different angles to the plane of the laser sheet in order to recover, independently by the two techniques, the shape of the gas-liquid interface along this section. The effect of the angle between the laser sheet and the PLIF camera axis is also investigated. In film regions where the gas-liquid interface is smooth and flat, the conventional approach used for interpreting PLIF data is affected by total internal reflection of the fluorescent light at the free surface, or "mirror effect", which leads to an overestimation of the film thickness that increases as the angle between the laser sheet and the camera axis is decreased. Nonetheless, local features such as light intensity maxima or minima are often located within the fluorescent signals that correctly identify the true interface, which in these conditions also coincides well with the BBLIF film-thickness measurement. When a correction for the mirror effect based on simple flat-film optical calculations is applied, this leads to PLIF results that correspond well to the true film thickness. Interestingly, it is further found that interfacial three-dimensionality, and in particular azimuthal/circumferential non-uniformity, can lead to underestimation of film thickness by PLIF that in some cases counteracts the overestimation due to the mirror effect. Smaller angles between the laser sheet and camera axis make PLIF less susceptible to this error. In regions where the film surface is rough, including on the surface of disturbance waves, the mirror effect is suppressed. The BBLIF measurement, on the other hand, is affected by loss of signal sensitivity or saturation in thick film regions, as well as distortions at complex or multiple interfaces in agitated flow regions with significant wave activity. Direct comparisons between PLIF and BBLIF measurement data confirm that local overestimations of the film thickness by the latter occur in film regions with higher interfacial curvature, especially at the front slopes of waves and around gas bubbles entrained in the liquid even in smooth films, while underestimations occur in regions with multiple interfaces and inside bubbles although it is often the case that features such as bubbles can be identified and corrected for in simple flows with smooth interfaces. Correction procedures are developed to compensate distortions caused by both methods that make these techniques more accurate for standalone employment. In highly complex, three-dimensional flows, the simultaneous application of both techniques is highly recommended to attain the most reliable information. [ABSTRACT FROM AUTHOR]

• Experimental assessment of air-water flow in a complete piping system is investigated for injector, upstream, U-bend and downstream sections. • The U-bend section includes two vertical 90-degree and one vertical 180-degree elbows. • Local void fractions and bubble numbers, measured with a dual optical probe, are presented. • A measurement error of up to 55% is estimated for immersion-type probes in sections near the injector part. • Local flow regime transitions due to the geometry are observed and evaluated. This paper presents an experimental investigation of air-water flow in a complete piping system, covering the two-phase flow development in injector, upstream, U-bend and downstream sections separately. Local void fractions and bubble numbers were measured with a dual optical probe. Two-phase flow conditions were established through a 40 mm diameter pipe with 180 l/min water and 30, 35 and 40 l/min air flow rates by the designed circular injector. In the horizontal upstream section, where a transition from bubbly to plug flow with elongated Taylor bubbles was observed, the fully-developed flow is reached at 37.5D. The average void fraction in the upstream region was acquired as 9.2%, 11.5% and 13.4%. The effect of two vertical 90-degree and one vertical 180-degree elbows is assessed in the U-bend section. While bubble break-up and locally annular flow were observed in the vertical upward section; bubble coalescence and accumulation resulting in higher average void fractions (30.8%, 33.8%, and 35.6%) were present in the downward section due to body forces and interphase interactions. After the bend section, disturbances were detected in the gas phase due to the increased water velocity and centripetal forces. Accordingly, a local transition to temporary churn flow is observed in the earlier downstream sections. The experimental assessment in our study shows that the system should be considered and analyzed as a whole to determine local transitions and differences in the flow regimes due to singularities in geometry. [Display omitted] [ABSTRACT FROM AUTHOR]

• New data for churn–annular flow using conductance probes placed at 3 entrance lengths are reported. • The comparison between the frequency obtained from conductance probes and wire mesh sensor (WMS) showed good agreement. • The comparison between the liquid film thickness obtained from conductance probes and local film thickness probes showed good agreement. • The experimental data have good agreements with empirical models. This paper presents the results of a series of experimental studies conducted to investigate the characteristics of churn and annular flows in an 11 m long riser with the internal diameter of 127 mm. A series of experiments in churn–annular flow regime were performed for a range of gas superficial velocities (3.5–16.1 m/s) and liquid superficial velocities of 0.02–0.33 m/s. Statistical analysis involving average, standard deviation and skewness, dominant frequency and structure velocity of the time series data was carried out as a quantitative approach to discriminate between churn and annular flows and to describe the transition between them. The variation of average liquid film thickness against gas superficial velocity showed a minimum at gas superficial velocity of 12.4 m/s. This is in close agreement with the minima at gas superficial velocity of 13 m/s reported in the literature. The results of the comparison between average liquid film thicknesses from the conductance ring probes agree well with those obtained by using local liquid film thickness probes and conductance ring probes in the literature. The results of the comparison of dominant frequency obtained from the conductance ring probes agree well with those obtained by using wire mesh sensor (WMS) in the literature. A comparative analysis of the current experimental data with those previously published experimental results confirms good agreement. [ABSTRACT FROM AUTHOR]

• Two-phase flow-induced vibration has been experimentally investigated. • Void fraction, flow field and vibration signals were analyzed simultaneously. • The maximum response was found for slug flow. • Flow pattern maps were created for the dynamic response. • Better evaluating criteria for two-phase flow induced vibration. In this paper, two-phase flow-induced vibration in a horizontal piping structure with flow restricting orifices has been experimentally investigated. The effect of flow patterns, void fraction, mass quality and orifice area ratio on vibration response is evaluated in detail. The two-phase flow measurements and vibration signals were analyzed simultaneously in order to understand the effect of two-phase flow behaviour on the governing factors responsible for flow-induced vibration mechanisms. The results show that the dynamic vibration response of the structure increased with the increase in the liquid mass flux and upstream void fraction in the case of intermittent flow patterns. The flow pattern change across the orifice is found to strongly affect the magnitude of the resulting vibrations. The maximum response was found in the region where a transition to slug flow occurred. Also, the RMS amplitude of vibration increased with an increase in the gas flux for both stratified and annular flows. Flow pattern maps were therefore created for both horizontal and vertical dynamic responses. These maps can be used in evaluating the dynamic response of piping structures with flow restrictions which are commonly found in the power generation industry. This will allow for a better design and safe operation of systems suffering from multiphase flow-induced vibration. [ABSTRACT FROM AUTHOR]

• The swirling flow patterns are 3-D reconstructed in a horizontal pipe. • Detailed information on the liquid film circumferential distribution is given. • The PDFs, CPDFs, time-trace of the liquid film thickness are analyzed. • The periodicity and frequency of the interfacial wave are analyzed by PSD and ACF. A comprehensive understanding of swirling flow liquid film characteristics is key to swirling flow applications. Previous studies lost sight of the swirling liquid film characteristics in horizontal tubes. In this paper, the liquid film characteristics of swirling flow in a horizontal tube with a diameter of 32 mm were investigated using water and air as process fluids. Three inlet flow patterns, including stratified flow, annular flow, and slug flow, are covered. A swirler, with a pitch of 40 mm giving a total of 5 pitches, was utilized to induce swirling flow, which has the advantages of uncomplicated structure, convenient installation and excellent rectification effect. The experimental data was collected via a pair of 16 × 16 conductivity wire mesh sensors (WMS). The results indicate that, in all three inlet flow patterns, the swirling flow liquid film at the pipe top is slightly thicker than that at the pipe bottom, breaking the gravitational effect. The circumferential and temporal distributions of the swirling liquid film thickness become more uniform as gas and liquid velocities increase. With increasing gas velocities, the peaks of power spectral density (PSDs) decrease, i.e., the interfacial wave perturbation is weakened. For inlet slug flow, the swirling flow interface wave disturbance is the largest and exhibits strong periodicity. The period decreases as gas velocities increase. These findings have enhanced the understanding of swirling flow, which provide new insights and scientific guidance for research related to pressure drop and heat and mass transfer in swirling flow. [ABSTRACT FROM AUTHOR]

• Liquid film thickness profiles in gas–liquid annular flow can be reconstructed from stereoscopic X-ray data. • Using X-ray data, the film thickness can be obtained at hundreds of points around the pipe perimeter. • The film thickness detection limit is approximately 30 µm with the presented setup. • The method is applicable to 2-phase or 3-phase systems, to horizontal or inclined pipes, and to steady or dynamic systems. • Experiments have been performed with high-density SF6 gas and Exxsol D60 oil. This paper presents a method for reconstructing the thickness profile of the thin liquid film being present in steady, fully developed gas–liquid annular pipe flow, taking advantage of nonintrusive X-ray measurements. Experiments have been performed with low liquid loading, annular flows, in a near horizontal pipe system (inclination +2.5°), using a high-density gas (SF 6) and Exxsol D60 oil. Special care was taken to ensure the flows were fully developed, and statistically steady. The superficial gas velocity was maintained at a constant value of 10 m/s, while the superficial liquid velocity was varied between 0.5 mm/s and 42 mm/s. The X-ray measurements were supplemented by gamma ray densitometer data and isokinetic data. An explicit mathematical method is presented, whereby the shape and thickness of very thin liquid films can be reconstructed if X-ray images of the flow have been obtained from two independent directions: one with a vertical view of the pipe, and another with a horizontal side view. The method can be straightforwardly applied to cases where the film volume dominates the total volume of droplets suspended in the gas phase. This condition is generally satisfied for the experiments presented in this paper, as verified by isokinetic measurements. Mean liquid hold-up values determined by gamma densitometer are in reasonable agreement with the X-ray results. It is found that the mean film thickness at any position on the pipe wall increases with the superficial liquid velocity. While the film thickness is maximal at the pipe bottom in virtually all experimental cases, its profile exhibits multiple local maxima and minima, with the smallest thickness apparently 20°–30° away from at the pipe top. The presented film reconstruction method is highly sensitive; even at extremely low liquid levels, with a superficial liquid velocity less than 0.01% of the mixture velocity, the film and its thickness profile are clearly detected. An analytical fit for the film profile as a function of perimeter angle and superficial liquid velocity is given. The fit captures the main features of the profile as derived from X-ray measurements, for thin films with maximum thickness less than 5% of the pipe diameter. [ABSTRACT FROM AUTHOR]

Two-phase gas–liquid annular flows are encountered in ventless aero-engine oil system pipes. The droplet size in the flow has an important impact on the performance of downstream equipment as breathers and de-aerators. However, literature studies present semi-empirical models that are not in the range of operating conditions of the oil system. To investigate the effect of the use of lubrication oil on the droplet sizes, this paper presents experimental results of annular flow with oil flow rates from 160 to 640 l/h and air flow rates from 60 to 120 Nm 3 / h . Comparison of the Sauter–Mean Diameter predicted by existing correlation show an error of minimum 30% compared to experimental values for higher oil flow rates, which are the most important in oil systems. To address this issue, correlations were adapted to fit experimental results. With the new set of parameters, the Sauter-Mean Diameter is estimated with an error of maximum 18% for higher oil flow rates. Results analysis illustrate that the main difference between existing and new correlations could be due do the surface tension and viscosity of lubrication oil, which are very different from water at low temperature. The results are also consistent with the transition between bag and ligament break-up droplet generation mechanism at a flow rate of 80 Nm 3 / h . [ABSTRACT FROM AUTHOR]

• Ring island array sensor for liquid film thickness measurement is developed. • Circumferential distribution of liquid film in helically coil tube is obtained. • Sub-flow regime map of annular flow in helically coil tube is proposed. • Prediction model of flow regime transition boundaries is established. The distribution of liquid film thickness in helically coiled tubes (HCTs) directly affects the heat transfer performance of the annular flow region. However, due to the limitation of measurement technology, accurate measurement data of liquid film thickness in HCTs is very rare. In this paper, a non-invasive contact liquid film thickness sensor (ring island array sensor) is successfully developed to obtain the refined three-dimensional spatial-temporal distribution data of liquid film thickness in HCTs. According to the results, four typical liquid film flow regimes are defined based on the circumferential position of the thicker liquid film appearing in the tube wall, i.e., the bottom distribution (BD) dominated by liquid phase gravity force, the outside distribution (OD) dominated by liquid phase centrifugal force, the inside distribution (ID) dominated by gas centrifugal force and the inside-outside distribution (IOD) dominated by the secondary flow. These circumferential distributions of liquid film thickness are mainly affected by the HCT structure (e.g., coil diameter and coil pitch) and superficial gas-liquid velocities. Based on the modified Dean number De * and the modified Lockhart-Martinelli parameter X *, which consider the influence of HCT structures, fluid properties and operating parameters, a general sub-flow regime map of annular flow is proposed. The prediction models of transition boundaries of different liquid film flow regimes are also established and verified with present and available data in the literature. [ABSTRACT FROM AUTHOR]

Flow pattern transitions in two-phase gas/shear-thinning fluid pipe flows represent a key aspect during oil transportation and in chemical industry. In this paper a unified model to build a complete flow pattern map is proposed and models to predict flow patterns are provided. The transitions from stratified flow regime are investigates performing a linear stability and well-posedness analyses, and a non-linear stability analysis. The steady and fully developed two-fluid model for the annular flow is proposed and validated; the flow pattern transition from the annular flow is discussed considering, also, the structural stability. The transition to dispersed flow, the slug stability approach, and criteria to obtain the slug/plug and the bubbly/plug boundaries are considered. A sensitivity analysis on the effect of the rheology of the shear-thinning fluid on flow pattern boundaries is carried out and, then, the complete flow pattern maps are validated with data taken from the literature showing a good agreement. [ABSTRACT FROM AUTHOR]

In this work, we extend our previous efforts on the effect of surfactants on air-water flow in a vertical pipe by also considering pipe inclinations between 20° (with respect to horizontal) and vertical. For air–water flow, independent of the inclination, there is a regular annular flow at large gas flow rates, and an irregular churn or slug flow at low gas flow rates. Closely related to the transition between regular and irregular flow, although not necessarily coinciding with it, there is a minimum in the pressure gradient as a function of the gas flow rate. In gas wells, surfactants are used to shift this minimum to lower gas flow rates, which allows a stable gas production up to lower reservoir pressures. In this work, we investigate how the pipe inclination affects air–water flow without and with surfactants. Surfactants generate foam, which decreases the density and increases the thickness of the film at the pipe wall. For vertical flow, we previously established that surfactants increase the pressure gradient at high gas flow rates, decrease the pressure gradient at low gas flow rates, shift the minimum in the pressure gradient to lower gas flow rates, and shift the transition between regular and irregular flow to lower gas flow rates. The new results described in this paper show that for large gas flow rates, both the flow with and without surfactants is unaffected by the inclination. At low gas velocities, however, in inclined pipes the surfactants are much less effective at shifting the transition between irregular flow and regular flow and at shifting the minimum in the pressure gradient than in vertical pipes. The foam causes a regular film morphology at the top wall of the pipe, but is unable to make the morphology of the bottom liquid film regular. As a result, at low gas flow rates the relative decrease of the pressure gradient due to surfactants is smaller for smaller inclinations from horizontal. This larger relative decrease for vertical flow compared to inclined flow is related to an increased foam formation and therefore a smaller mass density of the film in vertical flow. [ABSTRACT FROM AUTHOR]

• Flow patterns in helically coiled tube are classified by visual observation. • Void fraction for entire cross-section is measured by wire-mesh sensor. • k -Nearest Neighbors algorithm is utilized to identify flow patterns objectively. • Influence of centrifugal force and secondary flow on phase distribution and flow transition is revealed. Helically coiled tube (HCT) is used extensively in compact types of heat exchangers. Complete knowledge of gas-liquid flow behavior in HCT is essential in designing the heat transfer equipment involving phase change. Although the gas-liquid flow patterns in straight tubes have been widely studied, investigations on flow pattern recognition in HCT are still scarce. In this paper, based on the backlight imaging tomography and self-designed wire-mesh sensor, air-water experiments are carried out to study the characteristics of gas-liquid flow in a transparent HCT. Bubble, plug, slug, wavy and annular flow are observed, and their void fraction at the cross-section of the tube are measured and analyzed using the statistic parameters of time-trace, PDF and CPDF. Owing to the simplicity of implementation, the void fraction data are subsequently utilized along with the k -Nearest Neighbors algorithm (KNN algorithm) to identify the flow patterns objectively. Finally, a flow regime map is proposed and compared with the previous straight flow map and HCT map in the literatures. Accordingly, the influence of curvature-induced centrifugal force and secondary flow on the phase distribution and flow-regime transition is revealed in the present study. [ABSTRACT FROM AUTHOR]

• New model for pressure gradient during symmetric annular flow • Simultaneous evaluation of void fraction, entrainment ratio and phase velocities • Calibration of the friction factor for the liquid-vapor interface • Assessment of existing correlations and comparison with the present method This paper presents a new method for the evaluation of the pressure gradient during annular two-phase flow in the presence of entrainment, based on the momentum balance on the liquid film trapped between the wall and the vapor core. The entrained liquid fraction, together with the atomization/deposition rates and shear stresses, are considered with the use of closure equations, in which the interfacial shear stress is calibrated by means of an experimental database. Through the resolution of a simple iterative procedure, the outcomes of the method are both pressure gradient and void fraction. The accuracy of the present model is tested with independent experimental databases from the literature, showing a very good agreement also with respect to existing available methods for pressure gradient and void fraction. [ABSTRACT FROM AUTHOR]

The hydrodynamics of downwards gas–liquid annular flows and falling films in a pipe were studied experimentally using simultaneous planar laser-induced fluorescence and a combination of particle image and particle tracking velocimetry techniques. The investigated conditions covered the range of liquid and gas Reynolds numbers: Re L = 306 – 1532 and Re G = 0 – 84 600 . The results presented in this paper concern: (i) information on the local and instantaneous velocity fields underneath the interfacial waves and the appearance of recirculation zones within the liquid films under certain conditions, and (ii) mean velocity, velocity fluctuation rms and kinetic energy profiles within the liquid films. The results indicate that large waves contain multiple recirculation zones, which may play an important role in the gas and liquid phase entrainment mechanisms as well as in the mass and momentum transfer from the near-wall region towards the gas–liquid interface. [ABSTRACT FROM AUTHOR]

The first part of this paper surveys the distinctive features of trains of disturbance waves in high-speed annular two-phase flow. This data is then used to construct a mathematical model that predicts relations between the speed, height, and spacing of the waves, and the net liquid flow rate. These relations highlight the importance of the vorticity in the waves, a quantity that has received little experimental attention. [ABSTRACT FROM AUTHOR]

• Disturbance waves in annular two-phase flow are modeled using a four-field approach • A wave number density transport equation captures any non-equilibrium development • Constitutive models for the wave and base film interactions are developed • Model correlations are calibrated to high pressure steam / water (BWR) conditions • Non-equilibrium effects are found important to account in developing two-phase flows A new, phenomenological, model of disturbance waves in annular two-phase flow is presented. The model builds upon the three-field (gas, drops and film) one-dimensional model of annular two-phase flow by considering the disturbance waves as an additional fluid field, traveling over a thin liquid base film, along with its own mass, momentum and energy conservation equations. The waves are modeled as fluid particles characterized by basic macroscopic parameters such as the wave shape factor (amplitude/width) and the wave number density (spatial frequency of waves). Phase change is considered and relevant closure models are developed for the field mass and momentum exchange rates (drop entrainment/deposition and waves/base film interactions). A Boltzmann transport equation of wave number density is introduced to model the hydrodynamic non-equilibrium effects related to the wave creation, merging and splitting processes, characteristic of developing annular two-phase flows (due to e.g., inlet effect, phase change, geometrical change or transient). Finally, the base film mass and wave number density sink and source rates are modeled using a relaxation time approximation. The new model is motivated and fully documented in this paper and has been implemented in the latest version of the transient sub-channel analysis code MEFISTO-T. While the proposed model framework is generic for coherent disturbance waves, the required wave characteristic correlations are specifically developed and calibrated using high pressure steam/water experiments under equilibrium conditions relevant of Boiling Water Reactor operation. The model relaxation times are then estimated by comparing the model predictions against measured wave parameters downstream various flow perturbations (evaporation and film suction), along the channel recovery length toward equilibrium. For all considered experiments, the model predicts the averaged wave and base film parameters typically within +/-20% and is able to capture the axial development of disturbance wave characteristics. The consideration of non-equilibrium effects is found fundamental to the accurate simulation of axially developing annular two-phase flows. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents study to investigate the effect of the mixing method of a drag reducing additive into an air–water flow on the effectiveness of drag reduction. The air–water flow has an annular configuration in a horizontal 95.3mm pipe. The method of mixing of polymers can have a profound effect on their effectiveness in damping turbulence. Two different injection methods and different concentrations of the injected master solution were investigated. The downstream injection method at points located after the air water mixing section showed higher drag reducing effectiveness than the entrance injection method at a point right before the mixing section for the same concentration of the polymer in the liquid in the flow loop. The 1500 and 1000ppm master solution concentration starts reducing the pressure drop at lower loop polymer concentration than the 500 and 250ppm. The high master solution concentration (at the same concentration of polymers in the flowing liquid) reaches higher effectiveness for the same superficial gas and liquid velocities. The effectiveness of the drag reducing polymers for the case of the entrance injection decreases dramatically with increasing the superficial gas velocity, this phenomenon has not been noticed in the case of downstream injection. [Copyright &y& Elsevier]

Abstract: Entrainment fraction is one of the key parameters in many applications including wellbore and flow line design, separator design and wellbore loading and many others. Its prediction is important for the estimation of pressure drop, liquid holdup and dry-out in annular flow. The available and widely used maximum fraction of entrainment models have failed to predict the maximum entrainment fraction at low liquid Reynolds number. In this paper a new methodology of modeling the entrainment fraction, containing only two constants, is proposed. The proposed new methodology and procedure was found to predict the experimental data available in the literature very well. Furthermore, an equation for the maximum fraction of entrainment was developed. The proposed maximum entrainment fraction equation predicted more accurately than the equations found in the literature. [Copyright &y& Elsevier]

• Periodic oscillations of liquid flow rate were applied to control annular gas-liquid flow. • The flow becomes much more regular under excitation within a certain frequency range. • The effect is reached due to periodic enhancement / suppression of trains of short initial waves. • Amplitude and speed of excited disturbance waves grow and saturate much faster. • Minor increase of entrainment due to excitation is observed. In this paper, an analysis of susceptibility of wave structure in annular flow to small high-frequency oscillations is carried out. Downward adiabatic air-water flow at atmospheric pressure is studied in a vertical 11.7 mm pipe. The range of superficial gas velocities is 0 – 70 m/s and the range of wetting densities is 1.6 – 4.6 cm2/s. Spatiotemporal records of liquid film thickness are obtained with the Brightness-Based Laser-Induced Fluorescence technique. The region of interrogation is one longitudinal section of the pipe with a length of 470 mm, starting from the inlet. The oscillations are imposed by a solenoid-driven membrane causing periodic pulsations of liquid flow rate. The oscillation frequency range is 0 – 100 Hz. The amplitude of pulsations decreases with the oscillation frequency. The effect of oscillations is first tested on liquid films falling in absence of gas flow. In this case, the excited waves are of the same frequency as the imposed oscillations, and the stages of wave development are shifted closer to the inlet. In the case of liquid films under strong gas shear, the oscillations induce low-amplitude waves, which interfere with high-frequency initial waves, produced by the shear instability. Without excitation, groups of successive initial waves coalesce to produce the disturbance waves, which vary in speed and coalesce with each other. Within a certain range of excitation frequency, each period of low-amplitude wave produces one disturbance wave, so that coalescence nearly disappears and the waves become regular. Various criteria of susceptibility are proposed; the best ones are based on spectral analysis. The susceptibility range of excitation frequencies encompasses the 'natural' passing frequency of disturbance waves far downstream. The width of this range is of the order of 20 Hz but decreases at high frequencies when the amplitude of the pulsations decreases. Under excitation, the disturbance waves accelerate and grow in amplitude very fast, reaching saturated values near the inlet. The saturated values are not very different from the 'natural' ones. An increase in the excitation frequency within the range of susceptibility leads to a small decrease in the speed and amplitude of the disturbance waves. The entrained fraction shows only a minor increase due to the excitation. [ABSTRACT FROM AUTHOR]

In this paper, results for the interfacial friction factor and relative interfacial roughness on the gas–liquid interface are reported for an air–water annular flow in a small inner diameter tube (9.525 mm i.d.). The film structure was obtained through processing the time trace signal of film thickness that was measured using conductance probe technique. The interfacial friction factor and the relative interfacial roughness were altered through changing the gas mass flow rate. Changing gravity level was another way to alter the friction factor and roughness. It was found that wave height, hence the relative interfacial roughness (defined by the wave height measured from the substrate surface) decreased with increasing the gas Reynolds number. The roughness in microgravity is less than half of that in normal gravity, while the friction factor was about 10% smaller in microgravity than that in normal gravity. It was reasoned that the friction factor in annular two-phase flow decreased less significantly with the decrease of the relative interfacial roughness than that in single-phase flow, which could be explained by the flat wave shape in annular flow. The values of the interfacial shear stress at microgravity were also compared to those calculated at normal gravity. Based on the results, some similarities and differences between the single-phase flow and the gas core in annular flow were highlighted and discussed. [Copyright &y& Elsevier]

• Stability of an unsteady, two-phase flow with heat and mass transfer considered. • Work motivated by loss of coolant accidents in nuclear power plants. • Flow is mathematically unstable, regions of ǣengineering stabilityǥ identified. • Engineering stability quantified using solution growth behavior and doubling time. • Framework provided for application to other types of problems. This paper considers the stability of an unsteady, two-phase flow with heat and mass transfer. The model problem is motivated by loss of coolant accidents in nuclear power plants. For the example problem, two flow geometries are considered: inverted annular flow boiling and an annular mist flow. The model is comprised of coupled Mathieu equations so that stability can be determined using a Floquet analysis. The flow is found to be mathematically unstable to all perturbative wavenumbers, but for practical purposes there are regions of stability. Using the solution's growth behavior and doubling-time, the notion of practical stability, which is termed herein as "engineering stability," is quantified and a method is provided for application to other engineering stability problems. [ABSTRACT FROM AUTHOR]

• Condensation of R245fa in a 3.38 mm tube during vertical downflow is investigated. • Simultaneous film thickness and heat transfer measurements are performed. • Shadowgraphy and chromatic confocal imaging are used to measure film thickness. • The liquid film thickness and interfacial waves affect the heat transfer. • Statistical analysis on wave height, frequency and velocity is performed. The determination of the liquid film thickness during annular flow condensation in a small diameter channel is complex and very rare in the literature, although important to understand the heat transfer mechanisms involved during the phase change process. In the present paper, the liquid film thickness and heat transfer coefficients have been measured during vertical downflow condensation inside a 3.38 mm inner diameter channel. Condensation tests have been run with R245fa at 40 °C saturation temperature and mass velocity ranging from 30 kg m−2 s−1 to 150 kg m−2 s−1. The test section is composed of two heat transfer sectors connected with a glass tube. The glass tube has been realized with a special external shape that produces a magnification of the liquid film thickness and allows the use of a chromatic confocal sensor. The liquid film thickness is determined by coupling a shadowgraph technique with the measurements performed by the chromatic confocal sensor. An ad-hoc algorithm has been developed for the statistical characterization of the wave structures in terms of amplitude, frequency and velocity, with the aim to better understand how the interfacial waviness affects the heat transfer. [ABSTRACT FROM AUTHOR]

• Development of downward annular flow in 11.7 mm pipe over the first 400 mm is studied using BBLIF technique. • Strong change in wave and flow properties is found ~15 pipe diameters from the inlet. • Wave amplitude and entrained fraction keep growing beyond this point. • An average surviving disturbance wave absorbs two other waves in process of its evolution. • Effect of coalescence on wave speed and height is small in comparison to that of wave evolution. The behavior of the disturbance waves is studied in downward adiabatic air-water annular flow in a vertical pipe with an inner diameter of 11.7 mm. The range of liquid wetting densities is 2.5–4.6 cm2/s and the range of superficial gas velocities is 22–57 m/s. The paper is focused on the downstream evolution of the main properties of individual disturbance waves at the stage of flow development along with the process of coalescence of disturbance waves. Brightness-based laser-induced fluorescence technique enabled us to obtain spatiotemporal matrices of the local instantaneous thickness of liquid film along one longitudinal section of the pipe with a length of 400 mm. Dependencies of the main characteristics of the flow on the downstream distance are analyzed. Local shear stress, base film thickness, and longitudinal size of the disturbance waves are stabilized at the distance of 150–200 mm below the inlet. Disturbance wave velocity increases strongly up to this point; further downstream, the acceleration is reduced significantly. Disturbance wave amplitude grows approximately as the square root of distance within the whole region of interrogation. Entrained fraction grows linearly within this region. Disturbance wave frequency decreases exponentially due to the coalescence. It is found that an average disturbance wave surviving through the development stage absorbs two other waves. Individual acts of coalescence of disturbance waves are analyzed quantitatively to investigate their contribution to the change of disturbance wave properties. On average, when a disturbance wave absorbs another one, its speed increases by 2%, its amplitude – by 5–6% and its longitudinal size remains the same. Thus, the disturbance wave properties change downstream mainly due to their individual evolution, not due to coalescence. [ABSTRACT FROM AUTHOR]

• Flow patterns of decaying swirling gas-liquid flow are identified. • Void fraction of swirling flow patterns is measured. • Swirl length of swirling flow patterns is reported. • Pattern-based decay model for swirl length prediction is proposed. Decaying swirling flow is widely utilized in multiphase flow devices for separation, stabilization and heat transfer enhancement. In this paper, based on the image processing technique, the decay law and swirl length of swirling gas-liquid flow induced by a vane-type swirler in a vertical pipe is experimentally and theoretically studied. According to the visual observation, four swirling flow patterns including chain, swirling gas-column, swirling intermittent and swirling annular flow are recognized at the swirler outlet. Due to swirl decay, the chain, swirling intermittent and swirling annular flow revert to inlet non-swirling flows downstream of the swirler, whereas the swirling gas-column flow transforms to slug flow rather than inlet bubble flow. The swirl length of swirling gas-column and swirling intermittent flow increases with liquid superficial velocity increasing or gas superficial velocity decreasing. By contrast, the swirling annular flow exhibits a reverse trend. Based on the theory of angular momentum and the parameter of two-phase swirl number, comprehensive decay models are proposed to predict the swirl length of various swirling flow patterns. The prediction is correlated reasonably well with the experimental data within ±25% deviation in varied flow conditions. [ABSTRACT FROM AUTHOR]

• A comprehensive theory to describe flow regimes in bubble columns is proposed • Comprehensive experimental investigations are conducted • Global and local flow properties are experimentally obtained • Five gas spargers are tested • The influence of the aspect ratio and liquid phases are discussed When a gas phase is injected into a liquid phase, it gives rise to a rich, fascinating and mysterious fluid dynamic phenomenology. The lack of knowledge regarding this phenomenology, a shortcoming in the design and operation of multi-phase reactors, is related to the absence of an unique definition of the flow regimes. To date, different studies gave different definitions of the flow patterns and, subsequently they experimentally obtained some global and local flow properties, with no physical-based description of the flow patterns. Is there a theory able to determine a-priori the boundaries of different flow regimes (and, thus, the flow regime for a given set of boundary conditions, given the phases and the system design)? Answering this question requires changing the present point of view in defining and describing the flow regimes and it is the primary motivation of this paper. To achieve this goal, a new theory has been formulated, which changes the present way of approaching bubble columns and which is based on the following statement: the fluid dynamics in gas-liquid bubble columns is interpreted by means of a general relationship―built upon five flow regime transitions―between two global fluid dynamic parameters. The strategic path to formulate this theory is, first, to go back to the pioneering studies and formulate basic relationships on the basis of a general principle. Subsequently, in order to support and verify the theory a comprehensive and multi-scale experimental investigation has been performed and coupled with previous experimental studies. The different experimental studies have been conducted in a gas-liquid large-scale bubble column (height of 5.3 m; inner diameter of 0.24 m) operated in the batch and in counter-current modes; to study all flow regimes, the bubble column was tested with five gas spargers (viz., pipe sparger in open tube and annular gap configuration, spider sparger, two different perforated plates and needle spargers) with different values of the aspect ratios and different liquid phases. It was found that, in an air-water bubble column, the gas velocity is approximately 0.03 m/s either in the case of the destabilization of the mono-dispersed homogeneous flow regimes or in the case of the destabilization of the pseudo-homogeneous flow regimes. Increasing the gas sparger opening induces a narrowing of the boundaries between the transitional flow regimes; conversely, increasing the bubble column aspect ratio destabilizes the existing flow regimes up to some critical values. Also, increasing the superficial liquid velocity in the counter-current mode destabilizes the homogeneous flow regime. Finally, it has been observed that the prevailing effect of the liquid phase, in a bubble column operated with a " coarse " gas sparger, is to change the boundary of the homogenous flow regime. It has also been discussed how the change in coordinates of the flow regime transition between the homogeneous and the heterogeneous flow regimes is caused by the changes in the size distribution. This study is intended to outline a precise definition of the flow regimes in bubble columns and it poses a rational basis for future studies. [ABSTRACT FROM AUTHOR]

• The experiments of bubbly and cap-bubbly flow in a 66mm × 66mm square channel were carried out. A dual wire-mesh sensors (WMSs) was used to measure the bubbles in the two-phase flow. • The WMS's measurement results were verified the rotameters and get good agreements. • The experimental results include the void fraction projection, sectional averaged void fraction variation, time-averaged void fraction profile, gas velocity profile, and the bubble size distribution. • The void fraction predictions of two drift-flux correlations were compared against the experimental data and both models provided reasonable prediction accuracy. The experiments of co-current vertical upstream air-water two-phase flow in a 66 mm × 66 mm square channel were carried out. The superficial liquid velocity ranges from 0.765 m/s to 1.53 m/s and the superficial gas velocity ranges from 0.0215 m/s to 0.313 m/s, including bubbly flow and cap-bubbly flow. A dual wire-mesh sensors (WMSs) was used to measure the bubbles in the two-phase flow. The local superficial gas velocity from the dual WMSs was compared with the rotameters and most results were within the deviation of 5%. It can ensure the measurement accuracy of the WMSs. The experimental results include the void fraction projection, sectional averaged void fraction variation, time-averaged void fraction profile, gas velocity profile, and the bubble size distribution. Within the range of flow conditions of this paper, the void fraction was relatively uniformly distributed among the cross-section in the bubbly flow and core peak in the cap-bubbly. Both cross-correlation velocity and bubble velocity were used to calculate the gas velocity profiles. Since the bubble lateral displacement, the cross-correlation method trend to miss the correlation caused by small bubbles and get a higher velocity. Bubble velocity was recommended to be used for determining gas velocity profiles. The results of void fraction profiles, gas velocity profiles, and bubble size distributions can be used for the validation of two-phase models of CFD codes. The void fraction predictions of two drift-flux correlations were compared against the experimental data and both models provided reasonable prediction accuracy. [ABSTRACT FROM AUTHOR]

• High resolution, three-dimensional simulations of a Taylor bubble in an annular pipe was performed. • Previously developed vertical pipe closure models were manipulated to account for the annular intrusion. • Inclination effects of the annular pipe are simulated and key features of the flow can be observed. • Flowing fluid is introduced and the impact on the bubble dynamics is captured. A three-dimensional phase-field lattice Boltzmann method has been applied to investigate the rise of Taylor bubbles within annular pipes. The approach couples the conservative phase-field model with a velocity-based lattice Boltzmann scheme. The implementation uses 27 discrete velocities to resolve both the interfacial dynamics and the hydrodynamics. To assist numerical stability for the high-density ratio, two-phase flows a weighted multiple-relaxation-time collision operator is employed. This paper makes contributions in three areas. First, the model is employed to capture the behaviour of Taylor bubbles in five combinations of vertical annular pipes, with results compared to experimental findings from the literature for air-water flows. From this, shortcomings were identified in the ability of existing correlations to accurately predict rise velocities for bubbles in various liquids. Second, the effect of pipe inclination on the rise behaviour of the bubbles was investigated. From the findings, a preliminary correlation describing the rise velocity was proposed. The first two components of the study were conducted with the Taylor bubble rising in stagnant fluid. The final component of this study imposed liquid flow in a concentric annular pipe to determine the impact of this on the bubble's dynamics. The liquid velocity was defined through a Reynolds number based on the average inlet velocity up to an absolute value of 10. The viscosity was varied to examine Morton numbers from 2.56e-3 to 6.55e-5. To this end both co- and counter-current flow was analysed and a distribution parameter proposed to capture the liquid-gas interaction. To extend this work, future investigations will look to extend the parameter range assessed to ensure the universality of the correlations identified. [ABSTRACT FROM AUTHOR]

• Radial evolution of droplets in vertical annular flow was experimentally studied. • Radial variation of droplet flow rate, size, velocity and momentum were analyzed. • A few large droplets play a key role in momentum transfer between gas and droplet. In vertical gas-liquid two-phase annular flow, the radial evolution of droplets in the gas core plays a key role in affecting the characteristic of gas carrying liquids. In this paper, the droplets size and velocity at 11 radial positions were measured with the phase Doppler anemometry and then analyzed based on the recognition method of droplet role to figure out the radial evolution characteristics of droplets flow rate, size, velocity, and momentum. The results show that the droplet quantity flow rate decreases with the entrained droplets moving from the liquid film-gas core interface to the centerline, due to the fact that the droplet coalescence has a greater effect than the droplet breakup. The proportion of small droplets decreases while the proportion of relatively large droplets increases. The overall distribution characteristics of droplet diameter remain unchanged. The overall distribution characteristics of droplet axial velocity, radial velocity, and velocity size remain unchanged, although the droplets undergo the process of axial acceleration and radial deceleration during the radial migration. The distribution of droplet momentum varies greatly in the process of droplet radial migration. The contribution of small droplets to the total droplet momentum gradually decreases, while the contribution of large droplets to the total droplet momentum gradually increases. Although the number of large droplets is small, their contribution to the total droplet momentum cannot be neglected. [ABSTRACT FROM AUTHOR]

• A novel centrifugal gas liquid pipe separator for high velocity wet gas separation is proposed. • The centrifugal device of four semi-elliptic guide vanes can achieve a stable and uniform swirling core-annular flow. • Nearly all swirling film can completely discharge from pipe relying on its own kinetic energy and inertial through NTC. • With the increase of gas velocity, the abnormal change rules of separation efficiency are studied. • The effect of structure parameters of NTC on separation efficiency is studied detailly. Cyclones are the most widely used separators at present, but it's still difficult to reduce their size to serve as pipe separators. A new type of gas liquid pipe separator for high velocity wet gas is proposed in this paper. The separator is actually a short section of pipe, with a centrifugal device in the center portion and 3 narrow tangential conduits (NTC) in the pipe wall. As gas liquid mixture flows through the centrifugal device downwardly, a strong rotation flow is created. Liquid in the mixture is pushed to the pipe wall by centrifugal force and forms a uniform liquid film with high tangential velocity. Then nearly all the liquid film can directly enter the NTC and be discharged from the pipe relying on its own kinetic energy and inertial, because of the little resistance characteristic of NTC to the liquid film, therefore the whole separation process can be completed within the pipe. The swirl characteristics in the pipe were studied by numerical method, and the simulation results indicated that the swirling liquid film is fairly uniform and the suitable installation position for NTC is about 2.5 pipe diameters downstream of the centrifugal device. A separation model for the pipe separator was established and experiments were carried out to verify the proposed model. The superficial velocity ranges of gas and liquid were 22–72 m/s and 0.07–0.54 m/s, respectively. The experimental results showed that the separation efficiency always increases with the increase of both gas and liquid superficial velocity, generally it is over 0.81, and the maximum attainable is 0.97. However, the separation efficiency will begin to drop if liquid superficial velocity exceeds the critical value U CSL , due to the onset of unstable liquid film waves. Experimental results also showed that U CSL increases with the increase of gas velocity. [ABSTRACT FROM AUTHOR]

• Extensive data including local, chordal and area-averaged void fractions in a rod bundle was obtained respectively. • Pressure and mass flux had significant effects on void fraction distribution. • The non-uniform distribution of void fractions decreased in bubble and annular flow regimes. • The droplet entrainment occurred in the gas core of annular flow at high mass fluxes. An investigation of void fraction measurement and prediction in a triangular-array rod bundle geometry was carried out under high pressure conditions (P = 5–9 MPa and G = 100–350 kg·m−2·s−1). The aims of this work were to expand the existing database and assess and modify the predictive models. The present paper is the Part I of this study, where the experimental work was presented to measure local, chordal and area-average void fractions respectively by a comprehensive measuring system composed by optical probes and gamma-ray densitometry. Non-uniform distribution of void fraction was investigated by local-to-local and local-to-average comparisons of void fractions. The results indicated that non-uniform distribution was not only related to the geometry, but also affected by the transition of flow regimes in rod bundle. The effects of pressure and mass flux on void fractions were also analyzed in detail in consideration of variations of fluid properties and drift-flux parameters. Additionally, both void fraction and interface frequency were compared with the flow regime map in TRACE code. The variation of interface frequency indicated that the droplet entrainment would occur in annular flow at high mass flux. In the Part Ⅱ of this study, an assessment of existing models for void fraction prediction in rod bundles was carried out according to the present data and a newly-developed drift-flux model was proposed. [ABSTRACT FROM AUTHOR]

• We have demonstrated scale-up rules for fully-developed, steady-state, gas–liquid, two-phase flow, in a horizontal or near-horizontal pipe. • We have used our existing database and designed new, scaled laboratory experiments for verifying scaling laws. • The experimental results confirm that the gas-to-liquid density ratio plays an important role. Multiphase flow models are validated by comparison with a relatively good supply of high-quality laboratory data, and a relatively sparse supply of field data, which tends to have poorer quality. One of the principal challenges for multiphase flow models, in terms of uncertainty, is the difference in scale and some of the fluid properties between field and laboratory conditions. Therefore, the models may become unreliable when they are applied to conditions that are very different from those in the laboratory. IFE (Institute for Energy Technology) has recently developed and demonstrated scale-up rules for the most basic multiphase pipe flows. The objective of the work presented in this paper was to select appropriate data from our existing database and design new, scaled laboratory experiments, well-suited to demonstrate (or test) the scaling rules by comparing the results. The data include fluid properties, pipe configurations and flow rates. Besides the observed flow pattern, liquid holdup and pressure gradient are the two main parameters for comparison. IFE's CO 2 Flow Loop with a test section inner diameter (ID) of 44 mm, operates for two-phase flows over a large range of pressures and temperatures on the equilibrium line of pure CO 2. In order to verify scale-up principles, series of experiments were conducted according to the scaling rules to simulate similar conditions. The experiments were performed with gas–liquid two-phase CO 2 for fully-developed, steady-state flow, in a horizontal or near-horizontal pipe. The flow regimes include stratified and annular flows. The experimental results showed that measurements of liquid holdup, and pressure gradient in the CO 2 Flow Loop are in excellent agreement with appropriately scaled data from the larger-scale facilities. The results also confirm that the gas-to-liquid density ratio plays an important role. The experiments provide valuable data sets for verifying scaling laws, which are lacking in the literature. [ABSTRACT FROM AUTHOR]

• Flow patterns in swirling gas-liquid flow were identified by visual observation. • Characteristics at transition boundaries were discussed with flow-regime reconstruction technique. • Spatial and temporal distribution of gas core diameter was analyzed with image processing technique. • Swirling flow regime transition criteria were gained based on void fraction data. Swirling gas-liquid flow has a wide variety of engineering applications in petroleum, chemical, and nuclear industries. A fundamental understanding of swirling flow patterns and their transitions is essential for the proper modelling of pressure drop, mass transfer rate and interfacial area in swirling two-phase flow. Although the gas-liquid flows in straight pipes have been widely studied, little attention has been paid to characteristics at transition boundary of swirling gas-liquid flow. In this paper, based on high-speed camera method using back light source, flow patterns and transition criteria for swirling gas-liquid flow induced by a vane-type swirler were experimentally investigated in a vertical tube of 30 mm I.D., operating at ambient temperature and pressure. According to our observation, five typical flow patterns, i.e. swirling gas column flow, swirling intermittent flow, swirling annular flow, slug flow and churn flow, were identified and the detailed characteristics at the transition boundaries were discussed with flow-regime reconstruction technique. The spatial and temporal evolution of the gas core diameter in swirling flow regimes were also analyzed using image processing technique. Additionally, the flow-regime transition criteria were gained which reveal that the void fraction α g in swirling gas column flow is below 0.30∼0.32 and it attains values of 0.32 to 0.71 in the swirling intermittent flow. When α g is greater than 0.71∼0.74, the swirling intermittent flow transforms into swirling annular flow. [ABSTRACT FROM AUTHOR]

• Experimental bench with hydraulic fittings and directional flow changes for CAF. • Simultaneous evaluation of the biphasic flow in three sequential locations. • Photographs for all flow combinations in all test sections. • Pump power reduction factor on transporting oil with water. • Overall energy savings to be considered on project designs. Image, graphical abstract The shortage of new light oil reservoirs increases the need to explore heavy oil reserves and to deal with viscosity issues on their transportation through pipelines. An attractive procedure to reduce energy consumption of this operation is to pump oil using water as a wall lubricant in a core annular pattern. This method provides high efficiency and high fluid flow rates, being effective in decreasing pressure drop due to the friction reduction. This paper reports an experimental evaluation of the oil-water flow patterns on three consecutive test sections positioned as the sequence: horizontal vertical horizontal and connected by several hydraulic fittings. Also, photographs were taken to demonstrate the influence of those line irregularities and the directional flow changes on the flow patterns and on the attainable energy savings. The overall energy savings regarding the CAF method presented a factor of 2.2, which means that the same amount of oil could be transported at half the cost. The experiments have also shown that above 50% input oil cuts, no relevant energy reduction in order to compensate higher oil fooling risks was verified. [ABSTRACT FROM AUTHOR]

Abstract: The above referenced paper, published in International Journal of Multiphase Flow (), proposed an entrainment fraction correlation for annular flow in horizontal pipes. The entrainment fraction in annular flow is defined as the ratio of the mass flow rate of the liquid droplets in the gas to the total mass flow of liquid, FE = WLE/WL . The proposed correlation was verified with experimental data for liquids with viscosities close to that of water. The proposed entrainment fraction correlation includes another correlation for the critical film flow rate, WF,cr to estimate a maximum entrainment fraction FE,max . It is shown that the critical film flow rate correlation can result in negative maximum entrainment fraction values, for low liquid flow rates. [Copyright &y& Elsevier]