Search

The flow regime in geology is critical to the underground activity. In this study, axial seepage tests within a wide range of grain sizes were conducted to better understand the influence of pore size distribution and pore structure evolution on hydraulic characteristics in porous media. The effect of pore size distribution and evolution on flow regime transition was quantitatively analyzed. The pore size distribution was controlled by varying uniformity coefficient (Cu) (4 < Cu < 24) and curvature coefficient (Cc) (0.2 < Cc < 14). The results show that coefficient size appears to have a positive correlation with the permeability of the granular materials, but cannot impact the transition of flow regime that is mainly dominated by the heterogeneity of the pore structure. Compared to the pore space and the effect of medium, the heterogeneity of pore structure significantly impacts on the occurrence of pre-Darcy flow and directly correlates with the magnitude of inertial forces or fluid velocity. It demonstrates that the curvature coefficient (Cc) is an important characteristic parameter in flow transition. The particle size ratio d 60 d 30 could be considered as one of the reference characteristic particle sizes for assessing non-Darcy flow characteristics. Subjected to the axial stress, the critical hydraulic gradient for the transition from pre-Darcy to post-Darcy flow was reduced, and even the pre-Darcy flow disappeared. These findings will contribute to the identification of non-Darcy flow transition in groundwater and promote a better understanding of the nonlinear evolution behavior of fluids under underground extraction conditions. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of the flow regime transition is a prerequisite for designing and operating the rotating drum, especially when dealing with non-spherical particles. In this study, the multi-sphere method was used to construct cylindrical particles. The optimal number of spherical elements for particles with different aspect ratios was determined in terms of kinetic energy, rotational kinetic energy, collision number, collision force, deformation and angle of repose. And the effects of filling rate, rotational speed and particle size on the macro- and micro-behavior of cylindrical particles were systematically investigated. The results show that when the aspect ratio of cylindrical particle is 2, 3 and 4, the optimal number of spherical elements are 5, 8, and 10, respectively. Four flow structures, namely rolling, cascading, cataracting and centrifuging, were identified with the increase of the rotation speed, but the transition boundary from rolling to cascading is not clear due to the irregular particle shape. Three flow structures, namely slipping, transition, and cascading, were identified with the increase of the filling degree. The transition diagram of flow regime was established. The results show that the aspect ratio has a great influence on the flow transition from slipping to cascading regime. The area covered by the transition flow regime expands as the aspect ratio increases, necessitating a greater filling degree to achieve a stable rolling flow regime. [ABSTRACT FROM AUTHOR]

In an effort to reduce the charging time of electric vehicles (EVs), the thermal management of the charging system has emerged as one of the most urgent issues. The charging rate has been restricted to avoid thermal failure especially in charging cable. Recently, a direct contact cooling technique that utilizes subcooled flow boiling for charging cables has been proposed and has shown promising thermal management performance. However, there has been a lack of clear explanation regarding its thermal characteristics due to its intrinsically complicated flow features. This study investigates the thermal characteristics of subcooled flow boiling in a horizontally aligned concentric annular tube intended to simulate the direct contact cooling technique employed for charging cables. For the experimental investigation, the wall temperature measurements and high-speed flow visualization images acquired from the transparent test module annulus, with inner and outer diameters of 6.35 mm and 22.0 mm, were utilized. Three key axial thermal characteristics of subcooled flow boiling in annulus have been analyzed, and they are flow regime transition, variation of heat transfer mechanisms, and occurrence of critical heat flux (CHF). The transition of flow regimes and the variation of dominant heat transfer mechanisms in the axial direction are mutually influencing due to the coupled effects between the hydrodynamic and thermal characteristics of simultaneously developing flows. The flow regime changes from partially developed boiling (PDB) to fully developed boiling (FDB) as the intensities of nucleate boiling and bubble recondensation vary in the axial direction, while the dominant heat transfer mechanism shifts from single-phase convection to nucleate boiling, corresponding to the flow regime transition from PDB to FDB. The departure from nucleate boiling (DNB) type CHF is observed, manifested by the wavy interface propagating from the far downstream to the upstream region, and the intensities of nucleate boiling and bubble recondensation are also identified as the dominant parameters in determining the occurrence of CHF. Charging time estimation reveals that the proposed method can achieve an 80 % charge for 100-kWh EV batteries in 98 s, while maintaining the cable wire temperature below the safety limit of 80 °C. This strongly supports its potential as a promising thermal management technique for future ultra-fast charging systems.

Micro‐packed bed reactors (μPBRs) have the advantages of high heat and mass transfer efficiency and excellent safety, and they have been successfully applied to hydrogenation and oxidation reactions. However, the study of gas–liquid flow regimes in the μPBR, which is essential for the mass transfer modeling and reactor scale‐up, is still insufficient due to the limitation of micro‐scale and complexity of capillary force. In this work, the flow regimes in the two‐dimensional μPBR were systematically studied by visual method utilizing a high‐performance camera. Four typical flow regimes and characteristics were captured, and flow regime transition was revealed. Effects of gas and liquid superficial velocities, liquid physical properties, and particle sizes on liquid spreading areal fraction and pressure drop were investigated. Flow regime transition correlation of churn flow and pseudo‐static flow in the μPBR was provided for the first time based on the summary of the current and previous published results. [ABSTRACT FROM AUTHOR]

[Display omitted] • Interfacial evolution of droplet formation in a substantial mass transfer system is revealed. • Factors affecting droplet size and generation process are elucidated. • A novel theoretical model that predicts droplet size was constructed. • The influence of mass transfer on flow pattern changes is clarified. The size of droplets profoundly impacts the efficiency of mass transfer in heterogeneous extraction systems. Understanding the interface behavior of droplet formation is crucial for its size control. This study elucidates the complicated process of buoyancy-assisted droplet generation and the factors that influence droplet size in aromatics extraction system. Droplet production involves three distinct stages: interface rebound, droplet expansion, and fracture contraction. The expansion stage is the primary contributor to droplet volume. Notably, droplet generator pore size and paraxylene content considerably influence the final droplet size. Enlarging the pore size yields large droplets, but increasing aromatics in the dispersed phase which can lead to mass loss decrease the droplet size. Based on mechanical analysis, a novel droplet size prediction model is proposed, which considers the volume contribution of each part in the droplet formation process and demonstrates high accuracy (average error less than 5%) in both non-mass transfer and mass transfer systems. This study comprehensively analyzes the impact of various factors on droplet size, enhancing our understanding of droplet formation regime in systems with substantial mass transfer. Additionally, it assists in the development of strategies for dispersing droplets and controlling their size in such systems. [ABSTRACT FROM AUTHOR]

Dropshafts are commonly used as flood discharge structures in urban drainage systems and hydraulic structures. Plunging and vortex flows are the two main flow regimes occurring in the dropshaft with a tangential intake. However, the effects of the intake contraction on the flow regime transition have not been studied. This study aims to present the flow characteristics in the transition process between the plunging and vortex flows in dropshafts, through a series of three-dimensional computational fluid dynamic simulations. With the decrease in the intake contraction ratio, the ratio of rotation and falling velocities along the annular dropshaft wall increases gradually, and the vortex flow can remain a long distance in the vertical dropshaft. The flow regime transition from the plunging flow to the vortex flow is analyzed quantitatively based on two specific velocity ratios of rotation and falling velocities. In addition, for large flow discharges, the effects of intake contraction on the flow rotation performance are more obvious than that for small flow discharges. These findings provide reference for the hydraulic design in dropshaft structures. [ABSTRACT FROM AUTHOR]

• Effect of steam bubble length and height on CIWH onset is analyzed quantitatively. • Stratified flow and reverse flow of subcooled water in steam channel cause CIWH. • Dimensionless criterion has been developed to predict onset of CIWH. Condensation induced water hammer (CIWH) in equal-height-difference natural circulation systems (EHDNCS) can lead to significant damage to associated equipment. To investigate the mechanism and the prediction of the onset of CIWH in EHDNCS, a small-scale experiment of EHDNCS was conducted under different water temperatures, heat transfer powers, and resistance coefficients of the water-side inlet. This study analyzed the thermal hydraulics, flow regimes, and the onset mechanism of CIWH through temperature and pressure measurement, U-Net image segmentation, and analytical modeling. Four distinct flow regimes were observed in the horizontal pipe: single-phase flow, non-periodic bubble, periodic bubble, and CIWH. Flow regime maps based on water temperature and heat transfer power were obtained under different inlet resistance coefficients. When the steam bubble length is smaller than the pipe length, no CIWH occurs. When the steam bubble length reaches the pipe outlet to form stratified flow, CIWH occurs at a high steam channel height. The occurrence of stratified flow and reverse flow of subcooled water in the steam channel causes the onset of CIWH in EHDNCS. A dimensionless criterion has been developed to predict the onset of CIWH when the saturated steam-water mixture is discharged through a horizontal pipe in EHDNCS. This criterion effectively predicted the transition from periodic bubble to CIWH in the experiment. [ABSTRACT FROM AUTHOR]

Spatio-temporal void fraction in air-water two-phase flow regime transitions has been visualized by combination of convolutional neural network and long short-term memory implemented into multiple -current-voltage (MCV-CNN_LSTM). The MCV-ML is composed of two components, which are MCV-CNN_LSTM training and evaluation. In the first component, four steps are carried out as 1) true void fraction α measurement was conducted by wire-mesh sensor (WMS) as objective variables, 2) simulated MCV voltage U calculation by simulation of MCV measurement for explanatory variable, 3) CNN_LSTM training instance generation, and 4) CNN_LSTM model training and testing. In the second component, two additional steps are experimentally conducted which are 5) actual MCV voltage V measurement by MCV for input of trained ML, and 6) spatio-temporal void fraction α prediction. For the dataset generation, the experiment was conducted on air-water two phase flow regime transitions in vertical pipe with an inner diameter of 25 mm and a length of 350 mm from the elbow. The superficial velocity of liquid and gas phase are varied to present the flow regime transition of bubbly to slug flow. As a result, the estimated spatio-temporal void fraction α ˆ by MCV-CNN_LSTM enables the visualization of detailed gas distribution in vertical upward two-phase flow. The spatial averaged instantaneous void fraction measured by WMS and estimated by MCV-CNN_LSTM show qualitative agreement in normalized cross correlation (I N C C ) of 0.364 on its temporal variation. • Spatio-temporal void fraction visualization in air-water two-phase flow regime transition. • Combination of convolutional neural network and long short-term memory (CNN_LSTM). • Multiple current-voltage with embedded CNN_LSTM (MCV-CNN_LSTM). • True void fraction measured by wire-mesh sensor (WMS). • Comparison of spatial-averaged instantanious void fraction between MCV-CN_LSTM and WMS. [ABSTRACT FROM AUTHOR]

[Display omitted] • The fluidization of cohesive TiO 2 nanoparticles was studied in a conical fluidized bed. • Rigid- and soft-agglomerates are mainly formed in the partially fluidized and spouting flow regimes. • CFD-DEM simulation allows modelling interactions between simple- and complex-agglomerates. • The performance of elastic and inelastic contact models was analysed in the flow regimes. • Elastic models predict better pressure drop and bed expansion ratio than inelastic ones. The present study evaluates elastic and inelastic contact forces in predicting the fluidization characteristics of irregularly shaped Titania nanoparticle agglomerates in a bench-scale conical fluidized bed. At first, experiments were performed to identify the mutual effects of gas velocity and agglomerate size on the instantaneous pressure, bed pressure drop and bed expansion ratio. Rigid complex-agglomerates (∼100−200 μm) were mainly formed in the partially fluidized regime, while soft simple-agglomerates (∼20−70 μm) were primarily formed in the spouting regime. The simulations were performed through the CFD-DEM approach. The error analysis of results in the partially fluidized regime revealed that the combination of Hertz-Mindlin and Johnson-Kendall-Roberts (HM + JKR) model led to a better prediction of the bed pressure drop and bed expansion ratio than the spring-dashpot (LSD) model. In the transition flow regime, the Hysteretic model led to better results of the instantaneous pressure values than those proposed for other flow regimes. In the spouting flow regime, the HM and Thornton models led to over-estimation of bed pressure drop when compared to the LSD model. The comparative results of this study provide promising new insights into the collision mechanism of polydisperse agglomerates in different flow regimes for industrial aspects. [ABSTRACT FROM AUTHOR]

The gas–liquid–solid three‐phase moving beds could supply a potential solution for multiphase reactions with catalyst easily deactivated, and the flow regimes in it were studied by optical method and pressure drop measurement. Results showed that taking the trickle flow as the initial flow regime, the flow channels were more obvious as the particle velocity increased. When the initial flow regimes were pulse flow and bubble flow respectively, the pulse‐to‐trickle and bubble‐to‐pulse flow transitions mainly occurred at moderate‐to‐high particle velocities (0.01–0.04 m s−1 under conditions used in this work). Moreover, the flow regime map in the three‐phase moving bed was constructed and shown that the region of trickle flow increased and the region of bubble flow decreased. Finally, the application of three‐phase moving beds was discussed, and it could be suitable for those reactions, which had to operate in the pulse flow, bubble flow, and transition zone. [ABSTRACT FROM AUTHOR]

Magnetic fields were used to successfully improve the fluidization quality of magnetizable particles, forming the magnetized fluidized bed (MFB). Moreover, researchers found that the binary admixture of magnetizable and nonmagnetizable particles could also be used in the MFB, creating the admixture MFB. Consequently, the MFB technique is no longer restricted to the few magnetizable particles in nature and can be extended to numerous nonmagnetizable particles. Nevertheless, research on the admixture of MFB is far from sufficient, severely hindering its commercial application in the chemical and biochemical industries. To deepen our understanding in this area, this review summarizes the relevant findings, which mainly include (1) transport phenomena in the gas-solid admixture MFB with Geldart B particles; (2) elimination of the abnormal fluidization phenomena in the gas-solid admixture MFB with Geldart C particles; (3) flow regime transition of the liquid-solid admixture MFB under both the magnetization-FIRST and magnetization-LAST operation modes; and (4) application of the pure MFB in the fields of gas filtration and coal dry separation. Finally, critical comments are made on the shortcomings of the reported research with the hope that more efforts could be devoted to these aspects in the future. [ABSTRACT FROM AUTHOR]

Recent advancements in computing power allow utilization of state-of-the-art direct numerical simulations (DNSs), coupled with interface tracking techniques, to perform fully resolved simulations of complex two-phase flows, such as flow regime transitions. Studying the highly resolved temporal and spatial information produced from these virtual experiments can advance our understanding of the phenomenon and inform coarser models. With these improved models, better predictions of flow regime behavior and location in boiling water reactors can be made. The presented research uses the PHASTA code, which employs the level set method for interface tracking, to examine the mechanisms of flow regime transition, specifically the slug-to-bubbly and slug-to–churn-turbulent regime transitions. The DNS was validated using theoretical and experimental work found in open literature. Different geometries, including pipes and minichannels, were explored in order to improve the fundamental understanding of the complex flow phenomenon. Using advanced analysis techniques, the transient flow properties were analyzed at resolutions not available to other methods. The numerical data analysis allows for calculation of both time and spatially averaged properties as well as local instantaneous properties. Possible mechanisms for the transition are discussed. Examples include liquid kinetic energy/surface tension energy balance and interfacial shear forces in the liquid film. It is also noted that the transition out of slug flow can take at least two pathways: interfacial wave-induced instability development in the Taylor bubble, leading to its disintegration, or strong bubble shearing at the tail of the bubble. [ABSTRACT FROM AUTHOR]

Flow regime transition between dilute pneumatic conveying and fast fluidization is identified for two Geldart A particles and four Geldart B particles in a novel gas-solid fluidization system configuration without exit restrictions. The results indicate that both saturation carrying capacity G s ⁎ and saturated solid fraction ε s, ch strongly depend on superficial gas velocity and particle properties. Based on data from this study and literature, improved correlations for estimating G s ⁎ and ε s, ch were developed. The correlations predictions agreed much better with experimental data than previously reported correlations. Unlabelled Image • A novel riser without top and bottom constraints, with solids feed from the middle section. • Saturation carrying capacity and saturated solid fraction for gas-solids upflow are determined. • New correlations are developed using data from this study and literature. • New correlations give much improved performance than previously reported. [ABSTRACT FROM AUTHOR]

The objective of the present work is to experimentally study the pressure recovery process that occurs in supersonic ejectors. Pressure measurements are achieved using a specific sliding measuring system for the centerline static pressure. The axial displacement of this weakly intrusive system allows the measurement of axial distribution of static pressure inside the mixing zone where the shock train is formed. Experiments are realized both in mixed flow and fully supersonic regimes. They are carried out in a supersonic air-to-air ejector but may be applicable to other gases. The obtained results allow the detection between the different flow regimes and the understanding of the complex pressure recovery process which takes place in supersonic ejectors. [ABSTRACT FROM AUTHOR]

Dynamic phase tracking method was proposed by using the instantaneous local solids concentration signals to demarcate the regime transition from bubbling to turbulent fluidization. Compared with the conventional methods, this method can provide clear demarcation between bubbling fluidization, the transition process and turbulent fluidization. And some detailed characteristics about regime transition had been obtained. The tendency of bubble coalescence surpasses that of bubble breakup in bubbling fluidization, but the latter one is the primary phenomenon in turbulent fluidization. And in the transition process, neither of the tendencies is dominant. Moreover, the transition process is increasingly gradual, with radial position extending from the center to the wall region, with axial position moving from the top to the bottom of the fluidized bed, and with static bed height becoming higher. Whether the transition process is more gradual is positively correlated with the solids concentration of dense phase. Unlabelled Image • A method demarcating transition from bubbling to turbulent fluidization is proposed. • Transition process between bubbling and turbulent fluidization can be distinguished. • Neither tendency of bubble coalescence nor breakup is dominant in transition process. • Transition is studied at various positions and with different static bed heights. • Transition process is closely related to solids concentration of dense phase. [ABSTRACT FROM AUTHOR]

• Provided a non-subjective and quantitative analysis for flow regime identification. • Applied four signal analysis techniques on pressure signals to characterise flow. • Provided insights into dynamic behaviour of counter-current flows in annuli. We characterised different flow regimes and their transition through an objective and quantitative assessment of flow behaviour by applying four signal process techniques (autocorrelation, power spectral density (PSD), Shannon entropy, and permutation entropy) onto pressure signals of counter-current two-phase flows in a 170 mm × 70 mm concentric vertical annulus. Differential pressure values were measured using a 100 Hz sampling rate with 30 second sample sizes in 147 experiments at superficial gas velocities within 0.043–5.387 m/s and superficial liquid velocities within 0.016–0.093 m/s. Video images were also captured during all flow experiments. Using the signal analysis techniques, we could overcome the difficulties and uncertainties that accompany subjective visual observations in the characterisation and classification of flow regimes. Also, the signal analyses of the pressure signals allowed us to provide insights into the mechanisms responsible for formation of each flow regimes and their transitions. [ABSTRACT FROM AUTHOR]

A numerical research on flow regime of R134a in horizontal minichannels was conducted. The model was on the basis of the volume of fluid approach, and inserted user-defined routines which cover condensation mass and heat transfer. The observed annular flow, injection flow and intermittent flow of pure steam condensation, and annular (annular-wavy) flow, injection flow, annular-intermittent flow and intermittent flow of gas mixture condensation were qualitatively compared against experimental data. Flow patterns of lower non-condensable gas mole concentration show similar with that of pure steam, while the fluctuation of gas-liquid interface in annular flow and the noticeable annular-intermittent flow of higher non-condensable gas mole concentration were noted. Besides, injection flow characters with larger head volume and longer neck, and intermittent flow characters with larger slug sizes in existence of non-condensable gas were found. Furthermore, the influence of mole concentration of non-condensable gas and operating conditions on flow pattern transition lines were analyzed after validating the flow pattern transition. The results showed that the injection flow transition point has a relative fixed point at pure vapor condensation, while various in presence of NCG. Besides, lower non-condensable gas mole concentration, smaller inlet gas mass flux and larger wall heat flux could lead to earlier flow pattern regime. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Flow regime transition was compared between magnetization-FIRST and -LAST modes. • Magnetic stabilization state did no longer exist under magnetization-LAST mode. • Magnetic stabilization was a metastable equilibrium state. • Difficulty in forming a new phase resulted in forming magnetic stabilization state. Abstract Different flow regimes existed for the magnetized fluidized bed with purely Geldart-B magnetizable particles (pure MFB) or their admixture with Geldart-B nonmagnetizable particles (admixture MFB). Several unresolved issues about the flow regime transition were addressed first for both the pure and admixture MFBs. Then the flow regime transition was thoroughly compared for the first time between the magnetization-FIRST and -LAST operation modes. The magnetic stabilization flow regime was observed to no longer exist under the magnetization-LAST operation mode. The comparison further indicated that the operating range covered in the literature and this work could be roughly divided into four zones. Particularly in operating zone II, the MFB state was found to depend not only on the magnetic field intensity and superficial gas velocity but also on the operation mode. Such a dependence feature in essence referred to the dependence of the MFB state on the path taken to achieve it. The reason why the bed state was path-dependent in operating zone II lay in that the MFB therein could have two different equilibrium states and different paths led to the two states, respectively. Of the two equilibrium states, the magnetic stabilization was demonstrated to be a metastable equilibrium state. Such unique characteristics of the magnetic stabilization state explained why it could not be obtained under the magnetization-LAST operation mode. Furthermore, the reason why the MFB could have an additional metastable equilibrium state aside from the stable equilibrium state in operating zone II lay in the difficulty in forming a new phase under the magnetization-FIRST operation mode. [ABSTRACT FROM AUTHOR]

Abstract The development of heat transfer models of micro-channel boiling relies on understanding of all individual flow regimes as well as interactions and transitions between them. The presented paper performs a detailed study on the interactions between annular flow and its trailing bubbles in a cylindrical micro-channel with a diameter of 0.4 mm. Inlet mass flux is 400 kg/(m2∙s) and constant wall heat flux is 80 and 160 kW/m2, respectively. The growth rate of the bubble and the flow regime transition are validated against published experimental data. Comparisons of the results between transition flow and single annular flow show that the transition processes enhance bubble evaporation. A new phenomenon is observed: extreme high local heat transfer rates are observed under certain conditions. A detailed investigation of the thin film region unveil that this enhancement is a consequence of the interface distortion caused by the presence of the trailing bubble. This bubble interface deformation depends primarily on a few factors including wall heat flux, surface tension, trailing bubble. [ABSTRACT FROM AUTHOR]

The application of horizontal and hydraulically fractured wells for producing oil from low permeability formations has changed the face of the North American oil industry. One feature of the production profile of many such wells is a transition from transient linear oil flow to boundary-dominated flow. The identification of the time of this transition is important for the calibration of models that forecast the well's future production and the expected ultimate recovery. It is preferable that such models generally use data from the boundary-dominated flow regime for parameter calibration. Accurate well production forecasts are needed for operational decisions, long-term planning, commercial transactions, regulatory proceedings, and asset valuation. Petroleum engineers frequently make the call on the transition point based on subjective visual interpretations of log-log plots for individual wells. This is time-consuming and is generally not repeatable by other analysts. This note evaluates statistical approaches that can serve as alternatives to the subjective visual interpretations. Specifically, the predictive performance of production models calibrated with boundary-dominated data based on transition dates calculated with constrained nonlinear least squares and Bayesian regressions was very close to that obtained using the visual method, suggesting that statistical approaches may indeed be constructed to replace less objective visual approaches without loss of accuracy. [ABSTRACT FROM AUTHOR]

Highlights • Developed a new correlation for the liquid-side Nusselt number in the IAFB regime. • Applied a 1-D two-fluid model to predict the void fraction in the IAFB regime. • Predicted wall heat flux and temperature distributions in the current test section. Abstract Post critical heat flux (post-CHF) heat transfer may occur in loss of coolant accidents (LOCAs) in water-cooled nuclear reactors when makeup water quenches the uncovered fuel rods in the reactor core. The post-CHF regimes are characterized by low heat transfer coefficient and high wall temperature due to the vapor-wall contact. A good understanding of the hydrodynamics and heat transfer process in the post-CHF regimes, especially low-quality film boiling, such as inverted annular film boiling (IAFB) and inverted slug film boiling (ISFB), is imperative for accurately modeling the post-CHF phenomena and predicting the fuel rod cladding temperature and void fraction during accidents. This paper first discusses limitations of existing post-CHF hydrodynamics and heat transfer models/correlations and then identifies needs of post-CHF experimental data under high-flow and high-pressure conditions for model improvement. To narrow down the gap in the available experimental data, a Post-CHF Heat Transfer (PCHT) test facility is designed to perform quasi-steady-state low-quality film boiling experiments under high-pressure (ranging from 0.14 to 3.45 MPa), high-flow (ranging from 150 to 2000 kg/m2-s), and high inlet subcooling (up to 50 °C) conditions. In addition to the experimental study, a numerical model using COMSOL Multiphysics tool is developed to inform the test section design and to investigate wall heat flux and wall temperature distributions in the post-CHF regimes. The COMSOL Multiphysics simulation results show that quasi-steady-state post-CHF phenomena can be achieved in the PCHT test section. Considering thermal non-equilibrium conditions in the post-CHF regimes, a one-dimensional two-fluid model is implemented into a computer code to predict the void fraction using the wall temperature information obtained from the COMSOL simulation results. A correlation for the liquid-side Nusselt number is obtained for the IAFB regime based on previous experimental data and is implemented into the code, which results in an improvement in the agreement between the calculated void fraction and corresponding experimental data. With the new Nusselt number correlation, a parametric study of the effects of the heat flux, pressure, mass flux, and inlet subcooling on the liquid-side Nusselt number is studied. In addition, flow regime transition in the post-CHF regimes is studied by comparing the existing correlation and experimental data. Furthermore, the Weber number and liquid-side Nusselt number are calculated to examine whether there exists any correlation between these two non-dimensional numbers and the flow regime transition. The simulation results provide insights for design improvement of the PCHT test section and assist the development of a test plan for future IAFB and ISFB tests under high-pressure, high-flow, and high-subcooling conditions in the PCHT test facility. [ABSTRACT FROM AUTHOR]

The previous neglect of shale oil multi-component characteristics and the nanpore wall properties of real shale result in an insufficient understanding of shale oil flow mechanisms in nanopores. Meanwhile, research on the flow regimes of shale oil remains lacking. In this study, molecular dynamics simulations are employed to investigate the flow of shale oil in hydroxylated quartz nanopores and rough kerogen nanopores. Simulation results show that the flow regime changed as the pressure gradient (∇ p) increased to a critical value (∇ p c). The velocity profile was parabolic when ∇ p < ∇ p c , but gradually became piston-like when ∇ p ≥ ∇ p c. Because increasing ∇ p leads the adsorbed molecules desorbing, aggregating in the pore center, and forming clusters that are not easy to shear. Increasing vertical force from pore wall causes fluid aggregation in the pore center as ∇ p increases. The ∇ p c in kerogen nanopores is larger than that in quartz nanopores due to the rough kerogen surface and sticky layers. Multi-component fluids have higher ∇ p c than single-component fluids in quartz nanopores. However, they have the same ∇ p c in kerogen nanopores due to the rough kerogen surface. This investigation can provide theoretical basis for high-efficient production of shale oil. [Display omitted] • The flow regime of multicomponent shale oil in rough nanochannels was studied by MD. • Critical pressure gradients exist for fluid flow regime transitions in nanopores. • Determined governing factors of velocity profile changing from parabola to piston. • Determined governing factors of critical pressure gradients. [ABSTRACT FROM AUTHOR]

• Statistical, Hurst, Shannon entropy and chaos analysis methods are used. • Flow regime transitions in an aerated stirred tank are identified. • Standard deviation and largest Lyapunov exponent are effective indicators. • The relationship between the flow regime and the degree of chaos is discussed. A novel method was proposed to determine the flow regime based on the bottom pressure fluctuations of the stirred tank. Statistical, Hurst, Shannon entropy and chaos analysis methods were used to analyze the pressure time series in an aerated stirred tank with a Rushton impeller, and the relationship between the characteristic parameters and the flow regimes was established. The results showed that the bubble motion in the loading regime was obviously stronger than that in the flooding regime, and the homogeneity of the system was rapidly improved. Compared with the first two stages, the chaotic extent of the complete dispersion regime was slightly weaker, and the benefit generated by continuously increasing the speed was limited at this time. In addition, considering the sensitivity of pressure pulsations to the gas–liquid dispersion state, the transition points of the three flow regimes were successfully identified by the standard deviation and the largest Lyapunov exponent. The data were well consistent with the measurements from the visual method and power method, with a maximum deviation of 16%. This demonstrated that it was a reliable non-invasive method for flow regime identification in gas–liquid stirred tanks. [ABSTRACT FROM AUTHOR]

For the analysis of two-phase phenomena, this work addresses the implementation of the advanced multi-scale interface model of Tentner et al. (2006) into OpenFOAM-7 code, and its assessment for flow boiling in vertical pipes. The model considers the concentration and shape of the interfacial region, as well as the interphase interactions. Two-phase boiling-flow data were used to methodically evaluate this model and its overall propensity to predict the critical heat flux (CHF) was then assessed. Departure from nucleate boiling (DNB) was predicted based on a surface temperature rise, which served as the criterion. The impacts of exit quality, pressure and mass flow velocity on DNB were also addressed. The mean percentage discrepancy found between CHF lookup table and simulated results was 3%, indicating a satisfactory level of agreement. Findings indicate that the implemented two-phase multi-scale interface model in OpenFOAM-7 tool could be utilized in forecasting boiling flow involving flow topology transitions and adequately predict CHF. • A new solver was developed into the open source CFD tool OpenFOAM-7. • A multi-scale two-phase flow with various flow topology transitions. • Local flow topologies like bubbly flow, mist flow, and sharp interface may be distinguished. • Flow boiling and critical heat flux in vertical pipe were predicted using OpenFOAM-7. • CHF always occurs at the end of the heated pipe for uniform heat flux. • Most of the deviations between predicted CHF against CHF look-up table was within 10%. [ABSTRACT FROM AUTHOR]

Highlights • The vertical upward slug flow in the range of liquid superficial velocity (0.03< j l < 1.6m/s) were studied in tubular test section with the inside diameter of 25.4mm. • New structure of turbulent slug unit in strong relation with transition process was observed. • The flooding shows great influence on the issue that whether the flow regime evolves into churn-turbulent flow or directly into annular flow. • It was proposed that two structure parameters, i.e., void fraction of liquid slug and length ratio of Taylor bubble to liquid slug, are appropriate to be used as the transition criterion. Abstract In order to investigate the evolution characteristics of structure parameters during flow regime transition process in gas-liquid slug flow, the vertical upward slug flow in a wide range of liquid superficial velocity (0.03< j l <1.6m/s) are studied in a tubular test section with the inside diameter of 25.4mm. Impedance void meters are employed to measure the time series of cross-sectionally averaged void fraction of separated two parts corresponding to Taylor bubble and liquid slug. With increasing flow gas flow rates at constant liquid flow rate, the present research studies the evolution of length and void fraction of each part in slug unit. New structure of turbulent slug unit in strong relation with transition process is observed. It is found that such special structure plays an important role in transition from slug flow to churn-turbulent flow. It is also found that the flooding determines whether the flow regime evolves into churn-turbulent flow or directly into annular flow. By studying the evolution of flow regimes, two structure parameters, void fraction of liquid slug and length ratio of Taylor bubble to liquid slug, can be used as the transition criterion. New criterion for the transition from slug flow to churn-turbulent flow was also proposed based on these two parameters. It is found that the current results agree well with the existing flow regime map for relatively small diameter tubes in literature. [ABSTRACT FROM AUTHOR]

The Reynolds number for flow in a street canyon, Re = UrefH/ν (where Uref is a reference velocity, H the street canyon height, and ν the kinematic viscosity), cannot be matched between reduced-scale experiments and full-scale field measurements. This mismatch is often circumvented by satisfying the Re independence criterion, which states that above a critical Re (Rec), the flow field remains invariant with Re. Rec = 11,000 is often adopted in reduced-scale experiments. In deep street canyons with height-to-width aspect ratio ≥ 1.5, reduced-scale experiments have shown two recirculation vortices induced by the mean flows, but full-scale field measurements have observed only one vortex. We investigated this discrepancy by conducting water channel experiments with Re between 104 and 105 at three aspect ratios. The canyons with aspect ratio 1.0 have Rec = 11,000, the canyons with aspect ratio 1.5 have Rec between 31,000 and 58,000, while the canyons with aspect ratio 2.0 have Rec between 57,000 and 87,000. Therefore, the widely adopted Rec = 11,000 is not applicable for canyons with aspect ratio greater than 1.5. Our results also confirm that there is only one vortex in deep canyons at high Re. This single-vortex flow regime could change our fundamental understanding of deep canyons, which are often assumed to exhibit multiple-vortex flow regimes. Applications such as numerical model validation based on the multiple-vortex regime should be revisited. Our experimental data with Re up to 105 could be used to validate numerical models at high Re. [ABSTRACT FROM AUTHOR]

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 〈〈 v g 〉〉 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. [ABSTRACT FROM AUTHOR]

Hydrodynamic data obtained from laboratory‐scale trickle‐beds often fail to accurately represent industrial‐scale systems with high packing aspect ratios and column‐to‐particle diameter ratios. In this study, pressure drop, liquid holdup, and flow regime transition were investigated in a pilot‐scale trickle‐bed column of 33 cm ID and 2.45 m bed height packed with 1.6 mm × 8.4 ± 1.4 mm cylindrical extrudates for air‐water mass superficial velocities of 0.0023 – 0.094 kg/m2s and 4.5 – 45 kg/m2s, respectively, at atmospheric pressure. Significant deviation was observed from pressure drop and liquid holdup correlations at low liquid flows rates, corresponding to gravity‐driven flow limit. Likewise, liquid saturation is overestimated by correlations at high liquid flow rates, owing to significantly reduced wall effects. Lastly, trickle‐to‐dispersed bubble flow and trickle‐to‐pulsing flow regime transitions are reported using a combination of visual observations and analysis of the magnitude of local pressure fluctuations within the column. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2560–2569, 2018 [ABSTRACT FROM AUTHOR]

Volcanoes of Strombolian type are able to maintain their semi-permanent eruptive states through the constant convective recycling of magma within the conduit leading from the magma chamber. In this paper we study the form of this convection using an analytic model of degassing two-phase flow in a vertical channel. We provide solutions for the flow at small Grashof and large Prandtl numbers, and we suggest that permanent steady-state counter-current convection is only possible if an initial bubbly counter-current flow undergoes a régime transition to a churn-turbulent flow. We also suggest that the magma in the chamber must be under-pressured in order for the flow to be maintained, and that this compromises the assumed form of the flow. [ABSTRACT FROM AUTHOR]

It is known that the fluid dynamics and transport phenomena in bubble columns depend mainly on the bubble column design (i.e., the column diameter, aspect ratio, and gas sparger openings) and the liquid phase properties. In this communication, we contribute to present-day discussion through an experimental study concerning the combined effects of the gas sparger design and liquid phase properties on both the gas holdup and the main flow regime transition. The experimental study concerning gas holdup measurements was conducted in a large-diameter and large-scale bubble column (with a height of 5.3 m and inner diameter of 0.24 m) operated in the batch mode. Air was used as the dispersed phase (using gas superficial velocities in the range 0.004–0.20 m/s), and various water–monoethylene glycol (MEG) solutions were employed as binary liquid phases. The water–MEG solutions tested have viscosities between 0.9 mPa·s and 7.97 mPa·s, densities between 997.086 kg/m 3 and 1094.801 kg/m 3 , and surface tension between 0.0715 N/m and 0.0502 N/m. Two gas spargers were tested: (a) a spider sparger (“ coarse gas sparger ”) and (b) a needle sparger (“ fine gas sparger ”). The former produced a poly-dispersed homogeneous flow regime resulting in a concave gas holdup curve, whereas the latter produced a mono-dispersed homogeneous flow regime resulting in an S-shaped gas holdup curve. It was observed that the mono-dispersed bubble size distribution stabilized the homogeneous flow regime. The addition of MEG produced different effects depending on the gas sparger design. The addition of MEG in the “ coarse gas sparger ” configuration produced what is usually referred to as “ dual effect of viscosity ”: depending on the MEG concentration, the homogeneous flow regime was stabilized/destabilized, and thus, the gas holdup increased/decreased. Conversely, the addition of MEG in the “ fine gas sparger ” changed the shape of the gas holdup curve from an S-shape to concave, thus rendering it similar to the ones produced by “ coarse gas sparger ”. We speculate that viscous solutions reduce the influence of the inlet conditions in large-diameter and large-scale bubble columns; this is a matter of future research. [ABSTRACT FROM AUTHOR]

The influence of floating vessel motions on the hydrodynamic behavior of multiphase flows in porous media was studied using a hexapod ship motion emulator with an embarked packed column. The response of gas-liquid distribution, pressure drop, liquid saturation, and flow regime transition to column inclinations and roll motions was compared to those of the corresponding static vertical and inclined configurations. Two-phase flow patterns in terms of local liquid saturation distribution were visualized online by means of a capacitance wire-mesh sensor positioned firmly on the floating packed bed. The hydrodynamic performance of packed beds under roll motion deviates strongly from that of the static beds, indicating that the known characteristics of the conventional land-based trickle-bed reactors cannot be transposed on a one-to-one basis for design and scale-up of the floating reactor configurations. [ABSTRACT FROM AUTHOR]

The correct design, operation and scale-up of bubble columns rely on the knowledge of the fluid dynamics at the “ bubble-scale ” and at the “ reactor-scale ”. This paper contributes to the existing discussion on the multi-scale fluid dynamics of bubble columns, and provides a complete dataset concerning the influence of the liquid phase properties on bubble column fluid dynamics at “ bubble-scale ” and at the “ reactor-scale ”. The bubble column is large-diameter and large-scale (5.3 m in height, inner diameter of 0.24 m), it was operated with superficial gas velocities in the range of 0.004–0.20 m/s, and it was tested for different aspect ratios (in the range 5–12.5). Air was used as the dispersed phase and various fluids (tap water, aqueous solutions of NaCl, a water-ethanol mixture and solutions of water-monoethylene glycol of different concentrations) were employed as liquid phases. The gas holdup measurements were used to investigate the global fluid dynamics, the flow regime transition and the foaming phenomena. Subsequently, the image analysis was used to characterize the bubble size distributions and shapes in the homogeneous flow regime. Finally, the gas holdup and image analysis data were used to estimate the interfacial areas. The experimental data were extensively compared with previous studies and correlations and were used to propose novel correlations to estimate the bubble shapes and the interfacial areas, for the different liquid phases tested. It was found that a change in the liquid phase properties affects the bubble interfacial properties at the “ bubble-scale ”, thus changing the prevailing bubble size distribution because of the reduced/promoted coalescence phenomena. The variation in the prevailing bubble size distribution affects the “ reactor-scale ” (gas holdup and flow regime transition) and a lift-force-based approach is used to analyze the relationship between the “ bubble-scale ” and the “ reactor-scale ”. When the prevailing bubble size distribution shifts towards larger bubbles, because of the promoted coalescence (i.e., high viscous liquid phases), the lift force pushes the larger bubbles towards the center of the bubble column, inducing “ coalescence-induced bubbles ” and, consequently, destabilizing the homogeneous flow regime and decreasing the gas holdup. Conversely, when the prevailing bubble size distribution shifts towards smaller bubbles, because of the reduced coalescence (i.e., organic and inorganic active compounds, and low viscous liquid phases), the lift force pushes the small bubbles towards the wall, inducing cluster of bubbles and, consequently, stabilizing the homogeneous flow regime and increasing the gas holdup. In addition, foaming phenomena were observed for organic active compounds, in the case of high aspect ratios only: this observation suggests that the validity of the scale-up criteria is not ensured for all binary systems and future studies will be devoted to provide insights in the scale-up criteria. [ABSTRACT FROM AUTHOR]

This paper experimentally investigated the bubbly-intermittent flow (BI) transition in helically coiled tubes (HCT) with coil diameter of 20, 40 and 80 cm. A double sensor conductivity probe and a high-speed camera were used to obtain local phase distribution parameters and flow imagery. Because of the centrifugal force, buoyancy was redirected and enhanced, leading to a highly asymmetry and concentrated bubble distribution. As a result, BI transitions occurred at much lower gas flow rates in HCTs. Observation of the BI transition revealed two mechanisms: bubble accumulation in flow direction due to bubble velocity difference (liquid superficial velocity lower than 1.5 m/s), and bubble accumulation in radial and peripheral direction due to strong buoyancy (liquid superficial velocity higher than 1.5 m/s). The critical void fraction at which BI transition occurred (e.g., α = 0.065 for coil diameter of 40 cm) increased as the coil diameter increased, and was much lower than that of straight tubes ( α = 0.25–0.51). A correlation predicting BI transition was established, covering both straight inclined tubes and HCTs; which met the experimental results well when the coil diameter was larger than 0.2 m. [ABSTRACT FROM AUTHOR]

Different flow regimes are known to occur in the interaction of multiphase gas–liquid flows over packed beds of solid particles, such as those observed in trickle bed reactors (TBRs). There are four major flow regimes that are known to occur in downward cocurrent flow in TBRs, namely: trickle, pulse, bubble, and mist flow regimes. In this work, the focus is on macro-scale experimental visualizations and investigations of the flow regimes in a two-dimensional TBR. Experimental observations are made to investigate the development and transition of these flow regimes over a wide range of liquid and gas velocities. Cylindrical particles are placed between two glass plates that are sealed on the sides, and water and air are injected over them using an injection manifold to simulate multiphase flow in a TBR. A diffused light emitting device (LED) light table is used to illuminate the experimental window, while real time images are obtained using a high-speed camera. Flow maps are reported depicting all four regimes and the transition regions between them. Transition regions occur where the characteristics of more than one flow regime coexist. The 2D experimental results are then compared with the existing literature data of three dimensional results and found to be in good agreement. Emphasis is placed on the transition between the trickle and pulse regimes, since that is the most important mode of operation in industrial TBRs. It is observed that the change in diameters of the cylindrical particles in a two-dimensional TBR has little effect on the transition between the flow regimes when the porosity of the bed is kept constant. [ABSTRACT FROM AUTHOR]

An experimental study of the flow regimes and transitions in a fluidized bed (cross-section of 200 mm × 200 mm and height of 2000 mm) containing a mixture of cylindrical particles and silica sand is carried out. Six different flow regimes are identified: fixed bed (F), bubbling fluidization (B), transition fluidization (T), partial fluidization (P), complete fluidization (C) and unable to fluidize (N). The flow regime characteristics are described using schematic diagrams and photographic images. Based on the flow regime classification, three types of flow regime transition routes are explained. The effect of various operating parameters on the flow regime transition is determined and the resulting flow regime map is presented. Two unstable fluidization patterns are observed and their fluidization mechanisms are discussed. It is shown that the change of the Δ P - u f profile, where Δ P is the bed pressure drop and u f is the superficial gas velocity, indicates when a flow regime transition occurs and the reproducibility of the Δ P - u f curve identifies fluidization stability. Furthermore, a contact force model for a cylindrical particle in a bed material cloud is developed considering particle orientation, based on which a theoretic model for cylindrical particle fluidization termination velocity ( u b ) was derived and validated. [ABSTRACT FROM AUTHOR]

Some authors, in the last decades, have observed the dual effect of viscosity on gas holdup and flow regime transition in small-diameter and small-scale bubble columns. This work concerns the experimental investigation of the dual effect of viscosity on gas holdup and flow regime transition as well as bubble size distributions in a large-diameter and large-scale bubble column. The bubble column is 5.3 m in height, has an inner diameter of 0.24 m, and can be operated with gas superficial velocities in the range of 0.004–0.20 m/s. Air was used as the dispersed phase, and various water-monoethylene glycol solutions were employed as the liquid phase. The water-monoethylene glycol solutions that were tested correspond to a viscosity between 0.9 mPa s and 7.97 mPa s, a density between 997.086 kg/m 3 and 1094.801 kg/m 3 , a surface tension between 0.0715 N/m and 0.0502 N/m, and log 10 ( Mo ) between −10.77 and −6.55 (where Mo is the Morton number). Gas holdup measurements were used to investigate the global fluid dynamics and the flow regime transition between the homogeneous flow regime and the transition flow regime. An image analysis method was used to investigate the bubble size distributions and shapes for different gas superficial velocities, for different solutions of water-monoethylene glycol. In addition, based on the experimental data from the image analysis, a correlation between the bubble equivalent diameter and the bubble aspect ratio was proposed. The dual effect of viscosity, previously verified in smaller bubble columns, was confirmed not only with respect to the gas holdup and flow regime transition, but also for the bubble size distributions. Low viscosities stabilize the homogeneous flow regime and increase the gas holdup, and are characterized by a larger number of small bubbles. Conversely, higher viscosities destabilize the homogeneous flow regime and decrease the gas holdup, and the bubble size distribution moves toward large bubbles. The experimental results suggest that the stabilization/destabilization of the homogeneous regime is related to the changes in the bubble size distributions and a simple approach, based on the lift force, was proposed to explain this relationship. Finally, the experimental results were compared to the dual effect of organic compounds and inorganic compounds: future studies should propose a comprehensive theory to explain all the dual effects observed. [ABSTRACT FROM AUTHOR]

An understanding of the bubble properties, size distributions and shapes is of fundamental importance for comprehending flow dynamics and mass transfer phenomena in bubble column reactors. A large number of studies have focused on open tube bubble columns, and the knowledge concerning bubble columns with internals is still limited. This paper contributes to the existing discussion experimentally investigating a counter-current annular bubble column with 0.24 m inner diameter and two internal pipes. The experimental investigation consists in holdup measurements and image analysis. The former is used for identifying the flow regime transition and studying the bubble column hydrodynamics, whereas the latter is used for investigating the bubble shapes and size distributions. The definition of the transition point is important because the size distribution and bubble shapes depend on the operating conditions and a change of the bubble properties is expected near the transition. The image analysis is applied at different superficial gas and liquid velocities, corresponding to a gas holdup between 2.9% and 9.6%. It is difficult to measure bubble size distribution accurately in large-diameter bubble columns owing to the overlapping of bubbles, even at low void fractions, and—in an annular gap bubble column—the fact that cap bubbles have also been reported in the homogeneous flow regime. The use of a bubble image analysis method to study the bubbly flows in a large-diameter annular gap bubble column is described. In the proposed method, each bubble is approximated and reconstructed using an ellipse. The proposed approach is used to quantify the bubble size distribution, as well as to study the bubble shape and orientation as function of the superficial gas and liquid velocities. The experimental data obtained are used to develop a correlation between non-dimensional parameters and aspect ratios. Also, the experimental data are compared with non-dimensional diagrams from the literature, revealing good agreement. Finally, the image analysis is used for supporting the flow regime transition prediction in the stability analysis method: the virtual mass formulation is obtained by using the aspect ratio correlation provided by the image analysis. The stability analysis—supported by the image analysis—was able to predict the transition point in very good agreement with experimental data and performed better than literature correlations. [ABSTRACT FROM AUTHOR]

In this paper, we apply a variety of experimental techniques to investigate the influence of the counter-current mode on bubble column hydrodynamics. We study an air–water bubble column, which is 5.3 m in height and has an inner diameter of 0.24 m, and we consider gas superficial velocities in the range of 0.004–0.20 m/s and liquid superficial velocities up to −0.09 m/s. The experimental investigation consists of holdup, gas disengagement, image analysis and optical probe measurements. The holdup measurements are compared with the literature and are used to investigate the flow regime transition. The gas disengagement measurements are used to further investigate the flow regime transition and study the structure of the holdup curve. The image analysis is used to study the bubble shapes and size distributions near the sparger and in the developed region of the column; in particular, the image analysis is applied to different gas velocities in the homogeneous regime in both the batch and counter-current modes. The optical probe is used to acquire radial profiles of the local properties (i.e., local void fraction and bubble rise velocity) to study the flow properties and further investigate the flow regime transition. Comparing the results from the different techniques, the influence of the gas superficial velocity and the liquid superficial velocity is discussed considering all main aspects of the two-phase flow, from the local flow properties to the global flow features. The counter-current mode is found to increase the holdup, reduce the bubble rise velocity, destabilize the homogeneous regime and change the local flow properties. [ABSTRACT FROM AUTHOR]

Bubble columns are frequently studied without considering internals (open tube bubble columns). However, in most industrial applications, internal devices are often added to control heat transfer, to foster bubble break-up or to limit liquid phase back mixing. These elements can have significant effects on the multiphase flow inside the bubble column reactor and the prediction of these effects is still hardly possible without experimentation. In this paper, we study experimentally a counter-current gas–liquid bubble column in the open tube and annular gap configurations. In the annular gap bubble column, two vertical internal tubes are considered. The column has an inner diameter of 0.24 m, and the global and local hydrodynamic properties are studied using gas holdup measurements and a double-fiber optical probe. The gas holdup measurements are compared with the literature and used to investigate the flow regime transition. A double-fiber optical probe is used to acquire midpoint data and radial profiles of the local properties to study the flow properties and to further investigate the flow regime transition. The counter-current mode is found to increase the holdup, decrease the bubble velocity and cause regime transition at lower superficial gas velocity. The holdup curves in the annular gap and open tube configurations are similar in shape and values, suggesting that the presence of internals has a limited influence on the global hydrodynamic. In addition, it is found that the presence of the internals stabilizes the homogeneous regime in terms of transition gas velocity and holdup. [ABSTRACT FROM AUTHOR]

The hydrodynamics of trickle bed reactors operating near the trickle-bubbly flow regime transition have not been fully characterized in the literature. In this work, an air–water system is used to investigate the effects of particle size, particle shape, void fraction, and surface tension on the trickle-bubbly flow regime transition in trickle bed reactors. The flow regime transition is detected based on standard deviation of pressure drop with confirmation by visual observation. For all cases, the liquid superficial velocity required for the trickle-bubbly transition was found to be relatively independent of the gas superficial velocity ( v G = 4–40 mm/s). Literature models, defined for the trickle-pulse transition, are unable to predict this trend when extrapolated to low gas superficial velocities ( v G ⩽ 40 mm/s). To address this gap, a correlation is proposed based on the experimental data gathered in this work. [ABSTRACT FROM AUTHOR]

• Gas holdup at developing and developed regions was systematically measured. • Initial bubble size was estimated according to generator performance. • A semi-empirical correlation at dynamic equilibrium region was proposed. Gas holdup along with the superficial gas velocity was systematically investigated in a lab-scale bubble column. The experiments were conducted in a bubble column with a height of 6 m at superficial gas velocity of 0.01–0.13 m/s. Experimental results show that dynamic equilibrium region was achieved at a distance of 2800 mm above the bubble generator. Small initial bubble size will lead to higher gas holdup and slump trend. To overcome the trade-off between sufficient gas holdup and slump trend, a reasonable gas holdup without slump can be obtained by controlling the liquid mass flow in the nozzle. Based on previous empirical correlations and experiment data, a semi empirical correlation of gas holdup was established consider the flow regime transition and bubble generator performance. The semi empirical correlation in this work gives a fairly accurate prediction while several empirical coefficients were used. [ABSTRACT FROM AUTHOR]

As a part of characterizing the bubble interaction mechanisms and flow regime transition processes in horizontal gas–liquid two-phase flow, a flow visualization study is performed in an air–water test facility constructed from 3.81 cm inner diameter clear acrylic round pipes. The test section is approximately 250 diameters in length to allow for development of the flow. Flow visualizations are performed using a high-speed video camera at 80 and 245 diameters downstream of the inlet to observe the development of the flow structures. A total of 27 flow conditions including bubbly, plug, slug, stratified, wavy, and annular flows are characterized in the present study. In highly turbulent bubbly flow conditions it is found that the distribution becomes more uniform with increasing development length through a turbulence penetration process that counters the effect of buoyancy. It is also found that plug bubbles form below a layer of small bubbles rather than at the upper pipe wall where the bubbles are most packed. In fact, it is consistently found that turbulence-based bubble interactions do not occur in the most densely packed regions as the eddies there are not large enough to effect the bubbles. Rather, bubble packing-induced coalescence occurs in these regions and contributes to the formation of plug bubbles. The newly formed plug bubbles move faster than, and ultimately pass, the smaller bubbles above due to the effect of the wall. These small bubbles are subsequently overtaken by the following plug bubble and coalesce with the nose region through a process denoted as drag-induced coalescence. [ABSTRACT FROM AUTHOR]

Modern-day applications need mm-scale shear-driven flow condensers. Condenser designs need to ensure large heat transfer rates for a variety of flow conditions. For this, good estimates for heat-transfer rate correlations and correlations for the length of the annular regime (beyond which plug-slug flows typically occur) are needed. For confident use of existing correlations (particularly the more recent ones supported by large data sets) for shear-pressure driven internal condensing flows, there is a great need to relate the existing correlation development approaches to direct flow-physics based fundamental results from theory, computations, and experiments. This paper addresses this need for millimeter scale shear driven and annular condensing flows. In doing so, the paper proposes/compares a few new and reliable non-dimensional heat-transfer coefficient correlations as well as a key flow regime transition criteria/correlation. [ABSTRACT FROM AUTHOR]

The spouting of fine particles is significantly different from that of coarse particles and a stable spouting can only be obtained under very constrained operating conditions. However, the stability of a spouted bed with fine particles can be effectively improved by addition of coarse particles. In this work, the flow regime transition and hydrodynamics of spouted beds with binary mixtures were experimentally investigated under a broad range of operating parameters. Effects of particle size, density, mixing ratio, gas velocity and nozzle diameter on flow regime transition and minimum spouting velocity in spouted beds were explored. The results show that a moderate increase in the mean particle diameter and density due to addition of coarse particles is beneficial to spouting stability. However, segregation is also observed when adding excess amount of coarse particles, thus leading to decreased spouting stability. [ABSTRACT FROM AUTHOR]

Flow regime transition is observed in closed conduit when there is a transition between free surface and pressurized flows. This condition is often observed in stormwater storage tunnels undergoing rapid filling. Such tunnels provide stormwater control inurbanized areas, relieving the stormwater collection system during intense storms. Operational issues have been reported in such tunnels during intense rain events, some of which linked to air pocket entrapment. These issues motivated development of flowregime transition models and more recent models attempt to improve predictions by accounting for air effects in formulations. However, no comparative study has been performed between recent models that account for air pockets and earlier models that don't include this feature. This work aims to address this gap, focusing on an actual tunnel geometry that was proposed for the city of Washington, DC. Results indicate that including air in model formulation yielded more realistic predictions of low pressures. [ABSTRACT FROM AUTHOR]