Local Liquid Velocity Research Articles

Four-sensor droplet capable conductivity probe for measurement of churn-turbulent to annular transition flow

In churn-turbulent to annular transition two-phase flow, both gas and liquid phase exist in the form of either continuous field (gas core/liquid film) or dispersed field (bubble/droplet). The complexity of the flow structure makes it very difficult to obtain experimental data in the churn-turbulent to annular transition flow. A novel four-sensor conductivity probe has been designed to measure interfacial structures at the churn-turbulent to annular flow regime transition. The new probe incorporates a sensor close to the detection point to serve as a common electrode for the other four sensors. Relatively large droplets can be identified and distinguished from the continuous liquid phase such as a liquid ligament or a film attached to the wall. To satisfy the higher requirements of the probe signal quality for the churn-turbulent and annular transition flow, the probe circuit has been optimized for a faster signal response and less interference. The new probe was tested in a 200 mm × 10 mm channel with an inlet superficial gas velocity up to 19.91 m/s. The measurement results are cross-checked by area-averaging the local superficial liquid and gas velocities and comparing with the measurements from a vortex flow meter and rotameter. The error is less than 10% for the superficial gas velocity and less than 20% for the superficial liquid velocity. The measured droplet size distribution is compared with a previous model, and good agreement is obtained. The effect of superficial gas velocity on droplet and ligament volume fraction as well as interfacial area concentration are investigated using the local measurement results of the three churn-turbulent to annular flow conditions.

Modelling and validation of TBR Hydrodynamics: Local comparison between CFD and experiments

A Computational Fluid Dynamics (CFD) and validation experimental study was conducted for a packed column where two fluid phases flow downward in a trickle regime. In the model, the explicit description of the solid packing was incorporated in order to account for the textural characteristics of the bed through a resolved-particle approach, and the momentum balances for the two fluid phases were solved in the void space between the packing. The models were implemented in the commercial software COMSOL Multiphysics 5.3a. In the experimental setup, the local liquid velocity and liquid saturation diameter profiles at three different location in the column height were determined using optical fiber probes. These experimentally determined profiles were used to validate the local predictions of the CFD model, which considered the two-phase hydrodynamics and the three-phase interaction forces through interfacial momentum exchange closures. The results show that the CFD model can properly predict the local variation of the liquid velocity at different flow rate conditions, with an average absolute error below 18.6%. The CFD model properly predicted the liquid maldistribution observed in the experimental measurements. Furthermore, the CFD model results allowed to study other local phenomena, such as bypass channeling and backmixing; and also allowed to determine the variations in the interaction forces between the phases.

Predicting the sliding bubble velocity on the lower part of a horizontal tube heater under natural convection based on force balance analysis

The main objectives of this work are to examine the sliding bubble behaviour outside a horizontal tube through a visualisation experiment and to establish a force balance model for predicting the sliding bubble velocity. Of interest here, the sliding bubble velocity is included in a mechanistic boiling heat transfer model and it varies with the bubble volume, location of the nucleation site, generation frequency of the bubble, etc. In order to predict the sliding velocity of the bubbles generated under the various conditions, a model developed based on the physical mechanism is required. This paper presents experimental techniques and measurement results for observing vapour bubble sliding phenomena. A specially devised flexible heater was fabricated and attached on a rod in order to control the bubble generation location. With the heater and a digital image processing technique, accurate measurement of the bubble volume and velocity became facilitated. Through the experiment, the life cycles of a single bubble was clearly visualised including the birth, departure, sliding, and lift-off. The force balance model which includes the curvature effect in cylindrical coordinate was derived to predict the sliding bubble velocity in the experiment. The equation of the model was based on the balance of circumferential directional forces acting on a sliding bubble with a correction of the local liquid velocity to consider the wake generated by a preceding bubble. The force balance analysis showed that the dominant forces are the buoyancy, quasi-steady drag, and added mass forces. The proposed model was validated by comparing the predicted bubble velocity with the experimental data and it predicted the bubble velocity with satisfactory accuracy. In addition, the force balance analysis found that the sliding bubble velocity correlates well with the bubble generation location when it was non-dimensionalised in a form of Froude number.

Численное моделирование кристаллизации модифицированной металлической капли при растекании на подложке

The purpose of the research was to numerically simulate the processes when melting drops fall on a substrate. The paper deals with the solidification on the metal surface of a binary aluminum alloy modified by activated refractory nanosized particles, which are the centers of crystalline phase nucleation. We formulated a mathematical model which describes the thermo- and hydrodynamic phenomena in the drop upon interaction with a solid substrate, heterogeneous nucleation during melt cooling, and subsequent crystallization. The flow in a liquid is described by the Navier --- Stokes equations in the Boussinesq approximation. The position of the free boundary of the melt is fixed by marker particles moving with the local liquid velocity. On the melt --- substrate contact surface, thermal resistance is taken into account. The hydrodynamic problem is considered under conditions of crystallization of molten metal. The temperature conditions and the kinetics of the growth of the solid phase in the solidifying aluminum alloy are described for various sizes of formed splats. Satisfactory agreement was found between the shape of the splat obtained by the results of numerical simulation and the available experimental data. The adequacy of the crystallization model in the presence of ultradisperse refractory particles in a binary alloy is confirmed. It was determined that, regardless of the size of the drop, bulk crystallization of the metal takes place. It was found that at a high rate of collision of a drop with a substrate during the melt spreading, a small fraction of the solid phase can be formed.

Experimental study of the liquid velocity and turbulence in a large-scale air-water counter-current bubble column

Measuring the local liquid velocity and turbulence in large-scale bubble columns with optical methods is complex and usually limited to low gas holdups in thin geometries. Comprehensive datasets in large bubble columns are therefore seldom published. Since the importance of Computational Fluid Dynamics (CFD) is increasing for multiphase applications, such data is also important for validating models for dispersed bubbly flows. In the present work the liquid velocity and turbulence in a pilot-scale bubble column is studied and a CFD validation database is generated by completing previous measurements of the gas void fractions and bubble sizes. For this purpose, we used a Particle Shadowgraph Velocimetry (PSV) technique that was intentionally designed to study the fluid dynamics in large-scale facilities. The measurements were realized in the 5.3 m high and 0.24 m diameter counter-current bubble column at Politecnico di Milano. The superficial gas velocity ranged from 0.37 to 1.85 cm/s, the counter current superficial liquid velocity from 0 to 9.2 cm/s. All operation points are in the so-called pseudo-homogeneous flow regime, in which the integral gas holdup (ranging from 1.02 to 7.55%) increases linear with the superficial gas velocity but an inhomogeneous flow is present. The dominant frequencies of the bubbly flow, shear rates, and turbulence levels are increasing with increasing superficial gas velocity. With increasing superficial liquid velocities, the dominant frequencies are decreasing, the averaged liquid velocities are shifted downwards, but the overall turbulence levels remain constant. In order to investigate the smaller scales at which the bubble-induced turbulence is expected, a filtering process is proposed. As a result, the filtered turbulence levels of all operation points fall on a linear trend line when plotted over the local void fraction, which is the same result obtained in other studies in small, homogenous tabletop columns. The now available database for CFD validation contains the averaged liquid velocities, basic turbulence information, local void fractions, and the bubble sizes at two different heights. The data will be in particular useful to validate the capabilities of models to upscale bubbly flows from tabletop to the pilot-scale bubble columns.

Experimental study of liquid velocity profiles in large-scale bubble columns with particle tracking velocimetry

A complete knowledge of the bubble column fluid dynamics relies on understanding the global and the local fluid dynamic properties. Unfortunately, most of the previous literature focused on the “global-scale” fluid dynamics, whereas a limited attention was devoted to the “local-scale”. We contribute to present-day discussion by proposing an experimental study concerning the local liquid velocity profiles within the pseudo-homogeneous flow regime. The experimental study, based on a particle-identification and particle-tracking algorithm, was conducted in a large-diameter and large-scale bubble column (height equal to 5.3 m; inner diameter equal to 0.24 m) operated in the counter-current mode. We considered gas superficial velocities in the range of 0.37-1.88 cm/s and liquid superficial velocities up to −9 cm/s. Time-averaged and transient liquid velocity fields were obtained for five superficial gas velocities and four superficial liquid velocities at two measuring heights. Subsequently, the liquid velocity observations were coupled with previously measured bubble size distributions and local void fractions, to provide a complete description of the “local-scale” fluid dynamics. These data would help in the validation procedure of numerical codes, to support the prediction of industrial-scale relevant conditions.

Correlation Between Foam Flow Structure in Porous Media and Surfactant Formulation Properties

The optimization of foam injection in porous media for enhanced oil recovery or soil remediation requires a large screening of surfactant formulations. Tests of foam stability in vials often used quick criteria to accelerate selection and ensure performance in porous media. Using a selection of surfactant formulations of different chemistry and foam behaviors, the correlation between foam in vials and in porous media is investigated. Along with foam stability, foamability which quantifies the ability to create foam is shown to play a role in the maximum apparent viscosity. This is a first evidence that foamability is a key parameter for the maximum apparent viscosity reached in a steady state of apparent viscosity. To account for the relative contribution of foamability and foam stability, a parameter is inspired from the widely accepted model of population balance. These results support a workflow based on large foam screening in a first step and sandpack experiments in a second step, prior to more representative but longer coreflood tests. Finally, these experimental data emphasize the relevance of population balance simulations as a description based on experimental measurement. Second, the flow visualization in the sandpack allows the extraction of a local velocity of the liquid in the flowing foam. This parameter gives an experimental evidence that the transition between the high-quality and low-quality regime corresponds to a change in the efficiency of foam lamellae network to transport gas concomitantly to liquid. The local liquid velocity also represents an indirect and easy measurement of flow structure, and it is shown to change from one formulation to another. This observation highlights the complex relation between local microstructure and physical chemistry of surfactants.

CHF correlation development under ERVC conditions by using the local liquid velocity from PIV measurements

In-vessel retention through external reactor vessel cooling (IVR-ERVC) is a severe accident management strategy that removes the decay heat from the molten corium in the reactor vessel by flooding the reactor cavity. In ERVC conditions, the critical heat flux (CHF) on the outer wall of the vessel is a crucial factor that indicates a thermal margin of the system. Previous studies used averaged mass flux to predict CHF (Haramura and Katto, 1983; Lee and Mudawwar, 1988, Katto, 1990; Jeong et al., 2005; Park et al., 2013). The averaged value cannot indicate the degree of liquid supply which is the most critical factor in the CHF model. Local liquid velocity near the heated surface is one of the most crucial factors that should be quantified to develop a CHF model. In this study, the local liquid velocity near the surface has been measured. A curved rectangular flow channel was devised to simulate the gap between the external reactor vessel wall and the insulation. The boiling heat transfer and CHF were simulated with air injection at the inner wall of the test section. In the experiment, the flow field under the simulated ERVC conditions was measured using the particle image velocimetry (PIV) technique. The PIV measurement was validated by comparing the measured velocity data and the analytic solution under circular pipe flow. The liquid velocity profile at the middle plane of the curved rectangular test section was obtained in the multiple mass flux and inclination angle conditions. Based on the velocity data, a local velocity correlation was developed. The local velocity correlation was applied to improve the CHF prediction model.

A comprehensive CFD study on the effect of dense vertical internals on the hydrodynamics and population balance model in bubble columns

In this paper, the effects of dense vertical internals (rods) on gas holdup and local gas and liquid velocities were investigated by using the Eulerian-Eulerian model coupled with the population balance model. The inclusion of lift and wall lubrication forces was studied by applying different models. The results indicated that just by choosing the appropriate interfacial forces, the numerical model agrees well with the experimental data. A sharper gas holdup, a stronger gas velocity gradient, and a more intense liquid recirculation were observed as the important impacts of the internals. Moreover, three circular internals’ arrangements were considered to study the effect of wall and core clearance distances on the bubble column hydrodynamics. The results revealed that by increasing the wall clearance distance, flatter gas holdup and velocity distributions could be achieved. Also, the turbulence parameters were used to evaluate the capability of the model in the prediction of the bubble size distribution. A modification factor for the breakage and coalescence kernels was proposed; the factor enabled the model to reach equilibrium mean diameters for bubbles. The comparison of the probability distribution function (PDF) of bubble sizes in the bubble column with and without internals showed a narrower bubble size distribution with a smaller mean diameter in the presence of internals.

Gas holdup and liquid velocity distributions in the up flow jet-loop reactor

Hydrodynamics in an up flow jet-loop reactor (JLR) is experimentally investigated. The overall gas holdup, radial and axial distributions of local gas holdup and liquid velocity are presented. A modified drift-flux model was developed for correlating the “S-shape” relationship of overall gas holdup and gas velocity. Experiments show that the gas–liquid co-injection mode results in larger gas holdup and liquid circulation velocity than those of the gas jet alone mode. Liquid velocity distribution is more prone to reach the fully developed regime than the gas holdup, and the gas–liquid co-injection promotes the flow development. Comparison of JLR with the conventional bubble column (BC) and airlift reactor (ALR) shows that the JLR has a largest liquid circulation velocity, which is mostly beneficial to liquid mixing and solid suspension.

Study on the natural frequencies of pipes conveying gas-liquid two-phase slug flow

The natural frequencies of pipes conveying gas-liquid two-phase slug flow are explored, considering the significance of the intermittent characteristic of gas-liquid two-phase slug flow particularly. The widely used finite element method is employed to establish the theoretical model. The local flow parameters including local gas and liquid velocities and local liquid holdup varying with time and position along the pipe are calculated to obtain the global matrix equation of motion for the system. An experiment is conducted to verify the theoretical model. The results show that the time domain values of the natural frequency would keep periodicity during the process of slug flow. Root-mean-square of the natural frequency is employed to represent the characteristic of the piping system natural frequency. If the superficial gas velocity is given, root-mean-square of the natural frequency will decrease with the increase of the superficial liquid velocity. Yet if the superficial liquid velocity is given, root-mean-square of the natural frequency will increase firstly and then decrease with the increase of superficial gas velocity when the superficial liquid velocity is relatively large. Thus a critical superficial gas velocity exists for these conditions. The range of superficial gas velocity that could make the system become unstable may be predicted preliminarily by analyzing this critical value. Sufficient analysis of slug parameters and geometrical parameters of the pipes is necessary to obtain the critical superficial gas velocity. Finally, the divergent instability of the piping system is predicted based on the above analysis.

Hydrodynamics and mixing in airlift contactors: Experimental work and CFD modelling

Fluid flow was examined in an airlift contactor equipped with a central baffle operated at superficial air velocities between 0.07 and 0.34m/s. Measurements were made of the Bubble Size Distribution (BSD), profiles of the average local hold-up and upwards liquid velocity, the overall hold-up and the mixing time. Overall hold-up was similar to that for a bubble column while profiles of the average local hold-up and average upwards liquid velocity were markedly asymmetric (unlike those measured when this pilot-plant was operated as a bubble column). Mixing times ranged from 7 to 27s, decreasing as the superficial velocity increased. Neither the tracer addition point nor the measurement location had a major impact on the observed mixing time, this contrasts with work in bubble columns where the tracer addition and measurement locations had a large impact on the mixing time while the influence of the superficial velocity was relatively small. Associated CFD modelling generally offered good agreement with our experimental measurements of the average local hold-up and upwards liquid velocity profiles. However, it was found that the model underestimated the extent to which air is entrained into the downcomer section and the mixing time at low superficial velocities. This study examined an airlift system operated at high superficial gas velocities for which there are very limited data available in the open literature; comparing experimental and computational results is instructive in defining the strengths and limitations of employing a single bubble size CFD model.

Effect of aeration mode on the performance of center‐ and annulus‐rising internal‐loop airlift bioreactors

AbstractExtensive experimental studies on internal‐loop airlift reactors, including center‐rising (CR‐ALR) and annulus‐rising airlift reactors (AR‐ALR), have been reported in the literature. However, to the best of the authors’ knowledge, the effects of the aeration mode on the local hydrodynamics remain an under‐investigated area, especially for complex culture media. At present, it is difficult to select the best aeration mode for ALRs due to limited understanding of the pros and cons of the different modes. This study presents a detailed quantitative investigation of the overall gas holdup, local liquid velocity, liquid circulation time, shear rate distribution, and volumetric mass transfer coefficient in center‐ and annulus‐rising airlift bioreactors to better understand the effect of aeration mode on airlift bioreactor performance. Particle image velocimetry is employed to conduct local measurements. The results show that the overall gas holdup, liquid circulation time, and volumetric mass transfer coefficient are larger in the AR‐ALR than in the CR‐ALR. The local liquid velocity circulating into the downcomer of the AR‐ALR, which contributes to bubble entrainment and therefore to overall gas holdup, is higher than in the CR‐ALR. It was observed that a large circulation loop formed in the CR‐ALR, whereas two counter‐looping circulation cells appeared in the AR‐ALR. It was also found that the shear rate field was more uniform in the AR‐ALR than the CR‐ALR although the shear rates were similar in magnitude.

CFD Simulation on the Hydrodynamics in Gas-Liquid Airlift Reactor

Abstract Two-fluid model approach to simulate gas-liquid airlift reactors is widely implemented but have yet to reach a consensus on the closure model to account the gas-liquid interphase forces. Proper selection of a closure model is required in order to accurately capture the hydrodynamics in the complex of the two-phase system. Our work concerns the evaluation of the interfacial forces models (i. e. drag, lift and turbulent dispersion force) and their effects on local gas holdup and liquid velocity. A transient three-dimensional airlift reactor simulation was carried out using computational fluid dynamics by implementing the dispersed standard k-ε turbulence model. Four drag models governed by spherical bubble, bubble deformation and Rayleigh-Taylor were being evaluated in our work. The significance on the inclusion of the lift model on predictive accuracy on the flow field was also studied as well. Whereas, two turbulent dispersion force models were selected to evaluate on their performance in improving the predictive accuracy of the local hydrodynamics. Results showed that the drag governed by Rayleigh-Taylor which accounts the bubble swarm effect had better predictions on the gas holdup in the downcomer and improved predictions in radial gas holdup. The inclusion of the lift model improved local gas holdup predictions at higher heights of the reactor and shifted the bubble plume towards the centre region of the riser. Meanwhile, the turbulent dispersion models improved the overall results of predicted local gas holdup with closer agreement obtained when the drift velocity model was considered in the simulation. The axial liquid velocity was well predicted for all cases. The consideration of the drag, lift and turbulent dispersion forces resulted in a closer agreement with experimental data.

Transition of plug to slug flow and associated fluid dynamics

Transition of plug to slug flow is associated with bubble detachment from elongated bubble tail or bubble entrainment inside the liquid slug. The mechanism responsible for this transition was earlier identified by Ruder and Hanratty (1990) and Fagundes Netto et al. (1999) based on the shape of the hydraulic jump observed at elongated bubble tail region. The transition mechanism reported by Ruder and Hanratty (1990) and Fagundes Netto et al. (1999) was only based on their flow visualization study. Plug to slug transition and associated dynamics of bubble detachment from the elongated bubble is analysed in the present paper using flow visualization and local velocity measurements. Experiments are reported for 13 different inlet flow conditions of air and water phases. Images of plug/slug flow structures are captured at a rate of 4000 FPS using FASTCAM Photron camera and the local values of axial liquid velocity are measured using LDV system synchronised with a 3D automated traverse system. LDV measurement of local liquid velocity in the liquid slug and liquid film establishes the reason for detachment of bubbles from the slug bubble tail.

Influence of liquid phase hydrodynamics on biofilm formation on structured packing: Optimization of surfactin production from Bacillus amyloliquefaciens

This work gives a useful insight of hydrodynamic aspects within single-species biofilm reactor with metal structured packing and proposes an original methodology based on high energy X-ray tomography to characterize liquid and biofilm distribution. It has been observed that efficiency of initial liquid distribution controls the biofilm distribution within the packing. This parameter mainly depends on the liquid distributor design. The local liquid velocity within the packing element, depending of the recirculation flow rate, the structural properties of the packing and the bed void fraction distribution, have an effect on initial cell attachment and subsequent biofilm growth. However, once started the process efficiency depends mainly on the biofilm spreading on the packing, affecting further liquid distribution. The liquid flow rate is involved in a physicochemical effect leading to foam formation. The latter decreases surfactin recovery in the liquid phase and provokes biofilm detachment of the packing. Thus, the best surfactin yield production, up to 300mg/L, occurs for a liquid load of 17.6L/m2min supplied by a multiple point source distributor. From a technical point of view, these operating conditions are particularly interesting since foam formation is completely suppressed.

Experimental Investigation of Flow Development in Large-Scale Bubble Columns in the Churn-Turbulent Regime

Flow development in the bubble columns is of importance for the sparger design and reactor scale up. In the present study, axial development of local gas holdup and liquid velocity in two bubble columns (0.5 and 0.8 m in diameter, respectively) operated in the churn-turbulent regime was experimentally investigated. The results show that gas distribution type, superficial gas velocity and column diameter have remarkable influences on the flow development. Compared with the near-wall aeration, the uniform and central aeration will generate more rapid flow development. The observed strong influence of the sparger type on flow development shows an opposite trend to the reported findings. Furthermore, increasing superficial gas velocity or decreasing column diameter will reduce the entrance region length. The scale effect indicates that for the large-scale industrial bubble column reactors, a large portion of flow region may be covered by the distributor region rather than the well-developed region.

Investigation of Gas and Liquid Multiphase Flow in the Rheinsahl–Heraeus (RH) Reactor by Using the Euler–Euler Approach

In the present work, a mathematical model was developed to understand the multiphase flow behavior in a Rheinsahl–Heraeus (RH) reactor by using the Euler–Euler approach, and the effects of initial bubble diameter, nonequilibrium expansion of bubble caused by sudden thermal effect and sharp pressure drop, and various interphase forces were considered and clarified. The simulation results of mixing time, liquid circulation rate, and local liquid velocity in RH agree well with the measured results. The result indicates that the initial bubble diameter has a weak impact on the multiphase flow but that the bubble expansion has a tremendous impact on it for an actual RH. Meanwhile, the drag force and turbulent dispersion force strongly influence the multiphase flow, whereas the lift force and virtual mass force only have negligible influence on it. Furthermore, the turbulent dispersion force should be responsible for reasonable prediction of multiphase flow behavior in the RH reactor.

Effect of compensation error in drift-flux parameters on predictions of thermal-hydraulic parameters in nuclear safety system analysis codes

Void fraction in a nuclear reactor core is one of the most important parameters in a safety analysis using nuclear reactor thermal-hydraulics system analysis codes such as TRAC-BF1, RELAP5 and TRACE. Interfacial shear term governing void fraction in the two-fluid code is often estimated by Andersen approach which uses drift-flux type correlation to compute the interfacial shear term. The accuracy of two drift-flux parameters such as distribution parameter and drift velocity is anticipated to affect the accuracy of predicted void fraction significantly. In principle, the distribution parameter and drift velocity are independent parameters which should be determined by local gas and liquid velocities and void fraction. However, due to very limited local data, the distribution parameter and drift velocity are commonly determined by area-averaged void fraction and superficial gas and liquid velocities. This “approximate method” is acceptable when the distribution parameter and drift velocity are used together. However, in the Anderson's approach, the distribution parameter and drift velocity determined by the approximate method are used separately which may cause some compensation error in code calculations. In view of the great importance in accurately computing the interfacial shear term, the effect of the compensation error on the predicted void fraction is investigated. Intensive sensitivity analysis suggests the compensation error propagating to void fraction only up to 1% for steady state computations, whereas the effect of the compensation error on the predicted void fraction for transient computations becomes larger because temporal reduction of drag force may cause the increase in void fraction. A prototypic nuclear power plant analysis for ATWS scenario suggests that the overestimation of the void fraction may affect the neutron flux calculation.

Experimental investigation into the impact of sparger design on bubble columns at high superficial velocities

In this work we have quantified the impact of sparger design on the hydrodynamics of a pilot-scale bubble column (0.39m in diameter and 2m in height) at superficial velocities between 0.07 and 0.29m/s. It was found that increasing the sparger orifice diameter from 0.5mm to 3mm led to different behaviour in the homogenous flow regime, this being attributed to the different bubble size distributions (BSDs) generated. At higher superficial velocities (i.e., in the heterogeneous flow regime), it was observed that the sparger orifice diameter had little impact on the column behaviour (as characterised by the BSD, overall hold-up, local hold-up profile, liquid velocity profile and mixing time). Changing from a symmetric to an asymmetric sparger design was observed to have a minimal impact on the BSD and overall hold-up, but a large impact on the flow behaviour (i.e., the local hold-up and liquid velocity profiles). The change in flow patterns caused by the asymmetric sparger generally led to a decrease in the mixing time for all measurement locations and tracer addition points. Results from this work are of clear interest in the design of bubble columns.