Turbulent Two-phase Flow Research Articles

Study of the effect of particles on the kinetic parameters of a turbulent two-phase flow

Numerical simulations using a two-fluid Eulerian model were performed for two-dimensional axisymmetric jets from a circular 20 mm diameter nozzle. The particle size vary between 30 and 180 μm and the particle charge ranges from 0.1 to 1. The modulations of the flow structures and the turbulent characteristics of the gas flow due to the solid particles with different sizes and rates of particle loading are studied. The jet spread and the decay of the mean center line velocity are calculated for all sizes and particle loading rates considered in this study. Additions of solid particles in the gas flow significantly modulate the turbulence of the gas in the nozzle as well as the flow rates. Fine particles suppress turbulence, while coarse particles improve turbulence.

Electrochemical machining gap prediction with multi-physics coupling model based on two-phase turbulence flow

Considering the influence of hydrogen gas generated during electrochemical machining on the conductivity of electrolyte, a two-phase turbulent flow model is presented to describe the gas bubbles distribution. The k-ε turbulent model is used to describe the electrolyte flow field. The Euler–Euler model based on viscous drag and pressure force is used to calculate the two-dimensional distribution of gas volume fraction. A multi-physics coupling model of electric field, two-phase flow field and temperature field is established and solved by weak coupling iteration method. The numerical simulation results of gas volume fraction, temperature and conductivity in equilibrium state are discussed. The distributions of machining gap at different time are analyzed. The predicted results of the machining gap are consistent with the experimental results, and the maximum deviation between them is less than 50 μm.

Analysis and modelling of Reynolds stresses in turbulent bubbly up-flows from direct numerical simulations

Two-phase bubbly flows are found in many industrial applications. These flows involve complex local phenomena that are still poorly understood. For instance, two-phase turbulence modelling is still commonly based on single-phase flow analyses. A direct numerical simulation (DNS) database is described here to improve the understanding of two-phase turbulent channel flow at a parietal Reynolds number of 127. Based on DNS results, a physical interpretation of the Reynolds stress and momentum budgets is proposed. First, surface tension is found to be the strongest force in the direction of migration so that budgets of the momentum equations suggest a significant impact of surface tension in the migration process, whereas most modelling used in industrial application does not include it. Besides, the suitability of the design of our cases to study the interaction between bubble-induced fluctuations (BIF) and single-phase turbulence (SPT) is shown. Budgets of the Reynolds stress transport equation computed from DNS reveal an interaction between SPT and BIF, revealing weaknesses in the classical way in which pseudoturbulence and perturbations to standard single-phase turbulence are modelled. An SPT reduction is shown due to changes in the diffusion because of the presence of bubbles. An increase of the redistribution leading to a more isotropic SPT has been observed as well. BIF is comprised of a turbulent (wake-induced turbulence, WIT) and a non-turbulent (wake-induced fluctuations, WIF) part which are statistically independent. WIF is related to averaged wake and potential flow, whereas WIT appears when wakes become unstable or interact with each other for high-velocity bubbles. In the present low gravity conditions, BIF is reduced to WIF only. A thorough analysis of the transport equations of the Reynolds stresses is performed in order to propose an algebraic closure for the WIF towards an innovative two-phase turbulence model.

Mathematical Modelling of Gas–Liquid, Two-Phase Flows in a Ladle Shroud

Differential and macroscopic models of argon–steel flows in ladle shroud have been developed. In this, argon–steel, two-phase flow phenomena have been formulated via a transient, three dimensional, turbulent flow model, based on the volume of fluid (VOF) calculation procedure. While realizable k–e turbulence model has been applied to map turbulence, commercial, CFD software ANSYS-Fluent™ (Version 18), has been applied to carry out numerical calculations. Predictions from the model have been directly assessed against experimental measurements across the range of shroud dimensions and volumetric flow rates typically practiced in the industry. It is demonstrated that the two-phase turbulent flow model captures the general features of gas–liquid flows in ladle shroud providing estimates of free jet length and threshold gas flow rates (required to halt air ingression) which are in agreement with corresponding experimental measurements. In the absence of differential solutions, a macroscopic model has been worked out through dimensional analysis embodying multiple non-linear regression. It is shown that dimensionless free jet length in bloom and slab casting shrouds can be estimated reasonably accurately from the following correlation (in SI unit), viz., $$ \frac{{L_{{{\text{jet}}}} }}{{D_{{{\text{CN}}}} }} = 2.8 \times 10^{{ - 2}} \left( {\frac{{Q_{{\text{G}}} }}{{Q_{{\text{L}}} }}} \right)^{{1.14}} \left( {\frac{{gD_{{{\text{CN}}}}^{5} }}{{Q_{{\text{L}}}^{2} }}} \right)^{{0.8}} \left( {\frac{{\sigma D_{{{\text{CN}}}}^{3} }}{{\rho _{{\text{L}}} Q_{{\text{L}}}^{2} }}} \right)^{{ - 0.9}} \left( {\frac{{D_{{{\text{sh}}}} }}{{D_{{{\text{CN}}}} }}} \right)^{{2.0}} \left( {\frac{{\rho _{{\text{G}}} }}{{\rho _{{\text{L}}} }}} \right)^{{ - 0.30}} $$ in which, Ljet is the free liquid jet length (m), QG is the gas flow rate (m3/s), QL is the liquid flow rate (m3/s), Dsh is shroud diameter (m), DCN is the collector nozzle diameter (m), σ is the interfacial tension (N/m), and ρG as well as ρL are respectively density of gas and liquid (kg/m3). It is demonstrated that the proposed correlation is consistent with the laws of physical modeling and leads to estimates that are in good agreement with predictions from the differential models, for both air-water as well as argon–steel systems. Numerical simulations as well as macroscopic modeling have indicated that thermo-physical properties of the gas–liquid system are important and exert some influences on the gas–liquid, two-phase, flow in ladle shrouds, albeit not to a large extent. Despite dissimilar thermo-physical properties, full scale water modeling appears to be sufficiently predictive and provides reasonable macroscopic descriptions of the two-phase flow phenomena in industrial ladle shroud systems.

Influence of liquid properties on atomization characteristics of flow-blurring injector at ultra-low flow rates

Atomization and efficient evaporation at extremely small flow rates is an additional challenge for liquid fueled small-scale combustor and burner applications. In the present study, experiments are conducted to investigate the effects of liquid properties on atomization of flow-blurring atomizer at ultra-low liquid flow rate (2.78–41.7 μL/s). Shadowgraphy, backlight imaging technique and laser based spray visualization methodology were employed for atomization characterization. Formation of three different atomization regimes namely bubble bursting, weak spray and fully developed spray was observed with a gradual increase in airflow rate (8.33–83.3 mL/s). Prompt mode of atomization in weak spray regime produces coarse droplets, while a sudden reduction in droplet size is observed in fully developed spray regime. Fine atomization in fully developed spray regime is attributed to flow-blurring effect due to air recirculation and thereby formation of highly turbulent two-phase flow inside the injector. This investigation reports that the criterion proposed in literature is not sufficient to achieve enhanced atomization through flow-blurring phenomenon. Experiments demonstrate that viscosity has marginal effect on atomization, however, surface tension dominates the atomization characteristics in a flow-blurring atomizer. Meanwhile, Weber number appears an appropriate parameter to predict the atomization regime transition.

LES of turbulent non-isothermal two-phase flows within a multifield approach

Safety issues in nuclear power plant involve complex turbulent bubbly flows. To predict the behavior of these flows, the two-fluid approach is often used. Nevertheless, this model has been developed for the simulation of small spherical bubbles, considered as a dispersed field. To deal with bubbles with a large range of sizes, a multifield approach based on this two-fluid model has been proposed. A special treatment, called the Large Bubble Model (LBMo), has been implemented and coupled to the dispersed model. However, only laminar and isothermal flows were considered in previous papers. Thus, here, Large Eddy Simulations (LES) are investigated to model turbulence effects. For this purpose, the two-fluid model equations are filtered to highlight the specific subgrid terms. Then, an a priori LES study using filtered Direct Numerical Simulation (DNS) results is detailed. This analysis allows classifying these terms according to their relative weight and then concentrating the modelling efforts on the predominant ones. Five different turbulence models are compared. These results are finally used to perform true LES on a turbulent two-phase flow. Moreover, in order to tackle non-isothermal flows occurring in industrial studies, a new heat transfer model is implemented and validated to deal with phase change at large interfaces using the Large Bubble Model.

Ultrasonic Monitoring of Gas-liquid Two-phase Turbulent Flows

超音波がもつ特性を利用して，混相流の内部流動の定量的なモニタリングを実現した事例について紹介する．この技術の特徴は，高速流動する界面をもつ流れを時間分解して捕らえることにある．気液二相流においては，ボイド率を計測する手法，気液界面を検知する手法，各相の速度分布および流量を計測する手法を解説する．これらの応用として三相流ならび乱泥流の超音波可視化の事例を紹介する．

VALIDATION OF A BUOYANCY-MODIFIED TURBULENCE MODEL BY NUMERICAL SIMULATIONS OF BREAKING WAVES OVER A FIXED BAR

The surf zone dynamics are governed by important processes such as turbulence generation , nearshore sediment transport , wave run-up and wave overtopping at a coastal structure. During field observations , it is very challenging to measure and quantify wave breaking turbulence . Complementary to experimental laboratory studies in a more controlled environment , numerical simulations are highly suitable to understand and quantify surf zone processes more accurately.
 
 In this study, wave propagation and wave breaking over a fixed barred beach profile is investigated using a two­ phase Navier-Stokes flow solver. We show that accurate predictions of the turbulent two-phase flow field require special attention regarding turbulence modelling. The numerical wave flume is implemented in the open­ source OpenFOAM library. The computed results (surface elevations , velocity profiles and turbulence levels) are compared against experimental measurements in a wave flume (van der A et al., 2017) .

Numerical predictions of sand erosion in a choke valve

Two-phase turbulent flow of crude oil and sand in a choke valve is analysed in the present article using 3D computational fluid dynamics simulations. The discrete phase mathematical model is used to simulate the sand flow and its interaction with the oil flow in the system. Parametric study is done to identify the governing parameters to minimize the sand erosion in the system. The valve geometry and dimensions are taken from an industrial oil production project. The parameter considered in the present study are the percentage valve opening, flow rate of the sand and the pressure difference between the inlet and outlet pipes. The simulation results are presented to show the erosion rate variation with the valve opening, sand flow rate and the pressure difference. It is found that the erosion rate is high for small valve opening as well as large valve opening. Minimum erosion rate is found when the valve opening is between 20-30% for all the cases with various pressure differences. Locations of maximum erosion rate are predicted in the simulations.

A numerical analysis and experimental investigation of three oil grooves sleeve bearing performance

PurposeThis paper aims to reveal the cavitation characteristics of three oil wedges sleeve bearing and set the theoretical and experimental basis for defining the oil film boundary condition.Design/methodology/approachComputational fluid dynamics model of three oil wedges sleeve bearings based on the Navier–Stokes equation is set using Fluent considering turbulent situation and two-phase flow theory. The cavitation characteristics of bearing is investigated by taking pictures of experiment.FindingsThe rupture region of oil film and the contours of air volume fraction increase distinctly with the increase of rotating speed and the decrease of input pressure. The critical rotating speed of cavitation occurrence and oil film pressure increases with the increase of input pressure. The change trend of experiment cavitation with the rotating speed and input pressure is consistent with theoretical cavitation in general.Originality/valueThe finite element model of three oil wedges sleeve bearings is established based on the Navier-Stokes calculation equation of the fluid, two-phase flow theory and turbulent model. Sleeve bearing is transparent, the pictures of cavitation can be easily taken by high-speed camera, the cavitation characteristics of bearing is studied by experiment. The cavitation performance of three oil wedges bearings is studied with the change of input pressure and rotating speed, the change trend is basically consistent for theory and experiment. The study on critical rotating speed of cavitation occurrence is benefit for defining the oil film boundary condition.

Cavitation occurring in capillary tubes

Surface etches caused by cavitation often occur behind the blade of a rapidly rotating propeller or on a vibrating surface where liquid is subjected to suddenly reduced or oscillating saturation pressure. This phenomenon has never been reported in a continuous capillary pipe in which flow pressure varied gently without injection of external radiation energy. On the other aspect, acoustically tinkling signals were recognized decades ago during operation of the oscillation capillary heat pipe, but lacking of vigorous scientific understanding of the root causes. In this article, we report cavitation and its surface etches in meandering capillary tubes with the inner diameter of 1.8 mm. Numerous etching pits were observed on the interior face at the heat rejection region after over 200 hours operation. Irregular copper debris, with sizes ranging from 20 to 500 μm, is found in the reclaimed operating fluid. Analysis of temperature and acoustic data indicates that, driven by spiking temperature difference, highly turbulent two-phase flow carries the saturated vapor bubbles from the evaporator to the subcooled condensation region in a very short time of oscillation, turning vapor phase into oversaturated status. Rapid condensation accelerated shrinkage of the vapor bubbles causing strong micro jet impingement to damage the pipe wall, radiating acoustical signals.

A theoretical model describing bubble deformability and its effect on binary breakup in turbulent dispersions

Abstract This work addresses bubble deformability and binary breakup induced by turbulent fluctuations in liquid dispersion. In order to explore the dynamic process of bubble-eddy interaction, a theoretical model considering the surface deformation, is proposed through dimensional analysis in this work. Our model indicates that the bubble deformability depends on two key factors: the size ratio between the turbulent structure and the mother bubble, and the Weber number. Binary breakup of one single bubble is described through the degree of the surface deformation, and the effect of breakage constraints on it is investigated. It is found that equal-size breakage is dominated by the energy constraint, while most unequal-size breakage is dominated by the force constraint. The characteristic sizes of effective colliding eddies contributing to the bubble breakup process are computed based on the change of the surface energy. This work not only overcomes the sensitivity of the breakup models for turbulent dispersion systems to these parameters, but also gives good predication of bubble deformability in two-phase turbulent flow.

Validation of a closure model framework for turbulent bubbly two-phase flow in different flow situations

Abstract In the present paper a set of closure relations for interphase momentum exchange and for bubble-induced turbulence within the Euler-Euler framework is presented and validated against a set of tests performed at the HZDR-facility MT-Loop. The facility was equipped with wire-mesh sensors that allow cross sectional distributions of gas fraction, gas velocity, and bubble sizes to be measured at different distances from the gas injection. The here applied models were derived for the simulation of fully developed turbulent vertical upward bubbly flow in a pipe. The radial gas fraction profile for this kind of flow is the result of the ratio of the radial force components of the so-called non-drag forces and can be used for model validation. However, only the ratio, not the absolute value of the bubble forces can be tested in this way. In the present paper more detailed information from the experiment is exploited by consideration of the evolution of gas fraction distribution particularly after the gas injection region. The change of cross sectional gas volume fraction distribution is the result of the action of non-drag forces. In addition to vertical pipe flow tests further insight is obtained from the investigation of the effect of a slight tube inclination which shifts the gas distribution. Here the disturbance of the cylindrical symmetry of a vertical pipe gives hints on the absolute value of the non-drag force components. The results show that the presented model framework at least is able to describe the phenomena qualitatively. Possible reasons for quantitative deviations are discussed and require further investigations.

TWO-PHASE TURBULENT FLOW IN STRAIGHT HORIZONTAL CHANNELS WITH A SQUARE CROSS-SECTION TAKING INTO ACCOUNT THE INFLUENCE OF VERTICAL FORCES

The paper presents a numerical simulation of a two-phase turbulent flow in pneumatic transport through straight horizontal channels with a square cross-section. The transport of solid particles of ash and flour is taken as the two-phase flow, while air is chosen for the transporting fluid. The motion of solid particles occurs due to the aerodynamic forces of the transporting fluid. The paper considers the motion of solid particles by taking into account the influence of vertical forces, which act on the transported solid particles. In flow modelling, the transported solid particles are reduced to spherical shapes. The stress model of turbulence is corrected by taking into account the influence of the induction of secondary flows of the second kind in the gas phase. The full Reynolds stress model is used to model the turbulence, with the application of the complete model for turbulent stresses and turbulent temperature fluxes. The paper presents the results of the numerical grid with the highest resolution above which the increase of the mesh density does not affect the obtained results. The diagrams of the positions of the transported solid particles are provided for cross-sections normal and parallel to the flow direction.

Droplet-Laden Mist Film Cooling Effectiveness of Cylindrical Holes Embedded in a Transverse Trench

A three-dimensional numerical model of calculating in the Euler approach is developed to calculate a two-phase turbulent near-wall flow; simulation of thermal efficiency of a gas-droplet shielding injected into a transverse trench through inclined cylindrical holes is fulfilled. The influence of the main thermo-gas-dynamic characteristics of the two-phase flow on thermal efficiency is analyzed. Significant increase in thermal efficiency was obtained by adding droplets in the nearwall coolant flow (up to 2 times in comparison with a single-phase flow). A particular advantage of this method of coolant injection is achieved at high injection parameters. It is shown that the use of two-phase gas-droplet near-wall shielding is promising for protection of surfaces against thermal influence of the heated gas flows.

Fluid flow and heat transfer for a particle-laden gas modeled as a two-phase turbulent flow

PurposeThe purpose of this study is to examine the physical processes experienced by a particle-laden gas due to various types of collisions, different heat transfer modalities and jet axis switching. Here, attention is focused on a particle-laden gas subjected to jet axis switching while experiencing fluid flow and heat transfer.Design/methodology/approachThe methodology used to model and solve these complex problems is numerical simulation treated here as a two-phase turbulent flow in which the gas and the particles keep their separate identities. For the turbulent flow model, validation was achieved by comparisons with appropriate experimental data. The considered interactions between the fluid and the particles include one-way fluid–particle interactions, two-way fluid–particle interactions and particle–particle interactions.FindingsFor the fluid flow portion of the work, emphasis was placed on the particle collection efficiency and on independent variables that affect this quantity and the trajectories of the fluid and of the particles as they traverse the space between the jet orifice and the impingement plate. The extent of the effect depended on four factors: particle size, particle density, number of particles and the velocity of the fluid flow. The major effect on the heat transferred to the impingement plate occurred when direct heat transfer between the impinging particles and the plate was taken into account.Originality/valueThis paper deals with issues never before dealt with in the published literature: the effect of jet axis switching on the fluid mechanics of gas-particle flows without heat transfer and the effect of jet axis switching and the presence of particles on jet impingement heat transfer. The overall focus of the work is on the impact of jet axis switching on particle-laden fluid flow and heat transfer.

Three-dimensional numerical study of flow characteristics and membrane fouling evolution in an enzymatic membrane reactor

In order to enhance the understanding of membrane fouling mechanism, the hydrodynamics of granular flow in a stirred enzymatic membrane reactor was numerically investigated in the present study. A three-dimensional Euler-Euler model, coupled with k-e mixture turbulence model and drag function for interphase momentum exchange, was applied to simulate the two-phase (fluid-solid) turbulent flow. Numerical simulations of single- or two-phase turbulent flow under various stirring speed were implemented. The numerical results coincide very well with some published experimental data. Results for the distributions of velocity, shear stress and turbulent kinetic energy were provided. Our results show that the increase of stirring speed could not only enlarge the circulation loops in the reactor, but it can also increase the shear stress on the membrane surface and accelerate the mixing process of granular materials. The time evolution of volumetric function of granular materials on the membrane surface has qualitatively explained the evolution of membrane fouling.

Characterisation of transverse turbulent motion in quasi-two-dimensional aerated flow: Application of four-point air-water flow measurements in hydraulic jump

The measurement of turbulent velocity in highly aerated flow is difficult because of the presence of air bubbles. The characterisation of three-dimensional velocity field in highly aerated flows is even more challenging using existing phase-detection techniques. This paper presents an attempt on a quantification of transverse velocity and velocity fluctuations in quasi-two-dimensional hydraulic jumps with relatively high Froude and Reynolds numbers. A four-sensor phase-detection probe array was developed to measure the bubble convection in both streamwise and spanwise directions. A characteristic instantaneous transverse velocity component was derived together with a measure of its fluctuations. The transverse velocity component characterised the three-dimensional turbulent structures, although the time-averaged flow pattern was two-dimensional and the average transverse velocity was zero. Both the transverse velocity and velocity fluctuations were smaller than the longitudinal time-averaged velocity and velocity fluctuations in the shear flow, and were quantitatively comparable to those in the free-surface region, revealing different turbulent structures in the lower and upper roller regions. The approximate of Reynolds stresses was discussed together with the limitation of the method. The present work also provided some guidelines for the use of phase-detection probe array and correlation signal processing techniques in complex turbulent two-phase flows.

Flow field characterization in the vicinity of vertical plane electrodes in a bench-scale zinc electrowinning cell

Characteristics of flow fields in the vicinity of vertical plane electrodes in a bench-scale zinc electrolysis cell were visualized using particle image velocimetry (PIV). Two experimental conditions, fN (current density = 0 A m−2) and fW (current density = 440 A m−2), were applied to investigate and compare the pump-induced and bubble-induced flows. The flow patterns in the selected region changed from vortex topology in fN to smooth streamlines in fW. In the presence of rising electrogenerated bubbles, the average velocity of the electrolyte between electrodes increased around 9 times. Based on the spatial velocity variation in fW, the average thicknesses of the bubble diffusion regions near the anode and the cathode were 3.59 mm and 3.78 mm, respectively, where the local turbulence was significantly stronger than that in the bulk region. It is noteworthy that near the anode surface, the turbulence intensity increased linearly (R = 0.99) upwards from the anode bottom, indicating that the local turbulent two-phase flow might be attributed to the accumulated perturbations from individual bubbles. The drag effect of the bubble curtains was the main driving force to renew the electrolyte within electrode gap rather than the electrolyte feeding into cell in the zinc electrowinning system.

Studies of a combined way of flame stability in ramjet combustor

Abstract numerical simulation was conducted to study the influence of bleeding. The Euler-Lagrange method was used to investigate the two-phase turbulent combustion flow. Standard k-e turbulent model was adopted in the continuous phase simulation and particle-trajectory model was adopted in the dispersed phase simulation. The results demonstrates: air bleeding can improve the flow field after the strut and the stability of trapped vortex in the cavity; change of bleeding temperature has little effect on the total pressure recovery coefficient and significant effect on combustion efficiency; When fuel-air ratio changes, the combustor performs better in a lean oil state.