Dense Two-phase Flows Research Articles

Ultrasound imaging velocimetry in a dense two-phase swirling flow

Ultrasound imaging velocimetry in a dense two-phase swirling flow

Two-way coupled CFD-DEM model of a pre-mixed abrasive water jet and its application to the investigation of abrasive motion characteristics

This paper uses the weight function to modify the particle volume fraction algorithm. The VOF model is modified by adding the particle phase volume fraction and momentum source to the control equation, the semi-resolved VOF-DEM model is established, and PTV experiment is used to verify the model. Based on this model, the flow and distribution characteristics of the abrasive are obtained: 1) The water-abrasive flow near the outlet of the convergent section of the nozzle has a significantly dense two-phase flow characteristic, and the collision between particles dominates particle flow. 2) The particles at the nozzle outlet are distributed in the radial direction of “n”, and with the increase of the stand-off distance of the jet, the particle phase volume transitions from “n” to “m” distribution. The research on the flow characteristics of particles can provide a basis for the nozzle design of pre-mixed abrasive water jets.

A new DNS formalism dedicated to turbulent two-phase flows with phase change

Phase change in multiphase flows occurs in many natural and industrial applications, e.g., rain formation, internal combustion engines, heat exchangers, multiphase reactors, etc. In dense two-phase flows, quantitative experimental results are scarce due to the complexity of the configuration and experimental techniques limitations. Hence, the interest in the direct numerical simulation of such flows has grown recently to better understand and access quantitative data in this kind of flows.When simulating phase change in multiphase flows, advanced numerical method are needed to consider the jump conditions at the interface, and to ensure mass, energy, momentum and species conservation. In the literature, this problem is mainly investigated with an incompressible formalism. However, this assumption is not longer suitable for simulation of multiphase flows with phase change in enclosed environment or in atomized/aerated flows.The purpose of this work is to present a numerical formalism dedicated to turbulent two phase flows, including acoustics and compressible effects with a proper treatment of the jump conditions at the interface due to phase change. To achieve this task, first, the incompressible level-set method for vaporizing two-phase flows proposed by Tanguy et al. (2007) is revisited and adapted to a mass conservative interface representation: The Coupled Level-set/Volume of Fluid method. In this context, evaporating static cylinder with a constant vaporization rate and a droplet vaporization (D2 law) have been performed as validation cases. Both cases illustrate the method accuracy and robustness in presence of velocity discontinuities at the interface due to the presence of the Stefan flow. The D2 law configuration is used as validation of the implementation of the heat and mass transfer transport equations with their jump conditions and the coupling of the evaporation rate with the flow dynamic.Then, the numerical method is extended to compressible flows using the framework dedicated to the pressure based method proposed by Duret et al. (2018). The main advantage of this framework is the ability to consider acoustics effects, variable density and multiple gas inclusions with its own thermodynamic pressure. A modified Volume of Fluid transport equation is presented, including phase change and compressibility effects. Navier–Stokes and heat and mass transfer transport equation are solved using compressible assumptions. A validation case of a static evaporating cylinder in an enclosed environment is studied to illustrate and quantify the mass conservation properties of the method and the mass transfer between the two phases. Finally, a 3D two-phase Homogeneous Isotropic Turbulence (HIT) configuration is presented to demonstrate the potential of this method in presence of breakup, gas encapsulation, coalescence and evaporation processes.

Capturing both carrier and disperse phases in dense suspensions using harmonic ultrasound imaging velocimetry

In this study, a new ultrasound-based method for measuring both carrier-fluid and suspended-particle (disperse phase) motion is introduced. The method can extract flow quantities, such as slip velocity, from statistical information obtained in dense two-phase flows. The carrier fluid velocity is measured by leveraging the non-reflective properties of nearly-linear materials at harmonic frequencies to filter out the suspended particles. In contrast, the disperse phase velocity is measured using the ultrasound’s fundamental imaging mode and the slight impedance mismatch between the suspended particles and fluid. As a result, the carrier phase can be captured without cross-talk from the disperse phase. Finally, it is shown that the proposed technique allows for the measurement of suspension dynamics in vortex rings with high volume fractions on the order of 40%. Here, the proposed method is tested in dense suspensions, made from super-absorbent polymer beads with nearly-linear material properties mixed in deionized water, with volume fractions ranging from 20% to 40%.

Investigation on ultralight particle dispersions in dense two-phase flow using a kinetic friction stress model

The graphite particle dispersion characteristics with irregular shapes of dense gas-solid turbulent flow in downer reactor is numerically investigated using an improved kinetic friction stress model and a adopted drag coefficient describing momentum transfer between gas and non-spherical particles. Hydrodynamic predictions are in better line with experimental results. Redistributions of Reynolds normal and shear stresses with strongly anisotropic dispersions are captured. Comparisons to glass bead, ultralight particles elongate the second acceleration length, gather the concentration peaks at near wall region, and weak the maximum dense ring. Maximum kinetic energy at fully development region is approximately 1.2 times larger than that of second acceleration region. Lower particle sphericities are contribution to the enhancement of particle velocity, particle concentration, particle shear stress, particle collision frequency, and granular temperature, except for the reduction of turbulent kinetic energy. The negative amplitudes of shear stresses at fully development regions are approximately 4.0 times larger than those of second acceleration regions.

Simulated experiment on case overheating failure of solid rocket motor under flight overload condition

Under flight overload condition, the local heat transfer in a solid rocket motor (SRM) is strengthened due to the erosion of dense two-phase flow which may cause the case to overheat or even fail. To solve this problem, it is necessary to develop effective ground simulation experiment methods. In this study, an experimental device using HTPB/AP propellant and 30CrMnSi case material was set up to simulate the case overheating failure of SRMs under overload condition, meanwhile ablation and temperature measuring experiments were carried out. The case temperature response of the critical state, under which the insulation layer burned out but the case was not destroyed, was measured successfully, and the highest case temperature was estimated at 423 °C. The case temperature started to vary when the virgin insulation layer attained a thickness of 0.6 mm. If residual insulation layer is quite thin when the operation of SRM ends, the case temperature will continue to rise to a fairly high level. This heat transfer aftereffect might not only destroy the case, but also damage other equipment. The results of this study can provide a basis for evaluating the working safety of flight vehicles that employ SRM and undergo flight overloads.

Impact of draft plate on the inter-chamber interaction in a two-chamber spout-fluid bed

The widespread application of the spout-fluid bed in the processing of thermoplastic composites, coal gasification, municipal solid waste gasification, and biomass gasification necessitates a deeply understanding on the flow mechanism of dense two-phase flow in the system in order to improve its design optimization and operation. By tracking the gas and solid phases respectively in the Eulerian and Lagrangian framework, the dense gas-solid flow in the two-chamber spout-fluid bed with and without draft plate was carried out using the computational fluid dynamics coupled with discrete element method (CFD-DEM) to evaluate the impact of the presence of draft plates and the plate length on inter-chamber interaction. The results demonstrate that the interaction of rising bubbles with the spouting channel gives rise to the unfavorable dancing spout phenomenon, which can be alleviated by draft plates. Insertion of the plates lowers the pressure drop magnitudes of both the spouting and background inlets, and the correlation coefficient of the two signals. Furthermore, the draft plates enhance the turbulent intensity of the solid phase in the fountain region and the vertical dispersion in the spout region. Increasing the draft plate length has a significant influence on gas-solid hydrodynamics and chamber interaction.

A Physical Model to Study the Effects of Nozzle Design on Dense Two-Phase Flows in a Slab Mold Casting Ultra-Low Carbon Steels

The effects of nozzle design on dispersed, two-phase flows of the steel-argon system in a slab mold are studied using a water-air model with particle image velocimetry and ultrasound probe velocimetry techniques. Three nozzle designs were tested with the same bore size and different port geometries, including square (S), special bottom design with square ports (U), and circular (C). The meniscus velocities of the liquid increase two- or threefold in two-phase flows regarding one-phase flows using low flow rates of the gas phase. This effect is due to the dragging effects on bubbles by the liquid jets forming two-way coupled flows. Liquid velocities (primary phase) along the narrow face of the mold also are higher for two-phase flows. Flows using nozzle U are less dependent on the effects of the secondary phase (air). The smallest bubble sizes are obtained using nozzle U, which confirms that bubble breakup is dependent on the strain rates of the fluid and dissipation of kinetic energy in the nozzle bottom and port edges. Through dimensionless analysis, it was found that the bubble sizes are inversely proportional to the dissipation rate of the turbulent kinetic energy, e0.4. A simple expression involving e, surface tension, and density of metal is derived to scale up bubble sizes in water to bubble sizes in steel with different degrees of deoxidation. The validity of water-air models to study steel-argon flows is discussed. Prior works related with experiments to model argon bubbling in steel slab molds under nonwetting conditions are critically reviewed.

On modeling and physical laws of isentropic speed of sound in dense gas-particle two-phase compressible flows

Study of isentropic sound speed of two-phase or multiphase flow has theoretical significance and wide application background. As is well known, the speed of sound in fluid containing particles in suspension differs from that in the pure fluid. In the particular case of bubbly liquids (gas liquid two-phase flow), the researches find that the differences can be drastic. Up to now, the isentropic speed of sound in the flow field with a small volume fraction of bubbles (less than 1%), has been investigated fully both experimentally and theoretically. In this paper, we consider another situation, as the case with solid particles in gas, which is the so-called gas particle two-phase flow. Although many results have been obtained in gas liquid two-phase flow, there is still a lot of basic work to do due to the large differences in the flow structure and flow pattern between gas particle two-phase flow and gas liquid two-phase flow. Treating the gas particle suspension as the relaxed equilibrium, thermodynamic arguments are used to obtain the isentropic speed of sound. Unlike the existing work, we are dedicated to developing the computational model under dense condition. The space volume occupied by particle phase and the interaction between particles are overall considered, then a new formula of isentropic sound speed is derived. The new formula includes formulae of the pure gas flow and the already existing dilute gas particle two-phase flow as a special case. On the one hand, the correctness of our formula is verified. On the other hand, the new formula is more general. The variations of sound speed with different mass fractions of particle phase are analyzed. The theoretical calculation results show that the overall physical law of sound speed change is that with the increase of the particle mass fraction, the sound speed first decreases and then increases. The velocity of sound propagation in gas particle two-phase flow is far smaller than in pure gas in a wide range, so it is easy to reach the supersonic condition. When the particle volume fraction is below 10%, the result is consistent with Prandtl theoretical analysis. In this range, the influences of the particle phase pressure modeling parameters can be neglected. When the particle volume fraction is more than 10%, the particle phase pressure modeling parameters produce influences. Furthermore the corresponding physical principles and the mechanisms are discussed and revealed. The new formula and physical understandings obtained in this paper will provide a theoretical support for the researches of dense gas particle two-phase compressible flow and related engineering applications.

高濃度固気二相流中での粗大物体の挙動予測モデル開発

高濃度固気二相流中での粗大物体の挙動予測モデル開発

An efficient 4 way coupling CFD–DEM model for dense gas–solid particulate flows simulations

The goal of this paper is to illustrate how some recent numerical developments enable the simulation of dense two-phase flows. A coupled Computational Fluid Dynamic and Discrete Element Method (CFD–DEM) approach has been designed for the simulation of 3D fluidized beds. DEM is employed to model the granular particle phase while classical CFD is used to simulate the fluid flow by solving the Volume Averaged Navier-Stokes (VANS) equations. A special emphasize is put on the implementation of the velocity–pressure algorithm in the context of two-phase flows. By taking into consideration all particle–particle, fluid–particle and particle–wall interactions, a complete 4 way coupling model has been derived. Three dimensional computations of a single bubble formation as well as of the bubbly regime have been conducted. The simulations point out the existence of complex structures, such as the worm-like shape one, similar to those that have already been described in the literature.

Efficient simulation of particle-laden turbulent flows with high mass loadings using LES

The paper is concerned with the simulation of particle-laden two-phase flows based on the Euler–Lagrange approach. The methodology developed is driven by two major requirements: (i) the necessity to tackle complex turbulent flows by eddy-resolving schemes such as large-eddy simulation; (ii) the demand to predict dispersed multiphase flows at high mass loadings. First, a highly efficient particle tracking algorithm was developed working on curvilinear, block-structured grids. Second, to allow the prediction of dense two-phase flows, the fluid–particle interaction (two-way coupling) as well as particle–particle collisions (four-way coupling) had to be taken into account. For the latter instead of a stochastic collision model, in the present study a deterministic collision model is considered. Nevertheless, the computational burden is minor owing to the concept of virtual cells, where only adjacent particles are taken into account in the search for potential collision partners. The methodology is applied to different test cases (plane channel flow, combustion chamber flow). The computational results are compared with experimental measurements and good agreement is found.

Turbulence modulation of dense two-phase flows: Comment on “Modelling of dense gas–particle flow in a circulating fluidized bed by Distinct Cluster Method (DCM)”

In a recent paper, Liu X. and Xu X. [2009. Modelling of dense gas–particle flow in a circulating fluidized bed by Distinct Cluster Method (DCM). Powder Technology 195, 235–244] reported the results of numerical simulations of a circulating fluidized bed using Discrete Cluster Method (DCM). We comment on the veracity of the use of a one-way turbulence model to predict turbulence in the context of dense and moderately dense two-phase flows.

Large eddy simulation of dense two-phase flows: Comment on DEM-LES study of 3-D bubbling fluidized bed with immersed tubes

In a recent paper, Gui et al. (2008. DEM-LES study of 3-D bubbling fluidized bed with immersed tubes. Chemical Engineering Science 63, 3654–3663) reported the results of numerical simulations of a bubbling fluidized bed containing internal tubes using discrete element model (DEM) coupled to a large eddy simulation (LES). We comment on the correctness of the numerical set-up regarding the use of LES as well as the worthiness of using a turbulence model for the case of dense two-phase flows.

Modelling of dense gas-particle flow in a circulating fluidized bed by Distinct Cluster Method (DCM)

Computational Fluid Dynamics (CFD) is a powerful tool to study the dense gas–solid flow in a circulating fluidized bed. Most of the existing methods focus on the microscopic properties of individual particle. Therefore, the simulation scale is significantly limited by the huge number of individual particles, and so far the numbers of particles in most of the reported simulations are less than 10 5. The hydrodynamics behaviour of particle clustering in a dense gas–solid two-phase flow has been verified by several experimental results. The Distinct Cluster Method (DCM) was proposed in this paper by studying the macroscopic particle clustering behaviour, and comprehensive models for cluster motion, collision, break-up, and coalescence have been well developed. We model the dense two-phase flow field as gas-rich lean phase and solid-rich cluster phase. The particle cluster is directly treated as one discrete phase. The gas turbulent flow is calculated by Eulerian approach, and the particle behaviour is studied by Lagrangian approach. Using the proposed method, a three-dimension dense gas-particle two-phase flow field in a circulating fluidized bed with square-cross-section, with particle number up to 7.162 × 10 7 are able to be numerically studied, on which few results have been reported. Details on instantaneous and time-averaged distributions are obtained. Developing process of non-uniform particle distribution is visualized. These results are in agreements with experimental observations, which justified the feasibility of using the DCM method to model and simulate dense gas–solid flow in a circulating fluidized bed with large number of particle numbers.

Microfluidic Crystals: Dynamic Interplay between Rearrangement Waves and Flow

Microfluidic crystals are assemblies of miniature bubbles or drops flowing in channels. We explore here the flow of these crystals, when submitted to a given driving pressure. The flow velocity is linked to the finite number of elements in the channel width, and presents discontinuities when the crystal structure changes. At the transition from one structure to the other original dynamic features appear. The flow can self-regulate itself on a fixed velocity whatever the driving pressure, or, on the contrary, can spontaneously pulsate. All these features are predicted by simply considering the crystal's energy and friction, and looking at the propagation of structure rearrangements. We anticipate these results to improve the control over the structure of dense two-phase flows in microfluidic systems.

A second-order moment turbulence model for power law fluid with particles

The USM - θ model of power law fluid for dense two-phase turbulent flow was developed, which combines the unified second-order moment model for two-phase turbulence with the particle kinetic theory for the inter-particle collision. This model was used to simulate the turbulent flow of power law fluid single-phase in pipe. It is shown that the USM - θ model has better prediction result than the κ f - ɛ- f - k p - ɛ p - θ model. The USM - θ model was then used to simulate the dense two-phase turbulent up flow of power law fluid with particles. With the increase of the flow exponent, the velocities of power law fluid and particles increase near the pipe centre. Comparison between the two-phase flow of power law fluid-particle and of liquid-particle indicates that the axial fluctuation velocity of fluid phase and particle phase in liquid-particle two-phase flow is smaller than that in the power law fluid two-phase flow, but the two-phase velocities of power law fluid-particle and liquid-particle are close to each other.

Second-order moment model for dense two-phase turbulent flow of Bingham fluid with particles

The USM-ϑ model of Bingham fluid for dense two-phase turbulent flow was developed, which combines the second-order moment model for two-phase turbulence with the particle kinetic theory for the inter-particle collision. In this model, phases interaction and the extra term of Bingham fluid yield stress are taken into account. An algorithm for USM-ϑ model in dense two-phase flow was proposed, in which the influence of particle volume fraction is accounted for. This model was used to simulate turbulent flow of Bingham fluid single-phase and dense liquid-particle two-phase in pipe. It is shown USM-ϑ model has better prediction result than the five-equation model, in which the particle-particle collision is modeled by the particle kinetic theory, while the turbulence of both phase is simulated by the two-equation turbulence model. The USM-ϑ model was then used to simulate the dense two-phase turbulent up flow of Bingham fluid with particles. With the increasing of the yield stress, the velocities of Bingham and particle decrease near the pipe centre. Comparing the two-phase flow of Bingham-particle with that of liquid-particle, it is found the source term of yield stress has significant effect on flow.

EXTRACTION OF PARTICLE CONCENTRATION DISTRIBUTION FROM PLUG FLOW CT IMAGES USING 3D WAVELET MULTIRESOLUTION

Particle concentration distribution images of a dense solid-air two-phase (plug) flow have been obtained at 10 ms intervals at a bend pipe upstream in a horizontal pipeline by means of a capacitance computed tomography. The three-dimensional images (time and two-dimensional space images) have been decomposed to the wavelet time levels to extract the dominant particle concentration distribution using three-dimensional discrete wavelet multiresolution. As a result, the time dominant particle distribution with specific time frequency level is visualized in a cross-section. In detail, the high concentration of the particle spatial distribution at the dense flow front, which composes high-time frequency levels 6 and 7, is located at the center above the stationary layer.

Study of cluster formation in dense two-phase flow using a multi-lattice deterministic model

To simulate cluster formation in dense two-phase flow, a multi-lattice deterministic trajectory (MLDT) model is developed. The actual inter-particle collision and particle motion are treated by a Lagrangian trajectory model with three sets of lattices to reduce computation time. Initially, the MLDT model is used to simulate dense gas-particle flow in a vertical channel for validation with available simulated and experimental data. Using the MLDT model, effects of particle properties and flow parameters on cluster formation are studied. It is shown that cluster formation is enhanced by the reduction of particle restitution coefficient and increase of original particle volume fraction.