Turbulent Multiphase Flows Research Articles

Contribution to the wastewater by use of a numerical study of a secondary clarifier comsol multiphysics under

The environmental assessment of industrial effluents (containing organic pollutants and minerals without proper treatment), generated by the mineral fertilizer factory and emptied into the seacoast, revealed the concentrations intense of suspended solids (SS).The wastewater treatment is a multi-step process to remove contaminants. At first, large solid particles were removed by sedimentation, flotation and filtration. In a second step, the biological treatment is aggregated by forming the smaller particles, which is called flocs. These flocs can be easily removed by sedimentation. The present investigation studying the separation of water flocs in a circular secondary clarifier. A mixture model is used to model the multiphase turbulent flow in the tank.

Numerical modeling and simulation of the shedding mechanism and vortex structures at the development stage of ventilated partial cavitating flows

Abstract Ventilated partial cavitation is a complex multi-phase turbulent flow due to the strong interactions between gas and liquid. In the present work, we specially focus on the numerical modeling and simulation of the shedding mechanism and vortex structures. The Reynolds Averaged Navier–Stokes (RANS) method combined with a filter-based turbulence model (FBM) is proposed to explore the physical mechanism of the ventilated partial cavitating flows. Experimental results of cavity evolution and pressure are utilized to assess the prediction ability of the proposed method. Good agreements are observed between experimental measurements and numerical predictions, including the ventilated cavity growth, break off, shedding and the transient dynamic pressure inside the cavity. Based on the model strategy, the cavity dynamic evolution and shedding mechanism are analyzed. The results indicate that the re-entrant flow gives birth to the gas leakage at the cavity interface and is responsible for the ventilated cavity shedding. In addition, streamline vortex is presented to reveal the ventilated cavity shedding characteristics. Moreover, based on the vorticity transport equation, the influence of velocity gradient, fluid volumetric expansion/contraction, pressure gradient and the viscous dissipation factors on the vortex production in ventilated cavitating flows is examined. The present study can provide important basis to better understand the shedding mechanism and vortex structures on the development stage of ventilated partial cavitating flows.

Thermal Energy Processes in Direct Steam Generation Solar Systems: Boiling, Condensation and Energy Storage – A Review

Direct steam generation coupled is a promising solar-energy technology, which can reduce the growing dependency on fossil fuels. It has the potential to impact the power-generation sector as well as industrial sectors where significant quantities of process steam are required. Compared to conventional concentrated solar power systems, which use synthetic oils or molten salts as the heat transfer fluid, direct steam generation offers an opportunity to achieve higher steam temperatures in the Rankine power cycle and to reduce parasitic losses, thereby enabling improved thermal efficiencies. However, its practical implementation is associated with non-trivial challenges, which need to be addressed before such systems can become more economically competitive. Specifically, important thermal-energy processes take place during flow boiling, flow condensation and thermal-energy storage, which are highly complex, multi-scale and multi-physics in nature, and which involve phase-change, unsteady and turbulent multiphase flows in the presence of conjugate heat transfer. This paper reviews our current understanding and ability to predict these processes, and the knowledge that has been gained from experimental and computational efforts in the literature. In addition to conventional steam-Rankine cycles, the possibility of implementing organic Rankine cycle power blocks, which are relevant to lower operating temperature conditions, are also considered. This expands the focus beyond water as the working fluid, to include refrigerants also. In general, significant progress has been achieved in this space, yet there remain challenges in our capability to design and to operate high-performance and low-cost systems effectively and with confidence. Of interest are the flow regimes, heat transfer coefficients and pressure drops that are experienced during the thermal processes present in direct steam generation systems, including those occurring in the solar collectors, evaporators, condensers and relevant energy storage schemes during thermal charging and discharging. A brief overview of some energy storage options are also presented to motivate the inclusion of thermal energy storage into direct steam generation systems.

Buoyant finite-size particles in turbulent duct flow

Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) have been employed to investigate the dynamics of finite-size spherical particles, slightly heavier than the carrier fluid, in a horizontal turbulent square duct flow. Interface resolved Direct Numerical Simulations (DNS) have also been performed with the Immersed Boundary Method (IBM) at the same experimental conditions, bulk Reynolds number $Re_{2H}$ = 5600, duct height to particle size ratio $2H/d_p$ = 14.5, particle volume fraction $\Phi$ = 1% and particle to fluid density ratio $\rho_p/\rho_f$ = 1.0035. A good agreement has been observed between experiments and simulations in terms of the overall pressure drop, concentration distribution and turbulent statistics of the two phases. Additional experimental results considering two particle sizes, $2H/d_p$ = 14.5 and 9 and multiple $\Phi$ = 1, 2, 3, 4 and 5% are reported at the same $Re_{2H}$. The pressure drop monotonically increases with the volume fraction, almost linearly and nearly independently of the particle size for the above parameters. However, despite the similar pressure drop, the microscopic picture, the fluid velocity statistics, differs significantly with the particle size. This one-to-one comparison between simulations and experiments extends the validity of interface resolved DNS in complex turbulent multiphase flows and highlights the ability of experiments to investigate such flows in considerable details, even in regions where the local volume fraction is relatively high.

NUMERICAL STUDY OF AIR-ATOMIZED WATER MIST COOLING OF A HEATED FLAT PLATE

The present numerical study describes the phenomena of impingement cooling of a heated fl at surface by air and water mist-jet for mass loading fraction ƒ = 0.003−0.024 at Redhyd = 4500 and h/d = 5. A multiphase two-dimensional axisymmetric, incompressible and turbulent flow is considered in the present study. The results obtained using the standard k−ε, standard k−ω, RNG k−ε, realizable k−ε, and SST k−ω turbulence models were analyzed and discussed. The predictions of the local heat transfer coefficient show that the realizable k−ε turbulence model predicts heat transfer results better than other turbulence models. The comparative study was performed to select a suitable multiphase model among the volume of fluid, mixture, and Eulerian models. The Eulerian multiphase model predicts heat transfer from mist-jet impingement on a flat surface much better than the volume of fluid and mixture multiphase models. Normalized axial and radial velocities of the continuous and dispersed phase are presented to understand the fluid flow. The present numerical study also discusses turbulent kinetic energy near the nozzle exit and volume fraction over the heated flat surface.

A two-phase mixing layer between parallel gas and liquid streams: multiphase turbulence statistics and influence of interfacial instability

The two-phase mixing layer formed between parallel gas and liquid streams is an important fundamental problem in turbulent multiphase flows. The problem is relevant to many industrial applications and natural phenomena, such as air-blast atomizers in fuel injection systems and breaking waves in the ocean. The velocity difference between the gas and liquid streams triggers an interfacial instability which can be convective or absolute depending on the stream properties and injection parameters. In the present study, a direct numerical simulation of a two-phase gas–liquid mixing layer that lie in the absolute instability regime is conducted. A dominant frequency is observed in the simulation and the numerical result agrees well with the prediction from viscous stability theory. As the interfacial wave plays a critical role in turbulence transition and development, the temporal evolution of turbulent fluctuations (such as the enstrophy) also exhibits a similar frequency. To investigate the statistical response of the multiphase turbulence flow, the simulation has been run for a long physical time so that time-averaging can be performed to yield the statistically converged results for Reynolds stresses and the turbulent kinetic energy (TKE) budget. An extensive mesh refinement study using from 8 million to about 4 billions cells has been performed. The turbulent dissipation is shown to be highly demanding on mesh resolution compared with other terms in TKE budget. The results obtained with the finest mesh are shown to be close to converged results of turbulent dissipation which allow us to obtain estimations of the Kolmogorov and Hinze scales. The estimated Kolmogorov scale is found to be similar to the cell size of the finest mesh used here. The computed Hinze scale is significantly larger than the size of droplets observed and does not seem to be a relevant length scale to describe the smallest size of droplets formed in atomization.

Mass-conservation-improved phase field methods for turbulent multiphase flow simulation

The phase field method has emerged as a powerful tool for the simulation of multiphase flow. The method has great potential for further developments and applications: it has a sound physical basis, and when associated with a highly refined grid, physics is accurately rendered. However, in many cases, especially when dealing with turbulent flows, the available computational resources do not allow for a complete resolution of the interfacial phenomena and some undesired effects such as shrinkage, coarsening and misrepresentation of surface tension forces and thermo-physical properties can affect the accuracy of the simulations. In this paper, we present two improved phase field method formulations (profile-corrected and flux-corrected), specifically developed to overcome the previously mentioned drawbacks, and we benchmark their performance versus the classic one. The formulations are first tested considering the rise of a bubble in a quiescent fluid and the interaction of two droplets in laminar shear flow; then, their performances are compared in the simulation of a droplet-laden turbulent flow. The aim of this work is to review and benchmark the different phase field method formulations, with the final goal of laying down useful guidelines for the accurate simulation of turbulent multiphase flow with the phase field method.

Evaluation for VOF Simulation of Flow Field in Asphalt Foaming Chamber Based on Volume Fraction

Abstract Asphalt with high viscosity, air, and water constitutes multi-phase turbulent flow accompanied by complex dynamic phenomena such as heat transfer and phase transition in the foaming chamber during the asphalt foaming process. The foaming process is difficult to fully describe by mathematical model. In order to establish the correspondence between the approximate simulation flow field in the foaming chamber and the physical experiment, it is necessary to propose a reasonable evaluation method for the flow field simulation in the foaming chamber. Approximate numerical expression of asphalt foaming process is established by the volume of fluid (VOF) multiphase flow calculation method combined with self-defined phase and mass transfer functions. Based on the principle of asphalt foaming, effective foaming area is defined as the area where temperature is ≥100°C, volume fraction of asphalt is ≥0.5, and volume fraction of water vapor is between 0.1 and 0.4. The ratio of the effective foaming area to the total flow field is taken as the evaluation index. The proportion of effective foaming area obtained by asphalt foaming numerical simulation is compared with the expansion rate and half-life obtained by asphalt foaming experiments with three foaming chambers of different sizes. The comparison shows that ratio of effective foaming area has the same tendency as expansion rate and half-life. And the simulation evaluation index is verified. Suggestions are put forward on the structure design of asphalt foaming chamber based on the distribution of effective foaming area. It shows that this method achieves rapid theoretical evaluation of foaming chamber structure and could be used for the optimal design of foaming chamber.

Numerical Simulation of Slag Eye Formation and Slag Entrapment in a Bottom-Blown Argon-Stirred Ladle

A transient mathematical model is developed for simulating the bubble-steel-slag-top gas four-phase flow in a bottom-blown argon-stirred ladle with a 70-ton capacity. The Lagrangian discrete phase model (DPM) is used for describing the moving behavior of bubbles in the steel and slag. To observe the formation process of slag eye, the volume of fluid (VOF) model is used to track the interfaces between three incompressible phases: metal/slag, metal/gas, and slag/gas. The complex multiphase turbulent flow induced by bubble-liquid interactions is solved by a large eddy simulation (LES) model. Slag eye area and slag droplet dispersion are investigated under different gas flow rates. The results show that the movement of bubbles, formation and collapse of slag eye, volatility of steel/slag interface and behavior of slag entrapment can be properly predicted in the current model. When the gas flow rate is 300 L/min, the circulation driven by the bubble plume will stir the entire ladle adequately and form a slag eye of the right size. At the same time, it will not cause strong erosion to the ladle wall, and the fluctuation of the interface is of adequate intensity, which will be helpful for improving the desulfurization efficiency; the slag entrapment behavior can also be decreased. Interestingly, with the motion of liquid steel circulation, the collision and coalescence of dispersed slag droplets occur during the floating process in the vicinity of the wall.

An accurate and stable multiphase moving particle semi‐implicit method based on a corrective matrix for all particle interaction models

SummaryThe Lagrangian moving particle semi‐implicit (MPS) method has potential to simulate free‐surface and multiphase flows. However, the chaotic distribution of particles can decrease accuracy and reliability in the conventional MPS method. In this study, a new Laplacian model is proposed by removing the errors associated with first‐order partial derivatives based on a corrected matrix. Therefore, a corrective matrix is applied to all the MPS discretization models to enhance computational accuracy. Then, the developed corrected models are coupled into our previous multiphase MPS methods. Separate stabilizing strategies are developed for internal and free‐surface particles. Specifically, particle shifting is applied to internal particles. Meanwhile, a conservative pressure gradient model and a modified optimized particle shifting scheme are applied to free‐surface particles to produce the required adjustments in surface normal and tangent directions, respectively. The simulations of a multifluid pressure oscillation flow and a bubble rising flow demonstrate the accuracy improvements of the corrective matrix. The elliptical drop deformation demonstrates the stability/accuracy improvement of the present stabilizing strategies at free surface. Finally, a turbulent multiphase flow with complicated interface fragmentation and coalescence is simulated to demonstrate the capability of the developed method.

Large eddy simulation of particle agglomeration with shear breakup in turbulent channel flow

A systematic technique is developed for studying particle dynamics as induced by a turbulent liquid flow, in which transport, agglomeration, and breakup are considered. An Eulerian description of the carrier phase obtained using large eddy simulation is adopted and fully coupled to a Lagrangian definition of the particle phase using a pointwise discrete particle simulation. An efficient hard-sphere interaction model with deterministic collision detection enhanced with an energy-balance agglomeration model was implemented in an existing computational fluid dynamic code for turbulent multiphase flow. The breakup model adopted allows instantaneous breakup to occur once the transmitted hydrodynamic stress within an agglomerate exceeds a critical value, characterised by a fractal dimension and the size of the agglomerate. The results from the developed technique support the conclusion that the local turbulence kinetic energy, its dissipation rate, and the agglomerate fractal dimension control the kinetics of the agglomeration and de-agglomeration processes, and as well as defining with time the morphology of the particles and their resultant transport. Overall, the results are credible and consistent with the expected physical behavior and with known theories.

Solid distribution in fluidized and fixed beds with horizontal high speed gas jets

High speed jet penetration in fluidized beds can be used for particle size control or comminution processes. Thus, particle breakage is determined by the kinetic energy of the solids and the frequency of particle-particle impacts inside the flow. Both aspects are influenced by the solid concentration inside the jets and at the boundary. For understanding the fundamental breakage mechanism, the determination of the solid distribution in those complex flows is an important contribution.In order to access the turbulent and delicate multiphase flow, a modern fast X-ray computed tomography unit was used in this work. To obtain quantitative solid concentrations, the measurement system was optimized by means of hardware filtering, exposure time and reconstruction algorithm. These methods allowed the investigation of influencing parameters like jet velocity, fluidization velocity or particle size on the solid distribution in a single jet. With the obtained experimental data, correlations for the axial and radial solid distribution were developed. In addition, the influence of multiple jet injection with a common focal point is presented.

Combustion kinetic analysis of flame spray pyrolysis process

The flame spray pyrolysis (FSP) is a well-known process to produce nanoparticles and presents several advantages when compared to others, mainly regarding final product purity and operational flexibility. The correct modeling of the process is fundamental for the applicability of such techniques and, given that the temperature and chemical composition throughout the reactor are essential for the development of the nanoparticles, the correct representation of the chemical reactions is necessary. In this work, the production of zirconia (ZrO2) nanoparticles via FSP is modeled and the combustion of the precursor-solvent mixture is described through seven sets of different chemical reaction mechanisms to analyse their influence on flame temperature and particle evolution within the process. The reacting turbulent multiphase flow is described by an Eulerian-Lagrangian approach and ZrO2 nanoparticle growth is estimated by solving the population balance equations from a monodisperse model based on coagulation and sintering. Temperatures and primary particle diameters obtained from simulations are within 9% and 6% accuracies of experimental values, respectively. Although unmatching temperature profiles are found for the different mechanisms considered (mainly in the lower regions of the reactor), a small variation of primary particle diameter is observed when cases are compared.

Aerodynamic generation of electric fields in turbulence laden with charged inertial particles

Self-induced electricity, including lightning, is often observed in dusty atmospheres. However, the physical mechanisms leading to this phenomenon remain elusive as they are remarkably challenging to determine due to the high complexity of the multi-phase turbulent flows involved. Using a fast multi-pole method in direct numerical simulations of homogeneous turbulence laden with hundreds of millions of inertial particles, here we show that mesoscopic electric fields can be aerodynamically created in bi-disperse suspensions of oppositely charged particles. The generation mechanism is self-regulating and relies on turbulence preferentially concentrating particles of one sign in clouds while dispersing the others more uniformly. The resulting electric field varies over much larger length scales than both the mean inter-particle spacing and the size of the smallest eddies. Scaling analyses suggest that low ambient pressures, such as those prevailing in the atmosphere of Mars, increase the dynamical relevance of this aerodynamic mechanism for electrical breakdown.

Thermodynamically Constrained Averaging Theory: Principles, Model Hierarchies, and Deviation Kinetic Energy Extensions.

The thermodynamically constrained averaging theory (TCAT) is a comprehensive theory used to formulate hierarchies of multiphase, multiscale models that are closed based upon the second law of thermodynamics. The rate of entropy production is posed in terms of the product of fluxes and forces of dissipative processes. The attractive features of TCAT include consistency across disparate length scales; thermodynamic consistency across scales; the inclusion of interfaces and common curves as well as phases; the development of kinematic equations to provide closure relations for geometric extent measures; and a structured approach to model building. The elements of the TCAT approach are shown; the ways in which each of these attractive features emerge from the TCAT approach are illustrated; and a review of the hierarchies of models that have been formulated is provided. Because the TCAT approach is mathematically involved, we illustrate how this approach can be applied by leveraging existing components of the theory that can be applied to a wide range of applications. This can result in a substantial reduction in formulation effort compared to a complete derivation while yielding identical results. Lastly, we note the previous neglect of the deviation kinetic energy, which is not important in slow porous media flows, formulate the required equations to extend the theory, and comment on applications for which the new components would be especially useful. This work should serve to make TCAT more accessible for applications, thereby enabling higher fidelity models for applications such as turbulent multiphase flows.

A hybrid CFD RANS/Lagrangian approach to model atmospheric dispersion of pollutants in complex urban geometries

Lagrangian atmospheric dispersion models consist of tracking the trajectories of particles of pollutant emitted into the atmosphere. In this paper, the objective is to compare the Lagrangian and Eulerian dispersion models in the same computational fluid dynamics code (Code Saturne), therefore using the same wind and turbulence fields for both. The Lagrangian stochastic model used in this work is the simplified Langevin model (SLM) of Pope (1985, 2000) and pertains to the approaches referred to as probability density function methods. This model has been extensively used in turbulent combustion or multiphase flows, but to our knowledge, it has not been used in atmospheric dispersion applications. First, we show that the SLM respects the well-mixed criterion. Then, we validate the model in the case of a continuous point release with uniform mean wind speed and turbulent diffusivity. Finally, we validate the model with an experimental campaign involving a stably stratified surface layer.

A hybrid CFD RANS/Lagrangian approach to model atmospheric dispersion of pollutants in complex urban geometries

Lagrangian atmospheric dispersion models consist of tracking the trajectories of particles of pollutant emitted into the atmosphere. In this paper, the objective is to compare the Lagrangian and Eulerian dispersion models in the same computational fluid dynamics code (Code Saturne), therefore using the same wind and turbulence fields for both. The Lagrangian stochastic model used in this work is the simplified Langevin model (SLM) of Pope (1985, 2000) and pertains to the approaches referred to as probability density function methods. This model has been extensively used in turbulent combustion or multiphase flows, but to our knowledge, it has not been used in atmospheric dispersion applications. First, we show that the SLM respects the well-mixed criterion. Then, we validate the model in the case of a continuous point release with uniform mean wind speed and turbulent diffusivity. Finally, we validate the model with an experimental campaign involving a stably stratified surface layer.

Modeling Fuel Film Cooling on a Flat Plate

Cooling of liquid rocket engine combustion chambers and nozzles is a critical component to liquid rocket engine design. A common method of cooling is liquid-fuel film cooling. Liquid fuel is injected along the surface of the wall to act as a barrier against the core combustion gases. A numerical model is developed for simulating a physical experimental test setup meant to study liquid-fuel film cooling over a flat plate using a hydrocarbon fuel. The model incorporates turbulent multiphase flow with species transport within a commercial computational fluid dynamics software package. Conjugate heat transfer is simulated through walls containing embedded cooling channels. A novel user-defined function is written to incorporate reactions between the liquid fuel and the freestream gases. Comparisons are made between simulations with and without the phase interactions, as well as with simplifications to the cooling channel geometry. It was found necessary to model the cooling channel geometry in order to avoid artificially reducing cooling performance.

Review on Modeling and Simulation of Continuous Casting

Continuous casting is a mature, sophisticated technological process, used to produce most of the world's steel, so is worthy of fundamentally‐based computational modeling. It involves many interacting phenomena including heat transfer, solidification, multiphase turbulent flow, clogging, electromagnetic effects, complex interfacial behavior, particle entrapment, thermal‐mechanical distortion, stress, cracks, segregation, and microstructure formation. Furthermore, these phenomena are transient, three‐dimensional, and operate over wide length and time scales. This paper reviews the current state of the art in modeling these phenomena, focusing on practical applications to the formation of defects. It emphasizes model verification and validation of model predictions. The models reviewed range from fast and simple for implementation into online model‐based control systems to sophisticated multiphysics simulations that incorporate many coupled phenomena. Both the accomplishments and remaining challenges are discussed.

An Efficient Numerical Approach for Transient Simulation of Multiphase Flow Behavior in Centrifuges

AbstractSolid bowl centrifuges are applied for the separation of small particles in a variety of industries. However, the particles accumulate at the rotor wall, which leads to a time‐dependent separation efficiency. A numerical approach for spatial and time‐resolved simulations of the separation process in a tube centrifuge is developed and some numerical results are presented. The back coupling of the dispersed and sedimented particles is realized by locally defined viscosity functions. The open source computation software OpenFOAM was used to simulate the turbulent multiphase flow within the centrifuge. The numerical investigations demonstrate the significant influence of the growing sediment on the flow conditions and of the rheological behavior of the sediment on the sediment shape.