Numerical Simulation on Gas-Solid Two-Phase Turbulent Flow in FCC Riser Reactors(Ⅰ) Turbulent Gas-Solid Flow-Reaction Model

Gas-solid two-phase turbulent flows,mass transfer,heat transfer and catalytic cracking reactions areknown to exert interrelated influences in commercial fluid catalytic cracking(FCC)riser reactors.In the presentpaper,a three-dimensional turbulent gas-solid two-phase flow-reaction model for FCC riser reactors was devel-oped.The model took into account the gas-solid two-phase turbulent flows,inter-phase heat transfer,masstransfer,catalytic cracking reactions and their interrelated influence.The k-V-k_P two-phase turbulence modelwas employed and modified for the two-phase turbulent flow patterns with relatively high particle concentration.Boundary conditions for the flow-reaction model were given.Related numerical algorithm was formed and a nu-merical code was drawn up.Numerical modeling for commercial FCC riser reactors could be carried out with thepresented model.

Identifying the enhancement mechanism of Al/MoO_(3) reactive multilayered films on the ignition ability of semiconductor bridge using a one-dimensional gas-solid two-phase flow model

Energetic Semiconductor bridge(ESCB)based on reactive multilayered films(RMFs)has a promising application in the miniature and intelligence of initiator and pyrotechnics device.Understanding the ignition enhancement mechanism of RMFs on semiconductor bridge(SCB)during the ignition process is crucial for the engineering and practical application of advanced initiator and pyrotechnics devices.In this study,a one-dimensional(1D)gas-solid two-phase flow ignition model was established to study the ignition process of ESCB to charge particles based on the reactivity of Al/MoO_(3) RMFs.In order to fully consider the coupled exothermic between the RMFs and the SCB plasma during the ignition process,the heat release of chemical reaction in RMFs was used as an internal heat source in this model.It is found that the exothermal reaction in RMFs improved the ignition performance of SCB.In the process of plasma rapid condensation with heat release,the product of RMFs enhanced the heat transfer process between the gas phase and the solid charge particle,which accelerated the expansion of hot plasma,and heated the solid charge particle as well as gas phase region with low temperature.In addition,it made up for pressure loss in the gas phase.During the plasma dissipation process,the exothermal chemical reaction in RMFs acted as the main heating source to heat the charge particle,making the surface temperature of the charge particle,gas pressure,and gas temperature rise continuously.This result may yield significant advantages in providing a universal ignition model for miniaturized ignition devices.

NUMERICAL SIMULATION OF 3-D DENSE SOLID-LIQUID TWO-PHASE TURBULENT FLOW IN A NON-CLOGGING MUD PUMP

A mathematical model is set to evaluate the 3-D dense solid-liquid two-phaseturbulent flow in a non-clogging mud pump, the flow feature in the impeller channel is simulatedwith the tool of IPSA. Meanwhile, resort to TECPLOT as the post-processor, the simulation results isvisualized. The results show the main flow characteristics: There exists backflow and aberrantvelocities at inlet area and a relative velocity slip between two phases; A jet-wake flow pattern isdiscerned around the shroud-suction side area; The relative velocity vector of solid phase iscloser to the pressure surface than that of liquid phase and the trend is more obvious with theincrease of diameter; The kinetic energy of turbulence k and the dissipation rate e reach theirpeaks at the corner of pressure and suction surface. The simulation results show a good agreementwith the experimental flow features in the impeller channel, which prove the turbulent model used isvalid and provide a theoretical design basis to non-clogging pumps.

IMPROVED SUBGRID SCALE MODEL FOR DENSE TURBULENT SOLID-LIQUID TWO-PHASE FLOWS

The dense solid-phase governing equations for two-phase flows are obtained by using the kinetic theory of gas molecules.Assuming that the solid-phase velocity distributions obey the Maxwell equations,the collision term for particles under dense two-phase flow conditions is also derived. In comparison with the governing equations of a dilute two-phase flow,the solid-particle's governing equations are developed for a dense turbulent solid-liquid flow by adopting some relevant terms from the dilute two-phase governing equations.Based on Cauchy-Helmholtz theorem and Smagorinsky model, a second-order dynamic sub-grid-scale(SGS)model,in which the sub-grid-scale stress is a function of both the strain-rate tensor and the rotation-rate tensor,is proposed to model the two-phase governing equations by applying dimension analyses.Applying the SIMPLEC algorithm and staggering grid system to the two-phase discretized governing equations and employing the slip boundary conditions on the walls,the velocity and pressure fields,and the volumetric concentration are calculated.The simulation results are in a fairly good agreement with experimental data in two operating cases in a conduit with a rectangular cross-section and these comparisons imply that these models are practical.

Deduction and Validation of an Eulerian-Eulerian Model for Turbulent Dilute Two-Phase Flows by Means of the Phase Indicator Function-Disperse Elements Probability Density Function

A statistical formalism overcoming some conceptual and practical difficulties arising in existing two-phase flow (2PHF) mathematical modelling has been applied to propose a model for dilute 2PHF turbulent Hows. Phase interaction terms with a clear physical meaning enter the equations and the formalism provides some guidelines for the avoidance of closure assumptions or the rational approximation of these terms. Continuous phase averaged continuity, momentum, turbulent kinetic energy and turbulence dissipation rate equations have been rigorously and systematically obtained in a single step. These equations display a structure similar to that for single-phase flows. It is also assumed that dispersed phase dynamics is well described by a probability density function (pdf) equation and Eulerian continuity, momentum and fluctuating kinetic energy equations for the dispersed phase are deduced. An extension of the standard k-e turbulence model for the continuous phase is used. A gradient transport model is adopted for the dispersed phase fluctuating fluxes of momentum and kinetic energy at the non-colliding, large inertia limit. This model is then used to predict the behaviour of three axisymmetric turbulent jets of air laden with solid particles varying in size and concentration. Qualitative and quantitative numerical predictions compare reasonably well with the three different sets of experimental results, studying the influence of particle size, loading ratio and flow confinement velocity.

A USM-Θ two-phase turbulence model for simulating dense gas-particle flows

A second-order moment two-phase turbulence model for simulating dense gas-particle flows （USM-Θ model）, combining the unified second-order moment twophase turbulence model for dilute gas-particle flows with the kinetic theory of particle collision, is proposed. The interaction between gas and particle turbulence is simulated using the transport equation of two-phase velocity correlation with a two-time-scale dissipation closure. The proposed model is applied to simulate dense gas-particle flows in a horizontal channel and a downer. Simulation results and their comparison with experimental results show that the model accounting for both anisotropic particle turbulence and particle-particle collision is obviously better than models accounting for only particle turbulence or only particle-particle collision. The USM-Θ model is also better than the k-ε-kp-Θ model and the k-ε-kp-εp-Θ model in that the first model can simulate the redistribution of anisotropic particle Reynolds stress components due to inter-particle collision, whereas the second and third models cannot.

Comparison of a Full Second-Order Moment Model and an Algebraic Stress Two-Phase Turbulence Model for Simulating Bubble-Liquid Flows in a Bubble Column

A full second-order moment (FSM) model and an algebraic stress (ASM) two-phase turbulence modelare proposed and applied to predict turbulent bubble-liquid flows in a 2D rectangular bubble column. Predictiongives the bubble and liquid velocities, bubble volume fraction, bubble and liquid Reynolds stresses and bubble-liquidvelocity correlation. For predicted two-phase velocities and bubble volume fraction there is only slight differencebetween these two models, and the simulation results using both two models are in good agreement with the particleimage velocimetry (PIV) measurements. Although the predicted two-phase Reynolds stresses using the FSM are insomewhat better agreement with the PIV measurements than those predicted using the ASM, the Reynolds stressespredicted using both two models are in general agreement with the experiments. Therefore, it is suggested to usethe ASM two-phase turbulence model in engineering application for saving the computation time.

ON BASIC EQUATIONS OF TURBULENT SWIRLING GAS-SOLID FLOWS AND THEIR APPLICATION IN CYCLONES

The basic equations of turbulent gas-solid flows are derived by using the pseudo-fluid model of particle phase with a refined two-phase turbulence model.These equations are then applied to swirling gas-particle flows for analyzing the collection efficiency in cyclone separators.

Numerical simulation of the effect of void fraction and inlet velocity on two-phase turbulence in bubble-liquid flows

There are contradicted opinions on whether bubbles enhance or reduce the liquid turbulence. In this paper, the effect of void fraction and inlet velocity on the bubble-liquid two-phase turbulence of the multiple bubble-liquid jets in a two-dimensional channel is studied by using the two-phase second-order moment turbulence model. The results confirm the phenomena observed in experiments and reported in references that at a low void fraction and low inlet velocities the bubbles enhance the liquid turbulence, whereas at a high void fraction and high inlet velocities the bubbles reduce the liquid turbulence.

Numerical simulation of the gas-solid two-phase flow inside the multi-channel nozzle for the surface nanocrystallization induced by the ultrasonic particulate peening

Using a gas-solid two-phase model(a discrete phase model),the authors investigated the flow field inside the multi-channel nozzle for surface nanocrystallization(SNC)induced by the ultrasonic particulate peening(USPP).By computation,the velocity fields of both the gas and the solid phases were simulated and the track of the solid phase was analyzed in detail.It can be found that the velocities of the two phases are able to reach an ultrasonic level;meanwhile,the dispersion width of the solid phase at the nozzle exit is less than that of the gas phase.When particle diameters are less than 5 μm,there is a decreasing trend in the dispersion width of the solid phase with an increase in particle diameters.The trend becomes stable as the particle diameters are greater than 5 μm;in the meantime,the distribution of solid particles is near the axis of the jet flow.The optimal standoff distance between the nozzle and the substrate in the process of USPP is about 120 mm.Simulation results can help improve the design of mass-production-oriented multi-channel nozzles for SNC induced by USPP.

Numerical Simulation of the Gas-solid Two-phase Flows in a Precalciner

The gas-solid two-phase flous of the precalciner were simulated by different multiphase models,such as mixture model,the Enderium model,including mixture and dispersed,and discrete phase model（DPM）,The results of the different multiphase models were analyzed and compared.showing the rationality of the diffusion and mixture of the cenment raic meals and coal poroder some extent Moreover,the results also shose the rationality of the given inlets parameters of actual process of the precalciner.

PREDICTIONS OF 3-D STRONGLY SWIRLING GAS-SOLID TWO-PHASE FLOW WITH GAS COMBUSTION

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Experiments on two-phase flow in hydraulic jump on pebbled rough bed:Part 1–Turbulence properties and particle chord time and length

This study reported and discussed turbulence characteristics,such as turbulence intensity,correlation time scales,and advective length scales.The characteristic air–water time scale,including the particle chord time and length and their probability density functions(PDFs),was investigated.The results demonstrated that turbulence intensity was relatively greater on a rough bed in the roller length,whereas further downstream,the decay rate was higher.In addition,the relationship between turbulence intensity and dimensionless bubble count rate reflected an increase in turbulence intensity associated with the number of entrained particles.Triple decomposition analysis(TDA)was performed to determine the contributions of slow and fast turbulent components.The TDA results indicated that,regardless of bed type and inflow conditions,the sum of the band-pass(T'_(u))and high-pass(T″_(u))filtered turbulence intensities was equal to the turbulence intensity of the raw signal data(T_(u)).T″_(u) highlighted a higher turbulence intensity and larger vorticities on the rough bed for an identical inflow Froude number.Additional TDA results were presented in terms of the interfacial velocity,auto-and cross-correlation time scales,and longitudinal advection length scale,with the effects of low-and high-frequency signal components on each highlighted parameter.The analysis of the air chord time indicated an increase in the proportion of small bubbles moving downstream.The second part of this research focused on the basic properties of particle grouping and clustering.

An improved large eddy simulation of two-phase flows in a pump impeller

An improved large eddy simulation using a dynamic second-order sub-grid-scale （SGS） stress model has been developed to model the governing equations of dense turbulent particle-liquid two-phase flows in a rotating coordinate system, and continuity is conserved by a mass-weighted method to solve the filtered governing equations. In the cur- rent second-order SGS model, the SGS stress is a function of both the resolved strain-rate and rotation-rate tensors, and the model parameters are obtained from the dimensional consistency and the invariants of the strain-rate and the rotation-rate tensors. In the numerical calculation, the finite volume method is used to discretize the governing equations with a staggered grid system. The SIMPLEC algorithm is applied for the solution of the discretized governing equations. Body- fitted coordinates are used to simulate the two-phase flows in complex geometries. Finally the second-order dynamic SGS model is successfully applied to simulate the dense turbu-lent particle-liquid two-phase flows in a centrifugal impeller. The predicted pressure and velocity distributions are in good agreement with experimental results.

A Numerical Sirnulation of Gas-Particle Two-Phase Flow in a Suspension Bed Using DifFusion Flux Model

A mathematical modei of two-dimensional turbulent gas-particle twophase flow based on the modified diffusion flux modei (DFM) and a numerical simulation method to analyze the gas-particle flow structures are developed. The modified diffusion flux modei, in which the acceleration due to various forces is taken into account for the calculation of the diffusion velocity of particles, is applicable to the analysis of multi-dimensional gas-particle two-phase turbulent flow. In order to verify its accuracy and efficiency, the numerical simulation by DFM is compared with experimental studies and the prediction by k-ε-kp two-fluid modei, which shows a reasonable agreement. It is confirmed that the modified diffusion flux modei is suitable for simulating the multi-dimensional gas-particle two-phase flow.

Near-source characteristics of two-phase gas-solid outbursts in roadways

Coal and gas outbursts compromise two-phase gas-solid mixtures as they propagate as shock waves and flows from their sources.Propagation is influenced by the form of the outburst,proximity to source,the structure and form of the transmitting roadways and the influence of obstacles.The following characterizes the propagation of coal and gas outbursts as two-phase gas-solid flows proximal to source where the coupled effects of pulverized coal and gas flows dominate behavior.The characteristics of shock wave propagation and attenuation were systematically examined for varied roadway geometries using experiments and numerical models.The results demonstrate that the geometry of roadway obstructions is significant and may result in partial compression and sometimes secondary overpressurization in blocked and small comer roadways leading to significant attenuation of outburst shock waves.The shock waves attenuate slowly in both straight and abruptly expanding roadways and more significantly in T-shaped roadways.The most significant attenuation appears in small angle comers and bifurcations in roadways with the largest attenuation occurring in blocked roadways.These results provide basic parameters for simplifying transport in complex roadway networks in the far-field,and guidance for the design of coal and gas outburst prevention facilities and emergency rescue.

An experimental study of drag reduction by nanofluids in slug two-phase flow of air and water through horizontal pipes

This study investigates the effect of injecting nanofluids containing nano-SiO_2 as drag reducing agents(DRA) at different concentrations on the pressure drop of air-water flow through horizontal pipe.The test fluid used in this study was air-water with nano-SiO_2 particles at 0.1%-1%mass concentration.The test sections of the experimental set-up were five pipes of the same length of 9 m with ID from 0.0127m-0.03175m(0.5 to 1.25 in).Airwater flow was run in slug flow regime under different volumetric flow rates.The results of drag reduction(η%)indicated that the addition of DRA could be efficient up to some dosage.Drag reduction performed much better for smaller pipe diameters than it did for larger ones.For various nanosilica concentrations,the maximum drag reduction was about 66.8%for 0.75%mass concentration of nanosilica.

RESEARCH ON THE EFFECT OF CYLINDER PARTICLES ON THE TURBULENT PROPERTIES IN PARTICULATE FLOWS

The fluid fluctuating velocity equations which include the term of cylinder particles were established. The turbulent intensity and Reynolds stress of fluid were obtained by averaging fluctuating velocity based on the solution of the fluctuating velocity equations. Above approach was used to solve the channel turbulent flows, and computational results were compared with the experimental ones for the case of single phase flow. The effects of volume fraction of particles, the ratio of particle length to diameter and the particle relaxation time on turbulent properties were illustrated by changing cylinder particle parameters. It is shown that particles play a restraining role to turbulent properties in the flows. The degree of restraint is directly proportional to the volume fraction of particle, the ratio of particle length to diameter and inversely proportional to particle relaxation time.

Solid Particles Injection in Gas Turbulent Channel Flow

This paper represents a review of the recent researches that investigate the behavior of the gas turbulent flow laden with solid particles. The significant parameters that influence the interactions between the both phases, such as particle size, loading ratio and the gas velocity, have been extensively reviewed. Those parameters are presented in dimensionless numbers in which the applicability of studying its effect in terms of all circumstances of the gas turbulent channel flow at different condition is possible. The represented results show that the turbulence degree is proportional to the particle size. It was found that at the most flow conditions even at low mass ratio, the particle shape, density and size significantly alter the turbulence characteristics. However, the results demonstrate that the particle Reynolds number is a vital sign: the turbulence field becomes weaker if particle Reynolds number is lower than the critical limit and vies verse. The gas velocity has a strong effect on the particles settling along the channel flow and as a result, the pressure drop will be affected.

Critical Stokes Number for Gas-Solid Flow Erosion of Wind Turbine Airfoil

Wind turbine blades are inevitable to be eroded in wind-sand environment,so it is crucial to identify the flow conditions under which the erosion happens.Here,the effect of the sand diameter on wind turbine airfoil is first investigated.When the sand diameter is less than 3μm,the sands will bypass the airfoil and no erosion occurs.When the sand diameter is larger than 4μm,the sand grains collide with the airfoil and the erosion happens.Thus,there must be a critical sand diameter between 3μm and 4μm,at which the erosion is initiated on the airfoil surface.To find out this critical value,aparticle Stokes number is introduced here.According to the range of the critical sand diameter mentioned above,the critical value of particle Stokes number is reasonably assumed to be between 0.007 8and 0.014.The assumption is subsequently validated by other four factors influecing the erosion,i.e.,the angle of attack,relative thickness of the airfoil,different series airfoil,and inflow velocity.Therefore,the critical range of Stokes number has been confirmed.