Numerical simulation of dense particle-gas two-phase flow using the minimal potential energy principle

A simulation method of dense particle-gas two-phase flow has been developed. The binding force is introduced to present the impact of particle clustering and its expression is deduced according to the principle of minimal potential energy. The cluster collision, break-up and coalescence models are proposed based on the assumption that the particle cluster are treated as one discrete phase. These models are used to numerically study the two-phase flow field in a circulating fluidized bed (CFB). Detailed results of the cluster structure, cluster size, particle volume fraction, gas velocity, and particle velocity are obtained. The correlation between the simulation results and experimental data justifies that these models and algorithm are reasonable, and can be used to efficiently study the dense particle-gas two-phase flow.     

Fluid flow field synergy principle and its application to drag reduction

The concept of field synergy for fluid flow is introduced, which refers to the synergy of the velocity field and the velocity gradient field in an entire flow domain. Analyses show that the flow drag depends not only on the velocity and the velocity gradient fields but also on their synergy. The principle of minimum dissipation of mechanical energy is developed, which may be stated as follows： the worse the synergy between the velocity and velocity gradient fields is, the smaller the resistance becomes. Furthermore, based on the principle of minimum dissipation of mechanical energy together with conservation equations, a field synergy equation with a set of specified constraints has been established for optimizing flow processes. The optimal flow field can be obtained by solving the field synergy equation, which leads to the minimum resistance to fluid flow in the fixed flow domain. Finally, as an example, the field synergy analysis for duct flow with two parallel branches is presented. The optimized velocity distributor nearby the fork, which was designed based on the principle of minimum dissipation of mechanical energy, may reduce the drag of duct flow with two parallel branches.     

Progress in modeling of fluid flows in crystal growth processes

Modeling of fluid flows in crystal growth processes has become an important research area in theoretical and applied mechanics. Most crystal growth processes involve fluid flows, such as flows in the melt, solution or vapor. Theoretical modeling has played an important role in developing technologies used for growing semiconductor crystals for high performance electronic and optoelectronic devices. The application of devices requires large diameter crystals with a high degree of crystallographic perfection, low defect density and uniform dopant distribution. In this article, the flow models developed in modeling of the crystal growth processes such as Czochralski, ammonothermal and physical vapor transport methods are reviewed. In the Czochralski growth modeling, the flow models for thermocapillary flow, turbulent flow and MHD flow have been developed. In the ammonotbermal growth modeling, the buoyancy and porous media flow models have been developed based on a single-domain and continuum approach for the composite fluid-porous layer systems. In the physical vapor transport growth modeling, the Stefan flow model has been proposed based on the flow-kinetics theory for the vapor growth. In addition, perspectives for future studies On crystal growth modeling are proposed.     

Mixed Model for Silt-Laden Solid-Liquid Two-Phase Flows

The kinetic theory of molecular gases was used to derive the governing equations for dense solid-liquid two-phase flows from a microscopic flow characteristics viewpoint by multiplying the Boltzmann equation for each phase by property parameters and integrating over the velocity space. The particle collision term was derived from microscopic terms by comparison with dilute two-phase flow but with consideration of the collisions between particles for dense two-phase flow conditions and by assuming that the particle-phase velocity distribution obeys the Maxwell equations. Appropriate terms from the dilute two-phase governing equations were combined with the dense particle collision term to develop the governing equations for dense solid-liquid turbulent flows. The SIMPLEC algorithm and a staggered grid system were used to solve the discretized two-phase governing equations with a Reynolds averaged turbulence model. Dense solid-liquid turbulent two-phase flows were simulated for flow in a duct. The simulation results agree well with experimental data.     

Effect of particle loading on heat transfer enhancement in a gas-solid suspension cross flow

Heat transfer between gas-solid multiphase flow and tubes occurs in many industry processes, such as circulating fluidized bed process, pneumatic conveying process, chemical process, drying process, etc. (This paper focuses on the influence of the presence of particles on the heat transfer between a tube and gas-solid sus-pension. The presence of particles causes positive enhancement of heat transfer in the case of high solid loading ratio, but heat transfer reduction has been found for in the case of very low soliding ratio (Ms of less than 0.05 kg/kg). A usefial correlation ineorpomting solid lolling ratio, particle size and flow Reytmlds number was derived from experimental data. In addition, the κ-ε two-equation model and the Fluctuation-Spectrum-Random-Trajectory Model (FSRT Model) are used to simulate the flow field and heat transit of the gas-phase and the solid-phase, respectively. Through coupling of the two phases the model can predict the local and total heat transfer characteristics of tube in gas-solid cross flow. For the total heat transfer enhancement due to particles loading the model predictions agreed well wih experimental data.     

Active Cooling e-Textiles for Smart Clothing

Cooling function is definitely one of the most desirable attributes of clothing. In spite of the recent progress on phase changing material （PCM） research, the final products with sufficient amount of cooling capability have not yet to be developed in market. A new concept of cooling fabrics has been proposed by applying ＂Peltier effect＂ to textile materials. It occurs whenever electrical current flows through two dissimilar conductors t depending on the direction of current flow, the junction of the two conductors will either absorb or release heat. This effect has been tested on P-type and N-type conducting polymers. A P-type conductive polypyrrole coated fabric was synthesized by in- situ polymerization on plain weave PET to make conductive fabrics. And an N.typo electrically conductive material was synthesized by treatment of MWNT and polyethyleneimine （PEI）. A noticeable amount of temperature difference has been found in the fabric.     

Numerical study on the flow features of U-beam inertial separator

Detailed parametric study of three-dimensional gas-particle multiphase flow characteristics in Ubeam tube bundle ntertial separators was conducted by numerical simulation.The carrier phase was treated in the Eulerian frame,the particles were tracked in the Lagrangian frame,and particle-wall collision was considered.Simulation carried out at different inflow rate and mass loading ratios revealed the pressure losses in the separators,velocity field of the gas phase,and the trajectories of particles.The study results revealed the multiphase flow-dynamic features of the separators,and the relationship between separator pressure losses and different inlet velocity.The numerical simulation can provde basis both for optimal disign of mpacting-inertial separator used in circulating fluidized bed boiler;and for study of gas-particle multiphase circumfluence flow.     

Hydrostatic settling velocity of sediments

The settling of sediments is an accelerative process in which the concentration of the main sediments zone will heavily influence settling velocity, but the explicit relationship between the concentration and settling velocity has not been reported in literature. Here a theoretical function was built for the time dependent concentration and time dependent settling velocity of sediments ; then the entire settling process reflecting concentration was shown on the basis of sediments instant-settlement theory and mathematical method. Agreement of computed results and experimental data was found. Several governing parameters, including particle size,particle density,inltial suspended sediments concentration and suspension height, were discussed with a series of calculated velocity curves. The research indicated that (1) the presented concentration-velocity time relationship is rational, (2) settling process of the sediments group with variation of concentration consists of acceleration stage, uniform motion stage and deceleration stage, and (3) particle size, particle density and initial suspended sediments concentration have more influence on the settling velocity than the suspension height and water temperature.     

Molecular dynamics simulation and continuum modelling of granular surface flow in rotating drums

Numerical simulations of granular flows in rotating drums operated at medium to high rates (Fr=0.1― 0.2) have been carried out by using a Molecular Dynamics (MD) algorithm that incorporates inelastic particle interactions, sliding friction and rolling friction. The results indicate that the behavior of granular flow in rotating drums can be classified into two distinct zones: a shear active layer at the bed surface and a quasi-static plug flow region adjacent to the wall. The residence time of a tracer particle in the active layer is approximately a third or a half of that in the plug flow region. The thickness of the active layer at mid-chord is about 0.57―0.61 times that of the plug flow region. It is found that all cases simulated in this work are in the rolling-cascading intermediate regime instead of the pure rolling re-gime. The simulated tangential velocity at the mid-chord is also compared with experimental results reported in the literature and good agreement has been obtained. Based on the MD simulations and experimental results, a continuum approach has also been developed. It is shown that the behavior of granular solids in the plug flow region experiences plastic deformation along the radial direction from the wall with the velocity profiles well described by an exponential function, whereas the active layer velocity follows a simple expression for the Couette shear flow. Discussion has also been made on the granular temperature and concentration profiles.     

Columnar grains-covered small grains Cu–Sn alloy prepared by two-phase zone continuous casting

A new theory of two-phase zone continuous casting(TZCC) has been established in order to improve mechanical properties,corrosion resistance and conductivity properties of metals with wide solid-liquid two-phase zone.A Cu-Sn alloy with continuous columnar grains-covered non-columnar small grains of same phase microstructure containing many self-closed grain boundaries were produced by the self-developed TZCC process.Compared with water-cooled mold continuous casting Cu-Sn alloy,the tensile strength and ductility of the TZCC alloy are greatly improved,the corrosion resistance is improved up to fifteenfold,and the conductivity is improved by 12.2%.The excellent high strength may be due to the effective blockage of dislocation motion by numerous self-closed grain boundaries,which suppress the propagation of grain boundary corrosion,and the extremely low electrical resistivity and high ductility may be attributed to continuous columnar grains.     

The extremum principle of mass entransy dissipation and its application to decontamination ventilation designs in space station cabins

In terms of the analogy between mass and heat transfer phenomena, a new physical quantity, i.e. mass entransy, is introduced to represent the ability of an object for transferring mass to outside. Meanwhile, the mass entransy dissipation occurs during mass transfer processes as an alternative to measure the mass transfer irreversibility. Then the concepts of mass entransy and its dissipation are used to develop the extremum principle of mass entransy dissipation and the corresponding method for convective mass transfer optimization, based on which an Euler＇s equation has been deduced as the optimization equation for the fluid flow to obtain the best convective mass transfer performance with some specific constraints. As an example, the ventilation process for removing gaseous pollutants in a space station cabin with a uniform air supply system has been optimized to reduce the energy consumption of the ventilation system and decrease the contaminant concentration in the cabin. By solving the optimization equation, an optimal air velocity distribution with the best decontamination performance for a given viscous dissipation is firstly obtained. With the guide of this optimal velocity field, a suitable concentrated air supply system with appropriate air inlet position and width has been designed to replace the uniform air supply system, which leads to the averaged and the maximum contaminant concentrations in the cabin been decreased by 75% and 60%, respectively, and the contaminant concentration near the contaminant source surface been decreased by 50%, while the viscous dissipation been reduced by 30% simultaneously.     

A thermodynamic hypothesis regarding optimality principles for flow processes in geosystems

This paper proposes a new thermodynamic hypothesis that states that a nonlinear natural system that is not isolated and involves positive feedbacks tends to minimize its resistance to the flow process through it that is imposed by its environment. We demonstrate that the hypothesis is consistent with flow behavior in saturated and unsaturated porous media, river basins, and the Earth-atmosphere system. While optimization for flow processes has been previously discussed by a number of researchers in the literature, the unique contribution of this work is to indicate that only the driving process is subject to opti-mality when multiple flow processes are simultaneously involved in a system.     

Visualization of Coherent Structures in Acoustically Forced, Gas-Particle Turbulent Round Jet

To show the effects of the particles and forced disturbances on the instantaneous large-scale vortex structures in a gas-particle round jet, coherent structures in gas-particle turbulent round jets were investigated experimentally by flow visualization. The 45-μm and 350-μm diameter glass beads were used as the particles in the experiments. An acoustic speaker was used to introduce velocity perturbations at the jet exit. The Strouhal number based on the nozzle diameter, exit velocity, and forcing frequency was varied from 0,1 to 0.9. The Reynolds number was 9400. The coherent structures were visualized in unforced and forced single-phase jet flows and unforced and forced particle-laden jet flows with different diameter glass beads. The experimental results show that the particles have significant effects on the gas phase coherent structures. The coherent structures are controlled by the large 350-μm diameter particles, while the structures are mainly dominated by the forced disturbances in the flows with 45-μm diameter particles.     

Study on the Improvement in the One Dimensional Mathematical Model of the Air Drag in Melt Blowing Process

The one dimensional mathematical model of the air drag in melt blowing process, which had been put forward by Uyttendaele and Shambaugh is improved. The influence of the density and the specific heat capacity of polymer melt at constant pressure changing with polymer temperature on fiber diameter are studied. The power - law model is introduced as the constitutive equation. The influences of polymer volume flow rate, initial polymer temperature, initial air velocity and initial air temperature on fiber diameter are discussed.     

Concentration and angular velocity measurement in a cyclone separator dipleg using electrical capacitance tomography

Cyclone separator is one of the main parts of the circulating fluidized bed （CFB） boiler. The separation efficiency of the cyclone separator is very important to the whole boiler. Electrical capacitance tomography （ECT） is a unique measuring technique with great potential in multiphase flow measurement. Experimental studies are carried out on the measurement of volumetric concentration and angular velocity using ECT. The former is determined through image reconstruction method, and the latter is measured by cross-correlating the capacitance fluctuations caused by the conveyed solids. The distribution of void fraction in radial direction, the fluctuating characteristics, probability density function and the spectrum characteristics are analyzed. The feasibility and reliability of the method are verified by experimental results.     

Numerical study of flow pattern modulation in a vertical phase separation condenser tube

The passive phase separation concept was proposed to modulate flow patterns for heat transfer enhancement. By the flow pattern modulation, the gas tends to be near the wall and the liquid tends to be in the tube core. Experiment has been performed to verify the fresh idea and the flow pattern modulation mechanism was analyzed qualitatively. This paper focuses on the numerical simulation of the bubble dynamics for a single bubble in the vertical phase separation condenser tube to quantitatively explore the flow pattern mechanism, based on a multiscale grid system and the volume-of-fluid (VOF) method. It is found that: (1) the modulated liquid film thickness can be decreased by 70% compared to that in the bare tube region; (2) the modulated bubble traveling velocity can be doubled, causing the increased liquid velocity and velocity gradient in the annular region to weaken the fluid boundary layer; (3) the significantly increased bubble traveling velocity in the annular region promotes the mass and momentum exchange between the annular region and the core region, and yields the self-sustained pulsating flow in the core region. The above three factors are benefit for the performance improvement of the heat transfer facilities.     

Fluid Flow Behavior of Liquid in Cylindrical Vessels Stirred by One or Two Air Jets

Based on the two-phase model (Eulerian-Eulerian model), the three dimensional fluid flow in water and that liquid steel systems stirred by one or two multiple gas jets are simulated. In the Eulerian-Eulerian two-phase model, the gas and the liquid phase are considered to be two different continuous fluids interacting with each other through the finite inter-phase areas. The exchange between the phases is represented by source terms in conversation equations. Turbulence is assumed to be a property of the liquid phase. A new turbulence modification k-ε model is introduced to consider the bubbles movement contribution to k and ε. The dispersion of phases due to turbulence is represented by introducing a diffusion term in mass conservation equation. The mathematical simulation agrees well with the experiment results. The study results indicate that the distance of two nozzles has big effect on fluid flow behavior in the vessel. Using two gas injection nozzles at the half radii of one diameter of the bottom generates a much better mixing than with one nozzle under the condition of the same total gas flow rate.     

Numerical analysis of the two-phase pressure drop and liquid distribution in single-screw expander prototype

In this paper, the numerical simulations have been carried out to evaluate the two-phase pressure drop and liquid distribution in screw channel. The numerical models are validated against the present experimental data with statistical accuracy. The effects of pitch circle diameter （PCD）, inlet velocity and inlet sectional liquid holdup on the pressure drop and liquid distribution characteristics in the screw channel are illustrated. It is found that decreasing the PCD and increasing the inlet sectional liquid holdup can increase the liquid holdup on the outer side in screw channel. The PCD and the inlet sectional liquid holdup need to be considered in evaluating the two-phase frictional pressure drop per unit length in the screw channel. The PCD has an effect on the development of the average pressure in the various cross sections, and the inlet sectional liquid holdup has no visible impact on the changing process of the pressure drop in the screw channel. The correlation developed predicts the two-phase frictional pressure drop in the screw channel with great statistical accuracy.     

Numerical modeling of Jinlong CJD burner copper flash smelting furnace

Fluid flow, heat transfer and combustion in Jinlong CJD concentrate burner flash smelting furnace have been investigated by numerical modeling and flow visualization. The modeling is based on the Eulerian approach for the gas flow equations and the Lagrangian approach for the particles. Interaction between the gas phase and particle phase, such as frictional forces, heat and mass transfer, are included by the addition of sources and sinks. The modeling results including the fluid flow field, temperature field, concentration field of gas phase and the trajectories of particles have been obtained. The predicted results are in good agreement with the data obtained from a series of experiments and tests in the Jinlong Copper Smelter and the temperature error is less than 20 K.