Experiments and simulations of gas-solid flow in an airlift loop reactor

The hydrodynamics in a gas-solid airlift loop reactor was investigated systematically using experimental measurements and CFD simulation.In the experiments,the time averaged parameters,such as solid fraction and particle velocity,were measured by optical fiber probe.In the simulation,the modified Gidaspow drag model accounting for the interparticles clustering was incorporated into the Eulerian-Eulerian CFD model with particulate-phase kinetic theory.Predicted values of solid fraction and particle velocity ...     

Numerical simulation and experimental validation of gas-solid flow in the riser of a dense fluidized bed reactor

-Ladd models. As a result, the proposed drag force model can be used as an efficient approach for the dense gas-solid two-phase flow.     

3D full-loop simulation and experimental verification of gas-solid flow hydrodynamics in a dense circulating fluidized bed

Because of their advantages of high efficiency and low cost, numerical research methods for large-scale circulating fluidized bed （CFB） apparatus are gaining ever more importance. This article presents a numer- ical study of gas-solid flow dynamics using the Eulerian granular multiphase model with a drag coefficient correction based on the energy-minimization multi-scale （EMMS） model. A three-dimensional, full-loop, time-dependent simulation of the hydrodynamics of a dense CFB apparatus is performed. The process parameters （e.g., operating and initial conditions） are provided in accordance with the real experiment to enhance the accuracy of the simulation. The axial profiles of the averaged solid volume fractions and the solids flux at the outlet of the cyclone are in reasonable agreement with experimental data, thereby verifying the applicability of the mathematical and physical models. As a result, the streamline in the riser and standpipe as well as the solids distribution contours at the cross sections is analyzed. Computational fluid dynamics （CFD） serves as a basis for CFB modeling to help resolve certain issues long in dispute but difficult to address experimentally. The results of this study provide the basis of a general approach to describing dynamic simulations of gas-solid flows.     

On pressure and thermal flux in gas-mixture flows and in two-phase flows

To begin with, two different definitions of pressure, thermal flux, etc. in the diffusion model and two-fluid model are given. Then the physical interpretations of the pressure and the thermal flux are provided by introducing the momentum and energy fluxes,M and ε, through a surface dS in the flow field. The quantities defined in the diffusion model are suggested when the motion of the mixture is studied as a whole, while the quantities defined in the two-fluid model are suggested when the motion of individual species is studied. The collision pressure and thermal flux in dense gas-mixtures are also discussed in detail, i.e. their origin, their expressions in the momentum and energy equations, and their distinctions from the normal partial pressure and thermal flux. A gas-particle flow can be treated as a flow of dense gas-mixtures. The long-standing controversy whether the “inertial coupling term” should exist in the momentum equation can be clarified by the two different definitions of pressure.     

Parallelization of pseudo-particle modeling and its application in simulating gas-solid fluidization

dicating its scalability in future applications.     

Drag models for simulating gas-solid flow in the turbulent fluidization of FCC particles

in the turbulent fiuidization of FCC particles, and was validated by satisfactory agreement between prediction and experiment.     

A New modelling strategy for phase-change heat transfer in turbulent interfacial two-phase flow

The paper presents a modelling strategy for phase-change heat transfer in turbulent interfacial two-phase flow. The computational framework is based on interface tracking ITM (level set approach), combined with large-scale prediction of turbulence, a new methodology known as Large-Eddy & Interface Simulation (LEIS), where super-grid scale turbulence and interfaces are directly solved, whereas the sub-scale parts are modelled. Because steady-state flow conditions are difficult to attain, recourse is made of the Very Large-Eddy Simulation (V-LES) instead of LES, where the flow-dependent cut-off filter is larger and independent from the grid. The computational approach is completed by a DNS-based interfacial phase-change heat transfer model built within the Surface Divergence (SD) theory. The original SD model is found to return better results when modified to account for scale separation, i.e. to segregate low-Re from high-Re number flow portions in the same flow. The model was first validated for an experiment involving a smooth to wavy turbulent, stratified steam-water flow in a 2D channel (Lim et al., 1984, Condensation measurement of horizontal concurrent steam-water flow, ASME J. Heat Transfer 106, 425–432.), revealing that the original SD model performs better for high interfacial shear rates. This screening phase also demonstrated that the most critical issue is the accurate prediction of the interfacial shear using ITM. The model was then applied successfully to predict condensing steam in the event of emergency core cooling in a Pressurized Water Reactor (PWR), where water is injected into the cold leg during a postulated loss-of-coolant-accident. The simulation results agree fairly well with the COSI (short for COndensation at Safety Injections) data (Janicot and Bestion, 1993, Condensation modelling for ECC injection, Nucl. Eng. Des. 145, 37–45).     

A fractal analysis of subcooled flow boiling heat transfer

A fractal model for the subcooled flow boiling heat transfer is proposed in this paper. The analytical expressions for the subcooled flow boiling heat transfer are derived based on the fractal distribution of nucleation sites on boiling surfaces. The proposed fractal model for the subcooled flow boiling heat transfer is found to be a function of wall superheat, liquid subcooling, bulk velocity of fluid (or Reynolds number), fractal dimension, the minimum and maximum active cavity size, the contact angle and physical properties of fluid. No additional/new empirical constant is introduced, and the proposed model contains less empirical constants than the conventional models. The proposed model takes into account all the possible mechanisms for subcooled flow boiling heat transfer. The model predictions are compared with the existing experimental data, and fair agreement between the model predictions and experimental data is found for different bulk flow rates.     

Numerical predictions of flow and thermal fields in an enclosure with two periodically moving walls

Numerical predictions of transient flow and thermal fields in a rectangular enclosure with two periodically moving vertical walls are presented. The combined influence of the movement of the walls and the buoyancy as well on the flow pattern and heat transfer performance is evaluated. The compressible‐flow model is adopted, and governing equations are expressed in integral form and discretized on the moving grids, which deform in resonance with the walls to accommodate the variation in the volume of the enclosure. A two‐stage pressure‐correction scheme is applied for simultaneously determining the distributions of absolute pressure, density, temperature, and velocity of the compressible flow field in the enclosure during the periodically stable periods. Effects of the frequency, stroke, and the phase angle of the wall oscillations on the flow are of major concerns in this study. The frequency is ranged between 5 and 25 Hz and the dimensionless strokes (l/H) of the wall are varied from 0.4 to 1.0. Results for Nusselt numbers on the walls as well as the dimensionless input work required to excite the wall oscillation are provided. Copyright © 2006 John Wiley & Sons, Ltd.     

磁性靶向药物递送中铁磁流体的动力学建模

Among the proposed techniques for delivering drugs to specific locations within human body, magnetic drug targeting prevails due to its non-invasive character and its high targeting efficiency. Magnetic targeting drug delivery is a method of carrying drug-loaded magnetic nanoparticles to a target tissue target under the applied magnetic field. This method increases the drug concentration in the target while reducing the adverse side-effects. Although there have been some theoretical analyses for magnetic drug targeting, very few researchers have addressed the hydrodynamic models of magnetic fluids in the blood vessel. A mathematical model is presented to describe the hydrodynamics of ferrofiuids as drug carriers flowing in a blood vessel under the applied magnetic field. In this model, magnetic force and asymmetrical force are added, and an angular momentum equation of magnetic nanoparticles in the applied magnetic field is modeled. Engineering approximations are achieved by retaining the physically most significant items in the model due to the mathematical complexity of the motion equations. Numerical simulations are performed to obtain better insight into the theoretical model with computational fluid dynamics. Simulation results demonstrate the important parameters leading to adequate drug delivery to the target site depending on the magnetic field intensity, which coincident with those of animal experiments. Results of the analysis provide important information and suggest strategies for improving delivery in clinical application.     

Generalized relative permeability coefficients during steady-state two-phase flow in porous media,and correlation with the flow mechanisms

A parametric experimental investigation of the coupling effects during steady-state two-phase flow in porous media was carried out using a large model pore network of the chamber-and-throat type, etched in glass. The wetting phase saturation,S ₁, the capillary number,Ca, and the viscosity ratio,k, were changed systematically, whereas the wettability (contact angleθ _e ), the coalescence factorCo, and the geometrical and topological parameters were kept constant. The fluid flow rate and the pressure drop were measured independently for each fluid. During each experiment, the pore-scale flow mechanisms were observed and videorecorded, and the mean water saturation was determined with image analysis. Conventional relative permeability, as well as generalized relative permeability coefficients (with the viscous coupling terms taken explicitly into account) were determined with a new method that is based on a B-spline functional representation combined with standard constrained optimization techniques. A simple relationship between the conventional relative permeabilities and the generalized relative permeability coefficients is established based on several experimental sets. The viscous coupling (off-diagonal) coefficients are found to be comparable in magnitude to the direct (diagonal) coefficients over board ranges of the flow parameter values. The off-diagonal coefficients (k _rij /Μ _j ) are found to be unequal, and this is explained by the fact that, in the class of flows under consideration, microscopic reversibility does not hold and thus the Onsager-Casimir reciprocal relation does not apply. Thecoupling indices are introduced here; they are defined so that the magnitude of each coupling index is the measure of the contribution of the coupling effects to the flow rate of the corresponding fluid. A correlation of the coupling indices with the underlying flow mechanisms and the pertinent flow parameters is established.     

Lie group analysis of viscoelastic MHD aligned flow and heat transfer

Exact solutions for an incompressible, viscoelastic, electrically conducting MHD aligned fluid are obtained for velocity components and temperature profiles. Lie Group method is applied to obtain the solution and the symmetries used are of translational type.The English text was polished by Keren Wang and Yunming Chen.     

Hydrodynamic modelling and CFD simulation of ferrofluids flow in magnetic targeting drug delivery

Magnetic targeting drug delivery is a method of carrying drug-loaded magnetic nanoparticles to a tissue target in the human body under the applied magnetic field. This method increases the drug concentration in the target and reduces the adverse side-effects. In this article, a mathematical model is presented to describe the hydrodynamics of ferrofluids as drug carriers flowing in a blood vessel under the applied magnetic field. Numerical simulations are performed to obtain better insight into the theoretical analysis with computational fluid dynamics. A 3D flow field of magnetic particles in an idealised blood vessel model containing an aneurysm is analysed to further understand clinical application of magnetic targeting drug delivery. Simulation samples demonstrate the important parameters leading to adequate drug delivery to the target site depending on the applied magnetic field in coincidence with reported results from animal experiments. Results of the analysis provide the important information and can suggest strategies for improving delivery in favour of the clinical application.     

Numerical and experimental analysis of flow and heat transfer in a fuel assembly mock-up with transverse flow above the rods

Thermal-hydraulic conditions in a partially uncovered nuclear fuel assembly mock-up are studied with particular focus on the influence of the horizontal air flow above the rod bundle. The investigations are performed at the ALADIN test facility, which models a boiling water reactor fuel assembly at a 1:1 scale both axially and radially. In the scenario studied, the main heat transfer mechanisms – conduction, convection and radiation – are strongly coupled and all are of similar importance. A combination of measurements and CFD simulations serves to analyze the heat transfer processes in detail. Contrary to previous studies in this field, all heat transfer mechanisms were considered in the simulation with sophisticated models. The numerical results show a good agreement with the measurements, given the inevitable differences between the approaches. Although the successive evaporation of cooling water in a fuel assembly is a transient, multiphase process, the steady, single-phase simulation yields acceptable results. While single effects are overestimated in the simulation, the important dependencies are predicted similarly. A general result is that the maximum cladding temperature rises with decreasing water level. Further results indicate an impact of the horizontal air flow on the residual heat removal for moderate rod powers. Higher horizontal velocities above the fuel assembly lead to slightly higher temperatures inside. A characteristic flow field forms in the test facility that prevails for all studied water levels and horizontal velocities. However, it has only a minor effect on the temperature distribution in the central rod bundle. By combining experiments and numerical simulations, the study provides important information about the decisive parameters for the heat exchange in a spent fuel pool in case of an accident with loss of cooling. The exposed length of the fuel rods is of much more importance than the magnitude of the horizontal velocity above the fuel assembly.     

Heat transfer analysis according to condensation flow structures in a minichannel

An experimental investigation of complete condensation flow is undertaken for a range of mass flow rates between 3.4 and 13.8 kg/m² s. The associated flow regimes are visualized using an ombroscopic technique. Two major flows are observed (with or without release of bubbles). A critical value of the mass flow rate is obtained at the transition between these two regimes. The visualization also enables a local parameter to be determined: the void fraction. The influence of the presence of a bubbly zone is highlighted by the heat transfer and pressure drops. Finally, the dependence of the critical value of the mass flow rate on the temperature of the secondary flow is obtained.     

Coupled heat transfer and viscous flow,and magnetic effects in weld pool analysis

A finite element formulation and analysis is developed to study coupled heat transfer and viscous flow in a weld pool. The thermal effects generate not only buoyancy forces but also a variation in the surface tension which acts to drive the viscous flow in the molten weld pool. A moving phase boundary separates molten and solid material. Numerical experiments reveal the nature of the highly convective flow in the weld pool and the associated thermal profiles. The relative importance of buoyancy, surface tension, phase change, convection, etc. are examined. We also consider the sensitivity of the solution to the finite element mesh and related non-linear numerical instabilities. Of particular interest is the coupling of the thermal and viscous flow fields for the case when radial flow is inward or outward.     

Eulerian-Eulerian two-phase numerical simulation of nanofluid laminar forced convection in a microchannel

In this paper, laminar forced convection heat transfer of a copper-water nanofluid inside an isothermally heated microchannel is studied numerically. An Eulerian two-fluid model is considered to simulate the nanofluid flow inside the microchannel and the governing mass, momentum and energy equations for both phases are solved using the finite volume method. For the first time, the detailed study of the relative velocity and temperature of the phases are presented and it has been observed that the relative velocity and temperature between the phases is very small and negligible and the nanoparticle concentration distribution is uniform. However, the two-phase modeling results show higher heat transfer enhancement in comparison to the homogeneous single-phase model. Also, the heat transfer enhancement increases with increase in Reynolds number and nanoparticle volume concentration as well as with decrease in the nanoparticle diameter, while the pressure drop increases only slightly.     

The influence of fluid properties on electrohydrodynamic heat transfer enhancement in liquids under viscous and electrically dominated flow conditions

Fluid property effects on electrohydrodynamic (EHD) heat transfer enhancement were investigated. Heat transfer, pressure drop, electrical power requirements, and the transition between the viscous dominated and electrically dominated flow regimes as a function of fluid properties were examined using three cooling oils having widely varying physical properties. Low viscosity and low electrical conductivity gave the greatest heat transfer enhancement for a given electrical power input. The required electrical power to achieve a specified heat transfer enhancement was greater for working fluids that had a small charge relaxation time, defined as the ratio of the electrical permittivity to the electrical conductivity. These results correlate well with available experimental and analytical data. A theoretical prediction of the effect of fluid properties and forced flow rate on the onset of EHD enhancement was experimentally verified. The onset of significant EHD heat transfer enhancement occurs most readily in low viscosity liquids at low Reynolds number flows for a given electrical power input.