Bottom bed regimes in a circulating fluidized bed boiler

This paper extends previous work on the fluidization regimes of the bottom bed of circulating flyidized bed (CFB) boilers. Pressure measurements were performed to obtain the time-average bottom bed voidage and to study the bed pressure fluctuations. The measurements were carried out in a 12 MW_th CFB boiler operated at 850°C and also under ambient conditions (40°C). Two bubbling regimes were identified: a “single bubble regime” with large single bubbles present at low fluidization velocities, and, at high fluidization velocities, an “exploding bubble regime” with bubbles often stretching all the way from the air distributor to the surface of the bottom bed. The exploding bubble regime results in a high through-flow of gas, indirectly seen from the low average voidage of the bottom bed, which is similar to that of a stationary fluidized bed boiler, despite the higher gas velocities in the CFB boiler. Methods to determine the fluidization velocity at the transition from the single to the exploding bubble regime are proposed and discussed. The transition velocity increases with an increase in particle size and bed height.     

Dynamic modeling of soil–tool interaction: An overview from a fluid flow perspective

The study of tillage tool interaction centers on soil failure patterns and development of force prediction models for design optimization. The force-deformation relationships used in models developed to date have been considering soil as a rigid solid or elasto-plastic medium. Most of the models are based on quasi-static soil failure patterns. In recent years, efforts have been made to improve the conventional analytical and experimental models by numerical approaches. This paper aims at reviewing the existing methods of tillage tool modeling and exploring the use of computational fluid dynamics to deal with unresolved aspects of soil dynamics in tillage. The discussion also focuses on soil rheological behaviour for its visco-plastic nature and its mass deformation due to machine interaction which may be analyzed as a Bingham plastic material using a fluid flow approach. Preliminary results on visco-plastic soil deformation patterns and failure front advancement are very encouraging. For a tool operating speed of 5.5 m s⁻¹, the soil failure front was observed to be about 100-mm forward of the tool.     

复杂气固两相系统的微观结构

流化床中的气固两相流动是一个高度复杂的非线性混沌系统。本文利用激光粒子动态分析仪（ＰＤＡ）得到的循环流化床中颗粒脉动速度信号,采用ＦＦＴ分析了脉动信号的宽频谱特征,在此基础上应用小波法分析了脉动信号的动态特征,得到了颗粒脉动速度的微观结构,指出颗粒脉动速度的非线性特性是流化床具有混沌特性的根源,且在不同的尺度上颗粒脉动速度表现出各向异性的特征。     

Effect of Multi-Scale Porosity in Local Permeability Modelling of Non-Crimp Fabrics

The influence of multi-scale porosity of fibre reinforcements on local permeability is investigated, in order to determine the possibility of simplifying permeability models for more efficient permeability calculations. Unit cell models of a biaxial Non-Crimp Fabric are developed and used to investigate, whether or not the porous bundles can be excluded, when modelling the local permeability. Numerical accuracy of calculations is controlled to guarantee the quality of the results and the conclusions drawn from them. It is found that fibre bundles with high fibre density can be excluded from permeability models, while bundles with low fibre volume fractions need to be included. A new method to model the local permeability of multi-scale reinforcements is developed and verified for low fibre density in the bundles. In this method, the effects of the flow inside the fibre bundles are included through modifications of the boundary conditions of a single-scale model representing the interbundle regions. The local permeability of multi-scale reinforcements can, therefore, be calculated by models with simplified fluid domains for all fibre bundle porosities, instead of being calculated by models consisting of the entire multi-scale geometry.     

Counter-current three-phase fluidization in a turbulent contact absorber: A CFD simulation

A computational fluid dynamics study of three-phase counter-current fluidization occurring in a turbulent contact absorber was performed. A two-dimensional, transient Eulerian multi-fluid model was used, in which the dispersed solid phase was modeled employing a kinetic theory of granular flow. The grid independence of the model, the effect of wall boundary conditions, the choice of granular temperature model, the effects of order of discretization scheme and drag models were studied for a base case setting. The results of simulations were validated against experimental results obtained from the literature. Once the model settings were finalized, simulations were performed for different gas and liquid velocities to predict the hydrodynamics of the absorber. Computed bed expansion and pressure drop were compared with experimental data. Good agreement between the two was found for low velocities of gas and liquid.     

Nonlinear Waves and the Origin of Bubbles in Fluidized Beds

Early experiments in the mid-1940s established two different regimes of behavior of fluidized systems. These are broadly classified into systems that exhibit massive phase segregation, leading to particle-free regions called bubbles, and those that do not. Explaining the origin of bubbles and of these two regimes has represented both a technological and scientific challenge since then. The late 1960s through the 1970s saw a series of illuminating experiments that established many features of the flow regimes and their characteristics through both flow visualizations and quantitative measurements. Recent numerical and theoretical work has come close the resolving the problem. This paper represents the written version of the talk given at the Symposium in honor of Leen van Wijngaarden's retirement. In it, I review the history of progress on the problem in two giant 25-year steps.     

Orifice-induced Wall Pressure Fluctuations and Pipe Vibrations: Theory and Modeling of Fluid Excitations

The orifice-induced wall pressure fluctuations and pipe vibrations are theoretically studied in this paper. The formulations of pipe vibration responses are deduced using acceptance integral approach. Based on the previous experimental results, the empirical equations of the power spectral density of the wall pressure fluctuations are developed, while the mathematical models of the cross-spectral density are generated when the longitudinal and circumferential correlation lengths and the convection velocity are determined. The modeling of the fluid excitations makes the finite element prediction of the orifice-induced pipe vibration responses become possible.     

Eulerian-Lagrangian simulation of distinct clustering phenomena and RTDs in riser and downer

Numerical simulation of fully developed hydrodynamics of a riser and a downer was carried out using an Eulerian-Lagrangian model, where the particles are modeled by the discrete element method (DEM) and the gas by the Navier-Stokes equations. Periodic flow domain with two side walls was adopted to simulate the fully developed dynamics in a 2D channel of 10 cm in width. All the simulations were carried out under the same superficial gas velocity and solids holdup in the domain, starting with a homogenous sta...     

Aerodynamic characteristics of a two-dimensional supersonic biplane, covering its take-off to cruise conditions

Supersonic biplanes can possibly achieve low-boom and low-drag supersonic flights. In the present study, aerodynamic analysis and design of two-dimensional (2-D) biplanes were investigated with the help of computational fluid dynamics (CFD) tools. By utilizing an inverse-design method, a 2-D biplane configuration with lower wave drag than the single flat-plate airfoil at sufficient lift conditions (C _l > 0.14) was designed at its design Mach number (M _∞ = 1.7). In general, although, supersonic biplanes show superior aerodynamic characteristics at their design Mach numbers, unfortunately, they are characterized by poor performances under their off-design conditions. Flow choking occurs at high subsonic speeds, and continues to Mach numbers greater than the design Mach number in the acceleration stage due to flow hysteresis. Hinged slats and flaps were applied as high-lift devices to avoid the flow choking and concomitant hysteresis problems, and were also used as actual high-lift devices under take-off and landing conditions. As further improvement, morphing and Fowler motion were considered. Finally, a series of 2-D biplane configurations from take-off (and landing) to cruise conditions were studied by applying the slats and flaps, as well as morphing mechanisms, to our inversely designed biplane.      

A global approach to error estimation and physical diagnostics in multidimensional computational fluid dynamics

An approach for simultaneously assessing numerical accuracy and extracting physical information from multidimensional calculations of complex (engineering) flows is proposed and demonstrated. The method is based on global balance equations, i.e. volume-integrated partial differential equations for primary or derived physical quantities of interest. Balances can be applied to the full computational domain or to any subdomain down to the single-cell level. Applications to in-cylinder flows in reciprocating engines are used for illustration. It is demonstrated that comparison of the relative magnitude of the terms in the balances provides insight into the physics of the flow being computed. Moreover, for quantities that are not conserved at the cell or control volume level in the construction of the numerical scheme, the imbalance allows a direct assessment of numerical accuracy in a single run using a single mesh. The mean kinetic energy imbalance is shown to be a particularly sensitive indicator of numerical accuracy. This simple and powerful diagnostic approach can be implemented for finite-difference, finite-volume or finite-element methods.     

Analysis of Streamwise Velocity Fluctuations in Turbulent Pipe Flow with the Use of an Ultrasonic Doppler Flowmeter

Data collected from several studies of experimental and numerical nature in wall-bounded turbulent flows and in particular in internal flows (channel and pipe flows, Mochizuki and Nieuwstadt [1]) at different Reynolds numbers R ⁺(Ru _*/ν), indicate that: (i) the peak of the rms-value (normalized by u _*) of the streamwise velocity fluctuations (σ _u ⁺|_peak) is essentially independent of the Reynolds number, (ii) the position of the rms peak value (y ⁺|_peak) is weakly dependent of the Reynolds number, (iii) the skewness of the streamwise velocity fluctuations (S _u ) is close to zero at the position in which the variance has its peak. A series of measurements of streamwise velocity fluctuations has been performed in turbulent pipe flow with the use of an Ultrasonic Doppler Velocimeter and our results support those reported in [1]. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of drag laws on pressure and bed material recirculation rate in a cold flow model of an 8 MW dual fluidized bed system by means of CPFD

A cold flow model of an 8 MW dual fluidized bed (DFB) system is simulated using the commercial computational particle fluid dynamics (CPFD) software package Barracuda. The DFB system comprises a bubbling bed connected to a fast fluidized bed with the bed material circulating between them. As the hydrodynamics in hot DFB plants are complex because of high temperatures and many chemical reaction processes, cold flow models are used. Performing numerical simulations of cold flows enables a focus on the hydrodynamics as the chemistry and heat and mass transfer processes can be put aside. The drag law has a major influence on the hydrodynamics, and therefore its influence on pressure, particle distribution, and bed material recirculation rate is calculated using Barracuda and its results are compared with experimental results. The drag laws used were energy-minimization multiscale (EMMS), Ganser, Turton–Levenspiel, and a combination of Wen–Yu/Ergun. Eleven operating points were chosen for that study and each was calculated with the aforementioned drag laws. The EMMS drag law best predicted the pressure and distribution of the bed material in the different parts of the DFB system. For predicting the bed material recirculation rate, the Ganser drag law showed the best results. However, the drag laws often were not able to predict the experimentally found trends of the bed material recirculation rate. Indeed, the drag law significantly influences the hydrodynamic outcomes in a DFB system and must be chosen carefully to obtain meaningful simulation results. More research may enable recommendations as to which drag law is useful in simulations of a DFB system with CPFD.     

Diffuse-interface modeling of phase segregation in liquid mixtures

We simulate the phase separation of a binary mixture that is deeply quenched into the unstable range of its phase diagram. The mixture is described through the diffuse-interface model and the governing equations are integrated in 2D and 3D in a periodic box and in a channel using a pseudo-spectral method. Spinodal decomposition patterns for critical and off-critical mixtures are studied, revealing the scaling laws of the characteristic lengthscale and composition of single-phase microdomains, together with their dependence on the Peclet number. Comparison between 2D and 3D results reveals that 2D simulations capture, even quantitatively, the main features of the phenomenon. However, while the agreement between 2D and 3D simulations is excellent when the mixture is confined in a periodic box, it appears to be less pronounced in a channel-like geometry.     

Measurement of the Coefficient of Transverse Dispersion in Flow Through Packed Beds for a Wide Range of Values of the Schmidt Number

Experimental values of the coefficient of transverse dispersion (D _T) were measured with the system 2-naphthol/water, over a range of temperatures between 293K and 373K, which corresponds to a range of values of viscosity () between 2.83×10^–4 Ns/m² and 1.01×10^–3 Ns/m² and of molecular diffusion coefficient (D _m) between 1.03×10^–9 m²/s and 5.49×10^–9 m²/s. Since the density () of water is close to 10³ kg/m³, the corresponding variation of the Schmidt number (Sc=/D _m) was in the range 1000 – 50. More than 200 experimental values of the transverse dispersion coefficient were obtained using beds of silica sand with average particle sizes (d) of 0.297 and 0.496mm, operated over a range of interstitial liquid velocities (u) between 0.1mm/s and 14mm/s and this gave a variation of the Reynolds number (Re=du/) between 0.01 and 3.5.Plots of the dimensionless coefficient of transverse dispersion (D _T/D _m) vs. the Peclet number (Pe_m=ud/D _m) based on molecular diffusion bring into evidence the influence of Sc on transverse dispersion. As the temperature is increased, the value of Sc decreases and the values of D _T/D _m gradually approach the line corresponding to gas behaviour (i.e. Sc 1), which is known to be well approximated by the equation D _T/D _m=1/+ud/12D _m, where is the tortuosity with regard to diffusion.     

Large eddy simulation on the unsteady aerodynamic response of a road vehicle in transient crosswinds

A large eddy simulation method based on a fully unstructured finite volume method was developed, and the unsteady aerodynamic response of a road vehicle subjected to transient crosswinds was investigated. First, the method was validated for a 1/20-scale wind-tunnel model in a static aerodynamic condition; this showed that the surface pressure distributions as well as the aerodynamic forces and moments were in good agreement with wind-tunnel data. Second, the method was applied to two transient crosswind situations: a sinusoidal perturbation representing the typical length scale of atmospheric turbulence and a stepwise crosswind velocity corresponding to wind gusts. Typical transient responses of the aerodynamic forces and moments such as phase shifting and undershooting or overshooting were observed, and their dependence on the frequency and amplitude of the input perturbation is discussed. Thus, the utility and validity of the large eddy simulation was demonstrated in the context that such transient aerodynamic forces are difficult to measure using a conventional wind tunnel.     

Effect of coefficient of restitution in Euler-Euler CFD simulation of fluidized-bed hydrodynamics

Collision between particles plays an important role in determining the hydrodynamic characteristics of gas-solid flow in a fluidized bed.In the present work,earlier work(Loha,Chattopadhyay. Chatterjee,2013) was extended to study the effect of the elasticity of particle collision on the hydrodynamic behavior of a bubbling fluidized bed filled with 530-μm particles.The Eulerian-Eulerian two-fluid model was used to simulate the hydrodynamics of the bubbling fluidized bed.where the solid-phase properties were calculated by applying the kinetic theory of granular flow.To investigate the effect of the elasticity of particle collision,different values of the coefficient of restitution were applied in the simulation and their effects were studied in detail.Simulations were performed for two different solid-phase wall boundary conditions.No bubble formation was observed for perfectly elastic collision.The bubble formation started as soon as the coefficient of restitution was set below 1.0,and the space occupied by bubbles in the bed increased with a decrease in the coefficient of restitution.Simulation results were also compared with experimental data available in the literature,and good agreement was found for coefficients of restitution of 0.95 and 0.99.     

Numerical simulation of supersonic flow past reentry capsules

The flow fields over ARD (ESA's atmospheric reentry demonstrator), OREX (orbital reentry experiments) and spherically blunted cone-flare reentry configurations are numerically obtained by solving time-dependent, axisymmetric, compressible Navier–Stokes equations for freestream Mach numbers range of 1.2–6.0. The fluid dynamics are discretized in spatial coordinates employing a finite volume approach which reduces the governing equations to semi discretized ordinary differential equations. Temporal integration is performed using the multistage Runge–Kutta time-stepping scheme. A local time step is used to achieve steady-state solution. The numerical simulation is carried out on a structured grid. The flow-field features around the reentry capsule, such as bow shock wave, sonic line, expansion fan and recirculating flow in the base region are obtained. A good agreement is found between the calculated value of aerodynamic drag coefficient of the spherically blunted cone/fare reentry configuration with the experimental data. The effects of geometrical parameters, such as radius of the spherical cap, half cone angle, with sharp shoulder edge and with smooth shoulder edge on the flow-field have been numerically investigated for various reentry configuration which will be useful for optimization of the reentry capsule. PACS 47.11.Df, 47.40.Ki     

Modeling of share/soil interaction of a horizontally reversible plow using computational fluid dynamics

The horizontally reversible plow (HRP) is currently widely used instead of the regular mold-board plow due to its high operational performance. Soil pressure during HRP tillage generally has adverse effects on the plow surface, especially on either the plowshare or the plow-breast. This effect eventually shortens the tool’s service life. For this reason, this investigation used a three-dimensional (3D) computational fluid dynamics (CFD) approach to characterize the share/soil interaction and thus assess the effects of different tillage conditions on the interaction. To achieve this goal, a 3D model of the plowshare was first constructed in the commercial software SolidWorks, and soil from Xinjiang, China, was selected and subsequently characterized as a Bingham material based on rheological behaviors. Finally, 3D CFD predictions were performed using the control volume method in the commercial ANSYS code Fluent 14.0 in which the pressure distributions and patterns over the share surface were addressed under different tillage speeds in the range of 2–8 ms⁻¹ and at operational depths ranging from 0.1 to 0.3 m. The results show that the maximum pressure appeared at the share-point zone of the plowshare and that the increase in soil pressure was accompanied by either higher tool speed or greater operational depth. The calculated results qualitatively agreed with the preliminary experimental evidence at the same settings according to scanning electron microscopy (SEM). Once again, the CFD-based dynamic analysis in this study is demonstrated to offer great potential for the in-depth study of soil-tool interactions by simulating realistic soil matter.     

CFD modeling of pressure drop and drag coefficient in fixed beds: Wall effects

Simulations of fixed beds having column to particle diameter ratio (D/dp ) of 3, 5 and 10 were performed in the creeping, transition and turbulent flow regimes, where Reynolds number (dp VLL/L ) was varied from 0.1 to 10,000. The deviations from Ergun’s equation due to the wall effects, which are important in D/d p<15 beds were well explained by the CFD simulations. Thus, an increase in the pressure drop was observed due to the wall friction in the creeping flow, whereas, in turbulent regime a decrease in t...     

Investigation of dense slurry suspensions with coaxial mixers: Influences of design variables through tomography and mathematical modelling

The coaxial mixers enhance the suspension of concentrated slurries in an agitated reactor. In this research work, the complex slurry suspension and dissemination behavior in a coaxial slurry mixing system (comprised of a close clearance anchor rotating with a low speed and an inner axial impeller rotating with a high speed) was analyzed employing ERT (electrical resistance tomography, a non-intrusive flow visualization technique), and computational fluid dynamics (CFD). The numerical models were validated by comparing the axial solid concentration profiles generated using the ERT data and the CFD simulation results. The influences of various important parameters such as the diameter of the inner axial impeller, the inner impeller type, and the inner impeller spacing on the hydrodynamic characteristics of the slurry suspensions in a coaxial mixing vessel were thoroughly analyzed. The radial and axial velocity profiles of solid particles were generated using the validated mathematical models. The assessment of energy loss due to the solid–solid collisions, the particle–fluid frictions, and the particle–vessel wall collisions was conducted. The evaluation of optimum inner impeller clearance and inner impeller diameter is essential to attain a high degree of solids suspension and dissemination in a coaxial slurry mixing system.