Computational Particle Fluid Dynamics Research Articles

Optimization of a 50 MW bubbling fluidized bed biomass combustion chamber by means of computational particle fluid dynamics

An efficient utilization of biomass fuels in power plants is often limited by the melting behavior of the biomass ash, which causes unplanned shutdowns of the plants. If the melting temperature of the ash is locally exceeded, deposits can form on the walls of the combustion chamber. In this paper, a bubbling fluidized bed combustion chamber with 50 MW biomass input is investigated that severely suffers deposit build-up in the freeboard during operation. The deposit layers affect the operation negatively in two ways: they act as an additional heat resistance in regions of heat extraction, and they can come off the wall and fall into the bed and negatively influence the fluidization behavior. To detect zones where ash melting can occur, the temperature distribution in the combustion chamber is calculated numerically using the commercial CPFD (computational particle fluid dynamics) code, Barracuda Version 15. Regions where the ash melting temperature is exceeded are compared with the fouling observed on the walls in the freeboard. The numerically predicted regions agree well with the observed location of the deposits on the walls. Next, the model is used to find an optimized operating point with fewer regions in which the ash melting temperature is exceeded. Therefore, three cases with different distributions of the inlet gas streams are simulated. The simulations show if the air inlet streams are moved from the freeboard to the necking area above the bed a more even temperature distribution is obtained over the combustion chamber. Hence, the areas where the ash melting temperatures are exceeded are reduced significantly and the formation of deposits in the optimized operational mode is much less likely.

Experimental study and modelling on gas–solid flow in a lab-scale fluidised bed with Wurster tube

A lab-scale fluidised bed with a Wurster tube is designed for investigation of the gas–solid flow hydrodynamic behaviour by experiment and computational fluid dynamics (CFD) simulation. A unique electrical capacitance tomography (ECT) sensor is used to measure the particle concentration both inside and outside of the Wurster tube. CFD simulation is based on computational particle fluid dynamics (CPFD) and Eulerian–Eulerian-based two-fluid model (TFM). Coating processes are measured and key process parameters are analysed, including the solid concentration in a cross-sectional area, the time-averaged capacitance data, the average solid concentration in the chamber and the effect of the Wurster tube on the hydrodynamic behaviour of gas–solid flow. The CFD simulation results using both TFM and CPFD models are compared with the measurement results by ECT for the gas–solid flow in the fluidised bed but without spraying coating solution. The optimum operating range of the Wurster fluidisation process for different particles is discussed based on the CFD simulation and ECT measurement results, demonstrating that the combination of ECT measurement with TFM–CPFD simulation can provide useful information for the process design and operation for a Wurster fluidised bed.

Effect of particle–particle interaction on the bed pressure drop and bubble flow by computational particle-fluid dynamics simulation of bubbling fluidized beds with shroud nozzle

Computational particle-fluid dynamics (CPFD) simulations were carried out to determine the bed pressure drop and bubble behavior in bubbling fluidized beds produced using a shroud nozzle distributor. The fluidized bed had an internal diameter of 0.3m and height of 2.4m and was modeled using Barracuda, commercial CPFD software. The bed materials consisted of metal-grade silicon particles with dp, ρp, and Umf of 150μm, 2330kg/m3, and 0.02m/s, respectively. The total bed inventory and the static bed height were 75kg and 0.8m, respectively. Air was used as the fluidizing gas at room temperature and atmospheric pressure and was uniformly supplied at the inlet boundary below the distributor. The superficial gas velocity was controlled in the range between 0.07 and 0.17m/s, and the restitution coefficient of the collision model and the particle normal stress parameter, which affected the particle–particle interaction, were adjusted to obtain accurate simulation data. The results of each simulation were validated by comparing the pressure drop profile to that obtained through experiments under the same conditions. The values predicted for the bed pressure drop and the bubble volume fraction changed according to the restitution coefficient in the collision model and the particle normal stress. At Ps=5, the bed pressure drop and the bubble flow characteristics were similar to those obtained from the experimental data.

Effects of the riser exit geometries on the hydrodynamics and solids back-mixing in CFB risers: 3D simulation using CPFD approach

With the Computational Particle Fluid Dynamic (CPFD), which is based on the scheme of Multi-Phase Particle-In-Cell (MP-PIC), the present work studied the flow hydrodynamics and solids back-mixing behaviors in the riser with smooth and abrupt exit geometries. It was found that the flow hydrodynamics and solids back-mixing behaviors are quite different in the riser with different exit geometries. While particles can uniformly flow out of the riser with smooth exits with relatively lower residence time, part of the particles tend to be reflected back into the riser with abrupt exits. Different mechanisms were proposed to explain the distinct solids back-mixing behaviors in the riser with smooth and abrupt exit geometries. The moderate solids back-mixing in the riser with smooth exits is mainly due to the dynamic formation and dissolution of particle meso-scale structures/particle-clusters at the bottom part of the riser, while the intense solids back-mixing in the riser with abrupt exit geometries is the combined results of the particle downward flow from the riser top and the dynamic formation and dissolution of particle meso-scale structures at the riser bottom. The present work implied that the riser exit geometry should be carefully specified in the modeling study of the CFB riser.

Investigation of gas–solids flow characteristics in a circulating fluidized bed with annular combustion chamber by pressure measurements and CPFD simulation

For large scale circulating fluidized bed (CFB) boilers, a new type of annular combustion chamber was proposed to improve the secondary air penetration and provide adequate space to arrange the heat transfer surface. To investigate the gas–solids flow characteristics and optimize the structure of the annular combustion chamber, a “cold” test rig was built with a cross sectional area of 1.18m2 and a height of 8.0m. High frequency pressure transducers were used to obtain the pressure fluctuations characteristics. CFD simulation based on computational particle fluid dynamic (CPFD) was used to obtain pressure fluctuations and particles concentration. The time domain analysis, power spectrum analysis and Hilbert–Huang transform were applied to the experimental and simulation data. Different flow regimes including single bubble regime, exploding bubble regime and turbulent fluidization regime were identified. The flow characteristics were investigated in terms of standard deviation and higher-order moments, auto-correlation, power spectrum, and EMD energy entropy. The results show that the gas–solids flow in the bottom region of the annular combustion chamber was relatively uniform. The gas–solids flow in annular combustion chamber was proved similar to that in a conventional CFB riser.

Comparison of numerical approaches to model FCC particles in gas–solid bubbling fluidized bed

A comparative study has been carried out to determine the ability of computational fluid dynamics (CFD) and computational particle fluid dynamics (CPFD) codes to predict the hydrodynamics of FCC particles in a bubbling fluidized bed. Simulations were conducted in both 2-dimensional (2-D) and 3-dimensional (3-D) CFD configuration and in a 3-D CPFD model. The new structure based force-balance (FB) model of Ahmadi Motlagh et al. (2014. Chem. Eng. Sci. 120, 22–36.) was incorporated in CFD to simulate the behavior of FCC in the bubbling fluidization regime. This model modifies the conventional drag correlation by considering the effect of interparticle forces on the formation of agglomerates inside the bed, updating the drag calculations by replacing the particle diameter in the Wen and Yu correlation by an agglomerate diameter. The effects of superficial gas velocity, particle diameter, particle-particle restitution coefficient and specularity coefficient on voidage are examined. The sensitivity of different gas–solid drag closures in CFD are tested and compared. Experimental time-average axial and radial voidage profiles were simulated with varying degrees of agreement with experimental data obtained using different advanced experimental techniques in the “Traveling fluidized bed” (Dubrawski et al., 2013. Powder Technol. 235, 203–220; Tebianian et al., 2014. Extending the comparison of voidage measurement and modeling techniques in fluidized beds. In: Proc. 11th International Conference on Fluidized Bed Technology, (Eds.), J. Li, X.J. Bao and W. Wang. Chemical Industry Press, Beijing 137–142.). Both the FB and CPFD models were successful in resolving key issues in 3-D, whereas 2-D models tended to seriously underpredict particle volume fraction, especially near the wall.

A CPFD model for a bubbly biomass–sand fluidized bed

This study reports a computational particle fluid dynamics model for the numerical simulation of a sand fluidized bed with 8wt.% and 16wt.% biomass loadings. The proposed CPFD model, based on a Barracuda framework, is validated using experimental data obtained in a 15.2cm diameter bed with a 22.5cm static height. This fluidized bed unit is equipped with a pair of fiber optic sensors allowing one to obtain both the bubble size and bubble velocity frequency distributions in an ample range of experimental conditions. In addition to this, the first-, second-, and third-order statistical moments of the bubble size and the bubble velocity distributions are calculated for each one of the operating conditions studied. On this basis, the present study demonstrates that the proposed CPFD model reproduces the asymmetric character of both bubble size and bubble velocity distributions. In addition, the simulated and experimental bubble size and bubble velocity distributions yield close first-, second-, and third-order statistical moments. Accordingly, one can ascertain that the proposed CPFD model is quite satisfactory for establishing fluid flow patterns in sand–biomass fluidized bed gasifiers.

Numerical Simulation of Particle Transport and Deposition in the Pulmonary Vasculature

To quantify the transport and adhesion of drug particles in a complex vascular environment, computational fluid particle dynamics (CFPD) simulations of blood flow and drug particulate were conducted in three different geometries representing the human lung vasculature for steady and pulsatile flow conditions. A fully developed flow profile was assumed as the inlet velocity, and a lumped mathematical model was used for the calculation of the outlet pressure boundary condition. A receptor-ligand model was used to simulate the particle binding probability. The results indicate that bigger particles have lower deposition fraction due to less chance of successful binding. Realistic unsteady flow significantly accelerates the binding activity over a wide range of particle sizes and also improves the particle deposition fraction in bifurcation regions when comparing with steady flow condition. Furthermore, surface imperfections and geometrical complexity coupled with the pulsatility effect can enhance fluid mixing and accordingly particle binding efficiency. The particle binding density at bifurcation regions increases with generation order and drug carriers are washed away faster in steady flow. Thus, when studying drug delivery mechanism in vitro and in vivo, it is important to take into account blood flow pulsatility in realistic geometry. Moreover, tissues close to bifurcations are more susceptible to deterioration due to higher uptake.

Evaporation and condensation of multicomponent electronic cigarette droplets and conventional cigarette smoke particles in an idealized G3–G6 triple bifurcating unit

The present research study is intended to provide fundamental understanding of the dynamics and transport of aerosols from an e-cigarette in an idealized tubular G3–G6 respiratory tract model. A computational model has been developed that includes the effects of hygroscopic growth as well as evaporation from multicomponent aerosol droplets. The aerosols investigated usually contain carrier solvents such as propylene glycol (PG) and glycerol, along with water, nicotine, and flavors. An experimentally validated computational fluid-particle dynamics (CF-PD) model is presented, which for the first time is capable of simultaneously simulating interactive, multicomponent droplet-vapor dynamics with evaporation and/or condensation. As a first step to accomplish such complex numerical simulations, an idealized G3–G6 triple bifurcating unit (TBU) has been selected. The results are compared with the conventional smoke particles (CSPs) as well as solid particles. Parametric analysis and comparisons of the evaporation/condensation dynamics for EC-droplets vs. cigarette smoke particles were performed, including the effects of different droplet initial diameter, composition, temperature, and ambient relative humidity. The results indicate that EC-droplets, being more hygroscopic than cigarette smoke particles, tend to grow larger in maximum size in a typically highly humid environment. Additionally, a correlation for the growth ratio of EC-droplets in TBUs is proposed.

CPFD simulation of solids residence time and back-mixing in CFB risers

A Computational Particle Fluid Dynamics (CPFD) approach was applied to investigate the solids residence time distribution (RTD) and back-mixing behavior in a circulating fluidized bed (CFB) riser. The comparison between the simulation results and the experimental data indicates that the CPFD method was capable to predict the hydrodynamics and solids mean residence time. It was found that the solids residence time exhibited a non-uniform distribution both in the axial and in the radial directions of the riser. Even in the dilute phase transport (DPT) regime, the predicted solids RTD curves had the feature of an early peak and an extended tail, indicating that the solids flow deviates from plug flow and exhibits back-mixing. The solids back-mixing mainly occurred in the lower part of the riser, which provided significant implications for the industrial applications of the CFB reactors since most of the chemical reactions and heat/mass transport processes occur at the lower part of the riser. It is important to minimize the solids back-mixing at the lower part of the riser for the industrial CFB applications like the FCC.

Particle size distribution in CPFD modeling of gas–solid flows in a CFB riser

A computational particle fluid dynamics (CPFD) numerical method to model gas–solid flows in a circulating fluidized bed (CFB) riser was used to assess the effects of particle size distribution (PSD) on solids distribution and flow. We investigated a binary PSD and a polydisperse PSD case. Our simulations were compared with measured solids concentrations and velocity profiles from experiments, as well as with a published Eulerian-Eulerian simulation. Overall flow patterns were similar for both simulation cases, as confirmed by experimental measurements. However, our fine-mesh CPFD simulations failed to predict a dense bottom region in the riser, as seen in other numerical studies. Above this bottom region, distributions of particle volume fraction and particle vertical velocity were consistent with our experiments, and the simulated average particle diameter decreased as a power function with riser height. Interactions between particles and walls also were successfully modeled, with accurate predictions for the lateral profiles of particle vertical velocity. It was easy to implement PSD into the CPFD numerical model, and it required fewer computational resources compared with other models, especially when particles with a polydisperse PSD were present in the heterogeneous flow.

Experimental and CPFD Numerical Study on Hopper Discharge

This study investigated the applicability of computational particle fluid dynamic (CPFD) numerical scheme for simulating flows in a 3-D hopper. A glass bead with a particle size distribution was used as the experimental material, which is characterized on the limit between group A and group B powder. The discharge of glass bead particles was simulated using the CPFD model. The flow snapshots and the solid discharge rates were successfully captured by the CPFD calculations and compared well with the experimental results. It therefore confirmed a good feasibility of CPFD method to simulate the complex flow in the 3-D hopper. Consequently, the snapshots as well as the concrete values of the solid volume fraction, particle and gas velocities, and pressure in the hopper were given by the CPFD model. Based on the simulation results and the hopper structure, three flow regions were divided and flow characteristics in these regions were analyzed. It shows dense-flow in the hopper region, dilute-flue in the transi...

Effect of particle size distribution on hydrodynamics and solids back-mixing in CFB risers using CPFD simulation

Industrial CFB risers usually handle polydisperse mixtures with broad size distribution, which significantly influenced the performance of the reactors. However, traditional Computational Fluid Dynamics (CFD) models usually assumed that the particle followed the mono-disperse distribution. In the present work, the method of computational particle fluid dynamic (CPFD) was applied for simulating the complex hydrodynamics in the CFB riser with various particle size distributions (PSDs). Two kinds of PSDs, namely Gaussian and Lognormal distribution with various PSD widths, were implemented into the CPFD scheme. With the CPFD method, the present work extensively studied the effects of PSD on the hydrodynamics and on the solids back-mixing. The CPFD results showed that the PSD significantly affected particle's flow behaviors at the lower zone of the riser, while the PSD effects were negligible in the upper part of the riser. This is meaningful for the industrial riser reactors since most of the reaction and transport process occur in this lower zone of the riser. Besides, the simulation results showed that wider PSD dramatically weaken the particle's back-mixing behaviors in the riser. The significant effects of PSD predicted by the CPFD method imply that large errors will be introduced if the mono-disperse assumption is adopted to simulate the experimental CFB riser handling particles with broad size distribution.

Modelling and experimental investigation of the full-loop gas–solid flow in a circulating fluidized bed with six cyclone separators

In the literature, there are few reports on the full-loop gas–solid flow in a circulating fluidized bed (CFB) with large scale and complex cyclone arrangement. In this paper, a new approach based on computational particle fluid dynamic (CPFD) method combined with electrical capacitance tomography (ECT) is used to investigate the hydrodynamic behavior of gas–solid flow in a CFB with six cyclone separators in order to improve the design and performance of a large scale CFB boiler. The full-loop CFB system for the simulation includes the CFB riser, cyclone, standpipe and U-loop. Two types of cyclone arrangement, i.e. axis and point based symmetric arrangement, are used for the CPFD simulation and ECT measurements. To validate the CPFD simulation, ECT is applied to measure the solids concentration in the standpipe with eight electrodes mounted on the outside of the standpipe. Key parameters including pressure, solids recirculation flux and velocity profile along different positions based on the CPFD simulation are analyzed and compared with experimental results. The CPFD simulation shows that the gas–solid flow is non-uniform among the six parallel cyclones. The solids concentration of four cyclones at the corner of the riser is higher than that of the others. The location of cyclone as well as the inlet angle of the cyclone needs to be optimized. The study shows that the presented approach based on CPFD simulation and ECT measurements can be used to optimize the arrangement of cyclone separators in a supercritical pressure circulating fluidized bed system.

Micron-particle transport, interactions and deposition in triple lung-airway bifurcations using a novel modeling approach

Particulate suspensions inhaled by humans are typically dilute and hence interactions between particles can be neglected. In such cases conventional Euler–Lagrange or Euler–Euler methods are suitable to simulate micron- or nano-particle transport and deposition in human respiratory systems. However, when challenging conditions, such as large pressure differentials, high velocity gradients and/or intense particle collisions, exist, alternative approaches for numerical analysis are required to capture fluid–particle, particle–particle, and particle–wall interactions. In the present study, the dense discrete phase model (DDPM) in conjunction with the discrete element method (DEM) have been employed to simulate micron–particle transport, interaction and deposition dynamics in different triple bifurcations (i.e., G3–G6, G6–G9, and G9–G12), using ANSYS Fluent 14.0 enhanced by user-defined functions (UDFs). In light of the relatively high computational cost when employing DDPM–DEM for such simulations throughout the human respiratory system, it may be necessary to combine different computational fluid–particle dynamics (CF–PD) models based on the local intensity of particle–particle interactions. Thus, the validity and necessity of the DDPM–DEM approach for different lung airway generations were numerically investigated, considering new parametric criteria for the use of most suitable numerical models. Specifically, the relative intensities of three major particle deposition mechanisms (i.e., inertial impaction, secondary-flow effect, and particle–particle-interaction impact) in idealized lung-airway segments were investigated. As a result, a new criterion for CF–PD model combination in terms of a relationship between inlet-particle stacking-volume fraction, ϕ, and percentage-of-fate changing particles, Δβp, is proposed. Visualizations of the fluid–particle dynamics in bifurcating airways have been provided as well. Results of this study pave the way for accurate and cost-effective CF–PD simulations of lung-aerosol dynamics, aiming at the improvement of respiratory dose estimation for health risk assessment in case of toxic particles and for treatment options in case of therapeutic particles.

Drug-targeting methodologies with applications: A review.

Targeted drug delivery to solid tumors is a very active research area, focusing mainly on improved drug formulation and associated best delivery methods/devices. Drug-targeting has the potential to greatly improve drug-delivery efficacy, reduce side effects, and lower the treatment costs. However, the vast majority of drug-targeting studies assume that the drug-particles are already at the target site or at least in its direct vicinity. In this review, drug-delivery methodologies, drug types and drug-delivery devices are discussed with examples in two major application areas: (1) inhaled drug-aerosol delivery into human lung-airways; and (2) intravascular drug-delivery for solid tumor targeting. The major problem addressed is how to deliver efficiently the drug-particles from the entry/infusion point to the target site. So far, most experimental results are based on animal studies. Concerning pulmonary drug delivery, the focus is on the pros and cons of three inhaler types, i.e., pressurized metered dose inhaler, dry powder inhaler and nebulizer, in addition to drug-aerosol formulations. Computational fluid-particle dynamics techniques and the underlying methodology for a smart inhaler system are discussed as well. Concerning intravascular drug-delivery for solid tumor targeting, passive and active targeting are reviewed as well as direct drug-targeting, using optimal delivery of radioactive microspheres to liver tumors as an example. The review concludes with suggestions for future work, considereing both pulmonary drug targeting and direct drug delivery to solid tumors in the vascular system.

Application of CPFD method in the simulation of a circulating fluidized bed with a loop seal, part I—Determination of modeling parameters

The Computational Particle Fluid Dynamics (CPFD) method was applied in the simulation of the gas solid flow in a circulating fluidized bed (CFB) with a loop seal in this work, and the modeling parameters were mainly investigated in this part. The influences of some crucial modeling parameters in CPFD simulation, such as mesh size, particle close pack volume fraction, interphase drag model and particle size distribution (PSD) on the flow behaviors in the whole loop of CFB were carefully analyzed with the help of experimental data conducted on the same CFB test rig. A medium mesh size was proved to be appropriate and used for the CFB modeling in this work. The particle close pack volume fraction largely affected the particle packed condition in the dense loop seal and standpipe. A value of 0.58 was proved to be suitable to describe the particle circulation and pressure distribution characteristics. Different gas–solid drag models showed certain influence on the particle circulation behaviors, and the combination of Wen-Yu and standard Ergun model was determined as the optimum choice. The consideration of the real PSD in CPFD scheme would better predict the overall flow characteristics for CFB system than the mono-size simplifications. The determination of modeling parameters in this work provides the base for simulating the gas–solid flow in the whole loop of CFB reactors by CPFD method, especially for the dense flow behavior in the standpipe and loop seal.

Application of CPFD method in the simulation of a circulating fluidized bed with a loop seal Part II—Investigation of solids circulation

The Computational Particle Fluid Dynamics (CPFD) numerical method was used to study the gas solid flow characteristics in a circulating fluidized bed (CFB) with a loop seal in this work. The influences of operating parameters, including loop seal aeration rate (Ql), fluidized air velocity in the riser (Ur) and total bed inventory (Mp) on the solid circulation characteristics were mainly investigated in this part. The solid circulating rate (Gs), total pressure drop in the riser (ΔPr), particle packed height (Hs), pressure gradient (ΔPs/Hs) and gas flow rate (Gg) in the standpipe, as well as the particle concentration and pressure distributions in the loop were carefully analyzed under different operating conditions. The simulation results were compared with the experimental results also reported in this work and good agreements were obtained. The results showed that the increase of Ql resulted in a decrease of Hs and increases of Gs, ΔPr and ΔPs/Hs. By increasing Ur, the increase of Gs was limited and the maximum value was approaching. An increase of Mp also had a positive effect on Gs while an elevated Hs was obtained as a result of mass and pressure balance. No gas bypass flow phenomenon occurred in the operating cases of this work from the simulation results. The pressure drop in the riser could be properly predicted while the gas–solid interactions in the standpipe and loop seal were slightly underestimated by the CPFD method, where a relatively lower ΔPs/Hs was obtained for CFB modeling.

Solids Back-mixing Behavior and Effect of the Mesoscale Structure in CFB Risers

Solids back-mixing significantly influences the performance of circulating fluidized bed (CFB) risers. A better understanding of the solids back-mixing behavior is greatly essential for the design and operation of CFB risers. Computational particle fluid dynamics (CPFD) modeling based on the multiphase particle-in-cell (MP-PIC) method was applied to investigate the solids back-mixing behavior and its influencing factors in CFB risers operating in dilute phase transport (DPT) and fast fluidization (FF) regimes. The present work observed extremely long residence time and wide residence time distribution (RTD) curves for particles in the FF regime, indicating an extensive back-mixing of particles in the FF regime. The overall and local back-mixing behaviors of solids were further analyzed. The particles in the DPT regime have a little back-mixing in the lower part of the riser, while the particles in the FF regime experience a large-scale back-mixing throughout the riser. In addition, the factors influencing...

Experimental and CPFD study of axial and radial liquid mixing in water-fluidized beds of two solids exhibiting layer inversion

Time-responses of concentration of a saline tracer were determined during fluidization by water at 15°C in a column of 191mm diameter. Mono-component beds contained 1.85mm glass beads (density 2500kg/m3) or 0.550mm ceramic spheres (density 3800kg/m3). A binary fluidized bed, containing equal volumes of these two solids, displayed layer-inversion at superficial liquid velocities of 40.5 and 43.5mm/s at 10 and 20°C, respectively. Saline tracer pulses were injected just above the distributor, at the column centre. The salt concentration at the bed surface was measured at three radial positions for superficial water velocities up to 127mm/s. Concentration profiles, simulated by computational particle-fluid dynamics (CPFD) with Eulerian–Lagrangian methodology, showed reasonable agreement with the experimental data. An axial/radial dispersion model was also applied for the mono-component and binary fluidized beds.. The axial dispersion coefficients for the glass beads exceed those for the ceramic spheres. Those for the binary bed are less than expected based on the mono-component data, though a maximum appears at the inversion velocity when closed–closed boundary conditions are employed.