Development of an integrated multichannel inlet for improved particle classification in hydrocyclones

The inlet design of hydrocyclones determines the flow field symmetry and facilitates the formation of vortices. In this study, an integrated multichannel inlet based on the Archimedes spiral is developed to improve particle classification by combining the advantages of existing designs. Hydrocyclones with conventional tangent or novel spiral inlets are comparably studied to evaluate the feasibility and superiority of this design using the validated volume of fraction model and two-fluid model. Numerical results show that this novel spiral inlet dramatically improves the flow field symmetry in terms of radial velocity and air core as well as reduces the short-circuit flow and circulation flow. In addition, it also provides strong diversion and pre-separation effects on particles. Consequently, this novel spiral inlet provides superior classification performance than the conventional design, appearing in smaller cut-size, higher cut sharpness, and higher capacity. Such advantages become less evident with increasing channel number, due to the increased turbulence intensity caused by the additional feed streams. The spiral inlet with two channels can largely resemble the design with two tangential inlets in all indices, which makes it the most suitable in this study. This design method can be easily extended to other types of hydrocyclones.     

Investigation on dynamic movement of cylindrical biomass particles in a fast fluidized bed

The processing of biomass particles is complex in a fluidized bed due to their heterogeneous characteristics. To further understand biomass particles, the dynamic movement of cylindrical biomass particles were investigated in a fast fluidized bed. Cylindrical particles were studied via the impulse momentum theorem and the Discrete Element Method (DEM). Meanwhile, the contact action, the drag force and other conventional forces were considered. To validate the present method, the predicted orientation and the minimum fluidization velocities of cylindrical particles were compared with the relative results, and validate the present approach. Then the characteristics of biomass particle flow dynamics were analyzed in terms of the particle concentration, the orientation distribution and the residence time distribution (RTD). It was found that most particles present as the horizontal or nearly horizontal states (0°) during the fluidization, and the percentage can reach and exceed 16%. High concentration occurs near the wall due to the back flow. The lower fluidization velocity corresponds to a wider RTD. The particles with the same size leave the riser with various residence times. The applied method and the obtained results provide helpful consults to study the cylindrical and other non-spherical biomass particles in an extensive way.     

Air-dictated bottom spray process: impact of fluid dynamics on granule growth and morphology

Background: Growing interest in the use of the less-explored bottom spray technique for fluidized bed granulation provided impetus for this study. Aim: The impact of fluid dynamics (air accelerator insert diameter; partition gap) and wetting (binder spray rate) on granule properties were investigated. Method: In this 3³ full factorial study, the results were fitted to a quadratic model using response surface methodology. The air velocity at the spray granulation zone for the investigated conditions was measured using a pitot tube. Results: Air accelerator insert diameter correlated to measured air velocity at the spray granulation zone and was found to not only dictate growth but also influence granule morphology. The partition gap was found to play important roles in regulating particle movement into the spray granulation zone and optimizing process yields, whereas binder spray rate significantly affected granule morphology but not granule size. Conclusions: Unlike conventional fluidized bed granulation, ease of modulation of fluid dynamics and insensitivity of the bottom spray process to wetting allow flexible control of granule size, shape, and flow. Its good drying ability also indicated potential use in granulating moisture-sensitive materials.     

Simulated pulsed flow of gas and particles in a horizontal oppose-pulsed gas jets of bubbling fluidized bed

Flow behavior of gas and particles with a horizontal oppose-pulsed gas jets are simulated by means of a three dimensional Computational Fluid Dynamics (CFD) model with the kinetic theory of granular flow in a gas-particles bubbling fluidized bed. The effects of amplitudes and frequencies on the hydrodynamics of gas and particles are analyzed. The simulation results are presented in terms of phase velocity vector plot, volume fraction of phases, granular temperature, power spectrum and Reynolds stresses in the bed. Results show that the impingement caused by the oppose-pulsed gas jets oscillates with the variation of pulsed gas velocity. The impingement zone with the high solid volume fraction reciprocates from the left side to the right side through the bed center with the variation of pulsed jet gas velocities. The lateral velocity and gas turbulent kinetic energy, granular temperature and Reynolds stresses of gas and particles are larger near the pulsed gas jets than that at the center of the bed. The large dispersion coefficients of particles using the horizontal oppose-pulsed gas jets enhance the mixing of particles in gas-solid fluidized bed.     

Jamming in granular shear flows of frictional,polydisperse cylindrical particles

In this work, frictional, cylindrical particle shear flows with different size distributions (monodisperse, binary, Gaussian, uniform) are simulated using the Discrete Element Method (DEM). The influences of particle size distribution and interparticle friction coefficient on the solid phase stresses, bulk friction coefficient, and jamming transition are investigated. In frictional dense flows, shear stresses rise rapidly with the increasing solid volume fraction when jamming occurs. The results suggest that at the jamming volume fraction, stress fluctuation and granular temperature achieve the maximum values, and the rate of the stress increase with increasing solid volume fraction approaches the peak value. Meanwhile, the degree of cylindrical particle alignment approaches a valley value. In the polydisperse flows, the jamming volume fraction exhibits significant dependences on the fraction of the longer particles and the particle size distribution. Two models considering the effect of particle size distribution are discussed for predicting the jamming volume fractions of polydisperse flows with frictional, cylindrical particles.     

Effects of bottom base shapes on burden profiles and burden size distributions in the upper part of a COREX shaft furnace based on DEM

The shaft furnace plays a very important role in the quantity and quality indexes of the COREX process. However, research on burden distribution in the COREX shaft furnace is still immature and in need of further development and improvement. For instance, only a single ring charging process rather than multiple rings or a burden matrix process has been simulated for the shaft furnace in practical operation. Therefore, a three dimensional model of the upper part of COREX shaft furnace is developed in the present study. The model simulates the charging process with multiple rings and is then used to investigate the effects of different bottom base shapes on the burden profiles and radial size distributions. Results show that the last rings (the inner rings) in the burden matrix needs be carefully chosen, especially for the middle mode. The bottom base shape affects the burden size distribution a great deal but the surface burden profile very little at a fixed burden matrix. A bottom base of M shape is strongly recommended to obtain a uniform burden size distribution. The burden matrix and bottom base shape need to be well matched in order to obtain desired gas distributions.     

水力旋流器流场研究与其应用

旋流器流场研究是认识其分离机理的基础。通过对单相与两相流场的研究，获得了分离机理的新认识。基于流场的湍动性对分离有重要影响，进一步的湍流场研究获得了湍流度与d50的关系式。应用流场研究成果，研制成了淀粉精制与磷酸废水处理用的两种旋流器。     

Numerical study on deposition of non-spherical shaped particles in cascade impactor

This study investigated the deposition of non-spherical particles in a cascade impactor using numerical simulations based on computational fluid dynamics and a discrete phase model (CFD-DPM). An optimum drag force model of non-spherical particles was used to calculate the dynamic behavior of the needle-shaped particles. The trajectory of these particles in an elbow pipe was computed and measured using a high-speed video camera. The computed trajectory agreed well with the experimental trajectory, and it was confirmed that the drag force model of non-spherical particles correctly expressed the drag force in the CFD-DPM numerical simulation. Next, the motion of the needle-shaped particles in a cascade impactor was numerically simulated and compared with that in the experimental results. The simulated classification efficiency agreed well with the experimental results. Additionally, the relationship between the aspect ratio of the needle-shaped particles and their behavior in the cascade impactor was numerically analyzed. The cut-off diameter decreased with the aspect ratio at a 50% classification efficiency in the cascade impactor. This was because the drag force of the particle was assumed to increase with the aspect ratio, and longer particles fell at a lower stage in the cascade impactor.     

Effects of helical fins on the performance of a cyclone separator: A numerical study

This study aimed to investigate the separation performance of a cyclone separator after reshaping its cylindrical body by installing the helical triangular fins. A numerical simulation based on Fluent was adopted to perform an orthogonal test to optimise the structure of the cyclone separator with helical triangular fins. Three structural parameters of the helical triangular fins were selected as optimisation variables: base width, fin size, and fin pitch, and their influences on the evaluation indices of the cut-off diameter were investigated. The optimal combination scheme was determined by range analysis, and the cyclone separator performances before and after optimisation were compared and analysed. The significant influence of the structural parameters on the cut-off diameter was in descending order as the fin pitch, fin size, and base width. For particles with diameter of 0.1, 0.5, 1, 2, and 3 μm, the separation efficiency of the cyclone separator with optimized helical triangular fins increased by 7.4 %, 15.9 %, 20.1 %, 10.9 % and 14.8 % respectively. Moreover, the cut-off diameter of the finned cyclone separator is reduced by 30.7 %, while the pressure drop is only increased by 6.6 %. The short circuit flow and back-mixing were alleviated, thereby considerably enhancing the stability of the flow field. Therefore, the finned cyclone separator was found to play a critical role in increasing the separation of fine particulate matter.     

CFD modeling of erosion wear in pipe bend for the flow of bottom ash suspension

In the present study, erosion wear behavior of slurry pipeline due to solid–liquid suspension in the pipeline has been investigated using commercial computational fluid dynamics (CFD) code FLUENT. A multiphase Euler–Lagrange model was adopted to predict the solid particle erosion wear in a 90° pipe bend for the flow of bottom ash–water suspension. A standard k–ε turbulence modeling scheme was used to simulate the flow through the pipeline. Water and bottom ash were taken as liquid and as a dispersed phase of solid–liquid mixture, respectively. A simulation study for erosion wear in a pipe bend was carried out to investigate the influence of various parameters including velocity, solid concentration, and particle size. The velocity of the bottom ash–water suspension varied from 0.5 to 2.5?m/s for solid concentrations with a range of 2.5 to 10.0% (by volume). The particle diameters of the bottom ash were 162 and 300?µm. The simulation results agree with the results of previous studies.     

In depth characterisation of hydrocyclones: Ascertaining the effect of geometry and operating conditions on their performance

Hydrocyclones are used for densification of waste streams prior to drying or for classification of solid and liquids in two-phase streams. They are becoming popular in industrial units due to their simplicity, low energy consumption and high versatility. However, the effect of geometry and operating conditions on the cut diameter and solid recovery efficiency have been independently studied, and therefore there are no studies approaching the influence of all the parameters simultaneously. Thus, a detailed experimental study was conducted to ascertain the effect of the hydrocyclone body (diameter and angle) and the vortex finder and spigot size and shape, as well as operating conditions (inlet pressure and solid concentration) on the separation efficiency curve, cut diameter, solid and volume recovery and the main features of the outlet streams. It has been proven that separation efficiency and outlet stream composition are sensitive to both the geometry of the hydrocyclone and the operating parameters. Therefore, knowledge of their influence is essential for the design of industrial units where liquid reutilisation is a major concern.     

Numerical modeling of the effects of oil on annular laminar film condensation in minichannels

This paper presents a numerical model to predict laminar film condensation heat transfer in small channels of different internal geometries for miscible refrigerant-oil mixtures. The model includes the contributions of surface tension, axial shear stresses induced by the vapor to film interface, gravitational forces, wall conduction and the oil concentration dependency on the liquid's dynamic viscosity. For the same operative conditions and fluid, the presence of the oil has a significant negative impact on the thermal performance at high vapor qualities, with the degradation depending on the channel's shape. Presently, the performance of different channel shapes (circular and flattened shapes) are simulated and compared. It is concluded that the presence of oil has slightly less effect on capillary-dominated regimes (i.e. when the surface tension has a strong effect on the film dynamics) than on gravity-dominated regimes (i.e. annular stratified regime).     

Effect of particle size distribution on performance and particle kinetics in a centrifugal slurry pump handling multi-size particulate slurry

A significant variation in particle size distribution (PSD) is generally encountered in slurry transportation. The goal of this work is to establish the effect of variation in PSD on the centrifugal slurry pump (CSP) performance and particle kinetics. Computational fluid dynamics (CFD) modeling of a CSP with multi-size particulate slurry has been performed with a sliding mesh approach using the granular Eulerian-Eulerian model. The numerical model is validated with the experimental data of the pump performance for multi-size particulate fly ash slurry. The maximum deviations in the predicted head and efficiency compared to the measured values are of the order of ±2% and ±3.5%, respectively. Simulations with a single representative particle size for multi-size particulate slurry using median and weighted mean diameter approach are also carried out to understand the difference in performance prediction with equi-size and multi-size slurry. The predicted trend of pump performance variation with PSD is linear and non-linear with equi-size and multi-size slurries, respectively. The median and weighted mean approaches showed error in capturing the effect of variation in PSD on pump performance. The variation in PSD significantly affects the flow of particles inside the impeller and casing flow passages due to particle kinetics. Reduction in the intensity of granular pressure, maximum granular viscosity, and the head loss due to friction in impeller and casing flow passages are found with the increase in the fine size particles.     

Numerical simulation of a centrifugal slurry pump handling solid-liquid mixture: Effect of solids on flow field and performance

The effect of solids on a centrifugal slurry pump performance is a major concern to the design of slurry transportation system. In the present study, the multiphase modeling of centrifugal slurry pump is performed using two models, Mixture and Eulerian-Eulerian multiphase. Sliding mesh approach is employed for unsteady simulation of the pump. The accuracy of the simulations is ascertained by comparing the performance characteristics of the pump obtained numerically and experimentally. Experimental results are obtained by measurements in a pilot plant test rig with three different mean size sand particulate slurries. The Eulerian-Eulerian multiphase model predicted the effect of the solids on pump performance close to the experimental results as compared to Mixture model. The obtained accuracy with Eulerian-Eulerian model for predicting the effect of solids on head and efficiency is around ±2% and ±3%, respectively. The predicted results using Eulerian-Eulerian model confirm that the head and efficiency of the pump decrease with the increase in particle size and concentration. The particles of high specific gravity show less reduction in head and efficiency of the pump. Further, the effect of variation in particle size and concentration on the flow field in the impeller and casing has also been analyzed at best efficiency point operation. Non-homogeneous suspension of particles inside the blade channels and casing passages is examined. The particulate concentration is observed higher near the impeller back shroud, pressure side of the blades, and non-suction side of the casing as compared to other locations.     

Numerical investigation of gas species and energy separation in the Ranque-Hilsch vortex tube using real gas model

A three dimensional Computational Fluid Dynamics (CFD) model is used to investigate the phenomena of energy and species separation in a vortex tube (VT) with compressed air at normal atmospheric temperature and cryogenic temperature as the working fluid. In this work the NIST real gas model is used for the first time to accurately compute the thermodynamic and transport properties of air inside the VT. CFD simulations are carried out using the perfect gas law as well. The computed performance curves (hot and cold outlet temperatures versus hot outlet mass fraction) at normal atmospheric temperature obtained with both the real gas model and the perfect gas law are compared with the experimental results. The separation of air into its main components, i.e. oxygen and nitrogen is observed, although the separation effect is very small. The magnitudes of both the energy separation and the species separation at cryogenic temperature were found to be smaller than those at normal atmospheric temperature.     

Effect of model details on the predicted saturation profiles in condensation particle counters

We establish an analytical model, as well as perform Computational Fluid Dynamics (CFD) simulations to investigate the evaporation-condensation process in Condensation Particle Counters (CPCs). Via a systematic analysis of the effect of geometrical and operational parameters on the saturation profile in CPCs, we unveil that the insulation in these measurement devices is of key significance for the detection efficiency. Furthermore, we demonstrate that our analytical model can reliably predict the mass fraction of working fluid in the gas phase at the outlet of the saturator. This is of key importance for the efficient screening of working fluids, as well as the optimization of both geometry and operation parameters of CPCs.     

Comparative analysis of numerically derived drag models for development of bed expansion ratio correlation in a bubbling fluidized bed

For designing and operating fluidized bed reactors, bed expansion ratio is one of the most important parameters. In this research, a bubbling fluidized bed is simulated using three-dimensional Eulerian-Eulerian method that is incorporated with kinetic theory of granular flow (KTGF) to calculate the pressure drop, gas volume fraction (GVF) and bed expansion ratio. Grid optimization is firstly conducted to achieve suitable solution for further simulations. Subsequently, different numerically derived drag models are employed to investigate the effect of these models on gas-solid flow dynamics. Afterwards, the fluidized bed is simulated at different gas superficial velocities employing two different drag models respectively. Simulation results have been comprehensively validated against experimental data. Finally, an expression for bed expansion ratio has been formulated and compared with the empirical correlation. The proposed correlation holds reasonably well with various experimental values. This work provides a scalable way to aid in designing and operating process reactors.     

A modified numerical method for the accurate calculation of molecular flow transmission probabilities and density distributions of cylindrical tubes

Accurate numerical calculations of molecular flow transmission probabilities and density distributions of tubes are important to the benchmark problems of Monte Carlo solutions, dynamic expansion vacuum gauge calibration systems, and molecular beam formation studies. Although Nawyn and Meyer [published by van Essen and Heerens. J Vac Sci Technol 1976; 13:1183] have solved cylindrical tube problems by using the numerical method based on Clausing's equations, perhaps the calculated results still lack sufficient accuracy. In this paper, we propose a modified method that could achieve calculation accuracies of transmission probabilities as high as 10⁻¹¹–10⁻¹³ in the range of L′/R ≤ 100 (where L′ is the tube length, R is the tube radius), which are more accurate than the results recently reported by Mohan et al. [J Vac Sci Technol A 2007; 25:758] and Gómez-Goñi et al. [J Vac Sci Technol A 2003; 21:1452].     

Flow dynamics and wall shear-stress variation in a fusiform aneurysm

Pulsatile flow through a tube featuring a sinusoidal bulge is computed in order to determine the flow dynamics and wall shear-stress conditions encountered under conditions representative of blood flow through a human abdominal aortic aneurysm. A high-order spectral-element algorithm is employed to accurately determine velocity and vorticity fields, plus wall shear stresses, which are notoriously difficult to measure experimentally. A greater level of detail in the flow is revealed when compared to recent particle image velocimetry experiments. For both the mean and standard deviation of wall shear stress, minimum levels are found at the widest point of the aneurysm bulge, and maximum levels are recorded in the distal (downstream) region of the bulge. In an aneurysm with length and maximum diameter 2.9 and 1.9 times the artery diameter, respectively, peak instantaneous wall shear stress is 2.4 times greater than the peak wall shear stress recorded in a healthy vessel.     

Simulation and experimental study on the stone powder separator of a vertical shaft impact crusher

During the sand-making process, the stone powder produced by means of a vertical shaft impact (VSI) crusher affects the sand quality and pollutes the environment. This paper focuses on experiments and simulations with a stone powder separator (SPS) that is installed in a VSI crusher. Using FLUENT software, a coupling model of computational fluid dynamics (CFD) and the discrete phase model (DPM) is used to simulate the airflow distribution and particle traces in a VSI crusher. The stone powder separation and large particle retention performance are evaluated considering two important factors: the structure of the SPS and the air volume of the induced draft fan. The simulation results show that the air volume of the induced draft fan intuitively influences the particle traces and the distributions of particles of different sizes in the crushing chamber and the SPS chamber. There are many vortexes in the crushing chamber that cause the aggregate particles to be fully dispersed under the action of turbulence, and the SPS structure with radius decreasing from bottom to top can form an airflow velocity gradient in the SPS chamber and selectively remove particles according to size, thus improving the stone powder separation performance (SPSP). For this structure, when the air volume of the induced draft fan is set to approximately 40% of the maximum value, it can not only avoid large particles being massively removed but also ensure better SPSP of the device. Finally, the simulation results are verified by experiments. The results of this paper provide a reliable numerical model for the calculation of the flow field in a VSI crusher and provide a reference for the structural optimization of the stone powder separation device and for the selection of the best air volume of the induced draft fan.