Radial Flow Structure Research Articles

Morphodynamics of sine-generated meandering streams under variations of the width-to-depth ratio: A numerical investigation

Meandering is one of the most common planforms of rivers, however, the ecological and fluvial integrity of meandering rivers are under challenges, resulting from increased anthropogenic pressure and extreme climate events. The present study elucidates the morphodynamics of meandering streams under variations of width-to-depth ratio (B/h) by delving into the two dynamic processes underlying within the flow, i.e. cross-circulatory motion and convective redistribution of the longitudinal flow velocity. Series of numerical simulations are performed in 70° sine-generated meandering streams under increasing flow depth that necessitates a wide range of B/h, characterizing flume experiments to large scale natural rivers. The radial flow structures and the early bed deformation patterns are computed and elucidated. Among the classical cross-circulatory flow patterns, outer bank cell occurs for flows having B/h < 9. Next, the different bed deformation patterns reveal the shift of significance in the driving forces of the bed deformation regarding different B/h ratio. At B/h = 100, bed deformation is dominated by the convective redistribution of the downstream velocity that causes longitudinal adjacent erosion–deposition zones. This implies downstream migration tendency of meander loops. At B/h = 12, the intensified cross-circulatory flow deflects the sediment trajectory, causing extra laterally adjacent erosion–deposition zones that necessitate the lateral expansion tendency of the loop. Decrement of B/h ratio (from 100 on ward) critically enhances the non-equilibrium transport of sediment that can lead to active fluvial evolution of meandering rivers. Overall, the results can be used to improve river management and restoration.

Generalizing multi-branching radial symmetric flow structures

This study revisits the design of tree-shaped flow networks in disc-shaped bodies, focusing on thermal management applications. The key objectives are to achieve equal point distribution along the disk periphery and minimize flow resistance at each bifurcation point. The literature highlights the challenge of balancing fluid and thermal performance simultaneously. To address this, the authors propose a multi-branching approach and hydraulic diameter setting, allowing flexibility in network growth.The optimization process involves minimizing flow resistance as the objective function, employing a constrained minimization technique with an interior point algorithm. By solving this optimization problem, the geometric features of the network are determined. Notably, the research uncovers a memory effect in the geometry and overcomes computational time limitations. The Svelteness, defined using the concept of configuration area, aids in distinguishing between different design possibilities and identifying the evolutionary direction.Increasing the complexity of the network while minimizing the total branch length leads to the discovery of the optimal architecture configuration. These findings provide valuable insights for optimizing symmetric tree designs, improving thermal and hydraulic performance.

Characterizing Internal Flow Field in Binary Solution Droplet Combustion with Micro-Particle Image Velocimetry

Droplet internal flow participates in liquid-phase mass transfer during multicomponent solution droplet combustion. In this work, internal flow fields in the binary droplet combustion of two polyoxymethylene dimethyl ethers (CH3O(CH2O)nCH3, n ≥ 1, abbreviated as PODEn), i.e., PODE2 and PODE4, are characterized using micro-particle image velocimetry (Micro-PIV). The buoyancy-driven upward vapor flow around the droplet is found to initiate two opposite radial flows in the droplet, which form two vortex cores near the surface, while the gravitational effect and Marangoni effect resulting from the content and temperature gradients in the binary droplets can induce disturbance to the two flows. The binary droplets have comparable spatially averaged flow velocities at the stable evaporation stage to those of pure droplets, which are around 3 mm/s. The velocity curves are more fluctuant and tend to slightly increase and reach the peak values at around 250 ms, and then decrease until droplet atomization. The flow velocities in the droplet interior are generally higher than those near the droplet surface, forming a parabolic velocity profile along the horizontal radial direction. The peak velocity first increases to 5–9 mm/s as the radial flow and vortex structure start to form and then decreases to around 3 mm/s until droplet atomization. The radial flow with a spatially averaged velocity of 3 mm/s can run around one lap during the stable evaporation stage, which implies that the convection-induced mass transfer is relatively weak, and consequently, the content gradient of the binary droplet is still mainly controlled by mass diffusion.

Mesoscale model of radial hydrodynamics for fluidizing small particles with low solid holdup in gas-liquid-solid circulating fluidized bed

Gas-liquid-solid circulating fluidized bed (GLSCFB) is an important reactor. Radial hydrodynamics of GLSCFB are critical for the industrial design and scale-up, but the hydrodynamics have not been well described. To mechanically model the radial flow structures, a mesoscale method based on the energy-minimization multiscale principle is applied. A comparison between experimental data and model calculations shows that the radial mesoscale model can reasonably predict the radial distributions of the flow parameters and analyze the influence of the operating conditions on the radial hydrodynamics in GLSCFB for fluidizing solid particles of small diameters (dp ≤ 1 mm) and low solid holdup (εs ≤ 15%). Additionally, model predictions show that the peak positions of the suspension and transportation energy consumed per unit mass of solid particles and liquid shear stress range from 0.75 to 0.9, which is an obvious indicator of the radial flow structure.

Experimental and numerical studies on a bubble-induced inverse gas-liquid-solids fluidized bed

Hydrodynamics in a newly invented bubble-induced inverse gas–liquid-solids three-phase fluidized bed has been studied via both experimental and numerical methods. With experiments in a 3.0 m column of 0.153 m in diameter, four fluidization regimes including a fixed bed regime, a bed expansion regime, a complete fluidization regime, and a freeboard regime have been identified with the increase in the superficial gas velocity. A three-phase Eulerian-Eulerian CFD model was developed to simulate the hydrodynamics in the inverse three-phase fluidized bed and the simulation results have a good agreement with the experimental data. The effects of the particle property and solids loading on the transitions across the flow regimes were numerically studied. A higher solids loading and/or a larger particle density are reported to contribute to an easier fluidization and a faster flow development to the complete fluidization regime. The radial flow structure becomes less uniform with increased inner circulation of the liquid after introducing more bubbles into the column.

Computational study of particle distribution development in a cold-flow laboratory scale downer reactor

The use of downer reactors (gas-solid co-current downward flow) in the Fluid Catalytic cracking (FCC) process for the upgrading of heavy crude oil into more valuable products has gradually become more common in the last decades. This kind of reactor is characterized by having homogeneous axial and radial flow structures, no back mixing, and shorter residence times as compared with the riser reactor type. Although downer reactors were introduced a long time ago, available information in literature about the multiphase hydrodynamic behavior at FCC industrial scale is scarce. Therefore, it is necessary to conduct experimental and computational studies to enhance the understanding of the hydrodynamics of two-phase co-current downward flow. The Computational Fluids Dynamics (CFD) software, Ansys Fluent, is used to study two-dimensional gas (air) and solid (catalyst particle) flow in a downer section of a cold-flow circulation fluidized bed (CFB) system at a laboratory scale. The implemented computational model is validated by comparing numerical results for solid velocity and volume fraction with measurements carried out on a CFB system using a fiber-optic probe laser velocimeter. According to numerical results obtained for different gas velocity and solid flux, flow development cannot only be estimated by considering solid axial velocity changes along the reactor; it is also necessary to take into account solid volume fraction axial variations as radial profiles can change even when velocity profiles are developed.

Parametric study of particles homogenization in cold-flow riser reactors

Fluid Catalytic cracking (FCC) process are frequently used for the upgrading of heavy crude oil into more valuable products. Traditionally, the FCC process occurs in a riser reactor, where both vaporised hydrocarbon feed-stock and solid catalyst flow in the direction against gravity. This kind of reactor presents heterogeneous flow distributions that affect the axial and radial flow structures, as well as the product quality. To understand the effect of operational variables over riser flow distribution, a numerical study of the two-phase cold-flow in the pre-acceleration zone of a riser reactor fed with catalyst particles used in this kind of reactors is presented in this work considering a two-dimensional model. The Computational Fluids Dynamics (CFD) software ANSYS® Fluent® is used to study two-dimensional gas (air) and solid (catalyst particle) flow in a riser section of a cold-flow Circulation Fluidized Bed (CFB) system under different flow inlet conditions (i.e., either for the gas and solid flux), as well the particle diameter size. An Eulerian-Eulerian approach, including the turbulence ( κ − ε model) and the Kinetic Theory of Granular Flow (KTGF) models, are solved for the gas phase and the solid particles treated as a dispersed phase. The implemented computational model is validated by comparing numerical results for solid velocity and volume fraction distribution to values reported by peers. As expected, a higher concentration towards the axis of the reactor is obtained for the solid volume fraction values. The R N I index estimated for gas velocities of 3 m/s and 4 m/s are significantly lower than the others, indicating that as the inlet gas velocity is reduced, a more homogeneous profile is obtained. Finally, the numerical results obtained shown closer to the experimental data used for the validation as the steady-state is attained.

Numerical study on the hydrodynamics in high-density gas-solid circulating fluidized bed downer reactors

The hydrodynamics in a gas-solid circulating fluidized bed (CFB) downer reactor under high-density operating conditions is studied numerically using the Eulerian-Eulerian two-fluid model and compared with the low-density downers. Numerical results of both the axial and radial gas-solid flow structures have good agreements with the experimental data. The first and second acceleration regions, and a fully developed region along the CFB downer have been clearly shown. A correlation on the overall bed density is developed based on the numerical results as well as the experimental data, leading to 2D and 3D maps of the overall bed density profiles under different operating conditions. With the help of the two maps, the transition from a low-density operating condition to a high-density operating condition in the CFB downer is discussed. The scale-up effects are also numerically studied.

Numerical study on liquid-solid flow characteristics in inverse circulating fluidized beds

The flow behavior of particles in inverse liquid-solid circulating fluidized beds was simulated using the Eulerian-Eulerian approach incorporating the kinetic theory of granular flow. The Syamlal-O’Brien drag model was used to account for the interphase interaction between the liquid and solids phases. A detailed description of the model equations used in this work has been presented. Predicted results were compared with the experimental data obtained by other group members. A good agreement between the numerical simulations and experimental results has been achieved. The inverse flow characteristics of particles were studied using a single type of particles and multiple types of particles with different densities under different liquid velocities and solids flow rates. The results indicated that for the particles with different densities, 950 kg/m3, 850 kg/m3 and 640 kg/m3, the cross-sectional average solids holdup along the axis was uniform and the radial flow structures at different bed heights were identical. Particles moved from the walls to the center at the top region of the bed. Such tendency diminished as the particles move downwards. The axial velocities of solid particles were higher in the center and lower near the walls. For the particles with a density of 28 kg/m3, due to the randomly generated vortexes of particles, the solids holdup radial distributions and the velocities of particles were more irregular. For the mixed particles with two different densities of 850 kg/m3 and 950 kg/m3, the trend of solids holdup radial distribution, and the trend of the lateral and axial velocities of each particle group were similar to those in the simulation using the single-density particles.

A Multigene Genetic Programming approach for modeling effect of particle size in a liquid–solid circulating fluidized bed reactor

This communication presents the application of Multigene Genetic Programming, a new soft computing technique to investigate the effects of particle size on hydrodynamics behavior of a liquid–solid circulating fluidized bed (LSCFB) riser. The Multigene Genetic Programming based model is developed/trained based on experimental data collected from a pilot scale LSCFB reactor using two different size glass beads (500 & 1200μm) as solid phase and water as liquid phase. The trained Genetic Programming model successfully predicted experimental phase holdups of the LSCFB riser under different operating parameters. It is observed that the model predicted cross-sectional average of solids holdups in the axial directions and radial flow structure are well agreement with the experimental values. The statistical performance indicators including the mean absolute error (∼5.89%) and the correlation coefficient (∼0.982) also show favorable indications of the suitability of Genetic Programming modeling approach in predicting the solids holdup of the LSCFB system.

A comparison of flow development in high density gas‐solids circulating fluidized bed downer and riser reactors

Comparison of flow development in high density downer and riser reactors is experimentally investigated using fluid catalytic cracking particles with very high solids circulation rate up to 700 kg/m2s for the first time. Results show that both axial and radial flow structures are more uniform in downers compared to riser reactors even at very high density conditions, although the solids distribution becomes less uniform in the high density downer. Solids acceleration is much faster in the downer compared to the riser reactor indicating a shorter length of flow development and residence time, which is beneficial to the chemical reactions requiring short contact time and high product selectivity. Slip velocity in risers and downers is also first compared at high density conditions. The slip velocity in the downer is much smaller than in the riser for the same solids holdup indicating less particle aggregation and better gas‐solids contacting in the downer reactors. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1172–1183, 2015

Experimental studies of flow through radial channels using PIV technique

In the present work a confined, submerged, laminar water jet is issued axially from a short, circular nozzle and fed radially outward through the clearance of two parallel circular discs. The disc assembly was fully submerged into the water-filled tank. Water has been used as the experimental fluid and flow diagnosis has been performed using 2D particle image velocimetry (PIV) technique. Effect of inlet flow rates, disc radius and clearance between the two parallel discs have been investigated to understand the characteristics of submerged radial flow. It is observed that the flow field consists of several interesting features like toroidal recirculation, annular separation bubble around the inlet corner and flow reattachment, which are strongly influenced by the clearance between the two parallel discs and the flow rate. The present observations reveal distinct flow structure between two parallel discs and provide insights for understanding of radial flow structure. The numerical results simulated from PIV experiments are also included in the form of vector plots and from it a close consistency between the two results is clearly visible.

Application of Support Vector Machine Modeling on Phase Distribution in the Riser of an LSCFB Reactor

Abstract Support vector machine (SVM) modeling approach is applied to predict the solids holdups distribution of a liquid–solid circulating fluidized bed (LSCFB) riser. The SVM model is developed/trained using experimental data collected from a pilot-scale LSCFB reactor. Two different size glass bead particles (500 μm (GB-500) and 1,290 μm (GB-1290)) are used as solid phase, and water is used as liquid phase. The trained model successfully predicted the experimental solids holdups of the LSCFB riser under different operating parameters. It is observed that the model predicted cross-sectional average of solids holdups in the axial directions and radial flow structure are well agreement with the experimental values. The goodness of the model prediction is verified by using different statistical performance indicators. For the both sizes of particles, the mean absolute error is found to be less than 5%. The correlation coefficients (0.998 for GB-500 and 0.994 for GB-1290) also show favorable indications of the suitability of SVM approach in predicting the solids holdup of the LSCFB system.

Study of Phase Distribution of a Liquid-Solid Circulating Fluidized Bed Reactor Using Abductive Network Modeling Approach

Abstract This communication deals with the Abductive Network modeling approach to investigate the phase holdup distributions of a liquid–solid circulating fluidized bed (LSCFB) system. The Abductive Network model is developed/trained using experimental data collected from a pilot scale LSCFB reactor involving 500-μm size glass beads and water as solid and liquid phases, respectively. The trained Abductive Network model successfully predicted experimental phase holdups of the LSCFB riser under different operating parameters. It is observed that the model predicted cross-sectional average of solids holdups in the axial directions and radial flow structure are well agreement with the experimental values. The statistical performance indicators including the mean absolute error (~4.67%) and the correlation coefficient (0.992) also show favorable indications of the suitability of Abductive Network modeling approach in predicting the solids holdup of the LSCFB system.

Gas–solid flow behavior and contact efficiency in a circulating-turbulent fluidized bed

Abstract Circulating-turbulent fluidized bed (C-TFB) was characterized by high solid holdup, homogenous axial and radial flow structure, no net downflow of solids and high contact efficiency in this study. These flow dynamic properties were mainly represented by solid holdup profiles, the identification of flow regime and cluster dynamics in a riser, 100–150 mm in diameter and 10.06 m in height. To quantify the effect of novel diameter-expanding structure on flow dynamics, a parameter (contact efficiency) was introduced firstly. The effects of gas–solid interaction on flow performance were investigated by a fiber-optic probe to detect solid holdup. CO2 tracer injection and sampling system were used to characterize the flow structure and define the contact efficiency. The experimental results showed that flow regime in C-TFB is belonging to dense riser upflow (DRU) or dense-suspension upflow (DSU) regimes and transient region disappears in axial position. Flow structure is different from previous studies about traditional circulation fluidized bed (CFB) due to the effect of expanding structure and ring-feeder internal. A flow model based on the profiles of solid holdup and CO2 tracer concentration was proposed to account for the gas–solid contact efficiency in the reactor. The contact efficiency in C-TFB is much higher than that of high-density circulating fluidized bed (HDCFB), which means C-TFB reactor would exhibit better performance to optimize the product distribution under the same operating conditions.

Investigation of artificial neural network methodology for modeling of a liquid–solid circulating fluidized bed riser

Abstract An artificial neural network (ANN) approach is investigated to model and study the phase holdup distributions of a liquid–solid circulating fluidized bed (LSCFB) system. The ANN model is developed based on different operating parameters of the LSCFB including primary and auxiliary liquid velocities, and superficial solids velocity. The competency of the model is examined by comparing the model predicted and the experimental phase holdup of the LSCFB riser reactor. It is also found that the ANN model successfully predicted the radial non-uniformity of phase holdup that is observed in the experimental runs of the riser. When compared, the model predicted output and trend of radial flow structure for solids holdup are in well agreement with the experiments. The mean absolute percentage error is around 6% and the correlation coefficient value of the predicted output and the experimental data is 0.992.

Nonlinear quasisteady state benchmark of global gyrokinetic codes

Two global gyrokinetic codes are benchmarked against each other by comparing simulation results in the case of ion temperature gradient driven turbulence, in the adiabatic electron response limit. The two codes are the Eulerian code GENE and the Lagrangian particle-in-cell code ORB5 which solve the gyrokinetic equations. Linear results are presented, including growth rates, real frequencies, and mode structure comparisons. Nonlinear simulations without sources are carried out with particular attention to considering the same initial conditions, showing identical linear phase and first nonlinear burst. Very good agreement is also achieved between simulations obtained using a Krook-type heat source, which enables to reach a quasisteady state and thus to compare the heat diffusivity traces over a statistically meaningful time interval. For these nonlinear results, the radial zonal flow structure and shearing rate profile are also discussed. The very detailed comparisons presented may serve as reference for benchmarking other gyrokinetic simulation codes, in particular those which consider global geometry.

Hydrodynamic simulation of gas–solids downflow reactors

In this work, we investigate the radial flow structure in gas–solids downer using Euler–Euler computational fluid dynamics (CFD) models. Solids are modeled as pseudo-fluid using kinetic theory of granular flow. In addition to the mass and momentum conservation equations, transport equation for fluctuating kinetic energy of the solids (modeled as granular temperature) is solved. The main focus of this work is the systematic investigation of the most suitable closures for the various force interactions in the system of interest. Results are presented for mean solids velocity, volume fraction, granular temperature and slip velocities for various closure forms. Sensitivity of the predicted results to the choice of closure forms is presented. Finally we emphasize the idea of matching slip velocities and the trends thereof with solids fraction as the key to developing a robust CFD model which has predictive capability over a wide variety of flow conditions.

Hydrodynamics and scale-up of liquid–solid circulating fluidized beds: Similitude method vs. CFD

Hydrodynamics and scale-up of liquid–solid circulating fluidized beds (LSCFBs) are investigated using similitude method and computational fluid dynamics (CFD) technique. Similitude method is applied to establish the dynamic similarity among LSCFBs by tuning physical properties of liquids and solids, operating conditions and bed dimensions to match several scaling sets of dimensionless groups. The hydrodynamic behaviors in these constructed LSCFBs are simulated by a validated CFD model [Cheng, Y., Zhu, J., 2005. CFD modeling and simulation of hydrodynamics in liquid–solid circulating fluidized beds. Canadian Journal of Chemical Engineering 83, 177–185] and compared in terms of the axial and radial flow structures characterized by the solids fraction, particle and liquid velocities and solids mass flux. The results demonstrate that only the full set scaling parameters obtained from similitude method, i.e., five dimensionless groups together with fixed bed geometry, particle sphericity, particle size distribution as well as particle collision properties, can ensure the similarity of hydrodynamics in the fully developed region of different LSCFBs. Developing flow structures in LSCFBs are strongly influenced by some parameters such as turbulent kinetic energy at the inlet so that the proposed similitude method may not always be applicable.

Comparative Study of Flow Structure in Circulating Fluidized Bed Risers with FCC and Sand Particles

AbstractThe combined influences of particle properties and nozzle gas distributor design on the axial and radial flow structure in two 100 mm i.d., 15.1 m and 10.5 m long risers with FCC and sand particles were investigated by measuring the axial pressure gradient profiles, and the axial and radial profiles of solids concentration. The results show that the nozzle gas distributor design has significant effects on the axial and radial flow structure for the FCC and sand particles. At lower superficial gas velocity of less than 8.0 m/s, the upward gas‐solid flow of the sand particles decelerates in various degrees with the disappearing of the nozzle gas distributor effect. The upward gas‐solid flow of the FCC particles, however, occurs without noticeable deceleration within the range of this study. In the acceleration section, the radial distributions of the local solids concentration of the FCC particles are more uniform than those of sand particles under the same operating conditions; while in the fully developed zone, the sand particles have a more uniform radial distribution than the FCC particles. The gas‐solid flow is first developed in the center region, and then extends towards the wall. The overall flow development in the riser mainly depends on the local gas‐solid flow in the wall region.