Turbulent Bed Research Articles

Impact of emergent vegetation on three-dimensional turbulent flow properties and bed morphology in a partially vegetated channel

Impact of emergent vegetation on three-dimensional turbulent flow properties and bed morphology in a partially vegetated channel

Bed expansion in turbulent bed contactor: Experiments and prediction

In this work, turbulent bed contractor (TBC) hydrodynamics have been studied in terms of bed expansion (Hd/Hst) using a particular approach to predict this important property for the design of such equipment. The study is based on 1604 sets of experimental data on the bed expansion, obtained by varying the operating variables (gas velocity, liquid spray, packing characteristics, static bed height, and free opening of the supporting grid). The prediction of the bed expansion necessitates the estimation of gas and liquid holdups. To achieve this, we employed a variety of correlations derived from existing literature, comprising six equations for gas holdup and twenty equations for liquid holdup estimation. Out of a total of 120 cases, bed expansion was estimated, and the accuracy of the model was evaluated by calculating the mean absolute error in percentage (MAPE), root mean square error (RMSE), correlation coefficient (?XY), and explained variance (VECV). This study identified suitable correlations for gas and liquid holdups, leading to predictions with acceptable errors. Furthermore, statistical analysis was employed in a subsequent phase of the study to determine the most appropriate correlations for predicting bed expansion among those proposed by various authors.

Study on circulating characteristics of circulating fluidized bed

Based on the Euler-Euler two-fluid model, the circulating fluidized bed was simulated using the computational fluid dynamics (CFD) method. The physical model of the circulating turbulent fluidized bed had an inner diameter of 76mm and a height of 2000mm. The circulating material consisted of class B particles, with an initial bed material mass (Mp) of 0.188kg. Air was uniformly introduced at a velocity of 0.3m/s at the bottom of the riser, and secondary air was introduced at a velocity of 0.9m/s at the secondary air inlet. Numerical simulations were conducted to study the gas-solid flow characteristics of the entire circuit. The turbulent bed, fast bed particle concentration, material circulation rate, and pressure drop gradient were analyzed to evaluate the performance of the entire circuit. The results showed that the particle concentration at the bottom of the riser was higher in the overall circuit flow, and a turbulent state could be achieved at low gas velocities. The particle concentration in the transport bed was low, indicating good gas-solid interaction. Strong backmixing occurred at the connection between the return valve and the turbulent bed. Additionally, there was intense gas-solid reaction at the secondary air inlet.

Study on Condensed Matter Simulated by Multiphase Fluid-Solid Coupling Fluid Mechanics

The hydrodynamic characteristics of a three-phase gas-liquid-solid turbulent bed with light particle flow and intermittent liquid feeding were numerically simulated by CFD program Fluent6.3. The axial and radial motion of particles in a three-phase gas-liquid-solid turbulent bed and the variation of particle concentration in the bed are studied. The results show that the size and frequency of cavitation increase with the increase of the distance from the axis of the air distribution disk and the distance from the bed surface to the central region. The void varied from narrow to wide, while the void changed in the opposite direction. The bubble lift rate and bubble size are basically the same in axial and radial direction. With the increase of the surface gas velocity and the viscosity of the liquid phase, the average particle content in the bed decreases.

A corrected filtered drag model considering the effect of the wall boundary in gas-particle fluidized bed

The databases used to construct traditional mesoscale drag models is usually obtained from fine-grid simulation results in the full-periodic domains, ignoring the function of the wall boundary. In actual fluidized bed reactors, the presence of wall boundary can significantly affect the heterogeneous structures in near-wall areas, thereby affecting the drag force. Therefore, this work aims to further enhance the performance of the prediction results of existing filtered drag model by introducing the influence of the wall boundary. Based on the mesoscale drag force model published in previous literature, a correction coefficient considering the influence of the distance from the grid to the wall boundary is proposed. The applicability of the new corrected model is further enhanced by introducing the influence of solid volume fractions and slip velocities. Simple posteriori validations are systematically conducted in various common fluidized beds including the bubbling bed, the turbulent bed, and the fast bed to assess the predictive capability of the newly proposed correction drag model.

Steady flow at a breached levee

The present study uses a laboratory scale experimental model and an analytical model to study the steady flow at a breached levee. Flow past a zero height side weir is considered for the purpose. Water surface elevation and the three-dimensional velocity field are measured by a point gauge and an Acoustic Doppler Velocimeter (Vectrino), respectively. Experimental results include the velocity field, free surface elevation, turbulent kinetic energy and bed shear stress. Empirical relations to predict the minimum water depth and discharge at the breach are obtained. An analytical model to predict the approximate flow field at the breach is also developed. The model parameters are calibrated using the experimental results. The approximate flow conditions predicted by the analytical model can be used for field conditions. The 17th Street Canal breach is employed as a case study to demonstrate the performance of the analytical model.

Experimental and Numerical Study of Open Channel Flow with T-Section Artificial Bed Roughness

Experimental and numerical studies have been conducted on the effects of bed roughness elements such as cubic and T-section elements that are regularly half-channel arrayed on one side of the river on turbulent flow characteristics and bed erosion downstream of the roughness elements. The experimental study has been done for two types of bed roughness elements (cubic and T-section shape) to study the effect of these elements on the velocity profile downstream the elements with respect to different water flow discharges and water depths. A comparison between the cubic and T-section artificial bed roughness showed that the velocity profile downstream the T-section increased in smooth side from the river and decrease in the rough side from it compared with the case when a cubic artificial bed roughness is used. By comparing the results for the element shapes, it can be notices that the T-section bed roughness element more effective compared to cubic shape for both sides of the channel. The numerical method has been done using Computational Fluid Dynamic (CFD) method. A validation for the CFD model with the experimental study have been carried out for a specific flow discharge and water depth. The results indicated that the velocity distribution profiles downstream the bed roughness elements in both sides shown very good agreement for manning coefficients between the numerical and experimental studies. The range of errors between the experimental and numerical study have been calculated using Root Mean Square Error (RMSE) approach, which is found that the RMSE is approximately equal to 1 in case of cubic bed roughness and the RMSE is about 1.5 in case of T-section bed roughness for both smooth and rough sides. Furthermore, the influence of the velocity profile and the bed erosion downstream of the T-section element under the effect of tides have been investigated using the CFD method, which is commonly happened in Shat al-Arab south of Iraq. The results show that the tide of the flow has a reverse effect on the velocity profiles for both sides. Since the velocity profile downstream of bed roughness region increase in the rough side and decrease in the smooth side compared with the normal flow of the river.

Morphodynamics of vortex ripple creation under constant and changing oscillatory flow conditions

The morphodynamics of orbital vortex ripples due to oscillatory flow and induced sediment transport was studied numerically. The methodology was based on large-eddy simulations of the full coupling of turbulent oscillatory fluid flow, bed and suspended sediment transport, and bed evolution. The Immersed Boundary method was implemented for the imposition of fluid and sediment boundary conditions on the mobile bed surface. Results are presented for ripple creation starting from flat or rippled beds, as well as results of ripples adapting to changing flow (wave) direction. The computed equilibrium ripple dimensions are in agreement with existing empirical predictions; however, the computed ripple profile is not as steep in the vicinity of the ripple crest as the analytical one in Sleath (1984). The time needed for the bed geometry to reach equilibrium, in cases where the initial bed is almost flat or it is formed by ripples that shorten towards equilibrium, is longer than in cases where the initial bed is formed by ripples that widen towards equilibrium. Moreover, the ripple crests are reoriented when the flow changes direction by 45°; the new equilibrium ripple crestline is perpendicular to the flow direction. Finally, the numerical results are compared with corresponding results of flow above fixed ripples. The bed load flux does not present substantial differences between the fixed and mobile bed cases. On the contrary, a strong increase of the suspended load flux is observed for the mobile bed case in comparison to the fixed bed case. • LES Coupling between oscillatory flow, sediment transport, and bed morphodynamics. • Same flow forcing results into same bed equilibrium regardless of initial bed form. • Evolution of ripples longer than equilibrium ones is slower than for shorter ones. • Ripples are reoriented perpendicular to the flow direction. • Suspended load over mobile rippled bed is larger than over fixed ripples.

Gas Holdup in Turbulent Bed Contactor: Experiments and Prediction Model

Gas Holdup in Turbulent Bed Contactor: Experiments and Prediction Model

Using wavelet analysis to characterize the transition from bubbling to turbulent fluidization

Turbulent fluidized bed (TFB) regime has demonstrated many advantages such as a combination of enhanced gas-solids contact and increased gas throughput, making it an optimal reactor in many industrial chemical and metallurgical processes. While the transition from bubbling to turbulent bed has been extensively investigated, some details still remain unclear such as the local transition velocity and the corresponding flow characteristics during the transition process. In this work, experiments were conducted in a conventional fluidized column with 0.267 m in diameter and 2.46 m in height and solids holdup was measured using an optical fiber probe in a superficial velocity range of 0.1–1.0 m/s. A discrete wavelet analysis method was proposed to extract quantitative information of bubbles/voids including the count and the lasting time. The local transition details and the onset velocity to the TFB regime in the fluidized bed were more clearly identified based on the extracted bubble/void information.

An Eddy‐Resolving Numerical Model to Study Turbulent Flow, Sediment, and Bed Evolution Using Detached Eddy Simulation in a Lateral Separation Zone at the Field‐Scale

AbstractTurbulence‐resolving simulations elucidate key elements of fluid dynamics and sediment transport in fluvial environments. This research presents a feasible strategy for applying state‐of‐the‐art computational fluid mechanics to the study of sediment transport and morphodynamic processes in lateral separation zones, which are common features in canyon rivers where massive lateral flow separation causes large‐scale turbulence that controls sediment erosion and deposition. An eddy‐resolving model was developed and tested at the field‐scale, coupling a viscous flow and sediment transport solver using Detached Eddy Simulation techniques. A morphodynamic model was applied to the viscous flow/sediment solver to calculate erosion and deposition. A simulation of turbulence was performed at the grid resolution for a straight channel to determine the relative contributions of modeled and resolved diffusivity. The time‐dependent, energetically important, correlative, non‐stationary signals of the simulated quantities were captured at the lateral separation zone. Strong periodic signals featured by high amplitude were found at the separation zone, while low frequency pulsations were observed at the reattachment zone of the lateral separation zone. Interactions between the eddies and the loose bed boundaries resulted in erosion of sediment at the main channel followed by deposition at the primary eddy and eddy bars.

Two Dimensional Computational Fluid Dynamics Simulations of Three-Phase Hydrodynamics in Turbulent Bed Contactor

In recent years, Turbulent Bed Contactors (TBC) have become widely spread and used in the process industry; their prominence came as a result of their low capital, low operating costs and high efficiency. This paper aims to present a simulation of the hydrodynamic characteristics of a three phase (air-water-beads) turbulent bed contactor. The multi-phase Eulerian model is utilized to create a two-dimensional computational model, which has the ability of evaluating the hydrodynamic characteristics of bed contactors under various operational conditions. While incrementing the superficial velocity of the gas, the bed pressure drop is studied at varied operating conditions; static bed heights and liquid flow rates being two examples. Thereafter, the results of the simulated pressure drop were compared with and confirmed by previously obtained experimental data; furthermore, there was little to no difference in contrast to simulated results and the experimentally obtained data for varying operating conditions. This research adds to our understanding of the design and operational parameters that can affect the efficiency of turbulent bed contactors.

Measurement of Turbulent Flows and Shear Stress on Open Channels

The phenomenon of turbulent flows becomes a virtual object in any changes in open channel flow hydraulics. Turbulent flow and shear stress have a role in the geometrical changes of bed channel and sediment movement. The dynamics of turbulent flow are consequences of hydraulic channel dynamics. Turbulent flow has excessive kinetic energy resulting in resistance force because the increase of friction effect and infraction in turbulent flow creates a complex phenomenon. Shear stress is in the eternal pressure of flow against the deformation of the primary basic form of channel. The research aims to analyze turbulent flow, shear stress, and bed scours’ phenomena and potential. Measurement of turbulent flows is by measuring the flow velocity in four segments at a distance of 100 cm each. The channel’s cross-section is divided into nine parts and five measurement points in the flow depth of inner and outer regions. There are three variations of channel discharge and slope, i.e., low discharge (Q1), medium discharge (Q2), large discharge (Q3), and downward slope (S1), medium slope (S2), and high pitch (S3). The parameter of turbulent flow analysis, shear stress includes flow velocity average (U), flow depth (h), channel slope (S), viscosity (φ), the mass density of the liquid (ρ), the characteristic length or hydraulic radius (L/R) by using an empirical equation approach. Turbulent flow analysis used dimensionless Reynolds’ number equation approach. The effect of hydrodynamic on turbulent flow causes the distribution of shear and scour stress, transport, and sediment deposition. The increase in the slope of the channel affects the increase in the values of shear stress.

Modelling initial motion of non-spherical sediment particles on inclined and seeped beds

Analytical and numerical solutions are developed to compute the threshold stress for the initiation of motion of sediment particles, from spherical to ellipsoidal, on inclined beds under boundary layer flows. The numerical model is based on the discrete element method and simulates the micro-mechanics of the sediment as an aggregate of rigid ellipsoids interacting by contact, while analytical solution establishes the basic dynamics for initiation of motion. In both models the coupling of grain motion with the flow dynamics is accomplished via drag, lift, buoyancy and seepage forces under turbulent velocity fluctuations by a one-way technique, since incipient motion does not modify substantially the flow.The comparison of both models and their extension to compute the threshold stress in terms of seepage, turbulent velocity fluctuations, bed inclination and particle orientations provide a better understanding of the onset of sediment particles. The parametric analyses cover a wide range of natural and usual conditions for incipient motion, showing that most of results fall within the variability range of the experiments. Additionally, these models are able to compute the initiation of motion for configurations not still tested in laboratory.

Fast versus turbulent fluidization of Geldart Group B particles

AbstractBoth fast and turbulent fluidized beds exhibit entrainment, but the differences in the flow phenomena are not well understood. This study targeted a comparative analysis of the cluster (or streamer), mass flux, and segregation datasets from these two fluidization regimes. The particle systems were narrow particle size distributions (PSDs), binary mixtures, or broad PSDs of Geldart Group B particles. Relative to the fast fluidized bed, the turbulent bed exhibited (i) higher cluster probability and frequency, but lower cluster duration; (ii) lower local mass flux; and (iii) similar segregation extents. Regarding clusters, the relative dominance of the variables on probability was similar for both regimes, but there was a difference for probability and frequency. For overall mass flux, particle‐related properties were more dominant with the turbulent bed. As for segregation, the radial position was the most influential in the fast fluidized bed, but the least in the turbulent one.

Empirical models for the hydrodynamics of turbulent bed contactor with non-Newtonian liquids: (part 3)

Turbulent bed contactors (TBC) are three-phase fluidized beds which form an important class of process equipment used in many applications. In this work, experiments were conducted to investigate bed expansion and gas holdup behavior in TBC with non-Newtonian fluid for which limited information is reported in the literature. Aqueous solutions of carboxymethyl cellulose (CMC) with apparent viscosities ranging from 5 to 25 mPa.s were sprayed over beads packing at the top of the column. Forced airflow entered the bottom of the column to fluidize hollow plastic beads. The effect of rheological properties of CMC aqueous solutions on bed expansion and gas holdup was investigated upon variations of operating conditions. Two regions of bed expansion were existing when liquid flow rates were lower than 5.05 × 10−4 m3/s. Gas holdup decreased drastically for more viscous liquid solutions. Empirical models were developed for important hydrodynamic parameters of TBC such as pressure drop, liquid holdup, minimum fluidization velocity, and bed expansion as a function of several operational variables. Different dimensionless groups were formed from the available set of variables and empirical models were proposed and fitted based on maximum R2 criteria. The models developed can be used in the design and optimal operation of TBC.

Turbulent Flow Structures and Scour Hole Characteristics around Circular Bridge Piers over Non-Uniform Sand Bed Channels with Downward Seepage

In alluvial rivers bridge piers often cause local scour, a complex phenomenon as a result of the interaction between turbulent flow and bed material. In this paper, the results of an experimental study on the scour hole characteristics around single vertical pier sets on a non-uniform sand bed, under no seepage, and with downward seepage conditions, are described. In case of downward seepage, turbulent statistics, such as Reynolds stress, higher order moments, TKE-flux, and consequently sediment transport, decrease upstream of the pier, while increasing on both sides of it, where the enhanced erosive capacity of the flow results in an increase in the scour hole width. Moreover, the scour hole length shifts downstream. Empirical equations for the evaluation of scour hole characteristics, such as the length, width, area, and volume, including the downward seepage parameter, are proposed and experimentally tested. Model predictions give reasonably good agreement with the experimental data.

A general EMMS drag model applicable for gas-solid turbulent beds and cocurrent downers

Eulerian-Eulerian models incorporated with the kinetic theory of granular flow were widely used in the simulation of gas-solid two-phase flow, while the effects of mesoscale structures such as particle clusters and gas bubbles could not be considered adequately if traditional homogeneous drag models were adopted in the coarse grid simulations. The energy minimization multiscale (EMMS) model has been proved to facilitate calculating a structure-dependent drag coefficient by considering particle clustering phenomena, which can be coupled with the two-fluid model (TFM) to improve the accuracy of coarse-grid simulation of gas-solid circulating fluidized beds. However, the original EMMS drag model cannot be further applied to the simulation of gas-solid fluidized beds with solids flow rate smaller than zero, e.g., turbulent fluidized beds and cocurrent downward flow, because the original cluster diameter correlation gives rise to a value smaller than single particle diameter or even negative value at extremely low solids fluxes or downward gas-solid flow. In this study, a new proposed cluster evolution equation is proposed by quantifying local clustering dynamics to replace the original cluster diameter correlation. The newly formulated EMMS drag model can be used to avoid a negative cluster diameter to be involved in calculating interphase drag force in the overall fluidization regime. The improved EMMS drag law is incorporated into the Eulerian-Eulerian model to simulate gas-solid turbulent fluidized beds and cocurrent downer reactors, since they both were widely used in many industrial processes. By analyzing local hydrodynamics as well as the axial and radial heterogeneous distributions in the two kinds of fluidized beds, it is clarified that the simulation using the improved EMMS drag model shows a better agreement with the experimental data than the computation using the homogeneous drag law.

Circulating turbulent fluidized bed regime on flow regime diagram

Circulating turbulent fluidized bed (CTFB) regime was recently found under a low superficial gas velocity (Ug) system and contained the advantages of both a conventional turbulent fluidized bed, with an excellent mixing property between the gas and solid phases due to the high solid volume fraction (SVF), and a fast fluidized bed (FFB) or circulating fluidized bed (CFB) regime, which could be employed in a continuous mode. To observe the new flow regime, it was conducted in a plexiglas two-dimensional CFB reactor with a riser of 2.00 m height, 0.15 m width and 0.05 m depth. The interpretation of the SVF profiles along the riser height were used to determine the appearance the tubulent bed front where the turbulent bed was terminated. The identification of the CTFB was a profile without the appearance of a bed front in the riser. In conclusion, the development of a CTFB regime was caused by the turbulent bed expansion, which was affected by the magnitude of Ug. There were three important criteria to perform a CTFB: a solid recirculating rate of at least 300 kg/m2.s, the regulated Ug should be between a bubbling fluidized bed (Uc) and FFB (Utr), and lastly, the most important design parameter, the riser height must be shorter than the maximum turbulent bed expansion, which depends on the choice of solid particles. The functions that showed a good agreement with the experimental results were applied to obtain the correlations that later became the compulsory part of addressing the boundaries for each fluidized bed regime. The advantage of the new flow regime diagram was that it included the aspects of operation and design if CTFB was the desire.

Gas solid contacting measurements in a turbulent fluidized bed by oxidation of carbon monoxide

The conversion rate of the mass transfer controlled oxidation of CO over a Pt/?-alumina catalyst (d = 65 ?m) has been studied in a fluidized bed (internal diameter = 0.05 m) p operated close to and in the turbulent fluid bed regime. The objectives were to investigate the gas-solids contacting efficiency to evaluate the conversion data in terms of overall mass transfer coefficients and define the apparent contact efficiency. At high superficial gas velocities, the concept of formation of particle agglomerates and voids is more realistic than the two-phase model considering discrete bubbles and a dense phase. The two-phase model is not useless but has hardly any relation with the real flow pattern in the turbulent regime.