Modeling of Axial-Symmetric Flow Structure in Gas–Solids Risers

2015 ◽  
Vol 138 (4) ◽  
Author(s):  
Pengfei He ◽  
Dawei Wang ◽  
Rajesh Patel ◽  
Chao Zhu
Keyword(s):  
Flow Structure ◽  
Mechanistic Model ◽  
Turbulent Convection ◽  
Nonuniform Flow ◽  
Symmetric Flow ◽  
Radial Transport ◽  
The Core ◽  
Interparticle Collision ◽  
Model Predictions ◽  
Back Mixing

Pneumatic transport of solids in a riser has a unique nonuniform flow structure, characterized by the core solids acceleration and the wall solids deceleration along the riser, which causes the down-flow of solids and hence back mixing. To predict this nonuniform flow structure, this paper presents a mechanistic model that includes two controlling mechanisms: the interparticle collision damping for axial transport of solids and the effects of collision-induced diffusion and turbulent convection for radial transport of solids. The model predictions are partially validated against available measurements, such as axial and radial distributions of concentration and velocity of solids.

scholarly journals Building the City: From Slums to a Modern Metropolis

2020 ◽  
Author(s):  
J Vernon Henderson ◽  
Tanner Regan ◽  
Anthony J Venables
Keyword(s):  
Dynamic Evolution ◽  
Land Markets ◽  
Technology Choice ◽  
Formal Sector ◽  
Model Estimate ◽  
Unique Data ◽  
The Core ◽  
Welfare Losses ◽  
Model Predictions ◽  
The City

Abstract We model the building of a city, estimate parameters of the model, and calculate welfare losses from institutional frictions encountered in changing land-use. We distinguish formal and slum construction technologies; in contrast to slums, formal structures can be built tall, are durable, and non-malleable. As the city grows areas are initially developed informally, then formally, and then redeveloped periodically. Slums are modelled as a technology choice; however, institutional frictions in land markets may hinder their conversion to formal usage that requires secure property rights. Using unique data on Nairobi for 2003 and 2015, we develop a novel set of facts that support assumptions of the model, estimate all parameters of the model, and calculate welfare losses of conversion frictions. We track the dynamic evolution of the city and compare it with model predictions. In the core city formal sector, about a third of buildings were torn down over 12 years and replaced by buildings on average three times higher. For slums in older areas near the centre, even after buying out slumlords, overcoming institutional frictions would yield gains amounting to about ${\$}$18,000 per slum household, thirty times typical annual slum rent payments.

The structure and development of a wing-tip vortex

1996 ◽  
Vol 312 ◽  
pp. 67-106 ◽  
Author(s):  
William J. Devenport ◽  
Michael C. Rife ◽  
Stergios I. Liapis ◽  
Gordon J. Follin
Keyword(s):  
Flow Structure ◽  
Velocity Profiles ◽  
Tip Vortex ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
The Core ◽  
Show Evidence ◽  
Vortex Centre ◽  
Velocity Spectra ◽  
Naca 0012

Experiments have been performed on the tip vortex trailing from a rectangular NACA 0012 half-wing. Preliminary studies showed the vortex to be insensitive to the introduction of a probe and subject only to small wandering motions. Meaningful velocity measurements could therefore be made using hot-wire probes.Detailed analysis of the effects of wandering was performed to properly reveal the flow structure in the core region and to give confidence in measurements made outside the core. A theory has been developed to correct mean-velocity profiles for the effects of wandering and to provide complete quantitative estimates of its amplitude and contributions to Reynolds stress fields. Spectral decomposition was found to be the most effective method of separating these contributions from velocity fluctuations due to turbulence.Outside the core the flow structure is dominated by the remainder of the wing wake which winds into an ever-increasing spiral. There is no large region of axisymmetric turbulence surrounding the core and little sign of turbulence generated by the rotational motion of the vortex. Turbulence stress levels vary along the wake spiral in response to the varying rates of strain imposed by the vortex. Despite this complexity, the shape of the wake spiral and its turbulent structure reach an approximately self-similar form.On moving from the spiral wake to the core the overall level of velocity fluctuations greatly increases, but none of this increase is directly produced by turbulence. Velocity spectra measured at the vortex centre scale in a manner that implies that the core is laminar and that velocity fluctuations here are a consequence of inactive motion produced as the core is buffeted by turbulence in the surrounding spiral wake. Mean-velocity profiles through the core show evidence of a two-layered structure that dies away with distance downstream.

3D Numerical Simulation and PEPT Experimental Investigation of Pressurized Gas-Solid Fluidized Bed Hydrodynamic

2009 ◽  
Author(s):  
P. Fede ◽  
G. Moula ◽  
A. Ingram ◽  
T. Dumas ◽  
O. Simonin
Keyword(s):  
Numerical Model ◽  
Fluidized Bed ◽  
Particle Velocity ◽  
Particle Trajectory ◽  
Wall Region ◽  
3 Dimensional ◽  
The Core ◽  
Numerical Predictions ◽  
Model Predictions ◽  
Good Agreement

The present paper is dedicated to numerical and experimental study of the hydrodynamic of a non-reactive isothermal pressurized fluidized bed. Experimental data have been obtained using PEPT technique allowing to track a particle trajectory inside a dense fluidized bed. A specific post-processing approach has been developed to compute the Eulerian time-averaged particle velocity field. The comparison with 3-dimensional numerical model predictions shows a good agreement in the core of the fluidized bed. In contrast, in the near wall region the numerical model overestimate the downward particle velocity. The modification of particle phase wall boundary condition improves the numerical predictions.

Applicability of Choking Flow Models for Subcooled Flashing Flow Through Tube Cracks

2011 ◽  
Author(s):  
Brian Wolf ◽  
Shripad T. Revankar ◽  
Jovica R. Riznic
Keyword(s):  
Mechanistic Model ◽  
Channel Length ◽  
Two Phase ◽  
Experimental Conditions ◽  
Flow Rates ◽  
Flow Models ◽  
Model Predictions ◽  
Flow Through ◽  
The Difference ◽  
Non Equilibrium

Recently there is some database available on choking flow through cracks relevant to steam generator (SG) tubes to model the critical flow. These data are used in assessing the key choking flow models. Based on this assessment a mechanistic choking model is developed. The model is used to predict the choking flow rates for various experimental conditions for subcooled flashing flow through narrow slits with L/D varying from small values (∼5) to large values (100). Results are presented on the effects of thermal and mechanical non-equilibrium on the choking flow for small L/D channels. A mechanistic model was developed to model two-phase choking flow through slits. A comparison of model results to experimental data shows that the homogeneous equilibrium based models markedly under predict choking flow rates in such geometries. As subcooling increases, and channel length decreases the non-equilibrium effects play a greater role in the choking phenomenon, therefore the difference in model predictions and experimental results increases.

scholarly journals Consideration of Latent Infections Improves the Prediction of Botrytis Bunch Rot Severity in Vineyards

Plant Disease ◽  
2020 ◽  
Vol 104 (5) ◽  
pp. 1291-1297 ◽  
Author(s):  
Giorgia Fedele ◽  
Elisa González-Domínguez ◽  
Laurent Delière ◽  
Ana M. Díez-Navajas ◽  
Vittorio Rossi
Keyword(s):  
Decision Making ◽  
Mechanistic Model ◽  
False Negative ◽  
Growing Season ◽  
Weather Data ◽  
Growth Stages ◽  
Latent Infections ◽  
Vine Growth ◽  
Model Predictions ◽  
Bunch Rot

The current study validated a mechanistic model for Botrytis cinerea on grapevine with data from 23 independent Botrytis bunch rot (BBR) epidemics (combinations of vineyards × year) that occurred between 1997 and 2018 in Italy, France, and Spain. The model was operated for each vineyard by using weather data and vine growth stages to anticipate, at any day of the vine-growing season, the disease severity (DS) at harvest (severe, DS ≥ 15%; intermediate, 5 < DS < 15%; and mild, DS ≤ 5%). To determine the ability of the model to account for latent infections, postharvest incubation assays were also conducted using mature berries without symptoms or signs of BBR. The model correctly classified the severity of 15 of 23 epidemics (65% of epidemics) when the classification was based on field assessments of BBR severity; when the model was operated to include BBR severity after incubation assays, its ability to correctly predict BBR severity increased from 65% to >87%. This result showed that the model correctly accounts for latent infections, which is important because latent infections can substantially increase DS. The model was sensitive and specific, with the false-positive and false-negative proportion of model predictions equal to 0.24 and 0, respectively. Therefore, the model may be considered a reliable tool for decision-making for BBR control in vineyards.

Hydrodynamic Model of Gas-Solids Risers Flow With Continuous Axial and Radial Flow Structure

2014 ◽  
Author(s):  
Pengfei He ◽  
Rajesh Patel ◽  
Dawei Wang ◽  
Chao Zhu ◽  
Bo Zhang
Keyword(s):  
Transport Properties ◽  
Solid Phase ◽  
Complex Flow ◽  
Uniform Distributions ◽  
Wall Boundary ◽  
Proposed Model ◽  
Model Predictions ◽  
Continuous Modeling ◽  
Back Mixing ◽  
Phase Transport

The dynamic transport of gas-solids in a riser leads to highly non-uniform and complex flow distributions in both axial and radial directions. This study presents a continuous modeling approach that simultaneously computes the axial and radial non-uniform distribution of gas and solid phase transport properties in the risers. The radial non-uniform distributions of transport properties of gas and solids are approximated by the 3rd order polynomials, which have been validated by available experimental data from literatures. The radial heterogeneity is due to a combined effect of riser wall boundary, the radial transport by the collision-induced diffusion, and the turbulent convection of solids. Some important transport properties, such as core-wall boundary and back-mixing ratio, are flow-coupled and solved by the proposed model. The model predictions have been validated against some published experiment data, including the distributions of solid concentration, velocity and pressure gradient along the risers.

scholarly journals Computational simulation of flocculent sedimentation based on experimental results

2012 ◽  
Vol 65 (6) ◽  
pp. 1007-1013 ◽  
Author(s):  
Mafeni S. Ramatsoma ◽  
Evans M. N. Chirwa
Keyword(s):  
Empirical Model ◽  
Mechanistic Model ◽  
Computational Simulation ◽  
Data Sets ◽  
Random Errors ◽  
Wide Range ◽  
Model Predictions ◽  
Sedimentation Basins ◽  
Semi Empirical ◽  
Better Than

Computerised interpolation algorithms as well as the empirical model for analysing the flocculent settling data were developed. A mechanistic semi-empirical model developed from fundamental physical principles of a falling particle in a viscous fluid was tested against actual flocculation column data. The accuracy of the mechanistic model was evaluated using the sum of the squared errors between the interpolated values (real values) and the model predictions. Its fitting capabilities were compared with Özer's model using nine flocculent data sets of which four were obtained from literature and the rest were actual data from the performed experiments. The developed model consistently simulated the flocculation behaviour of particles in settling columns better than Özer's model in eight of the nine data sets considered. It is recommended that the model's performance be further compared with other models like the Rule based and San's model. The errors due to the use of interpolated values when determining the performance of the empirical models need to be investigated. Furthermore, a three-way rather than two-way interpolation should now be achievable using the interpolation algorithm developed in this study thereby reducing the effects of interpolation bias. The above work opens the way to full automation of design of flocculation sedimentation basins and other gravitational particle separation systems which at present are designed manually and are susceptible to a wide range of human and random errors.

Detection and Modeling of the Core-Annulus Transition in Industrial and Pilot Plant Risers

2003 ◽  
Vol 1 (1) ◽  
Author(s):  
Lauren Briens ◽  
Cedric Briens ◽  
David Nevicato ◽  
Jean René Bernard
Keyword(s):  
Pilot Plant ◽  
Signal Analysis ◽  
Optical Probe ◽  
Transition Analysis ◽  
The Core ◽  
Model Predictions ◽  
Vertical Pneumatic Transport ◽  
Probe Measurements ◽  
Good Agreement

The core-annulus structure is essential to the modeling and optimization of riser reactors such as used in Fluid Catalytic Cracking. This paper presents results of measurements taken with various probes in a pilot plant and in an industrial riser. The instantaneous probe signals were analyzed with sophisticated signal analysis methods based on the detection of cycles and the determination of the correlation dimension.In a pilot plant riser, a core-annulus structure was identified with optical and momentum probe measurements. Cycle analysis of the optical probe measurements showed that the annulus was unstable: its thickness fluctuated with an average cycle time of 0.3 s. There were waves at the core-annulus boundary. In an industrial riser, a similar core-annulus structure could be identified with temperature and momentum probe measurements. Local temperature measurements are much easier to perform in an industrial riser than momentum probe measurements but can provide, with cycle analysis, the location of the core-annulus transition. Analysis of the momentum probe and temperature signals showed that the thickness of the wavy transition layer between core and annulus was about the same in the pilot plant and the industrial riser, meaning that the relative range of fluctuations in annulus thickness was much smaller in the larger industrial riser.A model was developed to predict the time-averaged transition between core and annulus. This model, which had been successfully used to predict the annulus thickness in dilute-phase vertical pneumatic transport lines, assumes that the annulus thickness is such that the riser pressure drop is minimized. Measurements and model predictions were in good agreement.

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