Radial Particle Velocity Profiles Research Articles

Characterization of particle-laden jet flows in inertia-dominated regime

Point-particle large eddy simulations are used to characterize a vertical particle-laden round jet in inertia-dominated regime. Both particle–fluid and particle–particle interactions are considered, and a stochastic approach is adopted to model wall roughness. We fix the Reynolds number in the turbulent regime and vary the Stokes number based on the pipe bulk parameters over two orders of magnitude between 10 and 1000. We find that the decay and spreading rates of particle concentration grow with Stokes number and wall roughness. The particle concentration field, however, reaches a self-similarity in the jet far field, where the radial profiles follow a Gaussian-like function. We identify a regime change in the particle velocity field, which is characterized by a local Stokes number (Stl) defined based on the centerline particle velocity and its half-width. For Stl≳20, referred to as unresponsive regime, particles accelerate within the near field but retain their momentum within the far field, where the radial particle velocity profiles follow a self-similar basic exponential function. For Stl≲10, referred to as responsive regime, particles retain their momentum within the near field but decelerate within the far field, where the radial profiles follow a self-similar Gaussian-like function. Transition from unresponsive to responsive regime occurs within the far field when Stl reduces from 20 towards 10.

Bed symmetry in the fountain confined conical spouted beds with open-sided draft tubes

Bed symmetry has been analysed in fountain confined conical spouted beds operating with fine particles. Thus, vertical particle velocities and the spout shape have been determined in a wide range of spouting air velocities and bed configurations. Bed symmetry is widely accepted in the spouted beds without draft tubes or with nonporous ones, but this is not the case when open-sided draft tubes are used. Thus, radial particle velocity profiles differ depending on the cross-sectional angular plane considered in the open-sided draft tubes. The spout expands preferentially along the tube opened surface, and is wider as spouting air velocity and aperture ratio are increased. The literature correlations proposed for the average spout diameter have been analysed and no one is valid for estimating this parameter in the whole range of spouting air velocities and configurations analysed in these systems operating with fine particles.

Effects of Riser Diameter on Solids Holdup and Particle Velocity Profiles in Circulating Fluidized Bed Riser Systems

Abstract Tests were performed in a 0.1-m diameter small circulating fluidized bed (SCFB) and 0.3 m diameter cold flow circulating fluidized bed (CFCFB) riser systems located at the National Energy Technology Laboratory (NETL) to study the effects of riser diameter on the riser hydrodynamics. These tests were performed at solids circulation rates of Gs = 20 and 75 kg/m2 s and superficial gas velocities of Ug = 5.8 and 6.5 m/s using high-density polyethylene (HDPE) pellets with a density of 0.863 g/cm3, particle size range of 600–1400 µm (with a Sauter mean diameter of 871 µm, placing them in the Geldart B classification). Comparisons of riser axial pressure and solids fraction profiles, radial particle velocity profiles, and radial profiles of higher statistical moments and select chaos analysis parameters were considered. The results showed that for a given Ug and Gs, the smaller diameter riser exhibited characteristics associated with more dilute solids flow than that observed in the larger diameter riser. Additionally, the larger diameter riser exhibited a downward flow of solids near the wall under all test conditions, whereas the smaller diameter riser data exhibited little or no indications of solids downflow near the wall. These findings suggest that, from an industrial standpoint, a direct scaleup of small-scale tests cannot readily be accomplished as the solids holdup and the solids velocity profiles in small units (those normally tested in the laboratory) are not similar to those of large units and the performance of large units can therefore not be predicted from small-scale tests.

Experimental Study of Solids Motion in an 18 m Gas–Solids Circulating Fluidized Bed with High Solids Flux

A comprehensive experimental study of solids motion is conducted in an 18-m high circulating fluidized bed with extremely high solids circulating rate (Gs) up to 1000 kg/(m2 s), which is never achieved in such a tall riser in previous researches. The axial distributions of particle velocity show exponential types and slight changes in the fully-developed zone with Gs higher than 400 kg/(m2 s). The radial profiles of particle velocity are the parabolic shapes and become steeper with increasing Gs. The development of gas-solids flow structure becomes slower with increasing Gs or decreasing Ug. When Gs reaches 800 kg/(m2 s), the full development is achieved until to the axial height higher than 12 m. Better correlations between the local particle velocity and solids holdup and less down-flowing particles indicate better reactor performance with the reducing solids back-mixing and uniform residence times at extremely high Gs.

LDV measurements of particle velocity distribution and annular film thickness in a turbulent fluidized bed

Local particle velocity and annular film thickness in a 4.8m-high, 0.15m-ID turbulent fluidized bed (TFB) were determined with a Laser Doppler Velocimeter (LDV) at several axial heights. The effects of superficial gas velocities (0.47–0.94m/s), initial bed heights (0.3–0.5m), and particle sizes (mean diameter is 164μm and 89μm, respectively) on particle velocity and annular film thickness were investigated at ambient conditions. When superficial gas velocity or initial bed height was increased, the radial profile of particle velocity became significantly non-uniform. Additionally, the core-annulus flow pattern was observed in TFB. The annular film layer narrowed along the axis, but widened with the increase of initial bed height or superficial gas velocity. When compared particles with diameter of 164μm, the smaller particles were more sensitive to the change of operating conditions. The particles with diameter of 89μm showed more non-uniform radial profiles of particle velocity and a wider annulus. Based on effects of operating conditions, measurement height, and particle sizes, a new correlation was developed to predict the annular film thickness in turbulent fluidized bed. A good agreement was obtained between the predicted results and experimental data.

Comparison of particle velocity measurement techniques in a fluidized bed operating in the square-nosed slugging flow regime

The novel “travelling fluidized bed” (TFB), operated under identical conditions, was deployed to compare alternate experimental measurement techniques for the investigation of solid motion in gas-fluidized beds operating in the square-nosed slugging regime. Measurements of particle velocity obtained by radioactive particle tracking (RPT — non-invasive at the Ecole Polytechnique de Montréal), positron emission particle tracking (PEPT — non-invasive at University of Birmingham), optical fibre probes (invasive at UBC) and borescopic high speed particle image velocimetry (invasive at PSRI) are compared for sand particles of mean diameter of 292μm. Significant differences between the time-average radial profiles of particle velocity are observed in many cases. The results provide valuable insights into the merits and challenges of advanced particle velocity measurement techniques.

Detailed measurements of particle velocity and solids flux in a high density circulating fluidized bed riser

A comprehensive study of particle velocity and solids flux is conducted in a high density circulating fluidized bed with extremely high solids circulation rate up to 1000kg/m2s, for the first time in the lab-scale settings. Results show that the axial distribution of particle velocity becomes more uniform in such extremely high solids flux conditions than cases with lower solids flux. Radial profiles of particle velocity and solids flux become increasingly steep with increasing solids circulation rate. No net downward flow near the wall is detected, which is considered an important advantage of the high density riser leading to reduced solids backmixing. Correlations of particle velocity against solids holdup are stronger for low density conditions than for high density cases, suggesting that gas–particle interaction dominates in low density risers, while particle–particle interaction plays a key role for the motion of particles in the extremely high density ones.

CFD Modeling of Particulates Erosive Effect on a Commercial Scale Pipeline Bend

The computational fluid dynamics modeling of solid particles hydrodynamic based on the Lagrangian framework for diluted solid-gas flow through 90° gas pipeline bend is carried out to discover the effect of particles size distribution on particles flow pattern and their erosive effect on the bend. Particles size distribution has been obtained experimentally by measuring the sizes of solid particles that are flowing through the gas pipelines of Aghajari gas booster station. Also the erosion rate at the outer wall of the bend is predicted. The pipeline bend under study has a pipe diameter of 56 inches and ratios of the bend radius of the curvature to the pipeline diameter of 1.5. For the validation of computational model, firstly, the computational modeling is performed for a published experimental solid-gas flow data. The computational results include radial gas velocity and radial particle velocity profiles on planes which are at different angles through the bend. The comparison between the predicted numerical results and similar experimental data proves that the predictions of the computational model are acceptable. Finally, the particles' size distributions on each plane through the bend and the erosion rate on the outer wall of the bend have been obtained. The maximum rate of erosion is found to be 3.2 nm/s, occurring between 40 and 65° of the bend.

Distribution of particle velocity in a conical cylindrical spouted bed

AbstractInvestigations of particle velocities in a 200‐mm diameter conical cylindrical spouted bed were carried out. A direct measurement approach, coupled optical‐fibre endoscope with high‐speed photography, was used to determine the local velocity of glass bead and corn at various spouting velocities. The results showed that the radial particle velocity profiles when spouting of corns are similar to glass beads. The changes of particle velocities with increasing spouting gas velocity were more remarkable in the spout and fountain regions than in the annulus. The velocity of the glass beads is higher than the corns in the spout region, while they appear a reverse trend in the fountain region and annulus region at the same spouting velocity.

Coexistence of Solidlike and Fluidlike States in a Deep Gas-Fluidized Bed

Characterizing regime transition in gas-fluidized beds is of fundamental importance for the successful applications of fluidization technology. In this study, we show that a state-of-the-art two-fluid model has the ability to correctly predict the transition from packed bed to fully bubbling fluidized beds. To this end, we have studied a deep gas fluidized bed, in which the process of transition is shown much more clearly (see Figure 1) compared to relatively shallow fluidized beds. Particularly, we show that in the process of regime transition, the solidlike and fluidlike states can coexist. We conclude this on the basis of visual observation, global bed pressure drop signals, local solid volume fraction signals, cross-sectional averaged axial granular temperature profiles, and radial particle velocity profiles. The presence of the coexisting phases can be easily understood from the fact that the gas phase is compressible.

EMMS-based Eulerian simulation on the hydrodynamics of a bubbling fluidized bed with FCC particles

Although great progress has been made in modeling the bubbling fluidization of Geldart B and D particles using standard Eulerian approach, recent studies have shown that suitable sub-grid scale models should be introduced to improve the simulation on the hydrodynamics of Geldart A particles. In this study, the flow structures inside a bubbling fluidized bed of FCC particles are simulated in an Eulerian approach employing the energy minimization multi-scale (EMMS) model (Chemical Engineering Science, 2008, 63: 1553–1571) as the sub-grid scale model for effective inter-phase drag force, using an implicit cluster diameter expression. It was shown that the experimentally found axial and radial solid concentration profiles and radial particle velocity profiles can be well reproduced.

The influence of distributor structure on the solids distribution and flow development in circulating fluidized beds

AbstractThe influence of distributor structure on solids distribution is studied in two riser circulating fluidized bed reactors with different distributor structures but similar diameters. Optic fibre probes were used for the measurement of local solids distribution. The axial and radial distribution of solids holdup in the riser with a multi‐tube distributor is more uniform than that with a multi‐orifice distributor. The radial profiles of particle velocity in the riser with the multi‐tube distributor are also more uniform than that with the multi‐orifice distributor. In the riser with the multi‐tube distributor, both gas and particles are distributed more uniformly across the section, so that the flow acceleration is much more uniform and faster. The flow development is much faster and the fully developed region is reached early for the riser with the multi‐tube distributor. The distributor design is an important factor for the design of circulating fluidized bed reactor.

Effects of Draft Tubes on Particle Velocity Profiles in Spouted Beds

AbstractThe vertical particle velocity profiles in a full‐column cylindrical conical spouted bed, with or without a draft tube, are measured using a fibre optic probe system. The profiles have different characteristics for a draft tube spouted bed (DTSB) than for a conventional spouted bed (CSB). The spout of a CSB consists of a central flow where particle velocities fit exponential distributions, and a boundary layer where particle velocities are nearly uniform. The spout of a DTSB has no boundary layer and its radial particle velocity profiles are approximately linear. The particle velocities in the spout of a DTSB increase when superficial gas velocity increases, draft tube diameter decreases, or when entrainment height decreases. A kinematic model has been used to simulate the granular flow in the annulus of a CSB and DTSB, and they are compared with the experiments. The particle velocities in the annulus of a DTSB are much lower than that of a CSB. Their radial profiles are also different with a CSB. The dependence of particle velocities in the annulus of a DTSB on superficial gas velocity, draft tube diameter, and entrainment height are also discussed. One concludes that the draft tube diameter and entrainment height are two key factors for the solid circulation rate of a DTSB.

Scale‐up effect of riser reactors: Particle velocity and flow development

AbstractThe influence of riser diameter on the axial and radial particle velocity profiles and flow development is studied in a 10 m high twin‐riser system (76‐ and 203‐mm ID risers). Cross‐sectional average particle velocity is somewhat lower for the larger riser with a steeper radial particle velocity profile. The flow development in the riser center is nearly instant with the particle velocity remaining high. There is no significant difference for the two risers in the center region. In the wall region, the flow development is significantly slower and the particle velocity of the smaller riser is higher. The flow development slows down in the larger riser. In all locations measured, there was a clear dependency between the local particle velocity and solids concentration of both risers. Gas distribution and particle aggregation are considered the key factors that influence the local hydrodynamics in the twin‐riser system. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Velocity measurements in a cylindrical hydrocyclone operated with an opaque fiber suspension

Two novel techniques to measure velocity in opaque pulp fiber suspensions have been evaluated in a hydrocyclone. A pitometer equipped with a medical micropressure transducer was used to record the tangential velocity. The pressure opening of the pitometer was kept clean with a small purge flow. The small size of the probe minimized its influence on the flow field. The pitometer was able to measure the tangential velocity in the fiber suspension at different levels in the hydrocyclone. Additionally, a non-intrusive ultrasonic velocity profiler (UVP) was used to measure the radial particle velocity. The UVP proved to be capable of measuring radial particle velocity profiles.

Velocity measurements in a cylindrical hydrocyclone operated with an opaque fiber suspension

Two novel techniques to measure velocity in opaque pulp fiber suspensions have been evaluated in a hydrocyclone. A pitometer equipped with a medical micropressure transducer was used to record the tangential velocity. The pressure opening of the pitometer was kept clean with a small purge flow. The small size of the probe minimized its influence on the flow field. The pitometer was able to measure the tangential velocity in the fiber suspension at different levels in the hydrocyclone. Additionally, a non-intrusive ultrasonic velocity profiler (UVP) was used to measure the radial particle velocity. The UVP proved to be capable of measuring radial particle velocity profiles.

Radial Profiles of Particle Velocity in a Large Scale CFB Downer

Radial profiles of particle velocity in a large scale (418 mm I.D.) downward Circulating Fluidized Bed (CFB downer) were obtained via a Laser Doppler Velocimetry (LDV) system. Results show that particle velocity is gradually increasing along the radial direction and there exists a peak value in the near wall region. Such unique radial profile shape can be explained by the solids accumulating trend in the near wall region of the downer. Experiment results in this large scale downer are also compared with those obtained by other researchers in small scale units so as to investigate the scale-up effect on the radial particle velocity distribution in the downer.

Particle velocity and flow development in a long and high-flux circulating fluidized bed riser

In a 10 m tall and 76 mm i.d. high-flux riser, radial profiles of particle velocities were determined with a five-fiber optic probe on seven axial levels, with solids circulation rate (solids flux) up to 550 kg/m 2 s and superficial gas velocity up to 10 m/s . This study shows a more comprehensive set of radial profiles under higher fluxes than any previous literature. Four axial stages of flow development were distinguished based on radial distributions of particle velocity and concentration. Compared to low-flux operations ( <200 kg/m 2 s ), the radial profiles of particle velocity are significantly less uniform at high-flux operations ( >300 kg/m 2 s ). The superficial gas velocity has more influence on the radial velocity profiles than the solids flux at high solids flux. Net solids downflow only occurs near the wall under limited conditions in the bottom section.

Profiles of particle velocity and solids fraction in a high-density riser

Radial profiles of particle velocity and solids fraction in a high density circulating fluidized bed (HDCFB) at average cross-sectional solids fraction up to 0.21 were measured by an improved optical fiber laser doppler velocimeter and an optical fiber density sensor. The axial development of these radial profiles and the influence of operating conditions on the profiles were examined. The results showed that similar radial profiles of solids fraction exist in the HDCFB. The following Boltzman function can correlate well the solids fraction profile: (1 − ɛ)/(1 − ɛ) = 2.2 − (2)/(1 + exp(10· r/ R −7.665)). The radial profiles of particle velocity in HDCFB can also be described by the Boltzman function, that is, ( V p)/ ( U g) = (2.7)/(1 + exp(10· r/ R −10· X o) − 0.2). The Boltzman profiles of particle velocity in the high density operating regime was different with the parabolic shape operating in dilute phase regime. The local particle fluctuation velocity in the center of the riser increases with average solids concentration, while the fluctuation velocity decreases sharply as the radial position near the wall.

Flow study of a 0.05 m diameter downflow circulating fluidized bed

A number of flow measurements have been performed in a 0.05 m diameter pilot-scale downflow circulating fluidized bed including the axial pressure profile, the axial evolution of the average particle concentration and radial profile of the local particle velocity. A change in the material of construction of the reactor, thus changing the electrostatic effects, had little to no influence on the measured variables. Reduced radial particle velocity profiles near the reactor outlet were essentially the same, even though the gas still flows, on average, faster than the particles at this point.