Core-annulus Flow Structure Research Articles

An investigation of a gas–liquid swirling flow with shear-thinning power-law liquids

A gas–liquid swirling flow with shear-thinning liquid rheology exhibits complex behavior. In order to investigate its flow characteristics, experiments and computational fluid dynamics (CFD) simulations are conducted based on dimensional analysis. A Malvern particle size analyzer and electrical resistance tomography are applied to obtain the bubble size distribution and section void fraction. A Coriolis mass flowmeter is applied to obtain the mixture flow rate and mixture density for an entrance gas volume fraction smaller than 7%. The CFD coupled mixture multiphase model and large eddy simulation model are applied, considering the liquid shear-thinning power-law rheology. The results show that the swirling flow can be divided into developing and decaying sections according to the swirl intensity evolution in the axial direction. A gas–liquid swirl flow with shear-thinning liquid prohibits a core-annulus flow structure. A smaller index n contributes to maintaining the development of the swirl flow field and its core-annulus flow structure so that the swirl flow can form over a shorter distance with a stronger intensity. For a more uniform distribution of the apparent viscosity, the gas column in the pipe center is thinner. On the other hand, a larger consistency k enlarges the stress tensor. The amplitude of the velocity and the pressure of the core-annulus flow structure are reduced. A weaker swirl intensity appears with a wider gas column appearing as a consequence. Furthermore, the swirl number decays with an exponential behavior with parameters sensitive to the consistency k and index n of the decaying section of the swirling flow field. These are beneficial to gas–liquid separator design and optimization when encountering the shear-thinning power-law liquid phase in the petroleum industry.

Numerical simualtion of a circulating fluidized bed combustor and evaluation of empirical models for estimating solids volume fraction

The hydrodynamics of a circulating fluidized bed (CFB) combustor were investigated by conducting experiments on a 100 kWth CFB test rig and a 3D simulation using a computational particle fluid dynamics numerical scheme. Two empirical models were evaluated for estimating the solids volume fraction of the reactor by comparing the experimental and simulation results. The decay coefficients (a and K for the models) were derived from the simulation results to predict the decay of the axial solids volume fraction in the riser. From the viewpoint of accuracy, the two exponential functions model outperformed the one exponential function model. Empirical models for estimating the core-annulus flow structure in the transport zone were also investigated. Two existing empirical correlations reasonably predicted the wall layer thickness of the test cases except when the superficial velocity was 6.2 m/s. Under such high-velocity conditions, the empirical models overestimated the wall layer thickness by more than 100%.

Eulerian-Lagrangian simulation of the full-loop gas-solid hydrodynamics in a pilot-scale circulating fluidized bed

The full-loop gas-solid hydrodynamics in a pilot-scale circulating fluidized bed (CFB) is numerically investigated by using multi-phase particle-in-cell (MP-PIC) method. In this method, fluid motion is resolved under Eulerian framework while particle motion is tackled under Lagrangian framework. The numerical results agree well with experimental data, demonstrating the reasonability of the MP-PIC method in simulating dense gas-solid systems. The main features of gas-solid flows in the full-loop CFB are qualitatively predicted, with a typical core-annulus flow structure in the riser. A large pressure gradient of gas phase in the bottom region and top region of riser reflects the turbulent and chaotic gas-solid flows in these two regions. Enlarging superficial gas velocity leads to a more non-symmetrical pattern of axial gas velocity, indicating the enhanced influence of recycle structure on gas-sold uniformity under the higher superficial gas velocity. The solid residence time (SRT) distribution in the riser exhibits a feature of early peak and a long tail, which can be well fitted by a lognormal probability distribution function. Enlarging superficial gas velocity gives rise to a decreased mean value and narrower distribution of the solid residence time in the riser, reflecting the depressed solid back-mixing intensity in this condition.

Investigation of Entrance and Wall Dynamics of the High-Flux Gas-Solid Riser Using Statistical Analysis of Solids Concentration Signals

Statistical analysis of the entrance and wall dynamics of a high-flux gas-solid riser was done using solid concentration time series data collected from a 76 mm internal diameter and 10 m high riser of a CFB system with a twin-riser operated at 4.0 to 10.0 m/s gas velocity and 50 to 550 kg/m2s solids flux. Spent fluid catalytic cracking catalyst particles with 67 μm mean diameter and density of 1500 kg/m3 together with 70% to 80% humid air was used. Solid concentration data were analysed using code prepared using FORTRAN 2008 to get statistical parameters and plot their profiles. Results obtained show that the gas-solid suspension flow in the riser is dominated by low solid concentration in the centre region and high solid concentration in the wall region which forms a core-annulus flow structure. The mean solid concentration in the wall region decreases with riser height from the dense bottom section to less dense in the fully developed flow section at the top of the riser. The gas-solid suspension flow in the centre region is dominated with uniform flow structure while the wall region is dominated with high fluctuations in solid concentration. Further, it was found that the entrance and developing flow sections of the riser exhibit high flow non-uniformities than the fully developed flow section of the riser. The flow non-uniformities in the entrance and developing flow section increase with increase in superficial gas velocity at constant solid flux. The wall region, from the entrance to the top sections of the riser along the axial direction exhibits both dilute and dense suspension flow.

Experimental flow structure analysis in a 1 MWth circulating fluidized bed pilot plant

A water-cooled dual-channel capacitance probe system was developed for in-furnace measurements of solids concentration and solids velocity in a 1MWth dual fluidized bed pilot plant. Measurements were conducted in both reactors of the pilot plant during carbonate looping and combustion test campaigns. Solids velocities were derived by cross correlating the signals of the two channels of the capacitance probe. Thus, horizontal profiles of solids concentration and velocity were simultaneously determined in the reactors. These profiles show a strong core annulus flow structure in both reactors of the pilot plant. The influence of bed material characteristics, operating conditions as well as riser geometries on the particle flow structure is evaluated.

MP-PIC modeling of CFB risers with homogeneous and heterogeneous drag models

In this paper, the MP-PIC (multiphase particle-in-cell) approach is used for three-dimensional (3D) modeling of the gas-solid flows in two types of circulating fluidized bed (CFB) risers with Geldart group A and B particles by incorporating the homogeneous and heterogeneous drag force models in the MP-PIC method, respectively. First, the effects of the three important simulation parameters, namely, the grid cell number, numerical particle-parcel size and time step, are investigated. Having determined the appropriate values for the three parameters, the hydrodynamic characteristics predicted by different drag force models are rigorously analyzed. The homogeneous drag models considered are the six models, the Wen-Yu, Wenyu-Ergun, Syamlal-O’Brien, Gidaspow, HKL, and BVK models, while the four heterogeneous models considered are Sarkar and EMMS-based models (EMMS-Yang, EMMS-Matrix and EMMS-QL). For the riser 1 with the Geldart A particles, all the six homogeneous models predict extremely high solid fluxes and inconsistent void fraction distributions compared with experimental results. The heterogeneous Sarkar and EMMS-based models can effectively improve the simulation accuracy and predict a typical core-annulus flow structure. The lately-developed EMMS-QL model produces the most accurate solid flux. For the riser 2 with the Geldart B particles, both the heterogeneous and homogeneous drag force models can predict a reasonable flow structure. Further, there are no significant differences in the void fraction and velocity profiles due to the choice of a drag force model over the other. These drag force models also successfully capture the meso-scale local particle clusters. Of these drag-force models, the Wenyu-Ergun drag-forec model predicts comparatively accurate solid flux. Generally, MP-PIC combined with heterogeneous Sarkar and EMMS-based drag force models reasonably improve the simulation accuracy for the Geldart A particles, while these heterogeneous models have no superiority over the homogeneous drag models for the Geldart B particles.

Effect on the Flow Behaviors by Adding Internals in a Riser Reactor

Riser reactor is a key unit in the Fluid Catalytic Cracking (FCC), and it has important influences on increasing the yield coefficient of gas and oil. In this paper, the behaviors of gas-solid two-phase flow in the traditional y-type riser reactor are investigated by numerical simulation. The calculated particle concentration distribution is in good agreement with the experimental data, which verified the advanced models and calculating methods. The non-uniform distribution, such as core-annulus flow, may result in the unreasonable matching relationship of catalyst-to-oil ratio. An optimized riser with cuneal internals is proposed and the comparison of two different structures of riser reactor is presented. The comparison results show that the cuneal internals in the riser both can block effectively the slip down of the particles near wall region and weaken core-annulus flow structure due to the redistribution of particles. The results also prove that the particle concentration distribution becomes uniform along the axial and radial direction in the optimized riser by adding cuneal internals, which would be benefits for the catalytic cracking reactions.

Computational study of solid circulation in chemical-looping combustion reactor model

A three-dimensional computational fluid dynamic (CFD) model has been developed for simulating full-loop solid circulation in a dual circulating fluidized bed (DCFB) chemical loop combustion (CLC) reactor model. The standard k-ε turbulence model and kinetic theory of granular theory based Eulerian multiphase model were used to describe the gas and solid motions, respectively. The simulations focused on the investigation of solid circulation in a CLC reactor model which consists of an air reactor (AR, height of 1.36m, diameter of 0.05m) and a fuel reactor (FR, height of 0.97m, diameter of 0.054m). Key gas–solid flow behaviors related to gas–solid circulation behaviors, e.g. the transient flow regime, solid velocity, solid distribution and circulation characteristics, were numerically investigated. It was found that a core–annulus flow structure appeared in FR. The influences of FR and LLS fluidization flow rate on global and internal circulation rate were studied separately. With FR fluidization flow rate increasing, the time-averaged axial solid volume fraction in the top region of FR first increased and then decreased. The influences of FR and LLS fluidization rate on circulation dynamic characteristics, including circulation fluctuation strength and circulation stability, were also numerically investigated in this work.

High-fidelity simulation of the 3-D full-loop gas–solid flow characteristics in the circulating fluidized bed

The full-loop gas–solid motions in a three-dimensional circulating fluidized bed are numerically modeled using the computational fluid dynamics combined with the discrete element method. The time-averaged flow characteristics and the particle-scale details related to solid motion are discussed. The results show that solid motion shows an ‘S’ rising path in the riser but a spirally falling behavior in the cyclone. Typical core-annulus flow structure and non-uniform distribution of solid holdup appear in the riser. The inside and outside swirling flows of gas motion as well as the fishtailing phenomenon are captured in the cyclone. The solid residence time in the riser shows an early-occurred peak with a long tail. In the riser, details related to solid motion at the particle-scale level are obtained. The sudden changes of these particle-scale details when increasing the superficial velocity may be considered to be a new criterion of identifying the flow regime change.

Flow characteristics and axial bed composition according to mass ratio of binary solids in gas–solid circulating fluidised beds

The effects of the mass ratio of binary solids with fine and coarse particles on solid mixing and segregation were investigated in a gas–solid circulating fluidised bed with 0.1-m inner diameter and 3.7-m height. A sampling valve at the bottom of the bed was used to confirm the composition of the bed, and an extraction probe at the top of the bed was used to distinguish between the upward and downward flows. Iron ore (41-μm average particle size) of Geldart group A was used for the fine particles, and iron ore (130-μm average particle size) of Geldart group B was used for the large particles. The fluidisation regions of the riser were classified according to differential pressure into a dense bottom zone εs¯>0.3, a dense transition zone 0.01<εs¯<0.3, and a dilute zone εs¯<0.01. In the dilute zone, a typical core-annulus flow structure appeared, while the mixing of coarse particles positively correlated with the mass ratio of fine particles to the total amount of particles in the dense phase zone. Coarse particles were not easily entrained, and several entrained particles re-circulated through the downflow. This recirculation affected the dense phase flow.

Numerical and experimental investigation of an “S-shaped” circulating fluidized bed

It has been widely reported that traditional circulating fluidized beds (CFBs) suffer from core–annulus flow structure which is characterized by severe solid back-mixing and non-uniform distribution of solids in both axial and radial directions. In this work, an S-shaped design of risers is proposed and its performance is examined first by numerical and physical experiments. Both experiments show that the flow pattern in the S-shaped risers is significantly different from that in traditional straight risers. Generally speaking, the uniformity of the distribution of solid concentration is improved in the S-shaped risers and the high solid concentration regions are mainly in the center of the riser instead of the regions near the riser wall, with the core–annulus flow structure of a traditional design almost eliminated. The reason for the improvement is that in an S-shaped riser, the high fluid velocity regions move back and forth along the riser wall, which can prevent the boundary layer of solids from being established. This S-shaped design could be useful to improve the solid–fluid contacting in vertical particle–fluid flow systems.

Electrical Capacitance Volume Tomography Imaging of Three-Dimensional Flow Structures and Solids Concentration Distributions in a Riser and a Bend of a Gas–Solid Circulating Fluidized Bed

Electrical capacitance volume tomography (ECVT) is a newly developed imaging technique that can quantify three-dimensional (3D) multiphase flows in a complex, geometric flow field. In this study, the 3D phase distribution images inside a gas–solid circulating fluidized bed (CFB) are obtained using ECVT. Specifically, measurements are made at a riser section and a 90° bend-shape riser exit section of the CFB. Inside the vertical riser, a symmetric core–annulus structure with a low solids holdup in the riser center along with a high solids holdup near the riser wall is observed. The average volume solids holdup and the thickness of the annulus decrease with the superficial gas velocity. A core–annulus flow structure is formed both in the vertical and horizontal parts of the bend. The annulus structure is noncentro-symmetric in the horizontal part of the bend. The solids holdup in the annulus near the top wall area in the bend is higher than that in other locations of the annulus. At a higher superficial gas...

Fluctuation velocity correlation closure model for dense gas-particle turbulent flow

Turbulence model of kg-ɛg-kp-ɛp-kpg-θ is proposed. In the model, the two-phase velocity correlation turbulent kinetic energy kpg is modeled by transport equation. To close this turbulence model, algebraic expressions of two-phase Reynolds stresses and two-phase velocity correlation variable are established by considering both gas-particle interaction and anisotropy. This turbulence model is used to simulate dense gas-particle flow in a riser and in a downer. The predicted results show the core-annulus flow structure observed in the riser and the skin effect of particle concentration in the downer. The present model gives simulation results in much better agreement with the experimental results than those obtained by kg-ɛg-kp-ɛp-θ model which is simply closed using a semi-empirical dimensional analysis.

A two-scale second-order moment particle turbulence model and simulation of dense gas–particle flows in a riser

A two-scale second-order moment particle turbulence model is developed, based on the concept of particle large-scale fluctuation due to turbulence and particle small-scale fluctuation due to collision. It is then used to simulate dense gas–particle flows in a riser. The predicted results are in reasonable agreement with the experimental results and show the core-annulus flow structure observed in experiments. It is seen that the two-scale model is somewhat better than k f– ε f– k p– ε p– θ two-phase turbulence model and the single-scale second-order moment two-phase turbulence model (USM– θ model).

Characterization of the flow patterns in a pressurized circulating fluidized bed

To investigate the influence of the solid/gas density ratio ρ S/ ρ F on the flow pattern in a riser reactor, a pressurized circulating fluidized bed with an inner diameter of 0.194 m and 9 m height was used. The experiments have been carried out from ambient conditions to a static pressure p abs of 5 MPa in the regime of fast fluidization and/or pneumatic conveying. With the use of glass beads ( ρ S = 2500 kg/m 3, d P = 85 μm), density ratios ρ S/ ρ F from 2094 to 41 and solids net fluxes G S,Net from 65 to 200 kg/m 2s ( U G = 0.8 to 4.5 m/s) were found. Local information about solids concentrations, solids velocities, and solids mass flows has been obtained by a capacitance probe system. The results show a more uniform axial and radial solids distribution with increasing pressure p abs or decreasing density ratios ρ S/ ρ F, indicating flow conditions which are different to a well known core-annulus flow structure. This flow structure is defined by a dilute upflow in the core region and a dense downflow in the annulus region which is situated immediately near the wall. In agreement, the radial profiles of axial solids mass flux show flow patterns which are untypical for fast fluidization operating conditions. The results depict a distinct wall region that covers almost half of the riser cross-sectional area. Therefore, significant solids downflow occur, pointing out massive solids backmixing even at low solid/gas density ratios ρ S/ ρ F. Finally, the obtained information is used to develop an easy correlation for the prediction of internal solids reflux in a riser reactor as a function of solids/gas density ratios ρ S/ ρ F and the dimensionless superficial gas velocity Fr P.

Gas Mixing in the Reactor Section of Fluid Cokers

The gas flow structure in the reactor section of a fluid coker cold model was investigated using helium as the tracer gas. Gas residence time distributions indicate intensive gas mixing. The mean residence time from the feed nozzles decreased when the same flow of air was distributed from three rings of nozzles rather than six. When steady state measurements were carried out by injecting helium through a feed ring in the middle of the reactor section, the downstream radial profile of helium concentration formed a bell-shaped curve above the jet region near the tracer injection level and became nearly uniform with increasing height due to radial dispersion. Upstream, the helium concentration was higher in the wall region because of descending solids. The core-annulus flow structure disappeared toward the bottom of the reactor section.

A three-dimensional model for suspension-to-membrane-wall heat transfer in circulating fluidized beds

A three-dimensional model is proposed for both furnace-side and wall-side heat transfer in circulating fluidized beds with membrane walls. Following previous publications (Int. J. Heat Mass Transfer (2003a, b)), a core-annulus flow structure is employed in the model, with consideration of the membrane wall influence on bed hydrodynamics. The model couples radiation, conduction and convection on the furnace side to conduction and convection on the wall side. Radiation in the wall layer is simulated by the moment method. A finite-element method is employed to solve the set of non-linear, partial differential equations. The solution is demonstrated for a typical example. The model gives predictions of suspension-to-wall heat transfer which show satisfactory agreement with published experimental data.

Flow phenomena in the exit zone of a circulating fluidized bed

An experimental investigation of the gas and solids flow in a pilot-scale circulating fluidized bed (CFB) cold model with two different abrupt exit configurations (L-shape and extended top) has been carried out. Measurements of axial pressure profiles, high-speed video images of the flow phenomena at the wall as well as local optical probe measurements inside the exit zone are presented. Contrary to published results obtained in bench-scale CFB risers the axial profiles of the apparent solids volume concentration obtained by pressure measurements showed no indication of an increased solids hold-up in the vicinity of the exit, which confirms the conclusion by Pugsley et al. (Can. J. Chem. Eng. 75 (1997) 1001) that this is a scale effect. The local probe measurements showed the well-known core–annulus flow structure prevailing until the riser top. In the vicinity of the exit this flow structure is superimposed by a strong horizontal velocity component directed to the exit duct. In comparison to the conventional L-shaped abrupt exit the extended top does not increase the solids inventory in the riser.

Core heat transfer studies in a circulating fluidized bed

In some large capacity Circulating Fluidized Bed (CFB) boilers with water walls, additional heat transfer surfaces are necessary to control the furnace temperature while restricting the boiler height to about 40 m due to commercial considerations. These surfaces could be in the form of (1) External Heat Exchanger (EHE) in the solids recycle loop, (2) wing wall and omega panels at the very top of the furnace, or (3) a division or a curtain wall running through the core region of the upper dilute zone of the boiler. It is necessary to know the heat transfer characteristics for these core surfaces from design and operating considerations. In this context, experimental heat transfer studies have been conducted on a 0.6-m high single vertical tube, 0.5- and 1.0-m high U-tubes, and a 0.5-m high finned U tube, located in the top 10% height of a 100 mm×100 mm cross-section and 5.5-m tall CFB riser using silica sand of three mean sizes in the range of 143 to 363 μm as the bed material. The fluidizing air velocity and solid circulation flux were varied in the range of 4–8 m/s and 13–80 kg/m 2/s, respectively. The resulting cross-sectional average suspension density and measured total (gas convective plus particle convective) surface-average convective heat transfer coefficients for U-tubes were in the range of 9 to 25 kg/m 3 and 30 to 105 W/m 2/K, respectively. For single vertical tube at axis, they were in the range of 10 to 20 kg/m 3 and 60 to 128 W/m 2/K, and at off-axis positions, they were in the range of 10 to 20 kg/m 3 and 70 to 140 W/m 2/K, respectively. The results confirmed the increasing trend of the heat transfer coefficient from the axis towards the riser wall, thus reflecting the core-annulus flow structure of the CFB riser. It was observed that the length of the tube influences the heat transfer coefficient and fins augmented the heat transfer rate. Gas convective contribution was found to be substantial especially at high gas velocities and for large particles. Experimental data were compared with published heat transfer results under somewhat similar conditions. Using the experimental data points for the U-tubes, two empirical heat transfer correlations were developed in terms of the important design and operating parameters.

Wall-to-bed heat transfer in a circulating fluidized bed for the reduction of iron ore particles

A model is presented that describes the wall-to-bed heat transfer in a circulating fluidized bed (CFB) used for the prereduction of iron ore in the smelting-reduction iron-making process. The model incorporates the core-annulus type flow structure and the wall emulsion layer growing downward along the surface. Model predictions showed good agreements with measured data taken from the literature. The hydrodynamic behavior near the wall surface was able to be properly described by the core-annulus flow structure. A higher heat-transfer coefficient with higher solid circulation flux was obtained in the upper part of the bed because of the heat input caused by the lateral diffusion of particles from the core. The predicted and measured data also showed the minima in the heat-transfer coefficients in the lower part of the bed. Model predictions indicated that in the CFB for the reduction of iron ore particles, it is important to properly control the inlet temperature of the reducing gas, rather than that of the solid particles. The implications of the behavior of heat transfer in the CFB are discussed for the reduction of iron oxides.