Voidage Measurement of Gas-Oil Two-phase Flow

A new method for the voidage measurement of gas-oil two-phase flow was proposed.The voidage measurement was implemented by the identification of flow pattern and a flow pattern specific voidage measure- ment model.The flow pattern identification was achieved by combining the fuzzy pattern recognition technique and the crude cross-sectional image reconstructed by the simple back projection algorithm.The genetic algorithm and the partial least square method were applied to develop the voidage measurement models.Experimental results show that the proposed method is effective.It can overcome the influence of flow pattern on the voidage measure- ment,and also has the advantages of simplicity and speediness.

Voidage measurement based on genetic algorithm and electrical capacitance tomography

A new voidage measurement method based on electrical capacitance tomography (ECT) technique, Genetic Algo- rithm (GA) and Partial Least Square (PLS) method was proposed. The voidage measurement model, linear capacitance combi- nation, was developed to measure on-line voidage. GA and PLS method were used to determine the coefficients of the voidage measurement model. GA was used to explore the optimal capacitance combination which gave significant contribution to the voidage measurement. PLS method was applied to determine the weight coefficient of the contribution of each capacitance to the voidage measurement. Flow pattern identification result was introduced to improve the voidage measurement accuracy. Experi- mental results showed that the proposed voidage measurement method is effective and that the measurement accuracy is satis- factory.

Voidage Measurement of Air-Water Two-phase Flow Based on ERT Sensor and Data Mining Technology

Based on an electrical resistance tomography(ERT) sensor and the data mining technology,a new voidage measurement method is proposed for air-water two-phase flow.The data mining technology used in this work is a least squares support vector machine(LS-SVM) algorithm together with the feature extraction method,and three feature extraction methods are tested:principal component analysis(PCA),partial least squares(PLS) and independent component analysis(ICA).In the practical voidage measurement process,the flow pattern is firstly identified directly from the conductance values obtained by the ERT sensor.Then,the appropriate voidage measurement model is selected according to the flow pattern identification result.Finally,the voidage is calculated.Experimental results show that the proposed method can measure the voidage effectively,and the measurement accuracy and speed are satisfactory.Compared with the conventional voidage measurement methods based on ERT,the proposed method doesn't need any image reconstruction process,so it has the advantage of good real-time performance.Due to the introduction of flow pattern identification,the influence of flow pattern on the voidage measurement is overcome.Besides,it is demonstrated that the LS-SVM method with PLS feature extraction presents the best measurement performance among the tested methods.

Voidage correction method for DEM study of 3D pebble flows in a bed of cycloidal base

Voidage(porosity or void fraction)in packed particles(or pebbles)is of fundamental importance in calculating the pressure drop,obtaining the drag,predicting the bed permeability,estimating the neutron streaming,etc.For the case when particles are deformed,a method of voidage correction during the packing state is proposed using a Discrete Element Method(DEM)simulation of 3D pebble flow inside a bed of cycloidal base.A function to evaluate the remaining volume of a pebble intercepted by horizontal and vertical planes is proposed for voidage calculation.After that,the process of solving voidage distribution is provided in detail.Using this method,the voidage inside the cycloidal-base pebble bed is obtained to refer to reported similar data for validation.This method can be potentially used for dynamical voidage calculation in CFD-DEM simulation which can get suitable voidage distribution after the correction.

Direct calculation of voidage in the fine-grid CFD-DEM simulation of fluidized beds with large particles

Voidage is important in determining the hydrodynamic behavior of a fluidized bed and estimating the drag force,Exact calculation methods are limited,especially in terms of determining the intersection bf a particle and cell,This paper presents a method of directly calculating voidage.First,a judgment criterion of particle-cell overlap,which relies on the relationship of the distance from the particle to a cell face,edge,or vertex,is proposed.Eight cases of the overlap volume of a particle and cell are then ascribed to a unified formula in the framework of the cuboid cell,This formula relies on the volume of two kinds of segments named the hemispherical segment and quarter-spherical segment.The presented method is validated by calculating the voidage of simple cubic packing.Moreover,a three-dimensional fluidized bed with large particles is simulated and the results of numerical simulation are compared against experimental and simulation results reported in the literature.All numerical results are in good agreement with corresponding experimental data,and demonstrate the accuracy and reliability of the presented method in the three-dimensional simulation of fluidized beds.

Fluidization of particles in SCW fluidized bed:Voidage distribution of emulsion phase

Supercritical water(SCW)fluidized bed reactors convert biomass to fuels without pollutants emission.In this work,experimental studies were carried out to investigate voidage distribution in an SCW fu-idized bed by capacitance probes.Quartz sands with different particle sizes were fluidized by SCW under system pressure of 20-27 MPa and temperature of 410-570℃.The effect of operation conditions on voidage distributions of the emulsion phase(e.g.averaged voidage and probability density)is discussed.A predicting correlation between voidage and superficial velocity in emulsion phase is proposed.The relative error of the correlation is within+25%.These research results provide useful guidance for the optimization of supercritical water gasification technology.

Hydrodynamic Behaviour of a Gas-Solid Air-loop Stripper

In this paper, the principles of airlift loop reactor in gas-liquid and gas-liquid-solid systems are extended to gas-solid system. The models on bed average voidage in draft tube and the particle circulation velocity in a gas-solid loop reactor are deduced. The experiments are also conducted on a Φ600mm×7000mm reactor. The catalyst voidage and catalyst circulation velocity are measured at different radial and axial positions in draft tube and annulus, respectively. The experimental data are analyzed systemically and represented satisfactorily by the proposed models.

Effect of elevated temperature and silica sand particle size on minimum fluidization velocity in an atmospheric bubbling fluidized bed

The impact of temperature and particle size on minimumfluidizing velocity was studied and analyzed in a small pilot scale of bubbling fluidized bed reactor.This study was devoted to providing some data about fluidization to the literature under high temperature conditions.The experiments were carried out to evaluate the minimum fluidizing velocity over a vast range of temperature levels from 20℃ to 850℃ using silica sand with a particle size of 300-425μm,425-500μm,500-600μm,and 600-710μm.Furthermore,the variation in the minimumfluidized voidage was determined experimentally at the same conditions.The experimental data revealed that the Umf directly varied with particle size and inversely with temperature,whileεmf increases slightly with temperature based on the measurements of height at incipient fluidization.However,for all particle sizes used in this test,temperatures above 700℃ has a marginal effect on Umf.The results were compared with many empirical equations,and it was found that the experimental result is still in an acceptable range of empirical equations used.In which,our findings are not well predicted by the widely accepted correlations reported in the literature.Therefore,a new predicted equation has been developed that also accounts for the affecting of mean particle size in addition to other parameters.A good mean relative deviation of 5.473% between the experimental data and the predicted values was estimated from the correlation of the effective dimensionless group.Furthermore,the experimental work revealed that the minimum fluidizing velocity was not affected by the height of the bed even at high temperature.

Temperature influence on minimum fluidization velocity:Complexity,mechanism,and solutions

Fluidized-bed reactors are widely employed in various high-temperature industrial processes.Thus,it is crucial to understand the temperature effect on various fluidization phenomena,specifically the minimum fluidization velocity(U_(mf))that governs various aspects of fluidized bed behavior.In this study,we comprehensively analyze U_(mf) data from the literature to unravel the complexity and underlying mechanisms of temperature influence on this critical velocity.The research examines experimental data encompassing a wide range of temperatures,pressures,and solid particles.The analysis reveals that the influence of temperature on U_(mf) is fundamentally determined by the relative importance of hydrodynamic forces and interparticle forces within fluidized beds and is realized by three distinctive temperature-induced changes:gas properties,bed voidage,and physiochemical characteristics of particles.On this basis,an equation is derived to enable predictions of temperature influences on the minimum fluidization velocity under broad temperature conditions.

Study on force chain evolution of rice straw with different length during vibrational compression

This paper explores the mechanism of force chain evolution and voidage change under vibrational and non-vibrational compression conditions of rice straw of different lengths.Simulations were used to explore the force chain evolution and voidage variation mechanism under different conditions.The re-sults show that under non-vibrational compression,the strong force chain passes from top to bottom in vertical direction and from center to periphery in tangential direction.Under vibrational compression,the force chain passes from top and bottom to center in vertical direction and the force chain evolves from outer ring to interior and exterior in tangential direction.The number of strong chains,voidage and standard deviation of the mean pressure under vibratory compression are lower than the values under non-vibratory compression.Vibration promotes stress transfer and enhancement,velocity enhancement and density enhancement.This study analyzes the mechanical properties of different lengths straw during vibrational and non-vibrational compression from a detailed viewpoint.

Hydrodynamic characteristics of a rectangular gas-driven inverse liquid-solid fluidized bed

The hydrodynamic characteristics of a rectangular gas-driven inverse liquid-solid fluidized bed(GDFB)using particles of different diameters and densities were investigated in detail.Rising gas bubbles cause a liquid upflow in the riser portion,enabling a liquid downflow that causes an inverse fluidization in the downer portion.Four flow regimes(fixed bed regime,initial fluidization regime,complete fluidization regime,and circulating fluidization regime)and three transition gas velocities(initial fluidization gas velocity,minimum fluidization gas velocity,and circulating fluidization gas velocity)were identified via visual observation and by monitoring the variations in the pressure drop.The axial local bed voidage(e)of the downer first decreases and then increases with the increase of the gas velocity.Both the liquid circulation velocity and the average particle velocity inside the downer increase with the increase of the gas velocity in the riser,but decrease with the particle loading.An empirical formula was proposed to successfully predict the Richardson-Zaki index“n”,and the predicted e obtained from this formula has a±5%relative error when compared with the experimental e.

Validation of a discrete element model using magnetic resonance measurements

The discrete element model （DEM） is a very promising modelling strategy for two-phase granular systems. However, owing to a lack of experimental measurements, validation of numerical simulations of two-phase granular systems is still an important issue. In this study, a small two-dimensional gas- fluidized bed was simulated using a discrete element model. The dimensions of the simulated bed were 44mm × 10mm × 120 mm and the fluidized particles had a diameter dp = 1.2 mm and density ρp = 1000 kg/m^3. The comparison between DEM simulations and experiments are performed on the basis of time-averaged voidage maps. The drag-law of Beetstra et al. [Beetstra, R., van der Hoef, M.A., ＆ Kuipers,J. A. M. （2007b）. Drag force of intermediate Reynolds number flow past mono- and bidispersed arrays of spheres. AIChE Journal, 53,489-501 ] seems to give the best results. The simulations are fairly insensitive to the coefficient of restitution and the coefficient of friction as long as some route of energy dissipation during particle-particle and particle-wall contact is provided. Changing the boundary condition of the gas phase at the side-walls from zero-slip to full-slip does not affect the simulation results. Care is to be taken that the cell sizes are chosen so that a reasonable number of particles can be found in a fluid cell.

Liquid suspensions of single and binary component solid particles-An overview

In this paper theoretical approaches and experimental findings relative to the hydrodynamics of liquid suspensions of solid particles by liquids are reported and discussed. For the single particle specie systems, advantages and possible faults of well known empirical correlations are discussed. For binary-solid mixture suspensions, experimental evidence are reviewed and approaches capable of successfully describing observed behaviour are reported.

LOCAL SLIP VELOCITY IN A DOWNER

Based on the EMMS model, the local slip velocity between gas and solid is systematically analyzed and a theoretical correlation of local slip velocity with local voidage for a downer is derived as follows:U_s(r)/U_t=D^(8/7)(1-ε_mt)^(-2/7)[(1-ε(r)/(ε(r))]^(8/7)]ε(r)(47/14)((ε(r)-ε_(mt))/ε(r)) Using this correlation, the local gas-solid slip velocity in a downer is calculated. The calculated results are well consistent with experimental data. In addition, the variation of the local slip velocity with its corresponding solid holdup is also dis-cussed.

THE SERIAL MODEL IN LIQUID FLUIDIZATION AND ITS LIMITATIONS

The serial model of Epstein et al. （1981） is demonstrated to predict successfully the overall voidage of the mixed layer of incompletely segregated binary and ternary mixtures of liquid-fluidized solids that differ only in shape. Some speculations are then offered on the conditions for the success and failure of this model. Finally, methods proposed in the literature for estimating the deviations from the serial model for a binary-solids bed, which manifest themselves as a bed contraction, are discussed, with emphasis on packing models.

Modelling the bed characteristics in fluidised-beds for top-spray coating processes

A particle sub-model describing the bed characteristics of a bubbling fluidised bed is presented. Atomisation air, applied at high pressures via a nozzle positioned above the bed for spray formation, is incorporated in the model since its presence has a profound influence on the bed characteristics, though the spray itself is not yet considered. A particle sub-model is developed using well-known empirical relations for particle drag force, bubble growth and velocity and particle distribution above the fluidised-bed surface. Simple but effective assumptions and abstractions were made concerning bubble distribution, particle ejection at the bed surface and the behaviour of atomisation air flow upon impacting the surface of a bubbling fluidised bed, The model was shown to be capable of predicting the fluidised bed characteristics in terms of bed heights, voidage distributions and solids volume fractions with good accuracy in less than 5 min of calculation time on a regular desktop PC. It is therefore suitable for incorporation into general process control models aimed at dynamic control for process efficiency and product quality in top-spray fluidised bed coating processes.

Discrete element method modeling of corn-shaped particle flow in rectangular hopper

Discrete element method(DEM)was developed to simulate the corn-shaped particles flow in the hopper.The corn-shaped particle was described by four overlapping spheres.Contact force and gravity force were considered when establishing the model.In addition,flowing characteristic of particles in the hopper was studied.The effect of friction coefficient on the wall pressure,voidage and velocity distribution was analyzed.The results show that the discharge rate decreases with the friction coefficient increasing;and the"over-pressure"phenomenon occurs in the discharging process for two different friction coefficients.The voidage also increases as the friction coefficient increasing.And the velocity distribution is more uniformity and is closer to the mass flow with the friction coefficient deceasing.