焦油切割泵固液两相流数值分析

分析了不同颗粒大小、转速及转子安放角度对切割泵内流线及固体颗粒分布的影响。结果表明,小流量时进口区域涡流严重,流量对固相浓度分布影响不大;颗粒和浓度越大,转子工作面固相浓度越高,对转子表面的磨损与冲蚀越严重;增大转速可以提高粉碎效率,但是对设备的使用寿命有影响。     

CFD investigation of the pipe transport of coarse solids in laminar power law fluids

A numerical parametric study of the laminar pipe transport of coarse particles in non-Newtonian carrier fluids of the power law type has been conducted using an Eulerian-Eulerian computational fluid dynamics (CFD) model. The predicted flow fields have been successfully validated by experimental measurements of particle velocity profiles obtained using a positron emission particle tracking technique, whilst solid-liquid pressure drop has been validated using relevant correlations gleaned from the literature. The study is concerned with nearly-neutrally buoyant particles flowing in a horizontal or vertical pipe. The effects of various parameters on the flow properties of such mixtures have been investigated over a wide range of conditions. The variables studied are: particle diameter (2-9 mm), mean solids concentration (5-40% v/v), mean mixture velocity (25-125 mm s⁻¹), and rheological properties of the carrier fluid (k=0.15-20 Pa sⁿ; n=0.6-0.9). A few additional runs have been conducted for shear thickening fluids, i.e. n>1. Whilst the effects of varying the power law parameters and the mixture flowrate for shear thinning fluids are relatively small over the range of values considered, particle size and solids concentration have a significant bearing on the flow regime, the uniformity of the normalised particle radial distribution and of the normalised velocity profiles of both phases, and the magnitude of the solid-liquid pressure drop. The maximum particle velocity is always significantly less than twice the mean flow velocity for shear thinning fluids, but it can exceed this value in shear thickening fluids. In vertical down-flow, particles are uniformly distributed over the pipe cross-section, and particle diameter and concentration have little effect on the normalised velocity and concentration profiles. Pressure drop, however, is greatly influenced by particle concentration.     

Effect of particle gradation on flow characteristics of ash disposal pipelines

Pressure drop and concentration distribution studies for the flow of multi-sized solid-liquid flow through slurry pipelines has been carried out over a wide range of efflux concentrations and mixtures of solids having different particle size distributions. The particle size effect on pressure drop has been analyzed through the measured solid distribution pattern in the pipeline. An integral flow model has been used for prediction of the pressure drop and solids distribution under various conditions. The model has been used to predict the optimum particle size distribution that gives the minimum specific energy consumption.     

Hydrodynamic properties of a turbulent confined solid-liquid jet evaluated using PIV and CFD

Particle image velocimetry is used to evaluate liquid and solid velocities and turbulence levels in the developing region of a confined solid-liquid jet. The measurements are conducted utilizing the method of matching refractive indices together with digital phase separation. The diameters of the solids are and the maximum mean volume fractions for which measurements can be performed is 1.9%, a number estimated from image analysis. The experimental results are compared with those from numerical simulations using the mixture, dispersed and per-phase realizable k-ε models together with two models for the drag force. The results show that the differences in axial velocity between the two phases are small and the axial RMS velocities generally increases with increasing volume fraction and are larger for the dispersed phase compared to the continuous phase. The numerical simulations capture the flow structure well, but generally, the continuous-phase centreline velocities are underestimated close to the inlet and overestimated further downstream. Regardless of solid loading, the per-phase turbulence model in combination with a drag force modified by a correction factor as to take into account the turbulence of the carrier phase provides the best numerical results.     

A unified theory on solid-liquid separation: Filtration, expression, sedimentation, filtration by centrifugal force, and cross flow filtration

Based upon a new conception that the solid compressive pressure on a cake surface is not null, almost of all solid-liquid separation operations have been re-examined. For cake filtration, the phenomenon caused by the solid compressive pressure on a cake surface is discussed for thin cake. New expression and hindered sed-imentation theories are developed by above new conception using Darcy’s equation. Application of the new conception to centrifugal filtration and tangential filtration is also discussed. Above results lead to the conclusion that cake filtration, expression, hindered sedimentation, centrifugal filtration and tangential filtration can be described with a unified theory, and the main difference between the operations is only the boundary condition of cake.     

Using positron emission particle tracking (PEPT) to study the turbulent flow in a baffled vessel agitated by a Rushton turbine: Improving data treatment and validation

Positron emission particle tracking (PEPT) is a relatively new technique allowing the quantitative study of flow phenomena in three dimensions in opaque systems that cannot be studied by optical methods such as particle image velocimetry (PIV) or laser Doppler anemometry (LDA). Here, velocity measurements made using PEPT in two sizes of baffled vessel (∼0.20 m and ∼0.29 m diameter) and two different viscosity fluids agitated by a Rushton turbine are compared for the first time directly in depth with some studies reported in the literature made by LDA for the turbulent regime in the equivalent geometry. Initially, the paper considers how the Lagrangian data obtained by PEPT can be converted into Eulerian in order to make the comparison most effective. It also considers ways of data treatment that improve the accuracy of both the raw PEPT data and the velocities determined from it. It is shown that excellent agreement is found between the PEPT and literature results, especially for the smaller vessel, except for the radial velocity just off the tip of the blade in the plane of the disc of the Rushton turbine. This difference is attributed to the very rapid changes in both magnitude and direction that occurs in that region and also to the different way of ensemble averaging in the two techniques. In addition, the results for the absolute velocities normalised by the impeller tip velocity for all the rectangular cross-section toroidal cells in each size of vessel and each fluid and a range of agitator speeds are compared in the form of frequency histograms. In this analysis, the velocities for each run are obtained from PEPT based on tracking a particle for 30 min and the mean and mode of the velocities each decrease slightly with decreasing scale and Reynolds number. The possible reasons for this variation in the mode and the mean are discussed. Overall, it is concluded that for the radial flow Rushton turbine the PEPT technique can be used to obtain accurate velocity data throughout the entire complex three-dimensional turbulent flow field in an agitated, baffled vessel except very close to the impeller in the radial discharge stream.     

Choking and flow regime transitions: Simulation by a multi-scale CFD approach

The so-called “choking” phenomenon in fluidized systems lacks understanding from micro-scale hydrodynamics and hence always causes disputes. To solve this problem, a newly established multi-scale CFD (computational fluid dynamics) approach, which is based on drag correction from the EMMS (energy-minimization multi-scale) model, is used to simulate our experiments about choking and relevant flow-regime transitions. The computed flow-regime diagram is described with the functional relation between the imposed pressure drop and the solids flux at given gas flow rates. A bell-shaped area with an abrupt plateau, within which the solids flux equals the saturation carrying capacity and the dense upflow coexists with the dilute pneumatic transport, marks the choking transition in this diagram. The non-choking transition, however, is characterized by a smooth shift from the dilute flow to the dense flow, without an area for coexistence of flow regimes. The critical point, the summit of the bell-shaped area, demarcates between the choking and the non-choking transitions, near which the fluctuation of solids volume fraction reaches a maximum. In general, this CFD simulation of flow regime transitions suggests a good perspective of the EMMS based multi-scale approach.     

Hydrodynamics of gas-liquid slug flow in capillaries: Comparing theory and experiment

The hydrodynamics of slug flow in a horizontal capillary with a diameter of 0.92 mm is experimentally studied using water-air and aqueous glycerol solution-air systems. The experimental data (the velocity of bubbles, the volume fraction of the gas, the relative length of bubbles, and the pressure drop) are compared with the results of calculation by the previously constructed mathematical model of the slug flow of a gas-liquid mixture in capillaries. The developed model is in satisfactory agreement with experimental data in the range of capillary numbers from 0.05 to 0.12.     

In‐line jet mixing of liquid‐pulp‐fiber suspensions: Effect of fiber properties,flow regime,and jet penetration

Mixing effectiveness was determined experimentally for side jet injection into pipe flow for water and pulp suspensions for a range of fiber mass concentrations (0–3.0%), mainstream velocities (0.5–5.0 m/s), and side‐stream velocities (1.0–12.7 m/s). The mixing quality was measured in cross‐sectional planes along the pipe using electrical resistance tomography and quantified by a modified mixing index, derived from the coefficient of variation of conductivity. Mixing depended strongly on the flow regime and jet penetration. For turbulent flow, the criteria for in‐line jet mixing in water are applicable to the mixing in suspensions, with small differences likely due to differences in fiber network strength and influences of fiber‐turbulence interactions in modifying turbulent structures in the bulk. When a suspension flows as a plug, however, the mixing differs greatly from that in water, depending on the fiber network strength in the core of the pipe. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1420–1430, 2013     

Mixing and circulation of solids in spouted beds: particle tracking and Monte Carlo emulation of the gross flow pattern

Mixing and circulation of monosized particles in laboratory-scale tapered spouted beds have been characterized experimentally by measuring non-invasively the 3-D trajectory of a single tracer via a radioactive velocimetry technique. Processing the obtained Lagrangian trajectory allowed determination of the mixing dynamics in the longitudinal, radial and circumferential directions, of the return length and return time distributions, and of the mean Eulerian flow fields. A conceptual solids flow structure has been delineated. A four-zone 2-D axisymmetrical Monte Carlo model has been developed for emulating the elementary steps in play in the longitudinal mixing, i.e., the direction of slowest mixing, and in the return (or circulation) time and length of the solids phase. The four-zone solids flow structure is viewed as: (i) a spout region with a constant upward particle velocity, (ii) an annulus region above the conical base with a downward velocity radial profile, (iii) an annulus region within the conical base where the linear velocity, considered to be parallel to the cone wall, is equal to that of the incoming particles, (iv) a fountain in which the particle movement is characterized by the particle residence time, an exiting radius, and an average fountain height. The model proved successful in restoring the measured return time and return length distributions, and the mixing response curves.     

Jet breakup and droplet formation in near-critical regime of carbon dioxide-dichloromethane system

The jet breakup and droplet formation mechanism of a liquid in the near-critical conditions of a solvent-antisolvent system is examined with high-speed visualization experiments and simulated using a front tracking/finite volume method. The size of droplets formed under varying system pressure at various jet breakup regimes is measured with a Global Sizing Velocimetry, using the shadow sizing method. A stainless steel nozzle with 0.25 mm I.D and 1.6 mm O.D was used in this study. Experiments were performed at fixed temperature of 35 °C and system pressure in the range from 61 to 76 bar in the near-critical regime of the DCM-CO₂. At the near mixture critical regime for DCM-CO₂ mixture, the miscibility between the two fluid phases increases and the interfacial tension diminishes. This phase behavior has important applications in particle formation using gas antisolvent (GAS) and supercritical antisolvent (SAS) processes. The jet breakup and droplet formation in the near-critical regime is strongly dependent on the changes in interface tension and velocity of the liquid phase. An understanding of the droplet formation and jet breakup behavior of DCM-CO₂ in this regime is useful in experimental design for particle fabrication using SAS method.     

DEM simulations and experiments of pebble flow with monosized spheres

Pebble flow in the core of a pebble bed high-temperature reactor (HTR) is a typical granular flow with a huge number of monosized spherical pebbles, where the kinematics of the pebbles needs to be predicted more accurately. In this study the discharge of monosized glass beads from a semi-cylindrical silo has been investigated using the Discrete Element Method (DEM), in which the particle-particle interaction model is based on contact mechanics with measured friction coefficients for precisely predicting the flow behaviour. Particle flow experiments were carried out with a transparent semi-cylindrical silo. Two cases were considered: one is for tracking the trajectories of identified particles with 11,500 glass beads, and the other is for the ‘double-zone’ flow for estimating the mixing evolution with 10,460 glass beads. Comparisons between the experimental and DEM results show good agreement, which indicates the applicability of DEM for predicting the pebble flow in a HTR.     

Transport and deposition of inertial aerosols in bifurcated tubes under oscillatory flow

The transport and deposition of aerosol particles in a single bifurcation tube under oscillatory airflow condition are investigated via experiments and numerical simulations. In the experimental study, a novel stabilized laser-photodiode measurement technique is used to quantify the effects of frequency of oscillatory airflow and airflow rate on surface deposition of particles within the bifurcation tube. Surface deposition in the parent and daughter tubes is measured by orientating the laser-photodiode device parallel and perpendicular to the bifurcation plane and calculated using a transmission loss model. In the numerical simulation study, the same bifurcation tube is constructed using a two- and three-dimensional computer mesh in COMSOL^® to model particle mobility and deposition characteristics, considering the simultaneous effects of inertial impaction, gravitational settling and interception. The effects of frequency of oscillatory airflow, airflow rate and particle size on the particle trajectory and spatial deposition pattern are examined.     

Electrical resistance tomography (ERT) for flow and velocity profile measurement of a single phase liquid in a horizontal pipe

Electrical resistance tomography (ERT) is a novel, simple, and robust method of process imaging which uses non-invasive sensors located on the periphery of vessels to reconstruct a cross-sectional image of the vessel interior. This method of imaging when conducted on two adjacent planes on a pipe provides the ability to extract flow information. Previous studies have investigated this application on multi-phase flows in which the secondary phase provided the required pulse conductivity variation. In these studies the velocity profile and flow regime were identified using the common cross-correlation technique.     

Particle paths in three-dimensional flow fields as a means of study: Opposing jet mixing system

Particle paths can provide a tool for the analysis of mixing process. As an example, we will consider an opposing jet mixing system. Experimental, full-field, time-revolved, velocity vector data have been obtained using three-dimensional particle tracking velocimetry (PTV) technique. Using this field as a basis, hypothetical particles are injected anywhere and at any time into the field. The particles are then tracked as they move under the influence of the velocity field. The injected hypothetical particles can be neutrally buoyant or given a different density than the fluid. Color graphical representation of the results cannot be conveniently reported here; however, examples are provided in an equivalent gray-level coding. A more thorough presentation is possible by using color. These are provided at the URL (Uniform Resource Locator, i.e., the address for a location on the Internet) cited in Zhao and Brodkey [Y. Zhao, R.S. Brodkey, Particle paths in three-dimensional flow fields as a means of study: opposing jet mixing system, . See item on Paper to be published in a Special Issue of Powder, 1997].