The influence of inclined plates on expansion behaviour of solid suspensions in a liquid fluidised bed — A computational fluid dynamics study

A significant increase in the particle sedimentation rate can be achieved by introducing inclined plates into conventional fluidised beds. In turn, high suspension densities are possible at fluidisation velocities in excess of the particle terminal velocity. The installation of the inclined plates, however, alters the dynamic characteristics of the fluidised bed, in particular, impacting upon the expansion behaviour of the suspension. In the present work a Computational Fluid Dynamics (CFD) approach was employed to investigate the influence of inclined plates on the expansion behaviour of solids suspensions in liquid fluidised beds. The model is based on the solution of the Eulerian multiphase equations for up to two different particle sizes with a continuous phase of water. The momentum equations treat hindered settling behaviour via the inclusion of a volume fraction dependent drag law. The computational model was validated against our experimental data and compared with the predictions of a kinematic model developed in one of our earlier works. In general the predictions made by both the CFD and the kinematic models were found to be in good agreement with the experimental results.     

Combustion of coal-water suspensions

Combustion technologies using coal-water suspensions create a number of new possibilities for organising combustion processes so that they fulfil contemporary requirements (e.g., in terms of the environment protection-related issues). Therefore, an in-depth analysis is necessary for examining the technical applications of coal in the form of a suspension as a fuel. The paper undertakes the complex study of coal-water suspension combustion in air and in the fluidised beds. This, according to the author, best simulates the conditions that should be satisfied in order to use the new “fuel” efficiently and ecologically. An important element of the study was the identification of coal-water suspension drops, its complex morphology and evolution in the combustion process. The mathematical model enables the prognosis for change of the surface and the centre temperatures and a mass loss of the coal-water suspension during combustion in air and in the fluidised bed.     

Yield Stress of Multimodal Powder Suspensions: An Extension of the YODEL (Yield Stress mODEL)

The prediction of the rheological properties of concentrated suspensions is of great importance both in industrial processes (ceramics, cements, and pharmaceutics) and natural phenomena (debris flow, soil erosion). In a previous paper, we presented a new model (YODEL) that can predict the yield stress of concentrated particulate suspensions. The model is based on first principles and takes into account particle size distribution, interparticle forces, and microstructural features. It was validated using data from the literature on four different alumina powder suspensions. The current paper extends the application field of the YODEL, successfully, to multimodal distributions of much interest in the cement and concrete field. The key parameter governing the predictive capacity of the YODEL for multimodal distributions was shown to be the maximum packing fraction of the powder mixtures. The de Larrad compressive packing model was used to provide a maximum packing fraction for mixtures from their particle size distributions. The YODEL can predict yield stresses of multimodal suspensions within 10% of the experimental results. Further improvement of the maximum packing fraction prediction should help in our goal of yield stress prediction from basic powder and suspension characteristics.     

微旋流沉降器分离效率的准数模型

通过因次分析和实验数据回归建立了新型微旋流沉降器的无因次准数模型。用模拟悬浮液通过改变流体流量?原水浊度?絮凝剂量?沉降器波纹板斜槽的直径进行沉降实验,测定进水流量、絮凝体沉速、进出水及澄清器内各点溢流水的浊度。结果表明:絮凝体沉速对出水浊度的影响最大;流体流速较小时,增大流速出水浊度减小,流速较大时,增大流速出水浊度增大;波纹斜槽直径越小分离效率越高。实验数据回归的分离效率准数模型与实验及工业数据吻合良好。     

Role of inertial lift in elutriating particles according to their density

A Reflux Classifier involves the liquid fluidization of particles into a set of parallel inclined channels. Closely spaced inclined channels promote a combination of laminar flow and a high shear rate, which in turn promote the elutriation of the particles according to their density. The hydrodynamics of the particle transport within the inclined channels was examined theoretically by combining established equations for describing the fluid flow, the terminal velocity of a particle, and the shear induced inertial lift, with no adjustable parameters. The theoretical calculations provided excellent agreement with a comprehensive experimental data set, demonstrating the significance of the inertial lift force that arises at a high shear rate under the condition of laminar flow. Complex features of the experimental data were described theoretically. This work explains how it is possible to elutriate particles according to their density, with the effects of particle size suppressed. A remarkable convergence of several criteria was found to be necessary for achieving the reported phenomena.     

斜板沉降器处理负荷的计算

依据双膜模型和液滴在斜板层膜上的有效聚结 ,对层膜层流的临界雷诺数进行了关联 ,得出了斜板沉降器处理负荷的计算式。经实验数据验证 ,处理负荷的计算误差在 8%以内。     

Gas mixing in the cocurrent downflow circulating fluidised bed

An experimental study on the gas dispersion behaviour of a cocurrent downflow gas-solids suspension in a 140 mm i.d. circulating fluidised bed (CDCFB) using the steady-state tracer method is presented. The influence of gas velocity, solids circulating rate and particle density on radial gas dispersion has been examined. Gas dispersion can well be described by an eddy diffusion mechanism and a proposed two dimensional dispersed plug-flow model can fit the experimental data very well. Correlations of the radial diffusion coefficient were obtained. It is found that the axial diffusion coefficient obtained in the CDCFB is much lower than that in conventional circulating fluidised beds.     

用动态规划法确定倾斜矿床露天开采联合运输时转载站的合理位置

提出了倾斜矿床露天开采联合运输时用动态规划法选择露天矿各开采时期转载站设置的最佳方案。所建立的优选模型可用于新矿设计时和生产矿改扩建时联合运输系统的优化。     

斜板式气液分离器的设计

针时常规卧式和立式气液分离器的缺点和不足,论述了斜板式气液分离器的分离机理和结构特点,提出了采用斜板结构有利于提高气液分离效率的新思路,建立了斜板式气液分离器的理论计算公式,并以常规卧式分离器和立式分离器为例进行了对比计算。结果表明斜板式气液分离器可以提高气液分离器的分离效率,即在相同结构外径与处理量的前提下,可以缩短分离器的长度。或在相同结构条件下可以降低带出液体的粒径。     

Estimation of the batch-settling flux function for an ideal suspension from only two experiments

Modelling the sedimentation of suspensions with partial differential equations requires constitutive relations (material properties) to be known. Restricted to suspensions obeying Kynch's assumption (ideal suspensions), this paper deals with the inverse problem, which is to estimate the batch-settling flux function from experimental data. A new batch-settling test is suggested, from which it is theoretically possible to estimate a large part of the flux function for lower concentrations containing the extreme point. From a standard batch-settling test, a large part of the flux function for higher concentrations can be estimated with the famous method by Kynch. For these two parts, simple general explicit formulae are derived, which contain only the initial concentration and height variables, the interface height and its derivative as a function of time. The method is demonstrated on synthetic and experimental data. Further experimental development of the new test is required. The aim of the paper is to present a theoretical foundation for the method, including the explicit formulae as a solution of the inverse problem.     

Rheological behavior of polydispersed bubble suspensions in shear flows

Effects of the capillary number and size distribution of bubbles on rheological properties of bubble suspensions with different volume fractions were investigated through both experimental and theoretical analyses. Polydispersed bubble suspensions were prepared by using a bubble making device and characterized in steady and dynamic oscillatory shear flows to examine the effect of the bubble size distribution. To predict the rheological properties of the polydispersed bubble suspension, a constitutive equation was modified by applying superposition schemes, i.e., a probability density function was introduced to reflect the size distribution of bubbles. Dilute bubble suspensions showed shear thinning and linear viscoelastic behavior. At small capillary numbers, relative viscosity increased with increasing bubble volume fraction, whereas it decreased as bubble volume fraction increased at relatively large capillary numbers. It was found that rheological properties had been affected significantly by polydispersity of bubble suspensions and theoretical results were in good agreement with the experimental results. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

A new empirical viscosity model for ceramic suspensions

This paper presents a new predictive viscosity model for ceramic suspensions. Starting from Einstein's model (1906), various theoretical, empirical, and phenomenological models have been proposed for different suspension systems. However, there is still a lack of reliable model for ceramic suspensions used in colloidal ceramic shape-forming methods. Here, the rheological properties of ceramic suspensions comprising NiO/YSZ (nickel oxide/yttria stabilized zirconia) as the ceramic powder, and furfuryl alcohol as the suspending media were measured over a range of shear-rates (between 1 and 1000 s⁻¹) and different solid volume fractions from 0 to 0.4010. An empirical equation was then developed for the ceramic suspensions using the mobility parameter (?/(?_m−?)), which links Einstein's model with the more recent relative viscosity models. The proposed model was used to predict the relative viscosity data, showing excellent agreement to the experimental data from this study and with reported data in literature for other ceramic systems. The model was also used to estimate the maximum solid volume fraction for the ceramic suspensions (?_m=0.571), with better accuracy than those estimated by existing models.     

Wall slip of concentrated suspension melts in capillary flows

The effects of wall slip of concentrated suspension melts in capillary flows were investigated at elevated temperature. The modeled material is a mixture of polymer EVA (Ethylene Vinyl Acetate) and non-colloidal spherical powder (glass microspheres) with mean particle size within 53∼63 μm. The effect of particle concentration on wall slip was studied experimentally in a capillary rheometer. For suspensions with different particle loadings (35%, 40%, and 45% by volume), the slip velocity V_s increased with an increase of particle concentration at the same testing temperature. A master slip curve can be obtained by plotting slip velocity versus the product of wall shear stress and square root of particle concentration. As such, a new particle concentration-dependent slip model is proposed. A theoretical approach coupled with the new slip model and flow equation is employed to characterize the flow behavior of concentrated suspension in a capillary rheometer, with reasonable agreement obtained with experimental observations.     

A theoretical and experimental study of the crossflow microfiltration process of silica particles in an aqueous suspension

This work presents a theoretical and experimental analysis of a crossflow microfiltration process of silica particles in suspension. The silica suspensions were 0.001 M of NaCl with a pH of 6 (to maintain a constant ionic force within the medium to produce a stable silica particle suspension) for three different concentrations of silica particles: 100, 300, and 500 mg L⁻¹. The membrane used in the crossflow microfiltration experiments was a commercial polymeric membrane, microporous, asymmetric with a nominal pore diameter of 0.2 µm, manufactured by OSMONICS (Minnetonka, MN). The experiments were performed in a bench scale crossflow microfiltration system with a flat rectangular membrane cell. The permeate flux was obtained as a function of the transmembrane pressure, the crossflow velocities, and the silica particles concentration. The mathematical model describing the process takes into account the variation of the physical properties of the suspension (dynamic viscosity and mass diffusivity) with the silica concentration. The experimental data are used to predict the maximum silica concentration at the membrane surface as a function of the operating conditions.