引入气相对D类颗粒间歇卸料过程特性的影响

通过测量并分析引入气相前后D类颗粒间歇卸料过程中示踪颗粒的流动状态、压力分布和颗粒流率,发现床体内气固两相流动特性不仅随时间变化,还受到气速（正负压差）的影响。据此将卸料过程按时间分为3个阶段：初始蓄压（PS）阶段、稳定卸料（SD）阶段和非满管流（PP）阶段;并给出各卸料阶段不同正、负压差和重力条件下的气固流动特性。在时间较长、流场较为稳定的SD阶段,发现卸料口颗粒阻力是影响颗粒流率的关键参数,通过修正De Jong公式、Beverloo公式,依次建立卸料口颗粒阻力、D类颗粒卸料流率预测模型,与实验值吻合较好,有望为引入气相调控D类颗粒卸料流率的方法提供参考。     

Why the two-fluid model fails to predict the bed expansion characteristics of Geldart A particles in gas-fluidized beds: A tentative answer

It is well known that two-fluid models (TFMs) can successfully predict the hydrodynamics of Geldart B and D particles. However, up to now, TFM have failed to accurately describe the hydrodynamics of Geldart A particles inside bubbling gas-fluidized beds: Researchers have reported that bed expansions are over-predicted by as much as 70%. In this work we show—for the first time—that TFM can predict the correct bed expansion, without any artificial modifications, provided that a sufficiently fine grid size and small time step is used. This suggests that the previously reported failure of TFM is mainly due to the lack of scale resolution, and that from a modeling point of view there is no fundamental difference between Geldart A particles and Geldart B and D particles.     

鼓泡流化床内颗粒速度分布的研究

在内径0.185m 的鼓泡床内,采用PV-5A型速度仪考察不同表观气速下两种粒径GeldartB类物料 (玻璃微珠)在不同轴向位置颗粒速度径向分布规律。结果表明,在床层底部颗粒速度分布与分布器设计密切相关, 而远离分布器的上部区颗粒速度主要受气泡行为影响,表现为中心区域大而边壁附近小,且随着表观气速的增大这 种趋势变的更加明显;同时在这两个区域之间存在一个过渡区,该区域内分布器的影响明显减弱,导致颗粒速度的 径向分布逐渐趋于一定的规律;而在相同轴向位置处颗粒速度变化幅值的影响会随着表观气速的增大变得剧烈。     

D类颗粒节涌流态化的实验和数值模拟

应用双流体气固两相流模型,对均一粒径分布的Geldart D类颗粒的节涌流态化过程进行了模拟. 研究了流化过程中流化状态转变、床层膨胀比以及压力脉动功率谱,从Froude准数和颗粒动能的角度分析了模拟结果中的流化特性,并与实验数据进行了对比. 模拟结果准确预测了Geldart D类颗粒的节涌流态化特性,对床层膨胀高度和功率谱的预测与实验基本吻合. Froude准数和颗粒动能随时间的周期性脉动较好地反映了流化床内节涌流态化的气泡行为.     

Entrainment behaviour of high-density Geldart A powders with different shapes

Atomised and milled Ferrosilicon with average particle diameters of 38 and 50 µm respectively were fluidised with air at ambient conditions. The entrainment rate of the more spherical atomised particles was on average six times that of the irregularly shaped milled particles over the range of superficial velocities investigated. In an attempt to decouple the effect of particle size from shape, the bed was divided into theoretically isolated bins based on the distributions of particle sizes. This indicated that the atomised particles had a higher entrainment rate for particles smaller than approximately 25 µm whereas the opposite was true for particles greater than this size. None of the entrainment correlations investigated was able to predict the switch in entrainment behaviour as a function of particle sphericity and diameter. Furthermore, the traditional critical particle diameter associated with cohesive Geldart A particles was not observed for either of the two particle shapes. It is therefore concluded that neither the hydrodynamic nor Van der Waals forces acting on the particles can adequately explain the entrainment rate behaviour of differently shaped high-density Geldart A particles.     

Link between bubbling and segregation patterns in gas‐fluidized beds with continuous size distributions

Experiments involving a bubbling, gas‐fluidized bed with Gaussian and lognormal particle‐size distributions (PSDs) of Geldart Group B particles have been carried out, with a focus on bubble measurements. Previous work in the same systems indicated the degree of axial species segregation varies non‐monotonically with respect to the width of lognormal distributions. Given the widely accepted view of bubbles as “mixing agents,” the initial expectation was that bubble characteristics would be similarly non‐monotonic. Surprisingly, results show that measured bubble parameters (frequency, velocity, and chord length) increase monotonically with increasing width for all PSDs investigated. Closer inspection reveals a bubble‐less bottom region for the segregated systems, despite the bed being fully fluidized. More specifically, results indicate that, the larger the bubble‐less layer is, the more segregated the system becomes. The direct comparison between bubbling and segregation patterns performed provides a more complete physical picture of the link between the two phenomena. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Dependence of particle fluctuation velocity on gas flow, and particle diameter in gas fluidized beds for monodispersed spheres in the Geldart B and A fluidization regimes

In this paper we present new experimental data on the steady-state, mean squared, fluctuation velocity, or granular temperature, of Geldart B polymer, glass, nickel, and stainless steel monodispersed spheres averaged over the wall of a gas fluidized bed, as a function of gas flow and sphere diameter. The granular temperature is obtained by Acoustic Shot Noise technology—namely power spectral analysis of the steady state vibrational energy of the wall excited by random sphere impact, and calibrated by hammer excitation over the wall. The new data extends to polymer and metallic spheres the experimental discovery of a 1996 paper of Cody et al. that the fluctuation velocity of Geldart B glass spheres when scaled to the gas superficial velocity, U_s, is inversely proportional to sphere diameter, directly proportional to a fundamental length scale, D_o^B, and is a universal function of U = (U_s / U_mf). We also demonstrate that the new data is consistent with the diameter dependence of the fluctuation velocity that can be derived from both the 1997 paper of Menon and Durian, who measured random sphere motion near the wall through the spectroscopy of scattered laser light, and the 1992 paper of Rahman and Campbell, who measured the average granular pressure of random sphere impact on a porous steel membrane. While the inverse scaling of the fluctuation velocity with sphere diameter, and the existence of a fundamental length scale for gas fluidization, D_o^B, had not been a feature of any published fundamental model, or computer simulation, of the steady state granular temperature of spheres in gas fluidized beds, we show that it is a feature of two recent dense kinetic fluidization models published in 1999, by Buyevich and Kapbasov, and Koch and Sangani. Both theories implicitly define a fundamental length scale for the fluctuation velocity, D^? = (μ_f² / ρ_p²g)^1 / 3, where ρ_p is the sphere density, μ_f is the gas viscosity, and g is the laboratory gravitational field. The new data for polymer, glass, nickel and stainless steel spheres presented in this paper, defines D_o^B = (56 ± 2)D^?. We use the Anderson-Jackson stability model to show that the length scale D_o^B, also defines a stability length scale, such that for D < D_o^B(D > D_o^B), the uniform dense phase of the fluidized bed is stable (unstable), against one dimensional, first order fluctuations in sphere concentration. The length scale, D_o^B is thus the theoretical equivalent to the empirical scaling length introduced by Geldart, D_B/A, to distinguish spheres (D > D_B/A) that bubble at fluidization, from spheres (D < D_B/A) that fluidize before bubbling. Finally, we present new experimental data, on the remarkable changes in the granular temperature, bed expansion, and bed collapse time, between Geldart B and Geldart A monodispersed glass spheres, and compare that data to granular temperature, and bed expansion, for Geldart A rough, non-spherical, log-normal dispersed diameter catalytic particles.     

The effects of particle and gas properties on the fluidization of Geldart A particles

We report on 3D computer simulations based on the soft-sphere discrete particle model (DPM) of Geldart A particles in a 3D gas-fluidized bed. The effects of particle and gas properties on the fluidization behavior of Geldart A particles are studied, with focus on the predictions of U_mf and U_mb, which are compared with the classical empirical correlations due to Abrahamsen and Geldart [1980. Powder Technology 26, 35-46]. It is found that the predicted minimum fluidization velocities are consistent with the correlation given by Abrahamsen and Geldart for all cases that we studied. The overshoot of the pressure drop near the minimum fluidization point is shown to be influenced by both particle-wall friction and the interparticle van der Waals forces. A qualitative agreement between the correlation and the simulation data for U_mb has been found for different particle-wall friction coefficients, interparticle van der Waals forces, particle densities, particle sizes, and gas densities. For fine particles with a diameter , a deviation has been found between the U_mb from simulation and the correlation. This may be due to the fact that the interparticle van der Waals forces are not incorporated in the simulations, where it is expected that they play an important role in this size range. The simulation results obtained for different gas viscosities, however, display a different trend when compared with the correlation. We found that with an increasing gas shear viscosity the U_mb experiences a minimum point near , while in the correlation the minimum bubbling velocity decreases monotonously for increasing μ_g.     

Experiments on particle cluster behaviors in a fast fluidized bed

A three-dimensional (3D) fast fluidized bed with the riser of 3.0 m in height and 0.1 m in inner diameter was established to experimentally study the cluster behaviors ofGeldart B particles.Five kinds of quartz sand particles (dp =0.100,0.139,0.177,0.250 and 0.375 mm and ρp =2480 kg·m-3) were respectively investigated,with the total mass of the bed material kept as 10 kg.The superficial gas velocity in the riser ranges from 2.486 to 5.594 m·s-1 and the solid mass flux alters from 30 to 70 kg· (m 2· s)-1.Cluster characteristics and evolutionary processes in the different positions of the riser were captured by the cluster visualization systems and analyzed by the self-developed binary image processing.The results found four typical cluster structures in the riser,i.e.,the macro stripe-shaped cluster,saddle-shaped cluster,U-shaped cluster and the micro cluster.The increasing superficial gas velocity and particle sizes result in the increasing average cluster size and the decreasing cluster time fraction,while the solid mass flux in the riser have the reverse influences on the cluster size and time fraction.Additionally,clusters in the upper region of the riser often have the larger size and time fraction than that in the lower region.All these effects of operating conditions on clusters become less obvious when particle size is less than 0.100 mm.