A numerical study of fluidization behavior of Geldart A particles using a discrete particle model

This paper reports on a numerical study of fluidization behavior of Geldart A particles by use of a 2D soft-sphere discrete particle model (DPM). Some typical features, including the homogeneous expansion, gross particle circulation in the absence of bubbles, and fast bubbles, can be clearly displayed if the interparticle van der Waals forces are relatively weak. An anisotropy of the velocity fluctuation of particles is found in both the homogeneous fluidization regime and the bubbling regime. The homogeneous fluidization is shown to represent a transition phase resulting from the competition of three kinds of basic interactions: the fluid-particle interaction, the particle-particle collisions (and particle-wall collisions) and the interparticle van der Waals forces. In the bubbling regime, however, the effect of the interparticle van der Waals forces vanishes and the fluid-particle interaction becomes the dominant factor determining the fluidization behavior of Geldart A particles. This is also evidenced by the comparisons of the particulate pressure with other theoretical and experimental results.     

On the particle terminal velocity in a gas-liquid medium with liquid as the continuous phase

A mechanistic model was developed to describe the terminal velocity of spherical particles in a gas-liquid medium with liquid as the continuous phase. The model was established based on the momentum balance of a single particle taking into consideration the drag and buoyancy forces due to the liquid phase and the bubble-wake and particle interaction. The bubble-wake and particle interaction coefficient was shown to be a function of the particle properties only. A comparison of the particle terminal velocity between the model prediction and experimental data obtained in this study was also made over a wide range of particle size and gas velocity.     

A CORRELLATION FOR CYLINDER-TO-BED HEAT TRANSFER IN AERATED VIBRATED BEDS OF SMALL PARTICLES

ABSTRACT

Beds of alumina particles (d_p= 27 μm and 100 μm) were vibrated in the vertical direction at frequencies frdm 0–25 Hz and half-amplitudes from 0–4 mm. Air flow rate through a single-hole or multiple-holes bottom plate varied from 0 to 2 times the minimum fluidizing velocity. The contact heat transfer coefficients at resonance are much higher than those in packed beds and in vibrated fluidized beds (up to 1.2 times). The high heat transfer rates are due to enhanced particle mobility which reaches a maximum at the resonant point. A simple semi-empirical correlation is developed for contact heat transfer which is based on particle mobility. Heat transfer coefficients are correlated with frequency using amplitude, bed height and particle size as adjustable parameters. The correlation is found explain the observed trends in the data reasonably well over the range of parameters studied.     

Effects of acoustic vibration on nano and sub-micron powders fluidization

Fluidization of nano and sub-micron powders with and without acoustic vibration was investigated. The effects of sound pressure level and frequency were studied. Loudspeakers located under the distributor plate were used as the sound source to disintegrate larger agglomerates concentrated at the bottom of the bed. Nanoparticles showed fluid-like behavior similar to Geldart's A group and application of sound vibration improved their fluidization quality. Submicron particles were hard to fluidize and their fluidization quality was partially improved by sound excitation. Bed compaction, caused by rearranging of the agglomerates, was observed for submicron particles at low gas velocities while the bed was fixed. Nanoparticles did not experience any bed compaction. Sound vibration led to a decrease in minimum fluidization velocity and an increase in bed pressure drop and bed expansion for both types of particles. The fluidization quality of both particles increased at low frequencies, while the reverse was observed at higher frequencies. Fluidization of these particles was improved by increasing sound pressure level. There was a critical sound pressure level of 110 dB, below which the effect of sound vibration was insignificant. A novel technique was employed to find the apparent minimum fluidization velocity from pressure drop signals.     

Determination and comparison of rotational velocity in a pseudo 2‐D fluidized bed using magnetic particle tracking and discrete particle modeling

Modeling of dense granular flow has been subject to a large amount of research. Particularly discrete particle modeling has been of great importance because of the ability to describe the strongly coupled dynamics of the fluid and the solids in dense suspensions. Many studies have been reported on the validation of the translational particle velocities predicted by using these models. The rotational motion however has received far less attention, mainly because of the spherical nature of the particles under investigation and the lack of techniques with the capability to study the rotational behavior of the solid phase. In this study, we will for the first time present experimental data on the rotational behavior of particles in a pseudo two‐dimensional fluidized bed setup using Magnetic Particle Tracking. In addition the experimental results are compared to data obtained from discrete particle simulations. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3198–3207, 2015     

粘附性颗粒流化床内构件的选型

研究了多孔板式内构件和桨叶式内构件对粘附性颗粒流态化的影响. 实验表明,多孔板式内构件对流化质量的改善不好,桨叶式内构件的效果良好但仍然存在缺陷. 通过结合二者优点开发的孔桨式内构件克服了桨叶式内构件的缺点,显著地改善了粘附性颗粒的流化质量.     

大颗粒流态化特性的实验研究

通过对大颗粒流化曲线及床层高度的测试对大颗粒流化床的流化过程进行了研究。结果显示,大颗粒的流态化过程是一个渐进的过程,整个流化过程可以分为:床层高度恒定、颗粒位置调整、表面颗粒运动、节涌波动和完全流化5个阶段。由于颗粒自身特性的影响,导致大颗粒流化过程中的各个特征速度(如起始鼓泡流化速度和完全流化速度)产生了有别于小颗粒流化床的特性。     

Minimum fluidization velocity of binary mixtures

The fast defluidization method was used to measure the minimum fluidization velocities of binary systems. Based on the experimental data obtained from the published literature and from this work, different correlations used for predicting the minimum fluidization velocities of binary systems were evaluated and compared.     

压力脉动法预测硅粉颗粒最小流化速度的实验

应用压力传感器研究了不同筛分粒径的硅粉的流化性质,证实流化床层的压力脉动标准方差σp随着表观气速的增加而线性增大, 根据σp=0的条件即可确定流化床的初始流化气速Umf. 此Umf与传统压降变化法得到的实验结果基本一致. 对测得的不同筛分粒级的硅粉的Umf进行拟合,得到了Umf与相应粒级平均粒径的关联式Umf=0.014e10(d–0.28)–0.012e–10(d–0.28)+0.065. 对双粒级复配混合颗粒体系的σp进行的实验研究发现,其σp介于相关单粒级体系的σp之间,并且粗颗粒组份的比例对σp的影响较大.     

内置水平管的振动流化床中混合物的临界流化速度

在内置水平管的矩形振动流化床内研究了玉米粒和塑料珠混合物(Geldart D颗粒)中的流体力学性质。考察了颗粒质量分数、振动频率、振幅和内置水平管对混合颗粒的临界流化速度的影响。实验表明,振动的加入降低了混合物的临界流化速度。根据振动流化床的动力学特性,分析床层中颗粒受力情况,建立了混合颗粒物在振动流化床中临界流化速度的两类数学模型。通过实验数据建立两种模型的关联式所得的计算值和实验值误差控制在10%以内,吻合较好。     

生物质与煤混合颗粒流化特性的实验研究

在 D1 1 5 mm× 1 0 0 0 mm有机玻璃制成的圆柱型流化床中 ,对玉米秆、稻秆、煤及其混合物的流化特性进行了实验研究 .实验结果表明 ,单一生物质颗粒不能形成良好的流化状态 ,而加入一定量煤构成生物质和煤二组分混合颗粒可以实现稳定流化 .当生物质和煤混合颗粒中生物质的质量分率小于 5 0 %时 ,可以达到很好的流化状态 ,生物质和煤二组分混合颗粒的最小流化速度随生物质质量分率的增加而减小 .     

自动清洗式流态化换热器传热强化的运行参数优化研究

液-固流态化传热强化机理还不完全清楚,已有文献对流态化粒子的粒度与传热系数的影响关系有较大分歧。从传热强化角度对水-沙流态化自动清洗的运行参数进行优化试验研究,得到的结果是：沙子粒度不是愈细愈好,而是Dp2mm（8目）左右为好;粒子体积分数以2．4％比较好;流速在0．2～0．28m／s（Re=6000-9000）的范围较好。虽然浓度低,但是在优化条件下的流灰化传热强化幅度几乎可以达到一倍左右,并且阻力又很低。     

过热蒸汽流化床流化特性的实验研究

在底部直径为120 mm的锥型流化床中,以玻璃珠为流化颗粒,过热蒸汽为流化介质,研究了固体颗粒在过热蒸汽流化床中的流化特性,考察了操作温度和压力对临界流化速度(umf)的影响.结果表明,过热蒸汽流化床的流化行为与热空气相似,临界流化速度(umf)随床层温度的升高而减小,随床内压力的增大而减小;在相同温度条件下,过热蒸汽流化床的临界流化速度比热空气大.