纳米SiO2在添加磁性大颗粒磁场流化床中的流态化

研究了纳米SiO2在添加磁性大颗粒磁场流化床中的流化性能,实验中通过测量其床层膨胀曲线、床层压降曲线以及塌落曲线表明:磁性大颗粒在交变磁场下的振动作用可以破碎流化床中纳米SiO2的大聚团.使床层压降和床层膨胀明显增大,流化质量获得显著提高.实验同时考察了磁场强度、磁性颗粒的添加量以及不同的静床高度等参数对流化性能的影响.获得了最佳工艺操作条件.     

原生纳米颗粒SiO_2中添加大粒径组分的流化性能

以平均原生粒径为 10nm的SiO2 为原料 ,采用添加较大球形玻璃珠颗粒的方法 ,在内径为6 0mm的有机玻璃流化床中 ,考察了在不同添加量和不同添加颗粒粒径的情况下 ,纳米颗粒SiO2 的流化性能。实验表明 ,纳米颗粒SiO2 的流化经历了沟流、节涌、破碎和聚团 4个阶段 ,但当添加玻璃珠的量大于10∶1或添加玻璃珠粒径不大于 0 13mm时 ,SiO2 的流化质量较好 ,并可采用床层压降曲线来表示其流化性能     

原生纳米级颗粒的聚团散式流态化

在内径为 35mm的小型玻璃流化床中考察了平均原生粒径为 16nm的SiO₂ 颗粒的流化行为 .实验发现 :当操作气速远远超过原生纳米颗粒的最小流化气速时 ,这种超细颗粒可以通过自团聚实现稳定、均匀的散式流化 ,并可获得比GeldartA类颗粒更高的床层膨胀比和更宽的散式操作区 .由于原生纳米级SiO₂ 颗粒在流化时形成的团聚物具有较大的粒径和很小的密度 ,使得该气固体系的流化行为与液固体系有许多相似之处 ,该过程对于研究气固流态化的散式化具有重要意义     

磁性纳米Fe3O4在磁场协助作用下的流态化研究

磁性纳米Fe3O4的密度及黏性较大,透气性差,在传统流化床中很难实现“正常”流态化。实验采用外加磁场能的方法来改善这种状况,外加磁场、铁磁性大颗粒并与流化性能较好的非磁性物料混合流化。实验发现在一定磁场强度、添加质量分数及表观气速下,纳米Fe3O4的流诧性能得到改善,其在流化床中形成的大聚团得到有效的破碎,床层膨胀增加,压降趋子一个稳定值,最小流化速度降低,床层可以实现比较均匀的流化。     

C类颗粒添加纳米细粉后的流态化行为研究

为了研究纳米添加剂对细粉颗粒流化行为的影响,实验选用全床压降、最小流化速度、床层膨胀率、床层塌落曲线等表征细粉颗粒的流化特征。在直径为5.08 cm的小型流化床中研究了纳米添加剂SiO_2 (商品牌号R972)对细玻璃珠颗粒(Geldart A和C类颗粒)流化行为的影响。结果表明加入纳米添加剂后,C类颗粒的流动性得到明显的改善。与GeldartA类颗粒相比,经过表面改性的C类颗粒全床和浓相膨胀率更高,意味着更好的气固接触模式;在相同表观气速下,C类颗粒的流化更均匀,散式化程度更高。以上的发现对于C类颗粒的工业应用具有重要意义。     

纳米SiO2中添加大粒径组分的流化性能

以平均原生粒径为10nm的SiO2为原料，采用添加较大球形玻璃珠颗粒的方法，在内径为60nm的有机玻璃流化床中，考察了在不同添加量和不同添加颗粒粒径的情况下，纳米颗粒SiO2在流化性能。实验表明，纳米颗粒SiO2的流化经历了沟流、节涌破碎和聚团4个阶段，但当添加玻璃珠的量大于10：1或添加玻璃珠粒径不大于0．13mm时，SiO2的流化质量较好，并可采用床层压降曲线来表示其流化性能。     

纳米级SiO2颗粒流化床的塌落行为

在直径为 35mm的小型流化床中考察了纳米级SiO2 颗粒 (商品牌号R972 )的床层塌落行为 .该种经过表面改性的颗粒可以通过自团聚形成稳定、均匀的聚团散式流化 .实验证明 ,这种颗粒的床层塌落行为明显不同于普通的GeldartA类与C类颗粒 ,其塌落过程可分为恒速沉降阶段和减速压实阶段 .通过对塌落速率的分析 ,提出了聚团散式流化的床层塌落机制     

基于动态平衡预测纳米颗粒流化床内聚团尺寸分布

准确预测纳米颗粒流化床内聚团尺寸对研究纳米颗粒流化具有重要意义。鉴于目前文献中主要根据力平衡模型得出聚团尺寸平均值,然而聚团尺寸实际呈宽筛分分布,本工作从动态平衡角度,采用分子动理论中麦克斯韦气体速率分布律类比流化床内聚团速度分布,建立基于微观作用力的聚团解聚—重聚模型,并用以预测纳米颗粒流化床内聚团尺寸分布。通过比较聚团间结合力和分离力的相对大小,判断聚团结合和破碎;在分离力中,碰撞力起着至关重要的作用,且与聚团相对运动速度关系密切。预测结果显示,纳米颗粒流化床内聚团尺寸呈现卡方分布。将模型预测值和文献中的实验数据进行对比,包括TiO_2、Ni、SiO_2等颗粒的团聚尺寸,发现大部分偏差在20%以内。     

纳米级SiO2颗粒流化床的塌落行为

在直径为 35mm的小型流化床中考察了纳米级SiO₂ 颗粒 (商品牌号R972 )的床层塌落行为 .该种经过表面改性的颗粒可以通过自团聚形成稳定、均匀的聚团散式流化 .实验证明 ,这种颗粒的床层塌落行为明显不同于普通的GeldartA类与C类颗粒 ,其塌落过程可分为恒速沉降阶段和减速压实阶段 .通过对塌落速率的分析 ,提出了聚团散式流化的床层塌落机制     

基于DEM方法的旋转流化床纳米颗粒流动特性研究

基于DEM方法利用MFIX软件分别模拟了两种不同粒径纳米颗粒在旋转流化床内的流动。模拟结果与以往实验结果进行比较发现结果吻合较好，验证了旋转流化床模型的准确性和可靠性。对纳米颗粒在旋转流化床中的运动过程进行数值模拟，加入范德华力，得到了纳米颗粒分布、纳米颗粒速度矢量分布以及纳米颗粒床层压降在不同离心加速度下随气速的变化。结果表明，少部分纳米颗粒在旋转流化床上部游离，其余颗粒随旋转流化床运动，当纳米颗粒上升到60°旋转角附近时，颗粒出现回落，然后进行反复运动，由于纳米颗粒的运动受到颗粒堆积的影响，最终纳米颗粒在旋转流化床底部往复循环。纳米颗粒在旋转流化床内的压降均随离心加速度和气速的增加而增加，相同条件下TiO2颗粒的床层压降大于Al2O3颗粒的床层压降，而且与TiO2相比Al2O3颗粒先达到稳定状态。     

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.     

Fluidization of fine particles in a sound field and identification of group C/A particles using acoustic waves

Effects of sound field on the fluidization of fine particles have been comprehensively examined by using fine powders (4.8-65 μm average in size) including Al₂O₃, TiO₂, glass beads and FCC catalyst. It is found that the fluidization quality of fine particles can be enhanced with the assistance of a sound field, resulting in higher pressure drops and a lower u_mf. The effect of sound on the fluidization of fine particles is strongly dependent on the particle properties (Geldart type and particle size) as well as the parameters of the sound field such as sound pressure level (or intensity) and frequency. Given a fixed sound frequency, the effect becomes more significant at a higher sound pressure level. For the present sound-aided fluidized bed system, there is a resonant frequency at about 100-110 Hz, at which the effectiveness of the sound wave in improving fluidization of fine particles is most remarkable. In addition, based on the different attenuation features of sonic waves in the gas-solid suspension of group C and A particles, a novel acoustic method is explored to distinguish group C from group A particles.     

Parametric study of fine particle fluidization under mechanical vibration

Investigations into the effects of vibration on fluidization of fine particles (4.8-216 μm average in size) show that the fluidization quality of fine particles can be enhanced under mechanical vibration, leading to larger bed pressure drops at low superficial gas velocities and lower values of u_mf. The effectiveness of vibration on improving fluidization is strongly dependent on the properties (Geldart particle type, size-distribution and shape) of the primary particles used and the vibration parameters (frequency, amplitude and angle) applied. The possible roles of mechanical vibration in fine particle fluidization have been studied with respect to bed voidage, pressure drop, agglomeration, and tensile strength of particle bed. Vibration is found to significantly reduce both the average size and the segregation of agglomerates in the bed, thus improving the fluidization quality of cohesive particles. Also, vibration can dramatically reduce the tensile strength of the particle bed. Obviously, vibration is an effective means to overcome the interparticle forces of fine powders in fluidization and enhance their fluidization quality.     

Flow characteristics in an acoustic bubbling fluidized bed at high temperature

The pressure fluctuation of the quartz sand and SiO₂ particles was investigated using pressure transducer in high temperature fluidized bed with sound assistance. The effects of bed temperature, sound wave frequency, and sound pressure level (SPL) on the pressure fluctuation were examined. It indicates that the minimum fluidization velocity decreases with an increase in sound pressure level at the same sound frequency. At the same SPL and bed temperature, there always exists an optimal frequency range achieving good fluidization quality. As the sound frequency increases, the minimum fluidization velocity decreases firstly and then increases. Based on the statistical analysis of pressure signals, the effect of sound frequency on the fluidization quality at high-temperature fluidized bed was presented. On basis of discrete wavelet transform, an original signal was resolved into five-detailed scale signal. Furthermore, the peak frequency for Scale 3 detail signal represents the bubbling frequency.     

声场高温流化床流化特性

在内径50 mm、高1000 mm的声场高温鼓泡流化床中,研究Geldart A, B两类颗粒的流化特性,考察了床层温度、声波频率及声压级对流化床最小流化速度的影响. 结果表明,引入声场后,颗粒的最小流化速度随温度升高而下降;固定温度及频率,最小流化速度随声压级增大而减小;固定声压级与温度,颗粒最小流化速度随声波频率增大先减小后增大,存在一个最佳频率范围. 对床内压力波动信号进行分析,得出声场影响高温流化床流化质量的判据：当声压大于110 dB、频率在100~200 Hz范围内时压力波动偏差与最小流化速度值最小.     

大差异双组分混合颗粒的最小流化特性

在一套f260 mm′2000 mm的有机玻璃实验装置中,对大差异双组分混合颗粒的最小流化特性进行了实验研究,得到了混合颗粒的流化曲线,由此给出了其起始流化速度、最小流化速度、临界分离速度、完全流化速度等特征速度. 实验结果表明,流化过程可分为4个阶段,即完全流化、大小颗粒分离、大颗粒静止小颗粒流化、固定床阶段,对应混合颗粒的3个状态：完全混合、部分混合部分分离、完全分离状态;混合颗粒的特征速度随小颗粒质量分率的增加而减小,且在小颗粒质量分率达到0.4~0.5后其减小的趋势减缓;混合颗粒的固定床阶段和完全流化阶段的床层空隙率及混合颗粒的体积收缩比在小颗粒质量分率为0.4时达到极值.     

声场流化床流动特性研究进展

声场流化床是将声场引入普通流化床,采用颗粒为床层介质的流固相处理系统。声波可以有效降低颗粒聚团尺寸,显著改善超细颗粒的流化质量。本文介绍了声场流化床的基本原理以及近年来在基础研究和应用方面取得的进展及成果,综述了声场流化床在流体力学特性、颗粒特性、声场参数、流态化模型、颗粒团聚以及流化质量机理等方面的研究,并对声场流化床目前存在的问题及发展趋势提出了一些建议。     

Effect of sound on the fluidization of group B particles

The behaviour of several kinds of group B particles ranging from 100 μm to 600 μm was studied in a sound wave vibrated fluidized bed (SVFB). The fluidized bed consists of a transparent Plexiglas tube that is 54 mm i.d. × 1 m high. A speaker mounted at the top of the bed was supplied by a function generator with square waves and was used to generate the sound as the source of vibration of the fluidized bed. The influence of the particle size, density of particles and sphericity of particles on the minimum fluidization velocity, pressure fluctuations and bubble rise velocity in the SVFB was investigated. The minimum fluidization velocity decreased as the sound energy increased. When the sound energy was strong enough and greater than the critical power, the minimum fluidization velocity would approach the same value regardless of the degree of resonance (DOR) value if the particles were in spherical shape. For non-spherical shape particles the minimum fluidization velocity was the function of the DOR value if the power was greater than the critical power. For the middle particle size range, the standard deviation of pressure fluctuations in an SVFB became lower than the one without the effect of sound in high superficial gas velocity range, but the result was reverse for the low superficial velocity; for the large particle size range, the standard deviation of pressure fluctuations in an SVFB was larger than the one without the effect of sound. The sound could also reduce the bubble rise velocity in an SVFB.     

超细颗粒流化特性及团聚机理

在直径40mm的流化床中，考察了Al2O3、CaCO3等超细颗粒的流化特性，测定了床层压降变化、床层的坍落及膨胀规律.通过对超细颗粒自团聚现象的分析，探讨了超细颗粒稳定流化的机理.     

超细颗粒在声场流化床中的流化特性

在内径为130mm的声场流化床中,以原生纳米级SiO2超细颗粒为物料,在声压水平为0～140dB、声波频率为0～500Hz范围内系统地考察了声波对超细颗粒流化特性的影响。结果表明：当声波频率为100～150Hz、声压大于130dB时,声波可以有效地消除节涌、抑制沟流、降低临界流化速度,显著地改善纳米SiO2颗粒的流化质量。在频率一定的情况下,声压越高,超细颗粒的临界流化速度越低,流化质量越好。当频率低于100Hz或高于150Hz时,随着频率的进一步降低或增加,超细颗粒的临界流化速度都增大,甚至又出现节涌和沟流。声波的效果减弱甚至消失。