搅拌槽中固体颗粒离底悬浮机理的研究

The minimum fluid velocity to maintain particles just suspended was deduced, and the theoretical analysis shows that the minimum velocity is influenced by the properties of the solid and liquid, not by the operational conditions. For justification, the local minimum velocity at the bottom of the tank was measured by a bi- electrode conductivity probe, in a square-sectioned stirred tank (0.75 m×0.75m×1.0m) with the glass beads-water system. The experiments showed that the fluid velocities for the same suspension state were identical despite that the power dissipated per unit mass was not the same under different configuration and operation. Both theoretical analysis and experimental results indicate that the off-bottom suspension is controlled by the local fluid flow over the bottom of the stirred tank.     

Design factors affecting the dynamic performance of soil suspension in an agitated,baffled tank

The effects of particle size, impeller clearance and impel er speed are assessed to show how condition variations influence power consumption in the water-solid slurry suspension in an agitated tank. The energy efficiency of slurry height variation, impel er type and diameter, and solid movement speed has been investigated with six soil series stirred in a soil-water slurry. Coarser sand particles are observed to significantly increase power consumption, while finer particles, for instance clay, decrease the stirring power requirement. The 3-blade HR100 SUPERMIX? impeller manufactured by SATAKE general y performs more efficiently than a conventional 4-pitched blade turbine. The impeller's geometric design, including diameter and number of blades influences the impeller's energy efficiency, and HR100 impellers with greater diameters remarkably reduce power consumption. The tests demonstrated that the power required to provide off-bottom solid suspension and solid dispersion can be reduced dramatically by increasing the slurry height rather than by accelerating the impel er, if this option is possible.     

Gas dispersion and solid suspension in a three-phase stirred tank with triple impellers

The hydrodynamics is still not fully understood in the three-phase stirred tank equipped with multi-impeller due to the intensive interaction between phases. In this work, the solid critical suspension speed(NJSG), relative power demand(RPD) and overall gas holdup(ε_G) were measured in an air–water–glass beads stirred tank equipped with multi-impeller, which consists of a parabolic blade disk turbine below two down-pumping hydrofoils. Results show that either the NJSGor the specific power consumption increases when increasing the volumetric solid concentration or superficial gas velocity. RPD changes less than 10% when solid volumetric concentration ranges from 0 to 15%. ε_G decreases with the increase of solid concentration, and increases with the increase of both superficial gas velocity and the total specific power consumption. The quantitative correlations of NJSG,RPD and εGwere regressed as the function of superficial gas velocity, specific power consumption, Froude number and gas flow number, in order to provide the reference in the design of such three-phase stirred tank with similar multi-impellers.     

基于离散颗粒法模拟搅拌釜气含率分布

The discrete particle method was used to simulate the distribution of gas holdup in a gas-liquid standard Rushton stirred tank. The gas phase was treated as a large number of bubbles and their trajectories were tracked with the results of motion equations. The two-way approach was performed to couple the interphase momentum exchange. The turbulent dispersion of bubbles with a size distribution was modeled using a stochastic tracking model, and the added mass force was involved to account for the effect of bubble acceleration on the surrounding fluid. The predicted gas holdup distribution showed that this method could give reasonable prediction comparable to the reported experimental data when the effect of turbulence was took into account in modification for drag coefficient.     

Solid concentration and velocity distributions in an annulus turbulent fluidized bed

Solid concentration and particle velocity distributions in the transition section of a?200 mm turbulent fluidized bed (TFB) and a?200 mm annulus turbulent fluidized bed (A-TFB) with a?50 mm central standpipe were mea-sured using a PV6D optical probe. It is concluded that in turbulent regime, the axial distribution of solid concen-tration in A-TFB was similar to that in TFB, but the former had a shorter transition section. The axial solid concentration distribution, probability density, and power spectral distributions revealed that the standpipe hin-dered the turbulence of gas–solid two-phase flow at a low superficial gas velocity. Consequently, the bottom flow of A-TFB approached the bubbling fluidization pattern. By contrast, the standpipe facilitated the turbulence at a high superficial gas velocity, thus making the bottom flow of A-TFB approach the fast fluidization pattern. Both the particle velocity and solid concentration distribution presented a unimodal distribution in A-TFB and TFB. However, the standpipe at a high gas velocity and in the transition or dilute phase section significantly affected the radial distribution of flow parameters, presenting a bimodal distribution with particle concentration higher near the internal and external wal s and in downward flow. Conversely, particle concentration in the middle an-nulus area was lower, and particles flowed upward. This result indicated that the standpipe destroyed the core-annular structure of TFB in the transition and dilute phase sections at a high gas velocity and also improved the particle distribution of TFB. In conclusion, the standpipe improved the fluidization quality and flow homogeneity at high gas velocity and in the transition or dilute phase section, but caused opposite phenomena at low gas ve-locity and in the dense-phase section.     

Study on the solid–liquid suspension behavior in a tank stirred by the long-short blades impeller

We investigated the solid–liquid suspension characteristics in the tank with a liquid height/tank diameter ratio of 1.5 stirred by a novel long-short blades(LSB) impeller by the Euler granular flow model coupled with the standard k–ε turbulence model. After validation of the local solid holdup by experiments,numerical predictions have been successfully used to explain the influences of impeller rotating speed,particle density, particle size, liquid viscosity and initial solid loading on the soli...     

HYDRODYNAMIC MODEL FOR THE IMPELLER REGION IN AN AERATED AND STIRRED TANK

The radial and axial distribution of mean 1iquid velocity were measured by a.hot-filmanemometer at the impeller region in an aerated and stirred tank 0.287m in diameter.The tangentialjet model for impeller discharge flow used for single phase flow was modified to conform with thecharacteristics of gas-liquid flow.The radial and axial velocity profiles at the impeller region in thegas-liquid stirred tank were calculated by the model The results predicted by the model were in goodagreement with those obtained in experiment.     

Critical impeller speeds for a gas-inducing stirring tank loaded with solid particles

The influence of solid particles size,density and loading on the critical gas-inducing impeller speed was investigated in a gas–liquid–solid stirring tank equipped with a hollow Rushton impeller.Three types of solid particles,hollow glass beads with diameters of 300 μm,200 μm,100 μm,and 60 μm,silica gel and desalting resin,were used.It was found that the adding solid particles would change the critical impeller speed.For hollow glass beads and silica gel,whose relative densities were less than or equal to 1.5,the critical impeller speeds increased with the solid loading before reaching the maximum values,and then decreased to a value even lower than that without added solids.The size of the solids also had apparent influence on the critical impeller speed,and larger solid particles correspond to a smaller critical impeller speed.The experimental data also showed that the gasinducing was beneficial to the suspension of the solid particles.     

Numerical optimization for blades of Intermig impeller in solid–liquid stirred tank

The multiphase flow in the solid-liquid tank stirred with a new structure of Intermig impeller was analyzed by computational fluid dynamics(CFD).The Eulerian multiphase model and standard k-ε turbulence model were adopted to simulate the fluid flow,turbulent kinetic energy distribution,mixing performance and power consumption in a stirred tank.The simulation results were also verified by the water model experiments,and good agreement was achieved.The solid-liquid mixing performances of Intermig impeller with different blade structures were compared in detail.The results show that the improved Intermig impeller not only enhances the solid mixing and suspension,but also saves more than 20% power compared with the standard one.The inner blades have relatively little influence on power and the best angle of inner blades is 45°,while the outer blades affect greatly the power consumption and the optimized value is 45°.     

Numerical Study of Solid-Liquid Two-Phase Flow in StirredTanks with Rushton Impeller(Ⅱ) Prediction of Critical Impeller Speed

The critical impeller speed, NJS, for complete suspension of solid particles in the agitated solid-liquid two-phase system in baffled stirred tanks with a standard Rushton impeller is predicted using the computational procedure proposed in Part I. Three different numerical criteria are tested for determining the critical solid suspension. The predicted NJS is compared with those obtained from several empirical correlations. It is suggested the most reasonable criterion for determining the complete suspension of solid particles is the positive sign of simulated axial velocity of solid phase at the location where the solid particles are most difficult to be suspended.     

穿流-刚柔组合桨强化固液两相的悬浮行为

采用双向流固耦合计算方法对搅拌槽内刚性桨（RDT-PBDT）、刚性组合桨（R-RDT-PBDT）、刚柔组合桨（RF-RDT-PBDT）、穿流-刚性组合桨（PR-RDT-PBDT）以及穿流-刚柔组合桨（PRF-RDT-PBDT）体系中的固液两相悬浮特性和桨叶总变形量、等效应力进行了研究。结果表明,在相同功耗下,PRF-RDT-PBDT的最大变形量分别是RDT-PBDT、R-RDT-PBDT、RF-RDT-PBDT、PR-RDT-PBDT的1.984×10⁶倍、1.247×10³倍、1.169倍、1.041×10³倍。PRF-RDT-PBDT的应力大于RDT-PBDT、R-RDT-PBDT、RF-RDT-PBDT的,比PR-RDT-PBDT的应力分布更均匀。PRF-RDT-PBDT体系的固体颗粒最大U_z/U_tip值和最大ε/D²N³值分别比RDT-PBDT、R-RDT-PBDT、RF-RDT-PBDT和PR-RDT-PBDT体系提高了53.08%和80.84%,38.03%和28.42%,22.14%和20.16%,10.85%和5.725%。PRF-RDT-PBDT能够增大与流体之间的相互耦合作用,增大固体颗粒的轴向速度,提高体系的湍动能耗散率,减小搅拌槽底部固体颗粒的堆积程度,提高固体颗粒的悬浮程度。     

新型自吸式多相搅拌槽临界悬浮特性的实验研究

在不通气的情况下,对新型多相搅拌槽进行实验,得出了其固相含量w、颗粒密度r、搅拌槽内径D与临界悬浮转速Nc的关系,在相同条件下,与给定结构尺寸的标准搅拌槽对比了临界悬浮转速. 结果表明,Nc随固相分率j增加和粒径dp增大而增大,不利于达到良好的悬浮效果;固相颗粒密度越大,临界悬浮转速随之增大;槽径增大使达到悬浮状态的临界转速降低. 该新型搅拌槽临界悬浮转速的关联式是Nc=6.3dp0.21(gDr/rL)0.45w0.19/D0.55;同一固相分率下,新型多相搅拌槽的临界悬浮转速比标准搅拌槽降低了30%,在较低转速下就能使颗粒悬浮.     

无挡板搅拌槽中液－固体系的分散特性

在内径0.3 m、高0.45 m的无挡板搅拌槽内,采用直径0.15 m的三叶70o下推斜叶透平桨(PBTD, Pitched Blade Turbine Downflow)进行水-二氧化硅两相体系液固分散特性的研究. 应用PC-6A粉体浓度测量仪对体系中颗粒局部浓度进行测定. 考察了颗粒平均相含率为0.005的条件下,不同搅拌转速、搅拌桨离底高度对颗粒局部浓度分布的影响. 结果表明,采用较高搅拌转速、较低的搅拌桨离底高度有利于固体颗粒的悬浮. 本实验中,在搅拌转速为173 r/min、搅拌桨离底高度为0.08 m的操作条件下,颗粒悬浮效果最好.     

固-液导流筒搅拌槽内流体流动和颗粒悬浮特性

在直径0.8 m的导流筒搅拌槽内,对单相液体的三维速度分布、固-液两相的固体颗粒浓度分布和离底悬浮特性进行了系统的实验研究. 结果表明,导流筒内外的轴向液相速度远大于径向和切向速度,导流筒外壁附近存在一个与主体轴向流动方向相反的二次流区域;搅拌槽底部结构对固体颗粒的临界离底悬浮转速(NJS)有显著的影响,浅锥底的NJS比平底的低14%以上;NJS随固相浓度的增加而增加,但当浓度超过50%时,NJS略有降低;槽内固相浓度分布的均匀性随固相浓度的增加而得到改善. 本研究结果对导流筒搅拌槽的优化设计具有一定的指导意义.     

应用电导探针测定固－液两相流的局部速度

研制了用于测定固－液两相流液相局部速度的双电极电导探针，并在０．７５ｍ×０．７５ｍ×１ｍ方形固－液搅拌槽内测定了叶轮排出流区的速度分布。     

搅拌槽内固体颗粒对液相速度影响的研究

采用自行研制的双电极电导探针 ,在 0 .75m× 0 .75m× 1m方形搅拌槽中 ,测定了玻璃珠 水体系中固体颗粒对液相速度的影响。实验结果表明 :加入密度大于水的固体颗粒后 ,轴流式向下泵送液体搅拌桨的表观排出流量准数增大 ,但实际排液能力并不增加 ;近壁上流区液相速度衰减 ,衰减幅度随固相浓度不同而不同 ;由于固体颗粒的加入 ,搅拌槽底部液相速度衰减 ,与近壁上流区相比衰减幅度较小。近壁上流区液相速度与搅拌转速成正比 ;不同直径的搅拌桨 ,输送功率相同时 ,近壁上流区液相速度相等。近壁上流区液相速度可作为固液悬浮搅拌槽设计的重要参数     

通气条件下桨型对自浮颗粒悬浮的影响

考察了五十二种搅拌桨的组合对搅拌釜内自浮颗粒的气液固三相体系混合问题的功耗、气含率和釜底部的颗粒含量的影响。研究表明 ,对自浮颗粒三相体系的搅拌混合应采用多层桨 ,且上层最好用上推式桨 ;实验发现 ,当高径比为 1 .6时 ,三层桨的混合参数优于两层桨的 ;给出了优异的搅拌桨型。