螺带桨叶在粘弹性流体中的搅拌功率

本文研究了层流区内外单螺带-锚、四螺带-锚桨叶在粘弹性流体中的搅拌功率消耗。与牛顿型流体比较结果表明,流体的弹性对搅拌功率有较大的影响。Boger流体较牛顿型流体的功率消耗大50％以上。 本文采用了广义二阶流体本构方程导出搅拌功率计算式: 用实验数据确定f_3和F_(1av)~*,得到了适用于牛顿型流体、假塑性流体和粘弹性流体的功率计算式。计算结果与实验值比较接近。     

粘弹性流体在槽式反应器中的流动 功耗和混合

提出了粘弹性流体适于化工传递过程研究的流变模型和无量纲数,即Re~*=d~2Np/η_α,W_i=σ_1N/η_α,N_(VR)=σ_2/σ_1和r=W_i/Re~*。对比了牛顿流体、假塑性流体和粘弹性流体在数种搅拌器下的流型、流速分布、功耗和混合时间,并提出了适用于这三种流体的搅拌功率关联式: N_pRe~*f_s~(1-m)=K_p[1+(F_(1av)~*/k_s~2)f_s~((1-m-3)W_i]     

螺带式搅拌桨叶的功率消耗模型

<正>在化工生产中得到广泛应用的螺带式搅拌器,其大部分功率数据是以实验为基础进行关联的,也进行过半理论的关联.通常,一个物理模型要能满足大量实验数据的检验,并要求使用方便.本文旨在开发这样的模型,以便于计算螺带式搅拌器的功率消耗、循环流量以及Metzner常数,为工程设计放大提供参考.     

锚式搅拌槽中高粘弹性流体的流速分布及其功率消耗

用照相法测定了锚式搅拌槽中高粘弹性流体的流型和流速分布,另测定了搅拌功率消耗,结果发现:1.与牛顿流体相比,在低Re数下,粘弹性流体的切向速度较大,而径向速度则较小.2.转速相同时,在高剪切率区域,粘弹性流体的剪切率大于牛顿流体.由CEF方程导出功率计算式N_pRe_af_s~(1-n)=k_pf_vf_s~2[1+F_1avf_s~(m-n-3)Wi/K_s~2]用实验数据确定f_(?)和F_(1av),得到可适用于牛顿流体、假塑性流体和粘弹性流体的普适功率计算式,计算结果与实验值比较接近.     

流体弹性对圆盘反应器功率特性的影响

以工业聚酯终缩聚圆盘反应器开发应用为研究背景 ,分别考察了圆盘反应器在黏性和黏弹性介质中的功率特性 ,通过对两种实验介质中功率特性的比较 ,得出弹性对圆盘反应器功率消耗的影响程度 ,并给出相应的功率消耗关联式 ,为聚酯终缩聚圆盘反应器国产化开发应用提供功率设计依据 .     

高粘度假塑性流体搅拌功率的研究

本文研究了适用于高粘度假塑性流体的螺带螺旋组合式新型搅拌器的搅拌功率，并得出了搅拌功率并联式。     

具有黏结颗粒的搅拌槽功率特性

功耗是搅拌反应器设计和放大过程的重要参数。在圆柱形搅拌槽内通过实验研究了相对液体体积、转速和物料高度对功耗的影响,采用离散元方法(DEM),用Hertz-Mindlin with JKR模型模拟了不同相对液体体积颗粒的黏结力对功耗的影响。结果表明,随相对液体体积增大,功耗呈先增大后减小的变化趋势,在相对液体体积为0.0162时达到最大值。随转速增大,功耗增长变快,但随相对液体体积增大,转速对功耗的影响指数呈先减小后增大的趋势。不同物料高度的功耗最大值均为饱和颗粒的1.8倍,不同物料高度的单位质量功耗随相对液体体积的增长速率几乎一致。提出了新的间接测量颗粒间黏结力的方法,发现功耗与黏结力呈线性关系。     

假塑性流体搅拌槽内停留时间分布

在直径φ500 mm和高350 mm的无挡板半球底有机玻璃槽中,采用45°斜叶桨研究了流量、搅拌功率、全槽平均表观牯度和物料流变特性对停留时间分布(RTD)的影响.结果表明,对于假塑性流体(流变指数为0.654)和牛顿流体,流量对RTD的影响两者差别不大,搅拌功率的增加更容易改变假塑性流体的RTD,使其更趋于全混流;对于牛顿流体,当搅拌功率大到一定程度后,再增加搅拌功率收效不大.假塑性流体的相对全混釜数随着全槽平均表观粘度的增加而增加,在全槽平均表观粘度相同时,假塑性流体的相对全混釜效大于牛顿流体,随着流体假塑性的增强,假塑性流体与牛顿流体相对全混釜数的差值显著增大.     

柔性桨强化高黏度流体混合的能效分析

引言搅拌混合操作广泛应用于化工、食品、冶金及环保等过程工业领域[1-4]。搅拌槽内物料的混合程度及其功率消耗是影响产品质量和设备作业效率的关键因素。当物料黏度较高时,搅拌槽内的流     

Power Requirement when Mixing a Shear‐Thickening Fluid with a Helical Ribbon Impeller Type

The optimal design of close clearance impellers requires the knowledge of the power demand of the mixing equipment. In non‐Newtonian mixing, this can be readily obtained using the Metzner and Otto concept [1]. In this work, this concept and the determination of the K_s value for an atypical helical agitator (PARAVISC system from Ekato firm) have been revised in the case of shear‐thinning fluids and a shear‐thickening fluid. For poor shear‐thinning fluids, it has been shown that for our mixing system the K_s value does not vary strongly with the flow behavior index, and may be regarded as a constant for the mixing purpose design. By contrast, for the shear‐thickening fluid, power consumption measurements indicate that the relationship between the K_s values and the flow behavior index is much more complex due to a partial solidification of the product around the impeller.     

螺带型搅拌槽内异物性液体的混合性能

针对高固含量木质纤维素同步糖化与发酵生产燃料乙醇过程,研究了螺带型搅拌槽内层流流动条件下少量低粘低密度液体(a)加入大量高粘高密度主体液体(b)中的混合性能,考察了转速、两种液体体积比(Va/Vb)、粘度比(mb/ma)、密度差等影响因素. 结果表明,当Va/Vb>1%时,异物性物系混合时间远高于相同物性液体混合时间,可延长2~5倍;Va/Vb对混合时间影响最显著,Va/Vb=0.2%~3.7%范围内,无量纲混合时间Ntm随体积比增大先降低后升高存在一最小值;当Va/Vb≥2%,单位主体液体密度之差Dr/rb>0.2时,Ntm随着Va/Vb, Dr/rb增加线性增大;Ntm随着mb/ma增大而减小;加入相加在主体液体内所得混合时间比加在液体表面时短;Va/Vb>2%时,混合工况可分为Re数控制区(桨叶控制)及Ri数控制区(重力控制).     

螺带式搅拌器的循环时间分布及其混合特性

采用单个悬浮粒子法和脱色法分别测定了多种不同几何尺寸的螺带式搅拌器的循环时间分布以及混合时间。得到同频度C_5值和混合时间C_1计算式。考察了流体的弹性对循环时间分布和循环流量的影响。     

Viscoelastic Liquids in Stirred Vessels – Part I: Power Consumption in Unaerated Vessels

Different shear‐thinning and elastic fluids (STE fluids) have been stirred under unaerated conditions, in vessels equipped with Rushton disc turbines. Their power consumption has been evaluated over a wide range of stirring rates and their Metzner‐Otto constant (k_s) has been measured. A correlation has then been proposed to predict k_s values for a Rushton turbine operating in non‐Newtonian solutions. Power curves of STE fluids have been drawn and compared with reference curves (Newtonian, shear‐thinning inelastic and elastic with constant shear viscosity fluids). The STE fluids have thus been divided into two categories. The STE fluids of the first category (STE I fluids), which are concentrated viscous solutions of polymers (guar, CMC) reducing the power consumption at the beginning of the transitional region and connecting with the Newtonian reference at higher Reynolds numbers. In contrast, STE solutions of the second category (STE II fluids), which are solutions of drag reducing polymers (PAA), are less viscous and more elastic. They reduce the power consumption at the end of the transitional region and do not connect with the Newtonian reference, at least until Re = 6000. A general correlation has finally been proposed to model the power curve of STE fluids stirred by a Rushton turbine from the laminar to the turbulent regions, as a function of their elasticity.     

Power Consumption in Agitated Vessels with Dual Rushton Turbines: Baffle Length and Impeller Spacing Effects

The dependence of power consumption on baffle length, L, in vessels agitated by a dual Rushton turbine system was studied within the turbulent regime, and also in relation to the impeller spacing, ΔH. A dependence of varying strength could be observed. The presence of baffles in the agitated systems provided a stabilizing effect with regard to the dependence of the Newton number Ne on the Reynolds number Re. A sharp decrease in power consumption could be detected for baffle lengths L < 0.3 H, with H the liquid height in the vessel. The Newton number was not significantly affected by L in the range 0.3 H < L< 0.5 H. For L > 0.5 H a sharp increase in Ne with increasing L could be observed. The two Rushton turbines act independently at ΔH > 1.65 d, with d being the impeller diameter. As the baffle length decreased, an increased mutual interaction between the two impellers could be observed for a broad regime of ΔH/d values. Ne was not affected by ΔH for the unbaffled agitated systems studied.     

Power Consumption in Dual Impeller Gas‐Liquid Contactors: Impeller Spacing,Gas Flow Rate,and Viscosity Effects

The dependence of power consumption on impeller spacing, and also in relation to gas flow rate and viscosity, in unaerated and aerated gas‐liquid contactors agitated by dual Rushton‐ and by dual pitched blade turbines was comparatively studied. In tap water the two Rushton impellers acted independently for spacings greater than ΔH = 1.65d, while in glycerol solutions the impellers acted independently on reaching an impeller spacing equal to 1.20d; the corresponding values for the two pitched blade impellers were 1.50d for tap water, 1.07d for relatively high viscosities, and 0.53d for very high viscosity values. The Newton number Ne decreases with increasing viscosity for the dual Rushton turbine systems, while an increase of Ne can be observed with increasing viscosity for the corresponding pitched blade systems. For the dual Rushton turbines, gas flow number Q has no effect on Ne, at very high values of viscosity, while at low and relatively high viscosity values a small effect of Q on Ne can be detected. As observed for the dual Rushton turbine systems, Ne is also not affected by Q for the corresponding pitched blade systems at very high viscosity values. Flow number Q does not significantly affect the Newton number for the water‐glycerol solutions with a relatively high viscosity agitated by dual pitched blade turbines, while for the aerated water systems a decrease of Ne can be observed at relatively small gas flow numbers; high values of Q do not affect the Newton number.     

Using CFD and Ultrasonic velocimetry to Study the Mixing of Pseudoplastic Fluids with a Helical Ribbon Impeller

A commercial CFD package was used to simulate the 3D flow field generated in a cylindrical tank by a helical ribbon impeller. The study was carried out using a pseudoplastic fluid with yield stress in the laminar mixing region. Ultrasonic Doppler velocimetry (UDV), a noninvasive fluid flow measurement technique for opaque systems, was used to measure xanthan gum velocity. From flow field calculations and tracer homogenization simulations, power consumption and mixing time results were obtained. The torque and power characteristics remain the same for upward and downward pumping of the impeller, but the mixing times are considerably longer for the downward pumping mode. Overall, the numerical results showed good agreement with experimental results and correlations developed by other researchers. From the power and mixing time results, two efficiency criteria were utilized to determine the best pumping mode of the impeller.