刮壁搅拌桨的最优设计

运用罚函数Ｇａｌｅｒｋｉｎ加权余量有限元数值算法模拟了Ｖｏｔａｔｏｒ、Ｃ＆Ｒ反应器和刮壁槽式釜中轴截面的流速和剪切分布。计算结果表明，Ｖｏｔａｔｏｒ，Ｃ＆Ｒ反应器中流动和剪切分布较刮壁槽式釜更有利于流体的混合和壁面的传热。对Ｃ＆Ｒ反应器，采用无间隙刮板，其传热和混合性能优于有间隙刮板。建议刮壁搅拌反应器的设计采用类似Ｖｏｔａｔｏｒ和Ｃ＆Ｒ反应器的环隙结构和无间隙的刮板形式。     

Experimental determination of the optimal geometry of baffles for heat transfer in an agitated vessel

Experimental determination of the optimum values of geometrical parameters of baffles for heat transfer in a jacketed agitated vessel is the purpose of this research. The influence of the geometrical baffle parameters (number, width, length and distance between the lower edge of the baffle and the bottom of the vessel) upon both power consumption and heat transfer was examined for an agitated vessel with an inner diameter of 0.45 m. The experiments were performed with a disc turbine, a pitched-blade turbine and a propeller. An experimental design strategy was used in this research. The results of the experiments were approximated analytically. Based on the optimization criterion, an agitated vessel equipped with a disc turbine and optimally designed baffles is proposed as the most advantageous for heat transfer processes.     

高粘度流体在机械搅拌槽中的传热

研究了高粘度牛顿流体和非牛顿流体在螺杆-导流筒、螺杆-导流筒-直管组两种搅拌体系中的传热行为。结果表明,设置导流筒和直管组后,对夹套侧的传热膜系数α_j没有明显影响,导流筒侧和直管组直管外侧的传热膜系数α_d和α_c都较α_j大,传热面积比夹套增加一倍左右,从而增强了搅拌槽的传热能力。在所有几何因素中,搅拌桨径 d和导流筒内径 D_(ti)对传热膜系数影响最大。搅拌桨的转向对传热无明显影响。分析了传热关联式中 Re的方次的变化情况。用因次分析和回归分析得到的夹套、导流筒和直管组传热面的传热关联式和文献结果吻合甚好。     

Effects of the Stirred Tank's Design on Power Consumption and Mixing Time in Liquid Phase

The influence of the stirrer type and of the geometrical parameters of both tank and agitator (clearance of an impeller from tank bottom, impeller diameter, draft tube and geometry of the tank bottom) on power consumption and mixing time in liquid phase under turbulent regime conditions (Re > 10⁴) have been studied. Different types of agitators have been used, namely Rushton turbine, 45° pitched‐blade turbine, MIXEL TT and TTP propellers and 1‐stage or 2‐stage EKATO‐INTERMIG propellers. All these stirrers were tested with the same power consumption per unit mass of liquid. On the basis of measured power consumption per unit mass, which is required to achieve the same degree of mixing, the results obtained in the present work show that the TTP propeller is the most efficient in liquid phase. Recommendations on the optimum geometric configuration have been made for each type of stirrer.     

Role of sparger design on gas dispersion in mechanically agitated gas-liquid contactors

Critical impeller speeds for gas dispersion and gas recirculation were measured in 0.57, 1.0 and 1.5 m i.d. vessels, using visual observations, measurements of power consumption and liquid-phase mixing time. A pitched blade down-flow turbine impeller (PTD) was used. Design parameters of the PTD impeller such as diameter (0.22Tto 0.57T), blade width (0.25D to 0.4D) and blade thickness (2.8, 4.3 and 6.4 mm) were studied. The effect of sparger type and geometry on N_CD has been investigated. For this purpose, pipe, ring, conical and concentric ring spargers were employed. Design details of the ring sparger such as ring diameter, number of holes and hole size were also studied in detail. The sparger location with respect to the impeller was found to be the most important parameter. Correlations have been developed for N_CD and N_CR.     

Heat transfer coefficients in finned-coil stirred-tank systems

Steady-state and transient heat transfer coefficients were determined for a finned-coil, stirred-tank system using flat-blade turbine impellers in water. Fin spacing, flowrate, hot water temperature and agitator speed were varied for Re between 20 000 - 110 000 for the agitated side. Individual transfer coefficients were calculated from the experimental overall coefficients. The correlation developed for predicting external transfer coefficients in agitated systems with finned-tube coils was tested against transient data.     

刮壁浆在槽式釜的传热研究

针对结刮壁搅拌槽壁面的传热提出渗透－对流模型。运用在搅拌浆上安装热电偶的测温方法，考察了刮壁搅拌槽中的温度分布情况，实验结果支持了渗透－对流模型。当Ｒｅ＞１００，Ｒｅ·Ｐｒ＞１０６时，刮壁残余层对壁面传热有明显影响，随着Ｒｅ的进一步提高，刮壁槽壁面的传热系数增加缓慢，关联得到Ｒｅ＜１００，８０００＜Ｐｒ＜１８０００时刮壁槽壁面传热系数的经验关联式。     

多层桨搅拌槽内的液-液分散特性

在直径为0．476m的椭圆底圆柱形搅拌槽内,以水／航空煤油及水／环己烷为实验体系,研究了Rushton涡轮式搅拌桨(RT-6)、六半椭圆管涡轮式搅拌桨(HEDT)、折叶轴流式搅拌桨及翼形轴流式搅拌桨(CBY)的6种不同组合桨的液-液分散特性,用取样法测定了分散相体积分数的轴向分布及体系澄清时间。结果表明,组合桨中的底桨在液-液分散中发挥了主要作用。单位体积输入功率相同时,底桨为CBY的组合桨进行液-液分散后,液滴的平均滴径最小,体系澄清时间较长;底桨为HEDT的组合桨的分散效果次之;底桨为RT-6的组合桨因滴径分布较宽,虽然平均滴径最大,但澄清时间也较长。     

Real and pseudo-turbulence in the discharge stream from a rushton turbine

The trailing vortex system leaving the blade tips dominates the velocity field in the vicinity of a turbine agitator. This paper relates velocity measurements made with a stationary conical hot film probe to the known flow field defined relative to the impeller. It is shown that the ordered pseudo turbulence deviates so much from isotropic conditions that the usual interpretation in terms of the turbulence parameters of scale intensity is of doubtful value in this region.     

The Effects of Power Law Fluid Rheology on Coil Heat Transfer for Agitated Vessel in the Transitional Flow Regime

A CFD model of heat transfer from power‐law fluids to helical cooling coils in the transitional flow regime of a baffled tank mixed with a pitched blade turbine was developed with Fluent^TM. The model captured local temperature and velocity gradients. Simulations were run, varying Re, Pr, K and n. The results indicate that a Sieder‐Tate type correlation, with the exponent on and the coefficient in front of the Reynolds number being a function of n, is recommended for estimating ho. Also, a new two coil bank design was found to be more efficient when 450 < Re < 650.     

装有双层蛇管搅拌釜的传热

以水为传热介质，采用组合桨，测定了装有双层蛇管的搅拌釜的传热系数。结果表明，在三层圆盘涡轮搅拌下，外层蛇管外壁侧和釜内壁侧的传热系数分别比内层蛇管的下降了１０％和２８％，而用圆盘涡轮与折叶桨组合桨搅拌时，要分别高出３３％和５％，总传热效率提高了２０％以上；另外，釜内壁侧的传热系数受蛇管层数的影响较小。     

Characterisation of three different impellers using the LDV-technique

A characterisation of three commonly used impellers was made in this study by measuring local mean velocities and the fluctuations of these velocities with the LDV technique. The data was used to estimate volumetric flow, velocity fluctuations and turbulent intensity in the impeller region of the tank. The impellers investigated were a high flow impeller, a pitched blade turbine and a Rushton turbine. The cylindrical vessel used was made of Perspex, had a dished bottom (DIN 28013), was equipped with four baffles and had an inner diameter of 0.45 m. It was found that the bulk velocities could be scaled with the tip-speed of the impeller (ND). The flow rate at constant impeller speed increased in the order high flow impeller — Rushton turbine — pitched blade turbine. The corresponding order for the turbulence fluctuation is: high flow impeller — pitched blade turbine — Rushton turbine. The velocity profile of the flow out from the high flow impeller was furthermore, not as smooth as could be expected.     

Predicting Power for a Scaled‐up Non‐Newtonian Biomass Slurry

High‐solids biomass slurries exhibit non‐Newtonian behavior with a yield stress and require high power input for mixing. The goals were to determine the effect of scale and geometry on power number P₀, and estimate the power for mixing a pretreated biomass slurry in a 3.8 million L hydrolysis reactor of conventional design. A lab‐scale computational fluid dynamics model was validated against experimental data and then scaled up. A pitched‐blade turbine and A310 hydrofoil were tested for various geometric arrangements. Flow was transitional; laminar and turbulence models resulted in equivalent P₀ which increased with scale. The ratio of impeller diameter to tank diameter affected P₀ for both impellers, but impeller clearance to tank diameter affected P₀ only for the A310. At least 2 MW is required to operate at this scale.     

Experimental and CFD simulation studies on power consumption in mixing using energy saving turbine agitator

Experimental and simulation investigation on power consumption in new energy saving turbine agitator has been presented here. Modification was done by introducing single and double rectangular and V cuts in the conventional turbine agitator. The power consumption for new energy-saving turbine agitator has been found to be less than that of the conventional turbine agitator. Also, the power consumption for V cut has been found to be less than that of rectangular cut in turbine agitator. Power characteristics for different geometrical parameters of energy saving turbine agitators were obtained for both the transition and turbulent regimes of the flow in agitated vessel. CFD simulations have been used to predict power consumption. The simulation results have been compared with the experimental data obtained on a pilot-scale set up.     

Experimental Study of Influence of the Impeller Spacing on Flow Behaviour of Double‐Impeller Stirred Tank

The flow fields in the stirred tank with three different kinds of combined double‐impeller agitators: disc turbine + disc turbine (DT‐DT, radial impeller), pitched blade turbine + pitched blade turbine (PTD‐PTD, axial impeller) and pitched blade turbine + disc turbine (PTD‐DT), were investigated in detail by using laser Doppler anemometry. The two‐dimensional mean velocity field and the distribution of turbulence intensity were obtained for different impeller spacings. The experimental results show that the impeller spacing has a significant influence on the flow field. To improve flow homogeneity and agitator efficiency, the appropriate impeller spacing should be in the range of 1/2 to 2/3 of the tank diameter.     

Role of sparger design in mechanically agitated gas-liquid reactors. Part II: Liquid phase mixing

Liquid phase mixing time was measured in 0.57, 1.0 and 1.5 m i.d. mechanically agitated gas-liquid reactors. Transient conductivity technique was used for the mixing time measurement. Pitched blade downflow turbine was employed. The design details of PTD impellers such as diameter (0.22 T to 0.5 T) and blade width (0.25 D to 0.35 D) were studied. The influence of sparger types and their design on mixing time has been investigated. For this purpose, pipe, ring, conical, and concentric ring spargers were employed. The design details of the ring sparger, i.e. ring diameter, number of holes and hole size were also studied in depth. Sparger location with respect to the impeller was found to be the most important variable and, therefore, it was varied for practically all the spargers studied in this work. It was found that the liquid phase mixing time depends on the impeller design, sparger design, sparger location, impeller speed and superficial gas velocity. Correlations have been developed for the dimensionless mixing time.     

Investigation of feedpipe backmixing in an agitated vessel

Feedpipe backmixing in an agitated vessel was investigated using a newly developed conductivity technique. By this technique, the onset of feedpipe backmixing could be detected and the penetration depth of the vessel fluid into a feedpipe was determined. For a given feedpipe flowrate. critical agitator speeds to eliminate feedpipe backmixing were determined using Rushton six-bladcd disk turbine impeller (6BD) and high efficiency, axial-flow type 3-bladed impeller (HE-3) of 8.89 and 12.70 cm diameters in 11.2 liter reactor. The ratio of the feedpipe velocity to the critical agitator speed (v _f /v _i ) was determined as a function of feedpipe Reynolds number (N _R,T ). The conductivity technique was successful either in the laminar regime, the transitional regime, or in the turbulent regime in the feedpipe.     

Effect of the attack angle on the roll and trailing vortex structures in an agitated vessel with a paddle impeller

The purpose of the present study is to observe the effect of the blade attack angle on the roll and trailing vortex structures in a stirred vessel via laser-Doppler velocimetry (LDV). In this investigation, four-bladed paddle impellers with four attack angles, which were 45^°, 60^°, 75^° and 90^°, respectively, were used. By synchronizing LDV with a rotary encoder coupled to the impeller shaft, angle-resolved measurements of all three velocity components were performed. This experimental method made it possible to capture the details of the vortical structure both behind the impeller blade and discharge region. Our study on the mean flow structure generated by three types of pitched blade turbines (45^°, 60^°, and 75^°, respectively) found that a single trailing vortex was formed around each turbine blade. Roll-up of the vortex sheet issuing from the blade tip was also observed, which indicated a major roll of trailing vortex generation mechanism for each pitched blade turbine.     

Impact of impeller type on methyl methacrylate emulsion polymerization in a batch reactor

The emulsion polymerization of methyl methacrylate (MMA) was carried out in a lab‐scale reactor, which was equipped with a top‐entry agitator, four wall baffles, a U‐shaped cooling coil, and a temperature controller. Potassium per sulfate and sodium dodecyl sulfate as were used as the initiator and the surfactant, respectively. The experimental investigation demonstrated the impact of the impeller type (45° six pitched‐blade turbine and Rushton impeller), number of impellers (single and double impellers), and impeller speed (100–350 rpm) on the monomer conversion, polymer particles size, molecular weight, and glass transition temperature. The results revealed that the effect of the impeller speed on the characteristics of the polymer attained using the pitched‐blade turbine was more prominent than that for the Rushton turbine. It was also found that the impact of the impeller speed on the polymer characteristics was much more pronounced for the double pitched‐blade turbines rather than for the double Rushton turbines. However, more uniform size distribution was achieved with the Rushton turbine. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40496.     

Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Both the numerical and experimental approaches were used to study the effects of the gas recirculation and non‐uniform gas loading on the mass transfer rate for each impeller in a multiple impeller system. By combining the calculated gas velocity and local gas holdup, the gas recirculation rate around each impeller was estimated. The local mass transfer coefficients for systems equipped with various combinations of the Rushton turbine impeller (R) and pitched blade impeller (P) were determined by using the dynamic gassing out method. It is found that the Rushton turbine impeller has to be served as the lowest impeller in order to have a better gas dispersion and to give a higher overall K_La for a multiple impeller gas‐liquid contactor. The upper pitched blade impeller always enforces the circulating flow around the Rushton turbine impeller just beneath it and gives a higher overall average mass transfer rate. However, the system equipped with only the pitched blade impellers results in a much lower mass transfer rate than the other systems owing to the poor gas dispersion performance of the pitched blade impeller.