Effect of the shaft eccentricity and rotational direction on the mixing characteristics in cylindrical tank reactors

Strategy of the shaft eccentricity is introduced to enhance the mixing characteristics in a flat bottomed cylindrical vessel without baffles. The mixing is ensured by a six-curved blade impel er. Three solutions which are models of food emulsions are used as working fluids. These solutions have a shear thinning behavior modeled by the power-law. The effects of fluid properties, stirring rates, impeller rotational direction and impeller eccentricity on the 3D flow fields and power consumption are investigated. Three values of impeller eccentricity are consid-ered, namely 0%, 24%and 48%of the vessel diameter. It is found that the opposite clockwise rotational direction reduces the power consumption, compared with the clockwise rotational direction. Also, the obtained results show that an impeller placed at an eccentric position between 24%and 48%of the vessel diameter and at the third of the vessel height may ensure the best mixing characteristics.     

Liquid flow circulating within an unbaffled vessel agitated with an unsteady forward–reverse rotating impeller

Liquid‐phase mixing is a common operation, often performed in vessels using mechanically rotating impellers. To enhance axial mixing the vessels are generally equipped with baffles; however, in industries where cleaning the vessel interior is a major concern, i.e. food and pharmaceuticals, and crystallization, where baffles can disturb particle growth, unbaffled vessels are preferred. One method of agitation in unbaffled vessels is an impeller that periodically changes either the direction or rate of rotation: so‐called unsteady rotation. For use in an enhanced agitation vessel, an agitation technology using an unsteady forward–reverse rotating impeller in an unbaffled vessel was investigated. Such unsteady agitation is expected to enhance mixing. However, knowledge of the liquid flows in such an apparatus remains elusive. Thus an aim of this work was to characterize the circulation flow in such a system. Circulation by a disk turbine impeller with six flat blades was studied through examination of tracer particle trajectories. Images showing flow patterns with the forward–reverse rotating impeller resembled those obtained with a unidirectionally rotating impeller in a baffled vessel. The pattern was characterized by a circulation loop whose pathway exits from the impeller rotational region and returns to that region past the wall and bottom of the vessel. Time‐series particle tracking velocimetry (PTV) images obtained during one forward–reverse rotation of the impeller showed that the flow near the vessel wall reduced the periodic fluctuation downstream and that a flow that was almost independent of time was induced near the vessel bottom. For the flow from the bottom to the impeller, unsteadiness was provided by proximity to the impeller. Based on the intensity distribution of the unsteady flow produced by this type of forward–reverse rotating impeller within the vessel, the unsteady flow was shown to have the potential to reach the region near the vessel wall. Copyright © 2010 Society of Chemical Industry     

Experimental study by online measurement of the precipitation of nickel hydroxide:Effects of operating conditions

The objective of this work is using the online measurement method to study the process of precipitation of nickel hydroxide in a single-feed semi-batch stirred reactor with an internal diameter of D=240 mm. The effects of impeller speed, impeller type, impeller diameter and feed location on the mean particle size d43 and particle size distribution (PSD) were investigated. d43 and PSD were measured online using a Malvern Insitec Liquid Pro-cess Sizer every 20 s. It was found that d43 varied between 13μm and 26μm under different operating conditions, and it decreased with increasing impel er diameter. When feeding at the off-bottom distance of D/2 under lower impeller speeds, d43 was significantly smaller than that at D/3. PSDs were slightly influenced by operating conditions.     

双层组合桨搅拌槽内气液微观分散特性

采用双电导电极探针法对双层组合桨搅拌槽内气液相界面积特性进行了实验研究,考察了通气量、搅拌转速和桨组合对槽内相界面积的影响。结果表明:对于上层桨为上翻斜叶桨和下层桨为凹叶桨的组合,随着通气量的增加,搅拌槽内大部分区域的相界面积增大,但在槽底区域减小。随着搅拌转速的增加,在叶轮区域的相界面积增加明显,而在槽底和液面区域基本不变化。上下层桨的分散能力和气体分布器结构和操作条件密切相关。对于近壁管式气体分布器搅拌槽,在较低通气量下,上层桨对气液分散起着主要作用,而在高通气量下,下层桨的作用增强,起主要作用。带圆盘的搅拌桨对气体具有良好的阻缓作用,不同气速下均具有优异的气液分散能力。     

Fast-flowing LBE corrosion of the SiC pump impeller: Numerical and experimental investigations

In order to determine the stress state and the specific dynamic corrosion behaviors of the cooling pump impeller in the liquid lead-bismuth eutectic alloy (LBE), the numerical and the experimental investigations on the fast-flowing LBE corrosion behaviors of the SiC impeller were conducted. The distributions of stress and displacement of the SiC impeller were simulated, and then the microstructure and mechanical properties of the corroded impeller were characterized. The results indicated that the SiC impeller met the strength requirement of 1000 rpm in LBE, and the dynamic LBE corrosion had little effect upon structure safety of the SiC impeller. The maximum stresses of the SiC impeller without and with LBE loading were .452 and 9.736 MPa, respectively. The maximum displacements of the SiC impeller without and with LBE loading were 76.385 and 4.096 μm, respectively. The LBE loading reduced the axial displacement of the impeller and acted as an impeller stabilizer to some extent. Blade elongating and flapping were the main vibration mode of the SiC impeller. The mechanical properties of the SiC impellers before and after corrosion were basically the same.     

Behaviour of gas–liquid mixtures in an unbaffled reactor with unsteadily forward–reverse rotating impellers

The behaviour of gas–liquid mixtures in the vicinity of the blades of an unsteadily rotating impeller in an unbaffled agitated vessel was studied by observations made with a rotating camera. The impellers used were a disk turbine impeller with six flat blades (DT) and a novel cross‐type impeller with four delta blades (CD). The behaviour of gas–liquid mixtures near the blades of the forward–reverse rotating impeller was unsteady in terms of the formation of cavities behind the blades and their dispersion into gas bubbles, and differed from that near the blades of a unidirectionally, steadily rotating impeller. The differences in relative power consumption between the forward–reverse rotating impellers in the unbaffled vessel and the steadily rotating impellers in the baffled vessel are discussed in relation to the differences in the behaviour of gas–liquid mixtures near the blades of each rotating impeller. © 2002 Society of Chemical Industry     

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first observed at the vessel's wall and base, and at the free liquid surface. The effect of impeller geometry has been studied with a disk turbine (DT), a two bladed paddle (2BP), a pitched blade turbine (PBT) and a marine propeller (MP).

The presence of four baffles (10%) was found to increase the impeller speed at which the cavern reaches the vessel wall by 9% on average over that observed without baffles. After the cavern has reached the vessel walls, impeller type had a small effect upon the vertical expansion of the cavern with increasing impeller speed. Radial flow impellers (DT and 2BP), on average, performed better than an axial flow impeller (MP), with a mixed flow impeller (PBT) in between. Baffles significantly reduce the rate of this vertical expansion of the cavern. Clearance of the impeller from the vessel base had little effect upon the growth of the cavern above the impeller.     

Solid–liquid mass transfer characteristics of an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller

To develop an enhanced form of solid‐liquid apparatus, an unbaffled agitated vessel has been constructed, fitted with an agitation system using an impeller whose rotation alternates unsteadily in direction, i.e. a forward‐reverse rotating impeller. In this vessel, solid‐liquid mass transfer was studied using a disc turbine impeller with six flat blades. The effect of impeller rotation rate as an operating variable on the mass transfer coefficient was evaluated experimentally using various geometrical conditions of the apparatus, such as impeller diameter and height, in relation to the impeller power consumption. Mixing of gas above the free surface into the bulk liquid, i.e. surface aeration, which accompanied the solid‐liquid agitation, was also investigated. Comparison of the mass transfer characteristics between this type of vessel and a baffled vessel with a unidirectional rotating impeller underscored the sufficient solid‐liquid contact for prevention of gas mixing in the forward‐reverse rotation mode of the impeller. Copyright © 2008 Society of Chemical Industry     

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first observed at the vessel's wall and base, and at the free liquid surface. The effect of impeller geometry has been studied with a disk turbine (DT), a two bladed paddle (2BP), a pitched blade turbine (PBT) and a marine propeller (MP).

The presence of four baffles (10%) was found to increase the impeller speed at which the cavern reaches the vessel wall by 9% on average over that observed without baffles. After the cavern has reached the vessel walls, impeller type had a small effect upon the vertical expansion of the cavern with increasing impeller speed. Radial flow impellers (DT and 2BP), on average, performed better than an axial flow impeller (MP), with a mixed flow impeller (PBT) in between. Baffles significantly reduce the rate of this vertical expansion of the cavern. Clearance of the impeller from the vessel base had little effect upon the growth of the cavern above the impeller.     

Agitation requirements for complete solid suspension in an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller

Background: To develop a new type of solid–liquid apparatus, we have proposed the application of an agitation system with an impeller whose rotation alternates direction unsteadily, i.e., a forward–reverse rotating impeller. For an unbaffled agitated vessel fitted with this system, the suspension of solid particles in a liquid was studied using a disk turbine impeller with six flat blades. Results: The effects of the solid–liquid conditions and geometrical conditions of the apparatus on the minimum rotation rate and the corresponding impeller power consumption were evaluated experimentally for a completely suspended solid. The power consumption for a just suspended solid with this type of vessel was comparable with that for a baffled vessel with a unidirectionally rotating impeller, taking the liquid flow along the vessel bottom into consideration. Conclusion: Empirical relationships to predict the parameters of agitation requirements were found. A comparative investigation demonstrated the usefulness of the forward–reverse rotation mode of the impeller for off‐bottom suspension of solid particles. Copyright © 2007 Society of Chemical Industry     

刚-柔组合搅拌桨强化流体混沌混合

合理设计搅拌反应器的桨叶,强化流体流动与混合行为,是实现流体高效、节能混合的重要手段。柔性体与刚性体组合,可设计出具有多体运动行为的刚-柔组合搅拌桨。结合PIV流场观测和CFD模拟,对比分析了刚性桨和刚-柔组合桨对流场结构及流体混沌混合行为的影响。结果表明,与刚性搅拌桨相比,刚-柔组合桨的柔性端强化能量传递,流体流速衰减速率降低25%,有利于搅拌桨输入能量在流场结构内的有效分配。传统刚性六凹叶和六直叶涡轮桨搅拌反应器内流体形成的流线结构具有明显的周期吸引子,其时均流场的分形维数分别为1.9046和1.9138。刚-柔组合六直叶涡轮桨搅拌反应器内流体流线呈明显的准周期性吸引子性质,其流场分形维数为1.9337,而刚-柔组合六凹叶涡轮桨搅拌反应器内流体流线具有典型的混沌吸引子性质,其流场分形维数为1.9545。刚-柔组合搅拌桨可改变流体流线的吸引子来调控流场的多尺度结构,强化流体混沌混合,实现高效节能操作。     

LIQUID FLOW IN IMPELLER REGION OF AN UNBAFFLED AGITATED VESSEL WITH UNSTEADILY FORWARD-REVERSE ROTATING IMPELLER

For an unbaffled agitated vessel with an unsteadily forward-reverse rotating impeller whose rotation proceeds with repeated acceleration, deceleration, and stop-reverse processes, the liquid flow in the impeller region was studied based on photographs showing path lines of tracer particles. An image series taken during one cycle of the forward-reverse rotation was analyzed to characterize the internal stream inside the impeller rotational region and the discharge stream outside its region when a disk turbine impeller with six flat blades was rotated unsteadily. Because of the unsteady flow generated inside the impeller rotational region, the velocity vector of outflow from its region fluctuated periodically with the change of the impeller rotation rate. The circumferential velocity was almost in phase with the impeller rotation rate, oscillating periodically. The radial velocity exhibited larger values in the process for the impeller from decelerating to stopping and reversal. The radial flow, whose velocity decreased downstream outside the impeller rotational region, was clarified to be transformed into upward and downward axial flows that are almost uniform in the circumferential direction throughout the region near the vessel wall.     

LIQUID FLOW IN IMPELLER REGION OF AN UNBAFFLED AGITATED VESSEL WITH UNSTEADILY FORWARD-REVERSE ROTATING IMPELLER

For an unbaffled agitated vessel with an unsteadily forward-reverse rotating impeller whose rotation proceeds with repeated acceleration, deceleration, and stop-reverse processes, the liquid flow in the impeller region was studied based on photographs showing path lines of tracer particles. An image series taken during one cycle of the forward-reverse rotation was analyzed to characterize the internal stream inside the impeller rotational region and the discharge stream outside its region when a disk turbine impeller with six flat blades was rotated unsteadily. Because of the unsteady flow generated inside the impeller rotational region, the velocity vector of outflow from its region fluctuated periodically with the change of the impeller rotation rate. The circumferential velocity was almost in phase with the impeller rotation rate, oscillating periodically. The radial velocity exhibited larger values in the process for the impeller from decelerating to stopping and reversal. The radial flow, whose velocity decreased downstream outside the impeller rotational region, was clarified to be transformed into upward and downward axial flows that are almost uniform in the circumferential direction throughout the region near the vessel wall.     

Rushton桨气液搅拌槽内气泡流对搅拌轴弯矩的影响（英文）

The bending moment acting on the overhung shaft of a gas-sparged vessel stirred by a Rushton turbine, as one of the results of fluid and structure interactions in stirred vessels, was measured using a moment sensor equipped with digital telemetry. An analysis of the shaft bending moment amplitude shows that the amplitude distribution of the bending moment, which indicates the elasticity nature of shaft material against bending deformation, fol-lows the Weibull distribution. The trends of amplitude mean, standard deviation and peak deviation character-istics manifest an“S”shape versus gas flow. The“S”trend of the relative mean bending moment over gas flow rate, depending on the flow regime in gas–liquid stirred vessels, resulted from the competition among the non-uniformity of bubbly flow around the impeller, the formation of gas cavities behind the blades, and the gas direct impact on the impel er when gas is introduced. A further analysis of the bending moment power spectral density shows that the rather low frequency and speed frequency are evident. The low-frequency contribution to bend-ing moment fluctuation peaks in the complete dispersion regime.     

On the main flow features and instabilities in an unbaffled vessel agitated with an eccentrically located impeller

The hydrodynamics of an unbaffled vessel stirred by an eccentrically located Rushton turbine is investigated with both Laser Doppler Anemometry and flow visualisation techniques. The flow field is shown to be characterised by a strong circumferential motion which develops itself around two main vortices, one above and one below the impeller, both inclined with respect to the vertical plane. Such vortices are not steady but move periodically very slowly in comparison to the impeller rotational timescale. Accordingly, two low frequency components, whose values are linearly dependent on the impeller rotational speed, are identified across the vessel. The energetic contribution to the turbulent kinetic energy of such flow instabilities is significant so that they should be taken into account when evaluating micro-mixing information from turbulence quantities. Besides, an additional low frequency component is observed and related to vortex shedding phenomena from the flow-shaft interaction which occur in eccentric agitation operation. The flow discharged from the impeller is also measured and discussed.     

CLOUD HEIGHT IN SOLIDS SUSPENSION AGITATION

The focus of this work is the experimental determination of cloud height (the height to which solids are suspendedrpar; in an agitated slurry as a function of the agitation intensity, solid physical properties, impeller type (pitched-blade or high efficiency), and system geometry. Cloud height is not strongly dependent on impeller type or solid physical properties, except for extremely rapidly-settling particles. However, it is dependent on the impeller diameter to tank diameter ratio (D/T), impeller off-botiom clearance to tank diameter ratio (C/T), and solids loading. The use of multiple impellers allows solids to be suspended to higher levels in tall batches. An intermediate impeller separation leads to optimal performance in terms of the power requirement to achieve a desired cloud height. Lower impeller separations do not yield significant performance improvements over a single impeller, while higher impeller separations lead to poor performance caused by “zoning” between the impellers.     

轴流桨搅拌槽三维流场数值模拟

利用k -ε湍流模型预测了搅拌槽在不同操作条件下宏观速度场 ,模型成功预测了搅拌槽内速度分布 ,计算结果与实验结果吻合较好 .模型预测结果表明 ,搅拌槽内宏观流动场受搅拌桨槽径比影响较大 .对单层搅拌桨 -槽体系 ,挡板前后宏观流动场差别很大 ,在挡板以前区域 ,轴向流动较强 ,在整个r -z断面上形成一个整体循环 ;而在挡板后面区域 ,流体在桨叶安装位置高度附近转向轴心流动 ,槽体上半部区域形成二次循环区域 ,且二次循环区域内流体以向下流动为主 .     

POWER CONSUMPTION AND SOLID SUSPENSION IN COMPLETELY FILLED VESSELS

Mixing in a completely filled vessel, designed as a pressure vessel, has been investigated. The power demand of two pitched blade turbines of different sizes, and one high flow impeller was studied. The influence of side-mounted and bottom-mounted baffles, as well as the clearance between the bottom and the impeller was investigated. The experiments were carried out in a plexiglass vessel with a diameter of 288 mm. The Power number was calculated from measured data on the power demand and the impeller speed. Furthermore, preliminary studies on the critical impeller speed and power demand at the off-bottom suspension point for one set of glass particles were carried out. The Power number was mainly influenced by the impeller type, while the power demand at the off-bottom suspension point was more dependent of the impeller size and clearance. For bottom-mounted baffles a minimum in the Power number was noticed with increased clearance. It was found the minimum was located at the transition point between two different How patterns. This study shows that the configuration with the lowest Power number is not necessary the most efficient for suspending solid particles.     

Effect of impeller clearance on liquid flow within an unbaffled vessel agitated with a forward–reverse rotating impeller

For an unbaffled agitated vessel with an unsteadily forward–reverse rotating impeller whose rotation proceeds with repeated acceleration, deceleration, and stop–reverse processes, liquid flow was studied through visualisation and measurement using particle tracking velocimetry (PTV). A disk turbine impeller with six flat blades was used with varied height settings. The impeller clearance and its forward–reverse rotation cycle characterised the impeller region flow: the radially outward flow in the deceleration process for the larger clearance relative to the vessel diameter of 1/3, and the axially downward flow in the acceleration process for the smaller clearance relative to the vessel diameter of 1/8. The flow patterns within the vessel resulting from the impeller's larger and smaller clearances were outlined, respectively, by double loops and a single loop of circulation, resembling the pattern produced by unidirectionally rotating turbine‐type impellers. The discharge flow was revealed to contain a comparable level of periodic circumferential velocity component, irrespective of the impeller clearance.