Using CFD to predict the behavior of power law fluids near axial-flow impellers operating in the transitional flow regime

A computational fluid dynamics (CFD) model of flow in a mixing tank with a single axial-flow impeller was developed with the Fluent^TM software. The model consists of an unstructured hexagonal mesh (158,000 total cells), dense in the region from the surface of the impeller. The flow was modeled as laminar and a multiple reference frame approach was used to solve the discretized equations of motion in one-quarter of a baffled tank. A solution of 0.1% Carbopol in water, a shear-thinning fluid, was found to be clear enough to measure impeller discharge angles using laser Doppler velocimetry. This is the first time that impeller discharge angles have been reported in the literature for a shear-thinning fluid with a hydrofoil impeller. Rheological measurements indicated that the Carbopol solution can be characterized by the power law (K=9,n=0.2) under the range of shear conditions (0.1- expected near the impeller in the mixing tank. The CFD model accurately predicted the dependence of power number and discharge angle on Reynolds number (as predicted by Metzner and Otto), for an A200 (pitched blade turbine or PBT) and an A315 (Hydrofoil) impeller operating in the transitional flow regime (Reynolds numbers: 25-400) with glycerin and 0.1% Carbopol solutions. Subsequently, the results of a systematic CFD study with power law fluids indicated that the power number and discharge angle of an axial-flow impeller in the transitional flow regime depends not only on the Reynolds number (as determined by Metzner and Otto's method) but also on the flow behavior index n. Consequently, an alternative to Metzner and Otto's method was pursued. The results of converged CFD simulations indicate that the near-impeller “average shear rate” increases not only with increasing RPM (as proposed by Metzner and Otto), but also with decreasing flow behavior index (n) and discharge angle in the transitional flow regime. Considering this result, an improved method of estimating the power number and discharge angle for power law fluids in the transitional flow regime is proposed.     

Gas-liauid mass transfer in non-newtonian fluids in a tank stirred with a helical ribbon screw impeller

Oxygen transfer in aqueous solutions of CMC, xanthan gum, and polyacrylamide in a vessel equipped with a helical ribbon screw impeller and a multi-orifice ring sparger was studied. The K_La values obtained were comparable to those reported for radial flow impellers, but they are more representative of the oxygen transfer in the bulk of the liquid because of a more homogeneous mixing and absence of dead zones. Dimensionless correlations including the effect of the physical properties of the fluids, and those of operating conditions were developed. A dimensionless correlation for each fluid is proposed.     

Shear and skin friction on particles in power-law fluids agitated by flat-blade and fluid foil impellers

The shear rates that exert angular deformation on spherical particles have been measured. The particles are mimiced by a spherical probe. The probe has been immersed in various impeller-agitated power law fluids. The fluids are aqueous dispersions of polymers, e.g. CMC, xanthan gum and starch. The probe has been positioned in various points of a stirred vessel and at various angles. Angle-averaged shear rate distributions were produced. The distributions obtained are characteristic for the specific impeller flow patterns. The flow patterns have been identified by computational fluid dynamics (CFD). Two types of impellers representative for the flat and the fluid-foil blade design, i.e., a Rushton flat-blade turbine (RT) and a Narcissus impeller (NS) are studied. The effects of rheological properties and blade design on the ‘shear-rate-on-particles’ distribution are examined. The local shear field non-uniformity has been uncovered and compared in terms of the CFD-generated time-averaged velocity and deformation rate profiles. The ‘shear-rate-on-particles’ distribution apart from the impeller is found to follow qualitatively the time-averaged inner flow shear rate distribution. Referring to impeller speed 5-12.5 Hz, the dimensionless wall shear rate varied between 200 and 1000. In power law fluids, the shear rate on particles decreased up to 50%. The fluid-foil NS-generated shear field was found comparable to the shear field induced by conventional flat-blade turbines and appeared in cases less sensitive to polymer presence. The shear rate produced by the fluid-foil impeller in the highly shear-thinning model solution (n∼0.4) exceeded the flat-blade RT-imposed shear rate. The analysis has been extended to skin friction drag on particles. It is shown that, while exerting an undoubtedly greater angular deformation in water-like fluids, in polymer presence the conventional flat-blade turbine introduces a flow geometry that imposes particle drag that is close or in some cases even less than the one generated by the fluid-foil impeller. The fact implies a weak shape effect of radial turbines on shear-sensitive particles or particle dispersions in power law liquids.     

Process viscometry of complex fluids and suspensions with helical ribbon agitators

The viscosity of highly inelastic shear thinning fluids and aqueous suspensions of kaolin clay particles has been investigated using a helical ribbon impeller fitted to a rheometer. Viscosity data for the single phase non-Newtonian fluids adequately processed with a generalized Reynolds number based on the impeller tip speed are shown to superimpose very well to the results obtained with a cone and plate rheometer. In the case of the two-phase system, it is shown that the data treatment for single phase system can be extended. The helical ribbon impeller yields more stable viscosity values than with the traditional geometries and no spurious flow phenomena (i.e., sedimentation, slip at the wall, etc.) was observed, making this system a superior device for suspension rheology over cone and plate and Couette flow rheometers.     

黏度变化范围大的组合桨的数值模拟与应用

利用计算流体力学的方法研究了ZBK+BKS组合桨和螺带桨(LD)的流场和混合性质。对比研究了组合桨的流型、功率消耗、混合时间及混合能。结果表明:ZBK+BKS组合桨在过渡流区域,BKS起主要的流体混合作用,形成很好的整体轴向混合,而在层流区ZBK与BKS同等重要并且ZBK加强了轴向混合。ZBK+BKS组合桨的混合时间在过渡流区域要小于螺带桨,而层流区相差很小,并且二者的混合能相差较小。所以ZBK+BKS组合桨能够适用于黏度变化较大的混合过程。     

无油螺杆压缩机的节能降耗

针对LGW 25/16-38/8型无油螺杆压缩机在生产过程中电耗高的问题,从工艺和设备2个方面进行分析,并通过调整供风总管压力,对螺杆压缩机中后冷却器清洗以及调节二级排气温度,降低了功耗和机组频繁跳车次数,实现了高效运行和节约能源的目的。     

Synergistic and interference effects in coaxial mixers: Numerical analysis of the power consumption

This paper is concerned with the design and application of coaxial mixers with the aid of analysis of interaction between each individual impeller.Two types of coaxial mixers pitched blade turbine (PBT)-helical ribbon (HR) and inner-outer HR operated in laminar regime were studied experimentally and numerically.The interaction implies synergistic and interference effects,which was revealed through the investigation of axial circulation rate,energy dissipation rate and power consumption.The influence factors including rotational speed ratio,rotating mode and impeller configuration were explored systematically.Quantitative analysis of power consumption involves three parameters:rate of variation in power consumption,interactive mode and ratio of power consumption.Analysis indicated that some important properties were embodied in the power curve.These properties are one-way and two-way interactions,critical speed ratio and dominant impeller.Finally,a new suggestion for power estimation was given.     

Experimental and CFD investigation of power consumption in a dual Rushton turbine stirred tank

Power consumption of a mixing system is a key variable in chemical and bioprocess engineering, the determination of which is of interest of many processes. Besides, prediction of the flooding-loading transition in an aerated stirred tank is crucial for the correct design of aerated stirred tank reactors. In this research, laboratory investigation has been carried out on local and total power consumption of a single phase as well as gas-liquid phase systems in a fully baffled stirred tank equipped with dual six-blade Rushton turbines; moreover, the flow regime behavior of a gas-liquid system was investigated. Results have been compared with data obtained from CFD simulation of experimental setup and the data available in the literature. Reasonable agreement between the experimental and simulation results indicates the validity of the CFD model. Using predicted data some empirical correlations have been derived which present new relations in estimation of power consumption and flow regime transitions in stirred tanks with dual Rushton impellers.     

Preparation and power consumption of surfactant-fuel oil-water emulsions using axial, radial, and mixed flow impellers

Surfactant-oil-water emulsions could have applications in enhanced oil recovery and the bio-desulfurization process applied to crude oil and some fractions. A simple way to prepare oil in water (O/W) emulsions is using a tank and an agitation device. The aim of this work is to propose a technology to prepare surfactant-fuel oil-water emulsions by means of a system involving a tank equipped with baffles, and an agitation device. The employed fuel oil was a high-viscosity fraction, which makes it difficult to handle. Axial, radial, and mixed flow impellers were assessed in the preparation of O/W emulsions, with and without the presence of baffles. Sixteen commercial surfactants were evaluated on the O/W emulsion formation. The effect of the storage temperature on the emulsions stability was assessed. The presence of salt on the surfactant-fuel oil-water emulsion was also investigated. Power vs. Reynolds numbers, extremely important data for the scaling up of the process, were calculated in basis of the power drawn when preparing the emulsions. Total consumption energy applied to the system, as well as pumping capacity were measured and related to the quality of the O/W emulsions obtained.     

Solids distribution and rising velocity of buoyant solid particles in a vessel stirred with multiple impellers

The distribution of buoyant solid particles in agitated suspensions has been studied. The investigation was carried out in a baffled vessel characterised by an aspect ratio equal to four and stirred with four radial impellers. Dilute suspensions of single-sized spherical particles of expanded polystyrene (density equal to 90.7 kg/m³) in water were used. Solid concentration was measured with a non-intrusive optical technique. Measurements were performed along the axis of the reactor to obtain steady-state vertical profiles (that increase from the vessel base to the top) as well as at fixed elevations to determine their transient after a pulse of solids injected at the bottom.Both the steady-state profiles and the transient concentration curves were interpreted in terms of the axial dispersion model with sedimentation. By data treatment the rising velocity in the agitated system could be determined, which proved to be significantly smaller than the rising velocity in a still liquid. The ratio of these two velocities is in reasonable agreement with a correlation of the ratio of the settling velocities for heavy particles with the ratio of the Kolmogorov microscale to particle diameter established in the past.     

Power-draw analysis of a coaxial mixer with Newtonian and non-Newtonian fluids in the laminar regime

The power consumption of a new coaxial mixer composed of a wall scraping arm and a series of rods and a pitched-blade turbine mounted on the same axis of revolution and operated in a contra-rotating mode has been characterized. The work is based on experimental measurements and 3D numerical simulations in the case of homogeneous Newtonian and non-Newtonian fluids in the laminar regime. Very good agreements between experimental and numerical results have been obtained. It has been shown that the Metzner-Otto concept can be extended to account for the speed ratio between the impellers, which allows to represent the power consumption results of the coaxial mixer on a single power master curve like with a single agitator mixer.     

Numerical investigation of granular mixing in an intensive mixer:Effect of process and structural parameters on mixing performance and power consumption

Discrete element method (DEM) simulations of particle mixing process in an intensive mixer were con-ducted to study the influence of structural and process parameters on the mixing performance and power consumption. The DEM model was verified by comparing the impeller torque obtained from simulation with that from experiment. Impeller and vessel torque, coordination number (CN) and mixing index (Relative standard deviation) were adopted to qualify the particle dynamics and mixing performance with different parameters. A method based on cubic polynomial fitting was proposed to determine the critical mixing time and critical specific input work during the mixing process. It is found that the mixing performance and energy efficiency increases with the decrease of impeller offset. The mixing perfor-mance is improved slightly with the increase of blade number and the impeller with 3 blades has the highest energy efficiency due to its low input torque. Results indicate that the energy efficiency and the mixing performance increase with the decrease of filling level when the height of granular bed is higher than that of blade.     

Gassed and ungassed power draw in a pilot scale 550 litre fermentor retrofitted with up-pumping hydrofoil B2 impellers in media of different viscosity and with very high power draw

A number of modern impellers have been designed in the pursuit of an alternative to the traditional Rushton turbine, which has a number of well recognized deficiencies. A dual up-pumping combination of the Hayward Tyler B2 (former APV-B2 or simply B2), a high solidity ratio hydrofoil impeller, was retrofitted using traditional methods to a pilot scale fermentor based on cited reference studies of the impeller performance. Using water as a media and comparatively low power draws, the B2 impeller has previously been shown to have good gas handling properties and a low ungassed power number allowing for use at high impeller-to-tank diameter ratio. In the present study a power characterization of the B2 impeller was undertaken in order to extend the available data to viscous media resembling fermentation broths and to very high power draws. Power characterizations were carried out with specific energy input rates up to 12.9 kW/m³ using different shear-thinning media.The ungassed power number of the B2 impeller was determined (3.3 for power draws in the range 0-11.6 kW/m³) and our findings confirm those of previous studies and extend the trends to media of high viscosity. Upon aeration the B2 impellers loose little power compared to the traditional impeller even when applying a very high power draw (10-20% power loss at 450 rpm and 1.28 vvm corresponding to 11.6 kW/m³). Torque fluctuations are found to be small (<5%) for this impeller at high power draw and high viscosity media. Finally it is shown that the B2 impeller can be retrofitted to pilot scale fermentors using traditional methods with a high degree of confidence.     

Mixing of viscous Newtonian fluids in a vessel equipped with steady and unsteady rotating dual-turbine impellers

This paper presents investigation on the possibility to improve the efficiency of mixing of the highly viscous Newtonian fluids in a vessel with utilization of unsteady rotating dual turbine impellers. Flow visualization experiments were used to examine the size, positions and structure of the IMR (Isolated Mixing Regions) regions as a function of Reynolds number as well as mixing time. Additionally, the effect of frequency of impellers’ oscillation on mixing efficiency was examined. It was found that the use of unsteady forward–reverse mixing mode enhance the mixing efficiency in comparison to standard mixing (up to about eight times). The structure of IMR in the forward–reverse mixing is much more complicated than in standard mixing, because of liquid division into spiral-shape filaments. This had caused that dimensionless mixing time was up to about eight times shorter in comparison to standard mode.     

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.     

Power demand and mixing performance of coaxial mixers in a stirred tank with CMC solution

Experimental investigation was carried out in an el iptical based stirred tank with a diameter of 0.48 m to explore the power demand and mixing performance of coaxial mixers. Syrup and CMC solution (sodium carboxy methyl cellulose) were used as the Newtonian and non-Newtonian fluids, respectively. Four different coaxial mixers were combined with either CBY or Pfaudler impeller as the inner one, and anchor or helical ribbon (HR) as the outer one. Results show that Pfaudler-HR is the optimized combination among four coaxial mixers in this work, which provides the shortest mixing time given the same power consumption. Compared with the syrup solution, the increase of power input can make the mixing time decreasing more obviously in the CMC solution. The quantitative correlations for both syrup and CMC solutions were established to calculate the power draw and the mixing time of four coaxial mixers.     

A new investigation of the metzner-otto concept for anchor mixing impellers

Anchor impellers are commonly used for the homogenization of non-Newtonian fluids, often in association with a set of coaxial turbines. The optimal design of such mixers relies on the knowledge of power drawn by the individual impellers. In non-Newtonian mixing, this can be readily obtained using the Metzner-Otto (1957) concept. In this article, the Metzner-Otto concept and the determination of the constant K_s for anchor impellers have been revisited using numerical and experimental techniques for the case of shear-thinning and shear-thickening fluids. Contrary to literature findings, it is shown that the constant K_s does not vary strongly with the power law index and that, for mixer design purposes, the use of a constant value of K_s for each of the rheological behaviors considered is adequate.