Impeller Rotation Research Articles

Numerical Simulation of Gas-Liquid Dispersion in A Stirred Tank Agitated by Punched Rigid-Flexible Impeller

Abstract The hydrodynamics of gas-liquid dispersion process in a stirred tank with rigid impellers, rigid-flexible impellers, and punched rigid-flexible impellers were investigated using a combined computational fluid dynamics (CFD) and population balance model (PBM) approach. A classical Eulerian-Eulerian approach coupled with standard k-e turbulence model was employed to simulate gas-liquid turbulent flow in the stirred tank. The multiple reference frame (MRF) approach was used to simulate impeller rotation. The effects of impeller type, flexible connection piece length, aperture size/ratio, impeller speed and superficial gas velocity on the local gas holdup and bubble size distribution for the gas-liquid dispersion process were investigated. Results showed that a long flexible connection piece length was conductive to the gas-liquid dispersion process. The optimum aperture ratio and aperture diameter were 12 % and 8 mm, respectively, for the gas-liquid dispersion process. Punched rigid-flexible impeller could reduce the differential pressure ahead and behind the impeller blade, decrease the size of low-pressure zone, and improve the water shear strain rate compared with rigid impeller and rigid-flexible impeller at the same Pg,v. It was found that punched rigid-flexible impeller was more efficient in terms of gas dispersibility than rigid impeller and rigid-flexible impeller.

Study of the Geometrical Effects of Impeller on the Flow Field in Hybrid Mixing Process for Manufacturing Nanocomposites

Abstract For liquid-based processing of nanocomposite materials, mechanical stirring and ultrasonic processing are two different physical processes that are widely used for mixing of nanomaterials in a liquid medium such as molten metals or polymer solutions. The acoustic cavitation bubble effect generated during ultrasonic processing is highly advantageous in breaking apart aggregates or clusters of nanomaterials in the liquid, but the extent of this effect is confined within a limited volume of liquid adjacent to the source of the ultrasound energy. On the other hand, mechanical stirring using rotary impellers can achieve flow circulation in much larger volumes of liquids, aided by agitation of a liquid medium. However, the shear forces are not sufficient to overcome the inter-particle attraction of nanomaterials. In this work, a computational model of the mechanical stirring process was implemented to study the turbulent flow characteristics generated by a rotating radial flow impeller that can accommodate an ultrasonic probe concentrically located inside a hollow impeller shaft. Systematic investigations were performed using statistical analysis and design of experiments (DoE) to determine the influence of the different geometric parameters of the impeller, such as blade length, blade height, blade thickness, hub thickness, and number of blades, on the turbulent flow characteristics of the flow field in the mixing vessel with a large volume of liquid. The hybrid mixing process combined mechanical stirring and ultrasonic processing with the determined geometric configuration of the impeller has been used to prepare aqueous suspensions of carbon nanofibers (CNFs), and the experimental observations exhibited prolonged stability of the suspensions, compared to that of suspensions separately prepared through the mechanical stirring or the ultrasonic processing. This research provides a route towards overcoming the scientific and engineering challenges associated with scaling up the liquid-based manufacturing of high performance lightweight nanocomposite materials to sufficiently large quantities for use in practical industrial applications.

Local velocity scaling in upward flow to tooth impeller in a fully turbulent region

The hydrodynamics and flow field were measured in an agitated vessel using 2-D Time Resolved Particle Image Velocimetry (2-D TR PIV). The experiments were carried out in a fully baffled cylindrical flat bottom vessel 400 mm in inner diameter agitated by a tooth impeller 133 mm in diameter. Distilled water was used as the agitated liquid. The velocity fields were investigated in the upward flow to the impeller for three impeller rotation speeds – 300 rpm, 500 rpm and 700 rpm, corresponding to a Reynolds number in the range 94 000 < Re < 221 000. This means that fully-developed turbulent flow was reached. This Re range secures the fully-developed turbulent flow in an agitated liquid. In accordance with the theory of mixing, the dimensionless mean and fluctuation velocities in the measured directions were found to be constant and independent of the impeller Reynolds number. On the basis of the test results the spatial distributions of dimensionless velocities were calculated. The axial turbulence intensity was found to be in the majority in the range from 0.4 to 0.7, which corresponds to the middle level of turbulence intensity.

Nusselt number correlation for a jacketed stirred tank using computational fluid dynamics

AbstractThe heat exchange in stirred tanks occurs mainly due to the impeller rotation and overall circulation of the fluid that promotes a more effective heat transfer between fluid and heat transfer surfaces. The type of impeller used and the size of the vessel (that affects the relation heat transfer area and volume of the vessel) have significant effects on the heat transfer. The heat transfer coefficient is very much dependent on the impeller and the speed of rotation. Empirical correlations are usually used to estimate the process side heat transfer coefficient. However, its dependence on the geometrical parameters restricts the use of those correlations to specific tank configurations. In this respect, the use of computational fluid dynamics (CFD) has recently emerged as an alternative to experimental studies. The procedure proved to be faster and lower cost, and the results proved to be accurate. In this study, CFD was applied to obtain a Nusselt number correlation for a jacketed stirred tank equipped with a six‐blade Rushton turbine impeller. The Nusselt number correlation obtained from the simulated model agrees with experimental data providing a reliable representation of the heat transfer in the tank.

The Process of the Intensification of Coal Fly Ash Flotation Using a Stirred Tank

Pulp preconditioning using a stirred tank as a pretreatment process is vital to the flotation system, which can be used to improve the flotation efficiency of mineral particles. The kinetic energy that is dissipated in the stirred tank could strengthen the interaction process between mineral particles and flotation reagents to improve the flotation efficiency in the presence of the preconditioning. In this paper, the effect of the conditioning speed on the coal fly ash flotation was investigated using numerical simulations and conditioning-flotation tests. The large eddy simulation coupled with the Smagorinsky-Lilly subgrid model was employed to simulate the turbulence flow field in the stirred tank, which was equipped with a six blade Rushton turbine. The impeller rotation was modelled using the sliding mesh. The simulation results showed that the large eddy simulation (LES) well matched the previous experimental data. The turbulence characteristics, such as the mean velocity, turbulent kinetic energy, power consumption and instantaneous structures of trailing vortices were analysed in detail. The turbulent length scale (η) decreased as the rotation speed increased, and the minimum value of η was almost unchanged when the rotation speed was more than 1200 rpm. The conditioning-flotation tests of coal fly ash were conducted using different conditioning speeds. The results showed that the removal of unburned carbon was greatly improved due to the strengthened turbulence in the stirred tank, and the optimal results were obtained with an LOI of 3.32%, a yield of 78.69% and an RUC of 80.89% when the conditioning speed was 1200 rpm.

Mechanism of small bubble breakup in an unbaffled stirred vessel

The present study investigated the mechanism of small bubble breakup in an unbaffled vessel stirred by a 4-blade paddle impeller. Bubbles were injected underneath the rotating blades. Numerical simulation and experimental observations were performed to investigate the related phenomena. The Volume of Fluid (VOF) method was utilized to simulate a gas-liquid multiphase flow. The injected bubbles were broken behind the impeller blade, where they were transported due to a decreasing pressure gradient caused by the impeller rotation. At the behind of the blade, bubbles were elongated and fragmented due to the trailing vortices developing from the rear surface of blade. The fragmented bubbles were entrapped at the cores of trailing vortices, and then floated up in the space where the trailing vortices became weak. The size of the broken bubbles was dependent on the impeller rotation rate.

Improving uniform scattering device for straw-smashing, back-throwing, no-tillage planter under complete straw mulching condition

With the objective of obtaining a completely straw-mulched field, when no-tillage mechanical sowing is implemented with straw smashing, delivering, and back-throwing approaches, it may be difficult to scatter the smashed straw uniformly during a succeeding wheat sowing step. This is because the previous rice straw is substantial in quantity and has a high humidity and toughness, which may easily result in non-uniform straw mulching and thus sparse and weak seedlings of wheat. Therefore a force-dispersing and uniform-scattering device was designed. With the number of scattering impellers, impeller angle, and impeller rotation speed as the main factors and the percentage of pass for the scattering width and non-uniformity of the straw mulching as the assessment indices, single-factor experiments and orthogonal regressive tests were performed, and a dual-index (percentage of pass for the scattering width and non-uniformity of the straw mulching) fitted regression equation was established. The test results suggested that the main factors (from primary to secondary) that influence the indices were the impeller rotation speed, number of scattering impellers, and impeller angle. The optimal parameter combination for the uniform scattering device was four rows of impellers with an angle of 15°, rotation speed of 1015 r/min, percentage of pass of 72.65% for the scattering width, and a non-uniformity of 13.8% in the straw mulching. This combination can be used to realize a uniform scattering of the smashed straw along the seedling rows on the after-sowing ground. According to the field investigation of the wheat growth, the wheat emergence rate was 90.7%. The research results can provide a reference for improving the uniform scattering device for a straw-smashing, back-throwing, no-tillage planter for obtaining a completely straw-mulched field, enhancing the quality of the machinery operation, and ensuring good and strong seedlings after sowing. Keywords: complete straw mulching, no-tillage planter, uniform scattering of straw DOI: 10.25165/j.ijabe.20181106.4597 Citation: Gu F W, Gao X M, Wu F, Hu Z C, Chen Y Q, Zhang C. Improving uniform scattering device for straw-smashing, back-throwing, no-tillage planter under complete straw mulching condition. Int J Agric & Biol Eng, 2018; 11(6): 49–57.

In Vitro Characterization of the Pittsburgh Pediatric Ambulatory Lung.

Acute and chronic respiratory failure are a significant source of pediatric morbidity and mortality. Current respiratory support options used to bridge children to lung recovery or transplantation typically render them bedridden and can worsen long-term patient outcomes. The Pittsburgh Pediatric Ambulatory Lung (P-PAL) is a wearable pediatric blood pump and oxygenator (0.3 m surface area) integrated into a single compact unit that enables patient ambulation. The P-PAL is intended for long-term use and designed to provide up to 90% of respiratory support in children weighing 5-25 kg. Computational fluid dynamics and numerical gas exchange modeling were used to design the P-PAL and predict its performance. A P-PAL prototype was then used to obtain pressure versus flow curves at various impeller rotation rates using a blood analog fluid. In vitro oxygen exchange rates were obtained in blood in accordance with ISO standard 7199. The normalized index of hemolysis (NIH) was measured over a 6 hour period at blood flow rates of 1 and 2.5 L/min. The P-PAL provided blood flows of 1-2.5 L/min against the pressure drop associated with its intended-use pediatric cannulas. The oxygen exchange rate reached a maximum of 108 ml/min at a blood flow rate of 2.5 L/min and met our respiratory support design target. Device-induced hemolysis was low with NIH values of 0.022-0.027 g/100 L in the intended blood flow rate range. In conclusion, the current P-PAL design met our pumping, oxygenation, and hemolysis specifications and has the potential to improve treatment for pediatric respiratory failure.

Effect of mixing intensity on flocculation kinetics of polystyrene latex particles with high-charge density polyelectrolyte at various ionic strengths

Effect of mixing intensity on the flocculation kinetics of polystyrene latex (PSL) particles with a high-charge density polyelectrolyte was analyzed using a small stirred vessel equipped with a four-paddle impeller and four baffles. The intensity of mixing was estimated by the effective shear rate evaluated in terms of collision frequency between the colloidal particles. This estimation was performed by measuring the salt-induced rapid coagulation of PSL particles as a function of the rate of impeller rotation. Acrylamide-(dimethylamino)ethyl methacrylate methyl chloride with nominal molecular weight of 4.9×106 g mol−1 was used as a flocculant. An enhancement factor, determined by the ratio of the rate of flocculation with a polyelectrolyte in the initial stage to that of salt-induced rapid coagulation, was used as the evaluation index. It was found that the enhancement factor goes through a maximum with an increase in mixing intensity, thus implying the presence of an optimal mixing intensity for flocculation. Further, considerable fluctuation was observed for a low mixing intensity and the degree of fluctuation slightly increased as the ionic strength increased.

The CFD simulation of the flow structure in the sewage pump

Abstract The article presents the results of experimental and numerical tests of a sewage pump. As the object of the research, the single blade impeller was taken into consideration. The primary aim of the project was to investigate the flow structure in the submersible one channel sewage pump by means of the CFD methods. The numerical results were verified on the basis of comparison with experimental results obtained for the real pump. The numerical model was built with good accuracy to the reality and the flow was recognized, which allows creating a new explanation of the working process of the single blade impeller pumps and indicate the possibility of the improvement of the analyzed object. The head curve is characterized by smaller steepness than the results of the experimental tests and the difference, for the best point equals less than 10%. Transient simulation presented fluctuation of the head and power during the impeller rotation cycle. They are 4 m for head and mechanical power – 1 kW. The flow inside the pump was conscientiously analyzed. Pressure and velocity distribution as well as radial hydrodynamic force were presented in the paper.

CFD simulation model for mixing tank using multiple reference frame (MRF) impeller rotation

ABSTRACT In this work, CFD simulations have been conducted to investigate flow behaviour of water in fully baffled stirred tank with Rushton turbine as impeller. The aim of the paper is to develop CFD model of such tank and optimize the dimensions of inner rotating fluid zone for MRF model. The best inner rotating fluid zone is found when the simulation results for mean radial velocity, mean tangential velocity and mean axial velocity are reasonably matched with literature data of Wu and Patterson (1989). The global flow parameters such as flow number and power number predicted by CFD matched quite well with the data of Wu and Patterson (1989) and Rushton et al. (1950). The developed CFD model is able to predict key phenomenon of circulating loops inside the stirred tank. The trend of turbulence energy dissipation rate obtained from CFD simulations is in good agreement with the literature data of Wu and Patterson (1989). Abbreviations: CFD: Computational Fluid Dynamics; LDA: Laser Doppler Anemometry; PIV: Particle Image Velocimetry; LSI: Laser Sheet Illumination; MRF: Multiple Reference Frame; RANS: Reynolds averaged Navier–Stokes; LES: Large Eddy Simulation; DNS: Direct Numerical Simulation; SIMPLE: Semi Implicit Method for Pressure Linked Equations; RMSE: Root Mean Squared Error; PRESTO: Pressure Staggering Option

CFD study on double- to single-loop flow pattern transition and its influence on macro mixing efficiency in fully baffled tank stirred by a Rushton turbine

Abstract For a fully baffled tank stirred by a Rushton turbine (RT), the flow pattern will change from double- to single-loop as the off bottom clearance (C) of the RT decreases from one third of the tank diameter. Such a flow pattern transition as well as its influence on the macro mixing efficiency was investigated via CFD simulation. The transient sliding mesh approach coupled with the standard k-e turbulence model could correctly and efficiently reproduce the reported critical C range where the flow pattern changes. Simulation results indicated that such a critical C range varied hardly with the impeller rotation speed but decreased significantly with increasing impeller diameter. Small RTs are preferable to generating the single-loop flow pattern. A mechanism of the flow pattern transition was further proposed to explain these phenomena. The discharge stream from the RT deviates downwards from the horizontal direction for small C values; if it meets the tank wall first, the double-loop will form; if it hits the tank bottom first, the single-loop will form. With the flow pattern transition, the mixing time decreased by about 35% at the same power input (P), indicating that the single-loop flow pattern was more efficient than the double-loop to enhance the macro mixing in the tank. A comparison was further made between the single-loop RT and pitched blade turbine (PBT, 45°) from macro mixing perspective. The single-loop RT was found to be less efficient than the PBT and usually required 60% more time to achieve the same level of macro mixing at the same P.

PIV Experimental Study of the Axial Flow Blood Pump’s Internal Flow Field Under Pulsatile Condition

Taking the axial flow blood pump as the research object, we used the particle image velocimetry (PIV) to measure the flow distribution of inlet, outlet and impeller at the changing rotation speed. Effects of the acceleration and deceleration process on the change of internal flow field were compared and analyzed. The experimental results show that the flow field of inlet and outlet area of the blood pump is stable at the constant working speed, but the vortex and reflux exist in the impeller area. However, acceleration of the impeller increases the flow field’s volocity and disturbance. Deceleration of the impeller rotation speed reduces the flow velocity, and the flow state of overall flow field is relatively stable. Design of the pulsatile blood pump should be optimized in order to reduce the occurrence of hemolysis in the process of acceleration and deceleration.

Bench Validation of a Compact Low-Flow CO2 Removal Device

BackgroundThere is increasing evidence demonstrating the value of partial extracorporeal CO2 removal (ECCO2R) for the treatment of hypercapnia in patients with acute exacerbations of chronic obstructive pulmonary disease and acute respiratory distress syndrome. Mechanical ventilation has traditionally been used to treat hypercapnia in these patients, however, it has been well-established that aggressive ventilator settings can lead to ventilator-induced lung injury. ECCO2R removes CO2 independently of the lungs and has been used to permit lung protective ventilation to prevent ventilator-induced lung injury, prevent intubation, and aid in ventilator weaning. The Low-Flow Pittsburgh Ambulatory Lung (LF-PAL) is a low-flow ECCO2R device that integrates the fiber bundle (0.65 m2) and centrifugal pump into a compact unit to permit patient ambulation.MethodsA blood analog was used to evaluate the performance of the pump at various impeller rotation rates. In vitro CO2 removal tested under normocapnic conditions and 6-h hemolysis testing were completed using bovine blood. Computational fluid dynamics and a mass-transfer model were also used to evaluate the performance of the LF-PAL.ResultsThe integrated pump was able to generate flows up to 700 mL/min against the Hemolung 15.5 Fr dual lumen catheter. The maximum vCO2 of 105 mL/min was achieved at a blood flow rate of 700 mL/min. The therapeutic index of hemolysis was 0.080 g/(100 min). The normalized index of hemolysis was 0.158 g/(100 L).ConclusionsThe LF-PAL met pumping, CO2 removal, and hemolysis design targets and has the potential to enable ambulation while on ECCO2R.

Wet-granulation process: phenomenological analysis and process parameters optimization

Wet granulation is a size-enlargement process applied in many industrial fields, such as pharmaceutical, nutraceutical, zootecnichal, to improve flowability and compressibility properties of powders. In this work analysis of the particle size distribution (PSD) of granules was performed to understand the phenomena involved during the granulation process and to optimize the operating conditions. Hydroxypropyl methylcellulose (HPMC) granules were produced spraying distilled water as liquid binder on powders in a low-shear granulator. The experimental campaign was planned using the full factorial design statistical technique varying two factors (impeller rotation speed and binder flow rate), each at three intensities. PSDs of HPMC granules at different granulation times were obtained by an ad hoc dynamic image analysis device based on the free falling particle scheme. PSD measurements showed that wet granules size depends on the simultaneous presence of nucleation, agglomeration and breakage phenomena. The process parameters optimization was carried out using response surface methodology (RSM) and using the granulation yield (% w/w of wet granules within the size range 2000–10,000 μm) as the main variable of interest.

Physical Modelling of Aluminum Refining Process Conducted in Batch Reactor with Rotary Impeller

The refining process is one of the essential stages of aluminum production. Its main aim is to remove hydrogen, that causes porosity and weakens the mechanical and physical properties of casting aluminum. The process is mainly conducted by purging inert gas through the liquid metal, using rotary impellers. The geometry of the impellers and the processing parameters, such as flow rate of gas and rotary impeller speed, influence the gas dispersion level, and therefore the efficiency of the process. Improving the process, and optimization of parameters, can be done by physical modelling. In this paper, the research was carried out with the use of a water model of batch reactor, testing three different rotary impellers. Varied methods were used: visualization, which can help to evaluate the level of dispersion of gas bubbles in liquid metal; determination of residence time distribution (RTD) curves, which was obtained by measuring the conductivity of NaCl tracer in the fluid; and indirect studies, completed by measuring the content of dissolved oxygen in water to simulate hydrogen desorption. The research was carried out for different processing parameters, such as flow rate of refining gas (5–25 L·min−1) and rotary impeller speed (3.33–8.33 s−1). The obtained results were presented graphically and discussed in detail.

Numerical and experimental study on the pressure fluctuation, vibration, and noise of multistage pump with radial diffuser

Multistage pump can provide high-pressure liquid, which is widely used in various areas of national economy. In order to improve the stability and reduce the noise of multistage pump, the relationships among the pressure fluctuation, vibration, and noise were studied deeply by using computational fluid dynamics and experimental measurement. Based on the unsteady numerical calculation, the phase of the pressure fluctuation wave in the middle section of the impeller and the diffuser was obtained, and the unsteady velocity distribution was acquired in the rotor–stator interaction (RSI) region between the rotational impeller and the stationary diffuser. Moreover, the vibration and noise tests of a five-stage pump with radial diffuser were performed. The results show that the phase distribution of the pressure fluctuation wave in the impeller and diffuser can be divided into four regions: the impeller flow channel region, the impeller transition region, the diffuser transition region, and the diffuser flow channel region. In addition, the pressure fluctuation, vibration and noise of the multistage pump are strongly related to each other, that is, RSI induces strong unsteady flow and pressure fluctuation in the pump, which makes the pump produce serious vibration and cause the corresponding noise. The key to controlling the vibration and noise is to reduce the effect of RSI between the impeller and the diffuser.

A comprehensive study of the effect of chemical impurities on selection and sizing of centrifugal machines for supercritical carbon dioxide transport pipelines

Compressors and booster pumps constitute the “heart” of the supercritical carbon dioxide transport pipeline network because they consume most of the required energy input. In other words, most of the operating expenditure for the transport pipeline goes to the running of compressors and pumps. The long-term economic feasibility of running such pipeline networks is achievable only if operating costs linked to the energy consumption of both machines can be kept as low as possible. Energy consumption can be kept as low as possible by sizing compressors and booster pumps optimally to ensure that power losses in both machines are minimized.In this study, a quasi-dimensional model based on the laws of conservation was developed, validated with available experimental data and then used for a detailed investigation of the effect of various impurities on the performance of a centrifugal machine handling supercritical carbon dioxide of varying purity. Results of the study show that discharge pressure, power requirement and efficiency of a centrifugal machine are strongly dependent on certain key parameters; namely, the size and speed of its impeller rotor as well as the composition of the impure CO2 stream. More importantly, this study also demonstrates that the quasi-dimensional model can be used as a tool for appropriate selection and sizing of centrifugal compressors and booster pumps installed on a supercritical carbon dioxide transport pipeline.

EFECTO DEL TAMAÑO DEL MARCO DE REFERENCIA ROTATIVO PARA SIMULAR UN IMPULSOR DE MEZCLADO DE PALETAS RECTAS EN UN TANQUE CON BAFLES.

A computational fluid dynamics (CFD) study was performed to simulate the flow induced by a turbine of four flat blades in a stirred baffled-tank. The multiple reference frame approach is employed to model the impeller rotation. The work focuses specifically on investigating the influence of the surface position separating the rotating reference frame (RRF) and the static reference frame (SRF) regions over the numerical results reliability. Power number measurements were obtained to validate the CFD model. It was found that for laminar flow, the interface position between both regions does not have a substantial effect on numerical results. However, its position plays a substantial role as the flow regime increases; the higher this is, the larger the RRF must be. Finally, we employ the RRF-region dimensions obtained in turbulent conditions for reproducing satisfactorily experimental results of a published system.

Design and performance of low-pressure reversible axial fan with doubly curved profiles of blades

Reversible axial fans are used to achieve air (gas) flow in both directions. An axial fan only has an impeller that can achieve the reverse flow by changing the direction of the impeller rotation. In order to obtain the same performance for both directions, the profiles of the impeller blades should be completely symmetrical, in the shape of the ellipse. This paper presents the design and performance for a reversible axial fan with blades that have a slightly doubly curved profile. The shape of the profile, whose design is shown here, was previously obtained on the basis of the numerical simulation in the profile cascade. For the experimental tests, an axial reversible fan model with double curved profiles of the blades was made. Fan performance was obtained by testing a fan for three blade positions on a standard test installation. The performance curves were compared with the performance of the reversible axial fan blades with straight profiles. The fan with doubly curved profile blades achieved a greater increase in pressure and flow than the fan with straight profiles, with approximately the same efficiency. Low pressure axial reversible fans are used in drying chambers for wood and such a design can achieve higher drying efficiency.