Yuyun Bao

Particle Image Velocimetry Study of Turbulence Characteristics in a Vessel Agitated by a Dual Rushton Impeller

Abstract Particle Image Velocimetry (PIV) has been used to investigate turbulence characteristics in a 0.48 m diameter stirred vessel filled to a liquid height (H = 1.4T) of 0.67 m. The agitator had dual Rushton impellers of 0.19 m diameter (D = 0.4T). The developed flow patterns depend on the clearance of the lower impeller above the base of the vessel, the spacing between the two impellers, and the submergence of the upper impeller below the liquid surface. Their combinations can generate three basic flow patterns, named, parallel, merging and diverging flows. The results of velocity measurement show that the flow characteristics in the impeller jet flow region changes very little for different positions. Average velocity, trailing vortices and shear strain rate distributions for three flow patterns were measured by using PIV technique. The characteristics of trailing vortex and its trajectory were described in detail for those three flow patterns. Since the space-resolution of PIV can only reach the sub-grid rather than the Kolmogorov scale, a large-eddy PIV analysis has been used to estimate the distribution of the turbulent kinetic energy dissipation. Comparison of the distributions of turbulent kinetic energy and dissipation rate in merging flow shows that the highest turbulent kinetic energy and dissipation are both located in the vortex regions, but the maxima are at somewhat different locations behind the blade. About 37% of the total energy is dissipated in dual impeller jet flow regions. The obtained distribution of shear strain rate for merging flow is similar to that of turbulence dissipation, with the shear strain rate around the trailing vortices much higher than in other areas.

Hydrodynamic Behavior of a Single Bubble Rising in Viscous Liquids

Abstract The rising behavior of single bubbles has been investigated in six systems with different viscosity and Morton number (Mo) from 3.21×10−11 to 163. Bubbles with maximum equivalent diameter of up to 16 mm were investigated. The bubble Reynolds number (Re) ranged from 0.02 to 1200 covering 3 regimes in which two functions are obtained relating the drag coefficient, CD, with Re and Mo. It has been found that in the high Reynolds number regime the drag coefficient increases until the Reynolds number of about 1200. The classic expression of Jamialahmadi (1994) is improved and extended to high viscosity liquids. A new relationship for the aspect ratio of deformed bubbles in terms of Re, the Eotvos number and Mo, applicable to a wide range of system properties, especially in high viscosity liquids, is also suggested.

Effects of the Blade Shape on the Trailing Vortices in Liquid Flow Generated by Disc Turbines

Abstract Particle image velocimetry technique was used to analyze the trailing vortices and elucidate their relationship with turbulence properties in a stirred tank of 0.48 m diameter, agitated by four different disc turbines, including Rushton turbine, concaved blade disk turbine, half elliptical blade disk turbine, and parabolic blade disk turbine. Phase-averaged and phase-resolved flow fields near the impeller blades were measured and the structure of trailing vortices was studied in detail. The location, size and strength of vortices were determined by the simplified λ 2 -criterion and the results showed that the blade shape had great effect on the trailing vortex characteristics. The larger curvature resulted in longer residence time of the vortex at the impeller tip, bigger distance between the upper and lower vortices and longer vortex life, also leads to smaller and stronger vortices. In addition, the turbulent kinetic energy and turbulent energy dissipation in the discharge flow were determined and discussed. High turbulent kinetic energy and turbulent energy dissipation regions were located between the upper and lower vortices and moved along with them. Although restricted to single phase flow, the presented results are essential for reliable design and scale-up of stirred tank with disc turbines.

GAS DISPERSION AND SOLID SUSPENSION IN A THREE-PHASE STIRRED TANK WITH MULTIPLE IMPELLERS

ABSTRACT Despite much research on gas-liquid-solid systems and their widespread application in industry, gas dispersion with solid suspension in multistage stirred reactors equipped with multiple impellers has received little attention. We report here the critical just-suspension impeller speed for different concentrations of solid particles, gas holdup, and shaft power in a vessel of 0.48 m diameter with four baffles and dished base. Five agitator configurations, each with three impellers mounted on a single shaft, have been used in the experiments. Two novel impeller designs were used, a deep hollow blade (semi-ellipse) disc turbine (HEDT) and four-wide-blade hydrofoil impellers. The hydrofoils were used in both up-pumping (WHU) and down-pumping (WHD) modes. Glass beads of 50 ∼ 150 μm diameter and density 2500 kg · m−3 were suspended at solid volumetric concentrations of 1.5, 3, 6, 9, and 15%. Results show that these suspended solids have little effect on the relative power demand. Agitators using the HEDT radial dispersing impeller at the bottom have a higher relative power demand (RPD = PG/PU) than those with WHD or WHU as the lowest one. For all impeller combinations there is little or no effect on gas holdup with increasing solid concentrations. Of the five different impeller combinations, those with an axial flow bottom impeller have significantly higher just-suspension agitation speeds and power consumption, so mounting the hydrofoil impeller at the bottom is not the optimal configuration for particle suspension. Of these impeller combinations, at a given gas flow rate the arrangement of HEDT + 2WHU has the highest relative power demand, gas holdup, and power input for both the suspension of settling particles and gas dispersion.

Analysis of Turbulence Structure in the Stirred Tank with a Deep Hollow Blade Disc Turbine by Time-resolved PIV

Abstract The turbulence structure in the stirred tank with a deep hollow blade (semi-ellispe) disc turbine (HEDT) was investigated by using time-resolved particle image velocimetry (TRPIV) and traditional PIV. In the stirred tank, the turbulence generated by blade passage includes the periodic components and the random turbulent ones. Traditional PIV with angle-resolved measurement and TRPIV with wavelet analysis were both used to obtain the random turbulent kinetic energy as a comparison. The wavelet analysis method was successfully used in this work to separate the random turbulent kinetic energy. The distributions of the periodic kinetic energy and the random turbulent kinetic energy were obtained. In the impeller region, the averaged random turbulent kinetic energy was about 2.6 times of the averaged periodic one. The kinetic energies at different wavelet scales from a6 to d1 were also calculated and compared. TRPIV was used to record the sequence of instantaneous velocity in the impeller stream. The evolution of the impeller stream was observed clearly and the sequence of the vorticity field was also obtained for the identification of vortices. The slope of the energy spectrum was approximately (5/3 in high frequency representing the existence of inertial subrange and some isotropic properties in stirred tank. From the power spectral density (PSD), one peak existed evidently, which was located at f 0 (blade passage frequency) generated by the blade passage.

Void Fraction Distributions in Cold-gassed and Hot-parged Three Phase Stirred Tanks with Multi-impeller

Abstract Vertical distributions of void fraction in gas-liquid and gas-liquid-solid stirred tanks have been measured in a fully baffled dished base vessel of 0.48 m diameter, using a conductivity probe. The impeller configuration (a hollow half elliptical blade dispersing turbine below two up-pumping wide blade hydrofoils, identified as HEDT+2WH U ) recommended in previous work has been used in this work. The operating temperatures were 24°C and 81°C, identified as cold and hot respectively. The effects of superficial gas velocity, agitator speed and the corresponding power input on the local void fraction in two-phase systems are investigated and discussed. Results show that the increasing of agitator speed or gas flow rate leads to an increase in local void fraction at the majority of measurement points in both cold and hot systems. However, the uniformity of gas dispersion does not always increase as the raising of agitator speed and power input. In either cold or hot sparged conditions, the two- and three-phase systems have similar vertical profiles for void fraction, with maxima in similar locations; however, the void fractions are significantly lower in hot sparging than with cold. In cold operation the presence of particles leads to a lower void fraction at most points, although the local void fractions increase a little with the addition of solid particles at high temperature, in good agreement with the global gas holdup results, and the possible reasons are discussed in this paper. This work can give a better understanding of the differences between cold-gassed and hot-sparged three phase stirred tanks.

PIV Study of Flow in an Aerated Tank with a Hollow Blade Turbine

Liquid phase hydrodynamics in an aerated tank (ID = 0.19 m) stirred by a half elliptical blade disk turbine (HEDT) have been investigated using particle image velocimetry (PIV) under both aerated and unaerated conditions. The effect of the gas flow rate on the aerated mean velocity of the liquid phase, turbulent kinetic energy and turbulent kinetic energy dissipation are discussed, and their comparisons with those under unaerated conditions are also carried out. Under aerated conditions, the presence of gas does not remarkably change the velocity pattern of the liquid phase. When the direction of the liquid flow is the same as the buoyancy, the liquid is accelerated up to 10.4% by bubbles at position of z/T=0.8 and r/T=0.45; while the liquid flow reversely, the liquid is decelerated about 37.5% under the gas flow rate of 0.2 vvm at the position of z/T=0.25 and r/T=0.45. The turbulent kinetic energy in the bulk flow increases after introducing gas phase, and the average turbulent kinetic energy of the upper, middle and lower regions at 0.2 vvm are increased by 37.3%, 37.8% and 142.7%, respectively. Large-eddy PIV approach is carried out to estimate the distribution of the turbulence kinetic energy dissipation. The result shows that the distribution of the turbulent kinetic energy dissipation is similar as those of turbulent kinetic energy.

Power Demand and Mixing Performance of Helical Ribbon Coaxial Mixers with Newtonian Fluids

The power consumption and mixing performance of a coaxial mixer with different rotating speeds consisting of a helical ribbon (HR) agitator and four dispersion impellers (45° pitched blade impeller, Rushton turbine, 2-blade hydrofoil, and Pfaudler) in syrup solutions of different concentrations were evaluated in an elliptical dished-base cylindrical stirred vessel with a diameter of T=0.48 m and a filled aspect ratio H/T=1. The results show that stagnant zone is effectively eliminated and mixing time is significantly shortened with a coaxial mixer. The power consumption of HR is not affected by the inner dispersion impeller as greatly as the Anchor is, but the effect of the dispersion impeller increases with the increase of its diameter. On the other hand, the inner dispersion impeller is less affected by HR if the speed ratio RN between the high- and low-speed impellers increases. The power curves with different speed ratios fit well with each other when the modified definitions of generalized Reynolds number Re* and power number Np* are used considering the influence of the speed ratio. On the basis of the mixing time at a given power consumption, a Pfaudler-HR presents a better mixing performance than an Anchor as the outer agitator for a coaxial mixer.

PIV Investigation of Liquid Flow Field in Off-Centered Shaft Stirred Tanks with Floating Particles

The local behavior of continuous phase in a stirred tank with floating particles was investigated by using PIV technique. The effect of solid concentration and off-centered shaft position on the fluid field were studied. Experimental measurements were carried out in a fully baffled cylindrical vessel equipped with a flat bottom and an up-pumping 4-Wide-blade Hydrofoil impeller (WHU). The diameter of the vessel, the diameter of the impeller, and the vertical clearance of the bottom were 0.192 m, 0.075 m, and 0.064 m, respectively. The liquid phase was water, and low density polyethylene flat cylindrical particles were used as the floating particles at different concentrations (0.0, 0.6, 0.8, and 1.0 vol.%) and different shaft positions (0.0, 0.0096, 0.0192, and 0.0288 m away from the center of the tank, corresponding to the eccentricities of 0%, 5%, 10%, and 15%, respectively). PIV measurements showed that the velocity increases by approximately 50% in the tank with the most eccentric shaft. Axial and radial mean velocities for the carrier liquid phase were also measured by using fluorescent particles. Results indicated that as the increase of floating particle concentration, the velocity decreases by about 16% in the main flow direction. In addition, the flow behavior is not affected by the presence of floating particles, and the circulation pattern is similar in both the absence and the presence of floating particles.

Micromixing Characteristics in an Aerated Stirred Tank with Half Elliptical Blade Disk Turbine

Abstract The parallel-competing iodide-iodate reaction scheme was used to investigate the micromixing efficiency in an aerated stirred tank of 0.30 m diameter agitated by a half elliptical blade disk turbine. The mean specific energy dissipation rate Pm ranged from 0.5 to 2.2 W/kg, while the superficial gas velocity VS ranged from 0.015 to 0.047 m/s. Four sub-surface feed positions were considered. When the tank is fed just under the liquid surface or in the near-wall region, the micromixing efficiency can be enhanced by introducing gases with superficial gas velocities higher than 0.031 m/s. The effects of gas on the micromixing performance become complicated, while the tank is fed in the impeller discharging region. The increase of gas flow rate does not always have good effects on the micromixing performance. Moreover, the way to feed sulfuric acid can strongly affect the efficiency of the reaction scheme. For a single liquid phase, the micromixing time tm according to the incorporation model varies from 5 × 10−3 to 3 × 10−2 s. The dimensionless local specific energy dissipation rate Φ near the liquid surface is almost independent of Pm, while Φ in the impeller discharging area decreases with increasing Pm.