Hydrodynamics of gas-liquid flow in a round-bottom stirred tank is modelled at two gas flow rates, constant bubble size and agitator speed of 300 rpm. A round-bottom tank equipped with four baffles and a Rushton turbine was chosen to represent a typical reactor used in hydrometallurgical processes operating under pressure. The applicability of different momentum interchange models and the Realizable k-ε, SST k-ω, and RSM turbulence models was studied using CFD software. The results were compared and validated against experimental data from Particle Image Velocimetry measurements by means of liquid and gas velocity distributions. In addition, energy balance between power input and dissipation energy was compared for the different turbulence models. The CFD model was found to be in good agreement with the measurements. Of the turbulence models studied, the Realizable k-ε model showed best agreement with the measured velocity profiles. Popular drag force models proposed in the literature were assessed, as was the influence of inclusion of non-drag forces. Gas flow was found to affect the liquid phase flow in the tank by generating an additional secondary circulation loop in the upper part of the reactor.
In order to reveal the gas–liquid two-phase flow pattern of inverted-umbrella aerator, the high-speed photography technology, particle image velocimetry, and Volume of Fluid model are employed to capture the free-surface dynamics and velocity distribution. The Computational Fluid Dynamics simulations are validated by experimental data and the results are in good agreement with experiment. The simulation results of flow field, streamline distribution, velocity distribution, free-surface deformation, and turbulence kinetic energy are analyzed at in time and at radial profiles sampled at several vertical positions. Back surface of each blade revealed the area of low-pressure, which can drag air into water directly from surface and thus enhance liquid aeration and oxygenation capacity. Lifting capacity of the inverted-umbrella aerator is enough to get the liquid at the bottom of the aeration tank accelerated toward liquid surface generating the hydraulic jump. As a result, liquid phase splashes capture portions of air promoting aeration of the solution. A clear circulation whirlpool is formed during the process. The circulation whirlpool starts at the bottom of the impeller moving upward along the plate until the outer edge of the impeller, which is close to the free surface. The circulation whirlpool indicates that the inverted-umbrella aerator plays a significant role in shallow aeration. The turbulence intensity created by the impeller gradually reduces with depth. The position ( z = 0.65 H) is the watershed in the tank. The oxygen mass transfer mainly occurs in the layer above watershed.
Hydrodynamics of aerated slurry is studied experimentally and numerically using the example of thiosulphate leaching of gold concentrate. The studied milled concentrate has shear-thinning fluid rheology and it was imitated by water-based solutions of CMC. Presence of electrolytes, as in the case of the leaching slurry, has great influence on bubble size distribution. Primary phase flow is measured by Particle Image Velocimetry. Local gas hold-up in aerated CMC 50000 (0.15 w%) solution is measured by Electrical Impedance Tomography. Volumetric mass transfer is measured by dynamic method in different CMC solutions over a range of operational conditions in absence and presence of electrolytes. The experimental data was used in CFD modelling of the aerated slurry. Single phase hydrodynamics of shear-thinning fluid (CMC 50000 (0.15 w%)) have been modelled and validated against experimental data with reasonable agreement. Multiphase mixing of the thiosulphate slurry was modelled with the assumption of constant bubble size. The results of the simulations and measurements are presented and discussed.
Computational
fluid dynamics is a powerful method for scale-up
of reactors although it is still challenging to fully embrace hydrodynamics
and biological complexities. In this article, an aerobic fermentation
of Pichia pastoris cells is modeled in a batch OKTOP®9000
reactor. The 800 m3 industrial scale reactor is equipped
with a radial impeller, designed by Outotec Oy for gas dispersion
in the draft tube reactor. Measured Np of the impeller is used in hydrodynamics validation.
The resolved energy dissipation rate is compensated, and its influence
on mass transfer is analyzed and discussed. Gas–liquid drag
force is modified to simulate effects of liquid turbulence and bubble
swarms. Resolved steady state multiphase hydrodynamics is used to
simulate the fermentation process. Temporal evolution of species concentrations
is compared to experimental data measured in a small copy of the reactor
at lab scale (14 L). The effect of oxygenation on the P. pastoris cells cultivation is considered.
A novel auto-aspirated sparger is examined experimentally in a closed-loop reactor (CLR) at lab scale using particle image velocimetry, high-speed camera and oxygen mass transfer rate measurements. State-of-the-art 3D printing technology was utilized to develop the sparger design in stainless steel. An insignificant change in the bubble size distribution was observed along the aerated flow, proving the existence of a low coalescence rate in the constraint domain of the CLR pipeline. The studied sparger created macrobubbles evenly dispersed in space. In pure water, the produced bubble size distribution from 190 to 2500 μm is controlled by liquid flow rate. The bubble size dynamics exhibited a power-law function of water flow rate approaching a stable minimum bubble size, which was attributed to the ratio of the fast-growing energy of the bubble surface tension over the kinetic energy of the stream. Potentially, the stream energy can efficiently disperse higher gas flow rates. The oxygen transfer rate was rapid and depended on the water flow rate. The aeration efficiency below 0.4 kW/m3 was superior to the commonly used aerating apparatuses tested at lab scale. The efficient gas dissolution technology has potential in water treatment and carbon capture processes applications.
In this research, super-low specific speed centrifugal pump ( ns = 25, Chinese units: ns = 3.6 nQ1/2/ H3/4) is studied. The effect of the front streamline wrapping angles variation (135°, 139° and 145°) of the turbine on energy performance is considered. The pressure pulsation, interior and exterior noise characteristics and the performance of the impeller are thoroughly evaluated both experimentally and numerically. The pump has been modeled by means of computational fluid dynamics code of commercial software ANSYS CFX 11.0 to estimate energy performance and pressure pulsation. Boundary element method and finite element method are used to investigate the interior and exterior noise characteristics of the centrifugal pump by varying the front sweep angle. The front sweep angle variation was found to have insignificant influence on centrifugal pump performance characteristics. However, it influences fluid hydrodynamics around the volute tongue. In addition, the decreasing of the front streamline sweep angle slightly reduces the sound pressure level for the exterior acoustics, but the radiation distribution of the acoustic field does not change. In its turn, the modified trailing edge of the blades can reduce the peak value of the superposition decreasing the pressure pulsations at the blade passing frequency and its harmonic frequencies.
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