Turbulent dispersed multiphase flows are common in many engineering and environmental applications. The stochastic nature of both the carrier-phase turbulence and the dispersed-phase distribution makes the problem of turbulent dispersed multiphase flow far more complex than its single-phase counterpart. In this article we first review the current state-of-the-art experimental and computational techniques for turbulent dispersed multiphase flows, their strengths and limitations, and opportunities for the future. The review then focuses on three important aspects of turbulent dispersed multiphase flows: the preferential concentration of particles, droplets, and bubbles; the effect of turbulence on the coupling between the dispersed and carrier phases; and modulation of carrier-phase turbulence due to the presence of particles and bubbles.

Turbulent Dispersed Multiphase Flow

A finite element model has been used in order to study the mixing process of species in a 100-μm-wide zigzag microchannel integrating a “Y” inlet junction. The distribution of the concentration was obtained by solving successively the Navier−Stokes equation and the diffusion−convection equation in the steady state form. Because of the large range of Reynolds numbers studied (1 < Re < 800), the 2D diffusion−convection simulations are carried out with high diffusion coefficients. The results illustrated the effects of both flow rate and channel geometry on hydrodynamics and mixing efficiency. Below a critical Reynolds number of ∼80, the mixing is entirely ensured by molecular diffusion. For higher Reynolds numbers, simulations revealed the mixing contribution of laminar flow recirculations. This effect increases for lower values of diffusion coefficients. Experimental studies on the mixing of species at different flow rates are reported showing the same hydrodynamic tendency.

Mixing processes in a zigzag microchannel: finite element simulations and optical study

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Unit Operations Of Chemical Engineering

Organic pollutants removal in wastewater by heterogeneous photocatalytic ozonation

A direct numerical simulation (DNS) is used to study the effect of a freestream isotropic turbulent flow on the drag and lift forces on a spherical particle. The particle diameter is about 1.5–10 times the Kolmogorov scale, the particle Reynolds number is about 60–600, and the freestream turbulence intensity is about 10%–25%. The isotropic turbulent field considered here is stationary, i.e., frozen in time. It is shown that the freestream turbulence does not have a substantial and systematic effect on the time-averaged mean drag. The standard drag correlation based on the instantaneous or mean relative velocity results in a reasonably accurate prediction of the mean drag obtained from the DNS. However, the accuracy of prediction of the instantaneous drag decreases with increasing particle size. For the smaller particles, the low frequency oscillations in the DNS drag are well captured by the standard drag, but for the larger particles significant differences exist even for the low frequency components. In...

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Effect of turbulence on the drag and lift of a particle

For accurately measuring kLa in bioreactors the dynamic pressure method (DPM) was introduced by Linek et al. (1993). In this work a simplified version of the same method is discussed. With respect to the original DPM, the simplified version greatly simplifies data treatment. In fact final constant slope observable in the usual semi-log diagram of residual driving force versus time may be simply corrected to obtain the real kLa value with negligible inaccuracy. Experimental data obtained on a lab-size stirred tank reactor confirm all model predictions, including the feature that the adoption of large pressure changes may lead to a better accuracy.

kLa MEASUREMENT IN BIOREACTORS

Power savings is a problem of crucial importance nowadays. In process industry, suspension of solid particles into liquids is usually obtained by employing stirred tanks, which often are very power demanding. Notwithstanding tanks provided with baffles are traditionally adopted for this task, recent studies have shown that power reductions can be obtained in top-covered unbaffled vessels. In the present work experiments were carried out in a top-covered unbaffled vessel with a diameter T=0.19m and filled with distilled water and silica particles. Two different turbines were tested: a standard six-bladed Rushton Turbine (RT) and a 45° four bladed Pitched Blade Turbine (PBT). For the case of the PBT both the up-pumping (PBT-Up) and the downpumping (PBT-Down) operation mode were tested. Two different impeller sizes D (T/3 and T/2) and clearances C (T/3 and T/10) were investigated. The effects of particle size and concentration were also assessed. Investigations concern the assessment of the minimum impeller speed for complete suspension (Njs) along with the measurement of the relevant power consumption (Pjs) aiming at identifying the most efficient tank-turbine configuration among those investigated here. Results were also compared with corresponding ones pertaining to baffled tanks (obtained via correlations available in the literature). Results have shown that the RT with D=T/3 and C=T/3 and the PBT-Up with D=T/2 and C=T/10 appear to be the most convenient (least power demanding) options. Finally, a significant power saving with respect to the most efficient baffled configurations was observed thus confirming the convenience of operating solid-liquid suspensions in an unbaffled system for all those processes where the mixing time is not a limiting factor.

Comparison of Agitators Performance for Particle Suspension in Top-Covered Unbaffled Vessels

Abstract Computational Fluid Dynamics is a quite well established tool for carrying out realistic simulations of process apparatuses. However, as a difference from single phase systems, for multiphase systems the development of CFD models is still in progress. Among the two-phase systems, gas–liquid systems are characterised by an additional complexity level, related to the fact that bubble sizes are not known in advance, being rather the result of formation and breakage-coalescence dynamics and therefore of complex phenomena related to flow dynamics and interfacial effects. In the present work, Euler–Euler Reynolds-averaged flow simulations of an air-lift reactor are reported. All bubbles are assumed to share the same size, and a simplified approach is adopted for modelling inter-phase momentum exchange, that involves bubble terminal velocity as the sole parameter needed. Good agreement between simulation results and literature experimental data is found for all the gas flow rates simulated. This result implies that, despite the many simplifications that have to be adopted in order to make them viable, fully predictive CFD simulations of gas–liquid systems can give rise to reasonably accurate predictions of reactor fluid dynamics.

Modelling and Simulation of Gas–liquid Hydrodynamics in a Rectangular Air-lift Reactor

There is a growing interest in using unbaffled stirred tanks for addressing certain processing needs. In this work, digital image analysis coupled with a suitable shadowgraphy-based technique is used to investigate the shape of the free-surface vortex that forms in uncovered unbaffled stirred tanks. The technique is based on back-lighting the vessel and suitably averaging vortex shape over time. Impeller clearance from vessel bottom and tank filling level are varied to investigate their influence on vortex shape. A correlation is finally proposed to fully describe vortex shape also when the vortex encompasses the impeller.

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Franco Grisafi

Papers

kLa MEASUREMENT IN BIOREACTORS

Comparison of Agitators Performance for Particle Suspension in Top-Covered Unbaffled Vessels

Modelling and Simulation of Gas–liquid Hydrodynamics in a Rectangular Air-lift Reactor

Vortex shape in unbaffled stirred vessels: experimental study via digital image analysis

Lab-scale investigation of a pressure retarded osmosis module fed by "non-conventional" salinity gradient