An Overview of Flow Features and Mixing in Micro T and Arrow Mixers

An overview of the mixing performances of micro T mixers operating with a single fluid is presented. The focus is on the relationship between the flow features and mixing. Indeed, T mixers are characterized by a variety of regimes for increasing Reynolds numbers; they are briefly described, in particular in terms of the three-dimensional vorticity field, which can explain the different mixing performances. The effects of changes in the aspect ratio of the channels are also reviewed. The role of instability and sensitivity analyses in highlighting the mechanisms of the onsets of the different regimes is then described. These analyses also suggest possible geometrical modifications to promote mixing. We focus on that consisting of the downward tilting of the inlet channels (arrow mixers). Arrow mixers are interesting because the onset of the engulfment regime is anticipated at lower Reynolds numbers. Hence, the mixing performances of arrow mixers with varying Reynolds number are described

Mixing Improvement in a T-Shaped Micro-Junction through Small Rectangular Cavities

The T-shaped micro-junction is among the most used geometry in microfluidic applications, and many design modifications of the channel walls have been proposed to enhance mixing. In this work, we investigate through numerical simulations the introduction of one pair of small rectangular cavities in the lateral walls of the mixing channel just downstream of the confluence region. The aim is to preserve the simple geometry that has contributed to spread the practical use of the T-shaped micro-junction while suggesting a modification that should, in principle, work jointly with the vortical structures present in the mixing channel, further enhancing their efficiency in mixing without significant additional pressure drops. The performance is analyzed in the different flow regimes occurring by increasing the Reynolds number. The cavities are effective in the two highly-mixed flow regimes, viz., the steady engulfment and the periodic asymmetric regimes. This presence does not interfere with the formation of the vortical structures that promote mixing by convection in these two regimes, but it further enhances the mixing of the inlet streams in the near-wall region of the mixing channel without any additional cost, leading to better performance than the classical configuration

Green Plants in the Red: A Baseline Global Assessment for the IUCN Sampled Red List Index for Plants

Plants provide fundamental support systems for life on Earth and are the basis for all terrestrial ecosystems; a decline in plant diversity will be detrimental to all other groups of organisms including humans. Decline in plant diversity has been hard to quantify, due to the huge numbers of known and yet to be discovered species and the lack of an adequate baseline assessment of extinction risk against which to track changes. The biodiversity of many remote parts of the world remains poorly known, and the rate of new assessments of extinction risk for individual plant species approximates the rate at which new plant species are described. Thus the question ‘How threatened are plants?’ is still very difficult to answer accurately. While completing assessments for each species of plant remains a distant prospect, by assessing a randomly selected sample of species the Sampled Red List Index for Plants gives, for the first time, an accurate view of how threatened plants are across the world. It represents the first key phase of ongoing efforts to monitor the status of the world’s plants. More than 20% of plant species assessed are threatened with extinction, and the habitat with the most threatened species is overwhelmingly tropical rain forest, where the greatest threat to plants is anthropogenic habitat conversion, for arable and livestock agriculture, and harvesting of natural resources. Gymnosperms (e.g. conifers and cycads) are the most threatened group, while a third of plant species included in this study have yet to receive an assessment or are so poorly known that we cannot yet ascertain whether they are threatened or not. This study provides a baseline assessment from which trends in the status of plant biodiversity can be measured and periodically reassessed

A mechanistic model for re-entrainment in wave plate demisters

Among the separation devices used to remove minute droplets of liquid from a gas stream, wave plate mist eliminators, which separate the two phases by inertial impaction, are usually adopted in many applications. Re-entrainment limits the gas-liquid separator performance because it causes a sharp decrease in separation efficiency and therefore gas velocity cannot exceed a certain value. This work presents new experimental data for the critical gas velocity at which re-entrainment occurs in industrial wave plate mist eliminators operated at atmospheric conditions with horizontal flow. The results show how the critical gas velocity is influenced by physical properties and flow rate of the liquid and by separator geometry. This paper also presents a new model for predicting re-entrainment based on the calculation of the liquid load on each plate between two consecutive bends of the separator. To this scope, an efficiency model was used to evaluate liquid flow over each plate and a mechanistic model was developed to compute the re-entrainment from the trickles which are formed on the plates by the coalescence of liquid droplets that impinge on the blades of the wave plate mist eliminator. The model developed takes into account the parameters which influence re-entrainment according to experimental data, it could be used to predict the critical velocity for different wave plate mist eliminators and therefore it is useful to design new ones

Macro-Instabilities in Eccentrically Agitated Vessels

Laser Doppler anemometry and flow visualisation are used to shed light into the main turbulent flow features of an unbaffled vessel stirred by an eccentrically positioned Rushton turbine. Two main vortices, one above and one below the impeller, are present and the former vortex dominates the flow field, driving a strong circumferential flow around it. The vortices are not steady but oscillate slowly and periodically inducing a kind of flow instabilities, which may have a significant impact on macro-mixing. The characteristic frequencies of such flow instabilities were found to increase with reducing the impeller blade thickness, thus it is argued that their origin is related to the interaction between the impeller discharged stream and the vessel wall/bottom

Experimental measurements of solid concentration distribution in mechanically stirred solid-liquid systems

Solids concentration profiles at different heights, in a T=500 mm vessel mechanically stirred by a Lightnin A310 impeller, were measured by means of electrical impedance technique. A new probe was designed and employed. Its main advantage is that it is absolutely non-intrusive and that it can be used for laboratory as well as for industrial scale stirred tank reactors. Two sets of spherical glass particles (density of ρS=2500 kg/m3) of narrow size distribution (dp=90 and 600 μm) and solids load up to 40% (wt.solid/wt.liquid x100) were used. Three sizes of a Lightnin A310 impeller, set a clearance C=T/4 from the vessel flat bottom, were used: D/T= 0.346, 0.386 and 0.5. The liquids used were water and aqueous solutions of glycerine (Newtonian behaviour, viscosity 1 to 10 mPa s). Power consumption was determined with a strain gauge technique. For solid loadings greater than 10% and depending on the stirrer speeds, an interface between the suspension and a clear liquid layer was observed towards the top of the vessel. This is of critical importance in slurry catalyst reactor design, as mixing between the dense suspended layer and the clear liquid above it is very limited. Besides solid concentration distribution, both power consumption and height of interface were measured at each impeller speed

A capacitance probe and a new model to identify and predict the capacity of columns equipped with structured packings

A new probe based on the measurement of the electrical capacitance has been developed to measure liquid holdup in columns equipped with structured packings. The main advantages of this new probe are that it is nonintrusive, it allows fast and online measurements, and it can be used for laboratory-scale columns as well as for industrial applications. The probe is designed to measure local liquid holdup and it can therefore be used to evaluate the presence of the liquid hold-up gradient inside each packing element in the column. Evaluation of the liquid hold-up gradient is helpful to identify locations inside the packing element where flooding begins and to suggest packing design modifications. The relation between electrical capacitance and liquid holdup closely follows the predictions obtained using simple electrical models. The experimental results obtained with the probe have enabled a simplified model to be tuned to predict capacity limits of columns equipped with structured packings

Effect of shaft eccentricity and impeller blade thickness on the vortices features in an unbaffled vessel

Different techniques, i.e. laser Doppler anemometry, flow visualisation and decolourisation, were applied in order to gain insight into the main turbulent flow features of an unbaffled vessel stirred by an eccentrically positioned Rushton turbine. Attention was paid to the effect of geometrical parameters such as eccentricity and impeller blade thickness on the flow motion. Two main vortices, one above and one below the impeller, were identified in all configurations. Decolourisation experiments showed that the two vortices behave as segregated regions. The upper vortex dominates the flow field, driving a strong circumferential flow around it. The inclination of such vortex varies with eccentricity. Importantly, the vortex is not steady but oscillates slowly and periodically inducing a kind of flow instabilities, which may have a significant impact on macro-mixing. The characteristic frequency of flow instabilities was found to increase with reducing eccentricity or impeller blade thickness. Moreover, vortex shedding phenomena from the flow-shaft interaction were observed in different configurations. © 2008 The Institution of Chemical Engineers