Academic literature on the topic 'Fluid mixing'

PEPT (positron emission particle tracking) is a technique for tracking a small radioactive tracer in Lagrangian co-ordinates. The technique was used to study the flow patterns of non-Newtonian CMC (Carboxy Methyl Cellulose) solutions inside a vessel agitated by an axial flow impeller. The 'non-intrusive' PEPT technique uses two position-sensitive detectors to track a radioactive particle in space and time. The particle is labelled with a positron emitting isotope. Once emitted from the nucleus a positron annihilates with an electron releasing energy in the form of two 511 keV back-to-back gamma-rays travelling in opposite directions, 180 degrees apart. The tracer particle is introduced into the stirred vessel which is mounted between the two detectors of the positron camera. Three axial flow impellers produced by Lightnin Mixers Ltd were used to carry out the experiments. Results showed that the discharge from the three impellers was radial when agitating non-Newtonian viscous solutions of CMC. Trajectory analysis was used to compare the performance of the impellers using the agitation index and the efficiency of circulation. A limited number of experiments was carried out to compare the effect of baffles on the circulation of the fluids in a mixing tank. The results showed that mixing of these non-Newtonian liquids in an unbaffled tank is better than in a baffled tank when using axial flow impellers. Other experiments were carried out to suspend solid particles in viscous fluids. Results showed that the minimum speed required to suspend large particles is lower than that required to suspend small particles. There are many correlations and models in the literature to determine the minimum speed required to suspend all the particles in a fluid; some of these correlations and models were compared with experimental results from this work. The correlation of Zweitering (1958) agreed with experimental data after modification. The Geisler et al. (1993) model agreed with the data provided that the power consumption is correctly substituted. The last part of this work concerned the flow of non-Newtonian viscous materials through industrial equipment. Yoghurt was chosen as the test fluid as one of the companies sponsoring this project was Eden Vale, a yoghurt manufacturer. A method was proposed using rheological measurements to simulate the flow through the dispensing pipeline and distributing nozzles; this method allows the designer to predict the final properties of yoghurt after passing through the paching head. Measurements were also carried out to determine the final gel structure of yoghurt in the delivery pots. This data of this thesis is useful in designing stirred tanks when non-Newtonian fluid is present, either for agitation or when suspending solids. Also, a method was provided to design yoghurt manufacturing line.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2006.<br>Includes bibliographical references (leaf 64).<br>This thesis describes the processes of viscous dissipation in two generic situations for mixing flows. The objective is to illustrate, with simple examples, details of the entropy generation processes that are captured in an overall manner by a control volume analysis. The two situations are parallel mixing flows in a duct and the evolution of a wake in a centrifugal compressor. Results are given for the evolution of the velocity profile and for the dissipation function and stagnation pressure fields.<br>by François T. Le Floch-Yin.<br>S.M.

The work contained within this thesis is concerned with a theoretical investigatiop of both laminar and thermally driven types of cavity flow, together with an analysis of their associated mixing processes which find applications to Industrial mixing and also to the environment. The mixing efficiency has been viewed from two perspectives namely the tracking of a selection of fluid particles, and also the simulation of the dispersive mixing of a coloured fluid element as carried along by the flow. This thesis also incorporates features of both Newtonian and a wide range of non-Newtonian fluids.

Rapid mixing is crucial for the efficient and environment-friendly operation of many industrial and propulsion devices involving jet flows. In this dissertation, two methodologies, self-excited nozzles and radially lobed nozzles, are studied and presented in order to enhance mixing in the near field of coflowing, subsonic, turbulent, free jet flows. The characteristics of the concentration field and the mixing performance are examined, mainly in quantitative manner. Two new parameters, mixing index and mixing efficiency index, are defined for free jets, allowing quantitative analysis of the mixing performance and efficiency. The flow fields are studied with hot wire anemometry, and with CFD simulation for some of the radially lobed nozzles. Due to the large vectoring angle of the jet flows from these nozzles, a new definition for the entrainment ratio is also adopted in order to take the large radial velocity component into consideration. Self-excited nozzles, rectangular and square shaped, are examined at Reynolds numbers of 17,000 and 31,000. The self-excited square jet has fastest mixing and highest mixing efficiency, with 400% higher mixing index at 4 diameters downstream than the unexcited square jet. The mixing is improved as the excitation frequency or coflow velocity increases. The study of flow field shows the presence of one pair of periodic, coherent array of large-scale, streamwise, counter-rotating inviscid vortices shedding from each of the two flaps which dominate the mean flow and the mixing process. The coflow is primarily entrained into the jet in the minor plane while the jet fluid vectors in the major plane. Significant increase in turbulent kinetic energy immediately downstream the nozzle exit improves small-scale mixing. Radially lobed nozzles, a cross-shaped and a clover-shaped with four lobes each, are analyzed in comparison to a conical nozzle. In addition, a few modified radially lobe nozzles, including a 6-lobe nozzle and an 8-lobe nozzle, two type of fully penetrating nozzles, and a cross-shaped nozzle with centerbody, are examined in order to achieve better mixing than the cross-shaped nozzle. At 4 diameters downstream, the mixing index of the cross-shaped nozzle is 650% higher than that of the conical nozzle. The cross-shaped nozzle with centerbody, the 6- lobe and 8-lobe nozzles have slower mixing and lower efficiency than the cross-shaped nozzle,but the fully-penetrating nozzles are generally better than the cross-shaped nozzle, especially at low coflow velocities and in the far field. The flow field study shows that parallel lobe walls and deep penetration of the coflow are importance factors responsible for the observed mixing enhancement.<br>Ph. D.

An industrial rotor-stator mixer was fitted into a flow loop to carry out overall power balance, flow visualisation and residence time distribution experiments. These were performed on various rotor-stator geometries and a half-scale unit. The overall power measurements showed that a large amount of power was given to the fluid by the rotor and estimates of the local turbulent energy dissipation rate per unit mass, e, were made using these data. It was found that the pumping efficiency of rotor-stator mixers is 10 to 20% and an expression for the motor power (when the flow rate is controlled) was found. The flow pattern was characterised by a high tangential velocity in the rotor followed by an abrupt transformation to radial flow through the stator. It was suggested that the kinetic energy of the fluid in the rotor is transformed to pumping, ftiction and turbulence in the stator and that this is the region of greatest importance for mixing. The residence time distribution is characterised by a region of plug flow in series with a region of mixed flow. The flow through the volute has a dominant effect on the overall residence time distribution and the RTD is insensitive to operating condition (flow rate, rotor speed) or geometry. The knowledge gained from the above experiments was used to design diazo-coupling experiments (a mixing-sensitive competitive chemical reaction with well known kinetics) such that they gave qualitative information (e.g. best feed position) and quantitative information (e.g. turbulent energy dissipation rate) on the performance of a rotor-stator mixer. e was found to be proportional to the power given the fluid by the rotor and estimates for e of order 500 W kg were made using a micro-mixing model.

In this thesis we study four problems with potential biological and industrial applications which rely on fluid mixing and transport. The problem of simultaneous ultrafiltration, diffusion and osmosis across a membrane separating two fluids is studied, numerically and asymptotically, as a model for an artificial kidney dialyser. Couplings between the different transport mechanisms prove significant in determining overall transport rates. Our model appears to be the first to treat the three transport mechanisms in a spatially structured framework, and shows that previous, spatially averaged models can overestimate transport rates. Our results can be used to optimise dialyser geometry and to profile dialysis sessions. The remainder of this thesis concerns some fundamentals of fluid mixing and mixer design. Techniques for assessing the quality of fluid mixing are reviewed, and applied to a two-dimensional laminar chaotic flow. We find no outright optimum mixing method across the range of measures, suggesting that `sieving' a collection of mixing methods according to increasingly complicated mixing measures may fail to identify a global optimum. `Topological chaos' appears to allow good mixing stretch rate to be built-in to batch mixer design, avoiding the need to tune the mixer parameters, provided a correct flow topology is created. We show that the theoretical stretch rate predictions are achieved quite tightly, in practice in a significant fraction of the flow domain; we investigate the practicalities of topologically chaotic mixers. Finally, we discuss whether topological chaos may also apply to three-dimensional static mixer design, in a braided pipe mixer, in which pipe flow is mixed around carefully designed twisted inner pipes. We expect such a device to mix well if the inner pipes have appropriate topology. However, we demonstrate how three-dimensional flow features can undermine mixing performance.