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Creating mixing at low Reynolds numbers is a non-trivial challenge that has been approached from many different perspectives, using passive or active methods. This challenge been further highlighted with the rise of microfluidics. Based on the diminutive size of these devices, the Reynolds numbers are often less than 10, but have high Peclet numbers. Therefore, creating effective mixing is non-trivial and is a topic of active research, and is of paramount importance in order to improve performance of microfluidic devices in a wide range of applications. The objective of this research was to develop a novel active device for laminar mixing. The mixing device developed herein capitalized on Nafion ionic polymers, which are a class of active materials that are thin, flexible, inexpensive, and readily deployable in an aqueous medium and offer strains up to 5% under a small (<2V) applied voltage. The effect of these deflections on an incident flow is the mixing mechanism in a laminar channel flow explored in this effort. To the author's knowledge, the high-risk effort presented herein is the first attempt to exploit ionic polymers as an active mixing device. Several different configurations of ionic polymers were tested and Digital Particle Image Velocimetry (DPIV) measurements were obtained. Resulting analysis using a quantitative mixing metric shows that using cantilevered polymers create increases mixing potential in the flow for some actuation cases. Although these differences are present, they do not appear consistently in the results. However, only a partial set of flow information was obtained from DPIV, and an improved understanding of the effect of these polymers could be developed from additional experiments. Using cantilevered ionic polymers for laminar mixing could foster the development of a new generation of efficient micromixing devices, which will improve the capabilities and effectiveness of numerous microfluidic technologies that range across biomedical, lab-on-a-chip, separation and sorting technologies and many more. / Master of Science

This thesis presents the numerical and analytical study of the aerodynamic and heat transfer in laminar subsonic cascades along with the development of design guidelines and procedures to improve the design of microfabricated multistage radial turbines operating at low Reynolds number. Numerical analysis was performed on 24 cascade geometries using 2D computational fluid dynamics (CFD) for over 100 flow conditions for each cascade. Two dimensional correlations were extracted from CFD for profile and mixing losses, deviation and heat transfer. These correlations include Reynolds number and compressibility effects, and take into account incidence and various geometrical parameters (solidity, stagger, blade angles, thickness and mean-line distribution). Adaptation of losses to account for three dimensional effects and correlation for blockage were derived from analytical relationships. A turbomachinery simulation software based on mean-line analysis and conservation of rothalpy incorporating the developed correlations was programmed. The software can be adapted as for the physic it uses and the turbine configuration it analyses (axial, radial inward or outward, single or multi stage). The pressure profiles obtained from simulation were found to be in good agreement with experimental data for cold turbine tests. Design guidelines and charts are provided as well as cycle analysis considering microfabrication limitations. A considerable increase in stage isentropic efficiency compared to previous devices can result from the use of slender blades, lower solidity cascades and aspect ratios of 0.5, suggesting efficiencies as high as 85% for Re > 700. The study shows that higher power density and multistage matching can be achieved through the radial outward configuration. Two designs are presented, a single stage turbine for the next generation of microturbopump prototype and a turbine configuration with four rotors and 10 stages for closed Rankine cycle providing 50.7 W of net mechanical power.

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Resumo: Os sistemas de drenagem externa são componentes empregados para drenagem do excesso do fluido cefalorraquidiano (produzido no cérebro e contido nos ventrículos cerebrais) para um recipiente coletor, disposto fora do corpo humano. Em geral esses sistemas são compostos de um cateter ventricular, uma tubagem e uma bolsa de drenagem. Esses sistemas são utilizados em determinados procedimentos médicos e permitem, adicionalmente, a monitoração da pressão intracraniana, a coleta de materiais para análise e infusão de medicamentos. Entretanto, tais dispositivos não possuem um mecanismo que interrompa o escoamento do líquido cefalorraquidiano, quando o sistema é submetido a sensíveis variações de pressões hidrostáticas, seja pelo posicionamento indevido da bolsa de drenagem muito abaixo da cabeça do paciente, ou quando o paciente levanta-se subitamente, favorecendo a ocorrência do chamado efeito sifão e, como resultado, a hiperdrenagem desse fluido. A exposição do usuário à hiperdrenagem, mesmo que em curto intervalo de tempo, provoca lesões irreversíveis ao sistema nervoso do paciente. No presente trabalho, é proposto e ensaiado um mecanismo anti-sifão, com diferentes características construtivas, acoplado a um determinado sistema de drenagem externa, com o objetivo de fornecer maior proteção ao usuário. Experimentalmente, para a obtenção do desempenho hidrodinâmico do conjunto, utilizou-se a coleta automatizada de dados referentes à vazão de fluido com relação aos gradientes de pressão impostos. Parte dos modelos de mecanismo anti-sifão sugeridos demonstram adequada funcionalidade quando acoplados ao sistema de drenagem externa, oferecendo, assim, a possibilidade de uma proteção adicional ao usuário. / Abstract: Neurosurgical devices known as "External drainage systems" are hermetic devices employed to accomplish continuous or intermittent drainage of cerebrospinal fluid (CSF) excess caused by production /absorption imbalance. This hermetic system stores the CSF through a tubing and collection bag. In addition, this system allows for intracranial pressure monitoring (ICP), obtention of CSF sampling for biochemical analysis and intrathecal infusion of drugs. However, such devices do not incorporate a safe mechanism which would interrupt the flux in case of excessive drainage. This clinical situation is not uncommon, ocurring whenever the system is open and submitted to a sudden pressure drop, generated by the undue relative positioning of the collection bag, either by situating below the patientþs head or because the patient suddenly uprighting in the bed. Both conditions favors a physical phenomenon called syphoning effect, well known in neurosurgery. This undesirable side-effect, even for a short period of time, may compromise and even lead to irreversible damage to the brain. This project refers to the development of an anti-siphon device to prevent such side-effects. In order to accomplish this task, a special bench test has been developed. An automated data acquisition system has been employed in order to develop the experiments. Several models of anti-siphon device and pressure gradients were tested. Some models have shown a good performance and were suitable for clinical use, offering an additional protection to the patient. / Orientador: Edson Del Rio Vieira / Coorientador: Sérgio Said Mansur / Banca: José Luiz Gasche / Banca: Ricardo Augusto Mazza / Mestre

We consider the flow of two immiscible fluids in a thin inclined channel subject to gravity and a change in pressure. In particular, we focus on the effects of Navier slip along the channel walls on the long-wave linear stability. Of interest are two different physical scenarios. The first corresponds to two incompressible fluid layers separated by a sharp interface, while the second focuses on a more dense fluid below a compressible gas. From a lubrication analysis, we find in the first scenario that the system is stable in the zero-Reynolds number limit with the slip effects modifying the decay rate of the stable perturbation. In the case of the Rayeligh-Taylor problem, slip along the less dense fluid wall has a destabilizing effect. In the second scenario, fluid inertia is pertinent, and we find neutral stability criteria are not significantly affected with the presence of slip.

A theoretical treatment of the problem of unsteady turbulent free convection over a vertical flat plate is presented in this dissertation. An exhaustive review of the relevant publications revealed, that at the present time no solution of this problem has been given. The development of a method, by which the abovementioned problem could be tackled, is a substantial part of the dissertation. The equations of conservation of mass, momentum, and energy, written in a general form, were the starting point of the derivation. Various assumptions, simplifying the partial differential equations, were introduced. In the end, boundary layer equations were obtained. Turbulence was simulated by a phenomenological model, consisting of an algebraic law of the wall and a partial differential rate equation. The turbulence model is based on the concept of effective viscosity. Also, a constant turbulent Prandtl number was employed. The problem of an isothermal plate in a stagnant non-stratified fluid was treat; d, and appropriate initial and boundary conditions were formulated„ The system of equations was solved by an explicit finite- difference method. The numerical stability criteria were established. A computer programme, based on the numerical scheme, was developed and employed for calculations. The calculations were carried out for dry air, water, and mercury, representing gases, liquids, and liquid metals, respectively. In this way, a broad range of Prandtl numbers was covered. Temperature velocity, and effective viscosity profiles are presented here together with some other results of the calculations* An important observation is that the overall heat transfer coefficient goes through a temporary minimum before attaining its steady state value. The transient, which is extremely fast, can be divided into throe characteristic stages: the initial conduction regime, an intermediate stage, and the steady state. Our results were verified by comparison with data available from other independent sources. Due to the lack of data covering ■the whole transient, only the first and third stages were considered, The initial conduction regime was compared with an analytical solution and the final steady state results with experimental data of various authors, respectively. The agreement is good and no serious discrepancies were discovered. Although the present method produces reliable results, it cannot be widely employed, because the computing times are almost prohibitive with the present-day computers.

This study investigates the laminar burning velocity of premixed methane-air mixtures, having controlled supply of micron-sized (75-90 ¥ìm) coal dust and sand particles over a range of gas phase equivalence ratios (0.9-1.2), dust concentrations (0-250 g/m3) and reactant temperatures (297, 350, 400 K) using a novel Bunsen-burner type experimental design. The experimental results show that, the laminar burning velocity is enhanced by the increase in the reactant temperature, irrespective of the equivalence ratio of the mixture due to enhanced reaction rates. Addition of coal particles in fuel lean (ϕ < 1) mixtures increases the laminar burning velocity initially up to a certain coal dust concentration, but after that, the trend is altered; either it remains constant or shows a decreasing trend. The dust concentration value, which produces the initial or local maximum, increases with increase in reactant temperature. In other words, the reactant temperature plays a significant role in the trend of increase in laminar burning velocity with dust addition. For ϕ > 1, at a given reactant temperature, a linear decay of burning velocity with dust addition is observed. When a combustible dust particle interacts with the flame zone, it extracts energy from the flame (heat sink effect) and releases volatiles, thereby changing the local equivalence ratio around the flame zone. Both, increase in the equivalence ratio and the heat sink effect, are influenced by the reactant temperature. A mathematical model including these effects is developed and the model predictions are compared with the experimental results. The results are in a good agreement for fuel lean and stoichiometric mixtures; whereas the model is found to under predict results for fuel rich cases, and needs further improvements.