This paper investigates the role of end-effects in superhydrophobic microchannels for frictional reduction through COMSOL based modeling. Two precursor derivations, the Kim & Hidrovo and Enright model are discussed and expanded upon through analytical and numerical simulations. The author performed numerical models on superhydrophobic microchannels with planar, stationary and finite separation distance of surface roughness element with perfect Cassie-Baxter air-layers. The simulations indicate an asymptotic limit for the flow-rate, indicating an optimum air-layer thickness. Numerical post processing reveals that this phenomenon is due to the recirculation end-effects that are relevant when the surface roughness separation distance is on order of magnitude of the channel width. These results are the first that identify end-effects as inducing a plateauing flow-rate and can serve as a benchmark for future studies. / text

Renal failure or kidney failure is a medical condition when the kidneys fail to filter toxins and waste products from the blood. Most of the time, problems encountered in kidney malfunction include abnormal fluid levels in the body, increased acid levels and abnormal levels of Urea, Glucose, Endothelin, β2-Microglobulin, Complement Factor D. In medicine, dialysis is a method that is used to remove waste products from blood when the kidneys are in a state of renal failure. Parameters characterizing the structure of dialyzers are very important because they decide overall clearance of toxin molecules and at the same time should allow retaining useful molecules in the blood. It is however not clear how the changes of dialyzer parameters will affect the clearance. This can be found out by doing simulation of a dialysis process. In this thesis, a numerical model was developed to simulate the process that goes on inside a dialyzer to determine which parameters are important for getting better clearance of toxin molecules and how the changes of those parameters can improve the performance of dialysis. In order to do that, a model of dialyzer membrane with details of the porosity is necessary. The dialyzer membrane that was considered in this research was Polyflux 210H. Here the cross sectional images of Polyflux 210H dialyzer membrane were taken by FESEM (Field Emission Scanning Electron Microscope) to obtain the porosity values of different layers. Using these porosity values, a multilayered membrane model was developed in Finite Element Software- COMSOL Multiphysics 4.3. Then a blood flow containing - Urea, Glucose, Endothelin, β2-Microglobulin, Complement Factor D and Albumin was introduced. For a certain blood flow rate the toxins diffuse through the membrane and on the other side of the membrane a dialysate flow was introduced to remove the toxins. Two different definitions of effective diffusivity were considered for the phenomenon of the diffusion of the molecules in the membrane. Between the two, the better definition was found out by comparing the results with experimental data of the manufacturer of Polyflux 210H. Then for the chosen definition, further analysis was done and the results were compared with another set of experimental data to validate the model. Then different parameters - magnitude and direction of both blood and dialysate flow, length and diameter of the fiber, pore sizes were changed to simulate how these changes affect toxin clearance and the removal of useful molecules. The results suggest some very interesting points to achieve better dialysis performance. First of all, the clearance rate of both Urea and Glucose increase rapidly with the increasing blood flow rate. When a maximum allowable blood flow rate is attained, increasing the dialysate flow rate can ensure better clearance rate for Urea and Glucose. In both the cases of increasing radius or length of the dialyzer fiber, the clearance rate of Glucose increases more rapidly than the clearance rate of Urea. For Endothelin and β2-Microglobulin the clearance rate increases twice compared to the initial condition. Meanwhile, the clearance rate of Albumin does not change that much. Also increasing the pore diameter up to 20 nm (but not more than that) can ensure higher clearance rate of Urea and Glucose, moderate clearance rate of middle molecules and minimum loss of Albumin.

In a world where money often is the main controlling factor, everything that can be tends to be more and more optimized. Regarding electrical machines, developers have always had the goal to make them better. The latest trend is to make machines as efficient as possible, which calls for accurate simulation models where different designs can be tested and evaluated. The finite element method is probably the most popular approach since it makes it possible to, in an easy and accurate way, get numerical solutions to a variety of physics problems with complex geometries and non-linear materials. This licentiate thesis includes two different projects in which finite element methods have had a central roll. In the first project, the goal was to develop a simulation model to be able to predict demagnetization of permanent magnets. It is of great importance to be able to predict if a permanent magnet will be demagnetized or not in a certain situation. In the worst case, the permanent magnets will be completely destroyed and the machine will be completely useless. However, it is more probable that the permanent magnets will not be completely destroyed and that the machine still will be functional but not as good as before. In a time where money is more important than ever, the utilization has to be as high as possible. In this study the demagnetization risk for different rotor geometries in a 12 kW direct driven permanent magnet synchronous generator was studied with a proprietary finite element method simulation model. The demagnetization study of the different rotor geometries and magnet grades showed that here is no risk for the permanent magnets in the rotor as it is designed today to be demagnetized. The project also included experimental verification of the simulation model. The simulation model was compared with experiments and the results showed good agreement. The second project treated the redesign of the rotor in the generator previously mentioned. The goal was to redesign the surface mounted NdFeB rotor to use a field concentrating design with ferrite permanent magnets instead. The motivation was that the price on NdFeB magnets has fluctuated a lot the last few years as well as to see if it was physically possible to fit a ferrite rotor in the same space as the NdFeB rotor. A new rotor design with ferrite permanent magnets was presented together with an electromagnetic and a mechanical design.

Recent corrosion related failures of grouted post tensioned tendons, even after the introduction of improved grouts, have led to renewed interest in supplemental or backup means of corrosion control for these systems. A finite element model is presented to explore feasibility of impressed current cathodic protection of strand in grouted tendons. The model examines polarization evolution as function of service time and includes consideration of anode placement and size, grout porosity, pore water alkalinity, electrochemical species diffusivity and applied voltage on the polarization efficacy and durability of such a system. The exploratory model projections suggested that, within the context of the design parameters assumed, an impressed current cathodic protection system installed internally into a grouted duct for the purpose of cathodic protection of steel tensioning strand may be feasible for the case of initially passive steel.

Fluorine gas is produced industrially by electrolysing hydrogen fluoride in a potassium acid fluoride electrolyte. Fluorine is produced at the carbon anode, while hydrogen is produced at the mild-steel cathode. The fluorine produced has a wide range of uses, most notably in the nuclear industry where it is used to separate 235U and 238U. The South African Nuclear Energy Corporation (Necsa) is a producer of fluorine and requested an investigation into the hydrodynamics of their electrolysis cells as part of a larger national initiative to beneficiate more of South Africa’s large fluorspar deposits. Due to the extremely corrosive and toxic environment inside a typical fluorine electrolysis reactor, the fluid dynamics in the reactor are not understood well enough. The harsh conditions make detailed experimental investigation of the reactors extremely dangerous. The objective of this project is to construct a model that can accurately predict the physical processes involved in the production of fluorine gas. The results of the simulation will be compared to experimental results from tests done on a lab-scale reactor. A good correlation between reality and the simulacrum would mean engineers and designers can interrogate the inner operation of said reactors safely, effortlessly and economically. This contribution reports a time-dependent simulation of a fluorine-producing electrolysis reactor. COMSOL Multiphysics was used as a tool to construct a two dimensional model where the charge-, heat-, mass- and momentum transfer were fully coupled in one transient simulation. COMSOL is a finite element analysis software package. It enables the user to specify the dimensions of his/her investigation and specify a set of partial differential equations, boundary conditions and starting values. These equations can be coupled to ensure that the complex interaction between the various physical phenomena can be taken into account - an absolute necessity in a model as complex as this one. Results produced include a set of time dependent graphics where the charge-, heat-, mass- and momentum transfer inside the reactor and their development can be visualized clearly. The average liquid velocity in the reactor was also simulated and it was found that this value stabilises after around 90 s. The results of each transfer module are also shown at 100 s, where it is assumed that the simulation has achieved a quasi-steady state. The reactor, on which the model is based, is currently under construction and will be operated under the same conditions as specified in the model. The reactor, constructed of stainless steel, has a transparent side window through which both electrodes can clearly be seen. Thus the bubble formation and flow in the reactor can be studied effectively. Temperature will be measured with a set of thermocouples imbedded in PTFE throughout the reactor. The electric field will similarly be measured using electric induction probes. / Dissertation (MEng)--University of Pretoria, 2012. / Chemical Engineering / unrestricted

Pitting corrosion is one of the most destructive types of metal loss. The purpose of this study was to investigate the evolution, or in other words, the propagation, of a single pit in carbon steel after the initiation stage. In view of the chemical and electrochemical reactions inside a single pit in carbon steel, a two dimensional model that allows the prediction of pit evolution was developed. Eleven species in aqueous sodium chloride solution and two neutral complexes were considered in the model. Given that the active-passive transition of a metal is a key phenomenon in pitting, the equations used to construct a Pourbaix diagram for iron were incorporated in the model as rules to govern passivation behaviour. By using the finite element package COMSOL Multiphysics as a tool, the Nernst-Planck equations for the mass transport and potential variations were solved. In addition, the multiphysics model was extended with Moving Boundary (ALE) mode to predict shapes of pits. The results of the study were that the model was able to investigate migration of ionic species, account for the active-passive transition of metal and also able to show the effect of solid precipitation. The model was able to show movement of the boundaries of a pit and hence, predict the shapes of pit at a given range of time. The results were discussed in comparison to the Pourbaix diagram of iron and compared with the experimental results and published models reported in literature. The principal conclusion is that modelling corrosion activities with integrated thermodynamic equations based on Pourbaix diagram methods is an outstanding way to model any other corrosion activities.

The goal of this thesis was to get familiar with the basics of mathematical description of the thermal motion of particles in a given media, and with other possibilities of the software package COMSOL Multiphysics. A model for viscous and viscoelastic environments was created, a uniform and user friendly system for simulation and calculation of MSD and system for data conversion from FCS to MSD. Furthermore, the possibilities of the model for use in microrheology were assessed and another procedure in the implementation of the COMSOL program in the characterization of gels was proposed.

The aim of this thesis is to acquaint with the various types of the planar transmission lines and discuss their properties. Subsequently, the development of the real models of selected lines and verification their properties by simulations using COMSOL Multiphysics. The second part deals with the modeling of the semiconductor substrate, which replacing the dielectric substrate used for the transmission lines in the first part. The final part deals with the verification of calculation results in a specialized program TiberCAD.

Energy efficiency calculations and an economical analysis has been conducted on two different carbon regeneration kiln types. The purpose was do determine what economical winnings there could be by replacing the carbon regeneration system using propane burner with one heated by electricity. This project was conducted on a carbon regeneration kiln that stands at the G1A processing plant in Boliden. The two different heating solutions were implemented in the multiphysics software Comsol to determine their energy efficiency’s, and then an economical analysis was conducted by comparing the annual consumption of energy per kiln type. The models were built to be as accurate as possible, but some simplifications had to be made to make the simulations possible. The results from the study showed that the electrical heating solution was over four times more energy efficient, which would lead to an annual saving of 745 714 SEK/year. The results of this simulation must however be considered somewhat inconclusive due to implementation difficulties, but it might serve as a good predictor as to why electrical heating panels could be economically feasible.

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