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Dynamic modelling of a stented aortic valveVan Aswegen, Karl 12 1900 (has links)
Thesis (MScEng (Mechanical and Mechatronic Engineering))--Stellenbosch University, 2008. / Aortic valve replacements are frequently performed during heart surgery. However,
since this is quite a stressful procedure, many patients are turned down for
medical reasons. Stented valves, designed and manufactured for percutaneous
insertion, eliminate many of the risks involved in open-heart surgery, thus providing
a solution to patients not deemed strong enough for open-chest aortic
valve replacements. The aortic valve is a complex structure, and therefore numerical
simulation is necessary to obtain flow and stress data to support the
design of a prosthetic heart valve in the absence of viable physical measuring
methods.
To aid in the design of a prosthetic heart valve, various finite element valve
models were created, and the fluid structure interaction (FSI) between the
valves and the blood was simulated using commercial finite element software.
The effect of the geometry of the leaflets on the haemodynamic behaviour over
the cardiac cycle was investigated. It was found that leaflet dimensions should
be chosen judiciously, because of their considerable effect on the stress distribution
and performance of the valve. A simple leaflet geometry optimisation
was done for a 20 mm and 26 mm valve, respectively, by means of existing
geometry relationships found in the literature. Simulations were done to obtain the maximum leaflet attachment forces
that can be used by a stent designer for fatigue loading, or to investigate the
structural strength of the stent. These simulations were numerically validated.
The effect of leaflet thickness and stiffness on resistance to opening, stress
distribution and strain were investigated. Results showed that leaflet thickness
has a greater effect on the performance of the valve than leaflet stiffness, and
thereby validated the results of similar tests contained in the literature. After
simulating over-, as well as under-dilation of a stented valve, it was found that
problems associated with over-dilation can be minimised to a certain extent
by increasing the coaptation1 region of the leaflets.
A simple pulse duplicator was designed based on a four-element Windkessel
model. The pulse duplicator was used to study the performance of the prototype
valves by means of high-speed photography, the results of which were
fed into one of the numerical finite element models and compared to real valve
performance. Some of the prototype valves showed efficiencies of 88%.
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Critical evaluation of predictive modelling of a cervical disc designDe Jongh, Cornel 12 1900 (has links)
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2007. / This thesis is concerned with the simulation of the in vivo biomechanical performance of a
cervical disc replacement. A representative (averaged) maximum range of motion (ROM),
determined by measurement of 10 student participants (5 male, 5 female), was used as head
motion input to a simulation model of the cervical spine containing a disc implant at the
C5/C6 intervertebral level. Intradiscal pressure, relative applied force on the C5 vertebrae,
bending moments and vertebral rotations were recorded. The force and motion components
of the results obtained were critically evaluated against the ISO and ASTM experimental
protocol standards, probing the representativeness of these standards to the actual in vivo
behaviour of the cervical functional spinal unit. Further, the wear resulting from a lifetime
(10 million cycles) of the ISO prescribed -and simulation determined input cycles was
simulated using a linear wear model with a triangulation technique for volume lost due to
wear, and compared to in vitro results in the literature. The inputs used for the wear model
were determined from a validated non-linear static contact finite element method (FEM)
model. The simulation “chain” shows great potential as a comparative tool for the preexperimental
testing of spinal implant designs and may be used with relative success as an
alternative to expensive prototype testing.
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Performance evaluation of wet-cooling tower fills with computational fluid dynamicsGudmundsson, Yngvi 03 1900 (has links)
Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: A wet-cooling tower fill performance evaluation model developed by Reuter is derived in Cartesian coordinates for a rectangular cooling tower and compared to cross- and counterflow Merkel, e-NTU and Poppe models. The models are compared by applying them to a range of experimental data measured in the cross- and counterflow wet-cooling tower test facility at Stellenbosch University. The Reuter model is found to effectively give the same results as the Poppe method for cross- and counterflow fill configuration as well as the Merkel and e-NTU method if the assumptions as made by Merkel are implemented. A second order upwind discretization method is applied to the Reuter model for increased accuracy and compared to solution methods generally used to solve cross- and counterflow Merkel and Poppe models. First order methods used to solve the Reuter model and crossflow Merkel and Poppe models are found to need cell sizes four times smaller than the second order method to obtain the same results. The Reuter model is successfully implemented in two- and three-dimensional ANSYS-Fluent® CFD models for under- and supersaturated air. Heat and mass transfer in the fill area is simulated with a user defined function that employs a second order upwind method. The two dimensional ANSYS-Fluent® model is verified by means of a programmed numerical model for crossflow, counterflow and cross-counterflow. / AFRIKAANSE OPSOMMING: ‘n Natkoeltoring model vir die evaluering van pakkings werkverrigting, wat deur Reuter ontwikkel is, word in Kartesiese koördinate afgelei vir ‘n reghoekige koeltoring en word vergelyk met kruis- en teenvloei Merkel, e-NTU en Poppe modelle. Die verskillende modelle word vergelyk deur hulle op ‘n reeks eksperimentele data toe te pas wat in die kruis- en teenvloei natkoeltoring toetsfasiliteit by die Universiteit van Stellenbosch gemeet is. Dit is bevind dat die Reuter model effektief dieselfde resultate gee as die Poppe model vir kruis- en teenvloei pakkingskonfigurasies sowel as die Merkel en e-NTU metode, indien dieselfde aannames wat deur Merkel gemaak is geїmplementeer word. ‘n Tweede orde “upwind” metode word op die Reuter model toegepas vir hoër akkuraatheid en word vergelyk met oplossingsmetodes wat gewoonlik gebruik word om kruis- en teenvloei Merkel en Poppe modelle op te los. Eerste orde metodes wat gebruik is om die Reuter model en kruisvloei Merkel en Poppe modelle op te los benodig rooster selle wat vier keer kleiner is as vir tweede orde metodes om dieselfde resultaat te verkry. Die Reuter model is suksesvol in twee- en driedimensionele ANSYS-Fluent® BVD (“CFD”) modelle geїmplementeer vir on- en oorversadigde lug. Warmte- en massaoordrag in die pakkingsgebied word gesimuleer mbv ‘n gebruiker gedefinieerde funksie (“user defined function”) wat van ‘n tweede orde numeriese metode gebruik maak. Die tweedimensionele ANSYS-Fluent® model word m.b.v. ‘n geprogrameerde numeriese model bevestig vir kruis-, teen- en kruis-teenvloei.
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Aspects of waste heat recovery and utilisation (WHR&U) in pebble bed modular reactor (PBMR) technologySenda, Franck Mulumba 03 1900 (has links)
Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The focus of this project was on the potential application of waste heat recovery and utilisation
(WHR&U) systems in pebble bed modular reactor (PBMR) technology. The background theory
provided in the literature survey showed that WHR&U systems have attracted the attention of
many researchers over the past two decades, as using waste heat improves the system
overall efficiency, notwithstanding the cost of extra plant. PBMR waste heat streams were
identified and investigated based on the amount of heat rejected to the environment.
WHR&U systems require specially designed heat recovery equipment, and as such the used
and/or spent PBMR fuel tanks were considered by the way of example. An appropriately
scaled system was designed, built and tested, to demonstrate the functioning of such a
cooling system. Two separate and independent cooling lines, using natural circulation flow in a
particular form of heat pipes called thermosyphon loops were used to ensure that the fuel tank
is cooled when the power conversion unit has to be switched off for maintenance, or if it fails.
A theoretical model that simulates the heat transfer process in the as-designed WHR&U
system was developed. It is a one-dimensional flow model assuming quasi-static and
incompressible liquid and vapour flow. An experimental investigation of the WHR&U system
was performed in order to validate the theoretical model results. The experimental results
were then used to modify the theoretical heat transfer coefficients so that they simulate the
experiments more accurately.
Three energy conversion devices, the dual-function absorption cycle (DFAC), the organic
Rankine cycle (ORC) and the Stirling engine (SE), were identified as suitable for transforming
the recovered heat into a useful form, depending on the source temperatures from 60 ºC to
800 ºC. This project focuses on a free-piston SE with emphasis on the thermo-dynamic
performance of a SE heat exchanger. It was found that a heat exchanger with a copper woven
wire mesh configuration has a relatively large gas-to-metal and metal-to-liquid heat transfer
area. Tube-in-shell heat exchanger configurations were tested, with the working fluid flowing in
ten copper inner pipes, while a coolant flows through the shell tube.
A lumped parameter model was used to describe the thermo-fluid dynamic behaviour of the
SE heat exchanger. In order to validate the theoretical results, a uni-directional flow
experimental investigation was performed. The theoretical model was adjusted so that it
simulated the SE heat exchanger. It was found that after this correction the theoretical model
accurately predicts the experiment. Finally, a dynamic analysis of the SE heat exchanger
experimental set-up was undertaken to show that, although vibrating, the heat exchanger setup
assembly was indeed acceptable from a vibrational and fatigue point of view. / AFRIKAANSE OPSOMMING: Die hoofoogmerk met hierdie projek was die moontlike aanwending van afvalhitteherwinningen-
benutting-(WHR&U-) stelsels in modulêre-gruisbedreaktor-(PBMR-) tegnologie.
Agtergrondteorie in die literatuurondersoek toon dat WHR&U-stelsels al menige navorser se
belangstelling geprikkel het, hetsy vanweë die moontlike ekonomiese voordele wat dit inhou óf
vir besoedelingsvoorkoming, bo-en-behalwe die koste van bykomende toerusting. Die PBMRafvalhittestrome
is ondersoek en bepaal op grond van die hoeveelheid hitte wat dit na die
omgewing vrystel.
Om in die prosesbehoeftes van WHR&U-stelsels te voorsien, moet goed ontwerpte,
doelgemaakte hitteherwinningstoerusting in ʼn verkoelings- en/of verhittingsproses gebruik
word, dus is die PBMR as voorbeeld gebruik vir die konsep. ʼn Toepaslik geskaleerde
WHR&U-stelsel is dus ontwerp, gebou en getoets om die geldigheid van die stelselontwerp te
toon. Twee onafhanklike verkoelingslyne, wat van natuurlike konveksie gebruik maak, in die
vorm van hitte-pype of termoheuwel lusse, was gebruik om te verseker dat verkoeling verskaf
word wanneer die hoof lus breek of instandhouding nodig hê.
ʼn Teoretiese model is ontwikkel wat die hitteoordragproses in die ontwerpte WHR&U-stelsel
simuleer. Dié model was ʼn eendimensionele vloeimodel wat kwasistatiese en
onsamedrukbare vloeistof- en dampvloei in die WHR&U-stelsel-lusse veronderstel. ʼn
Eksperimentele ondersoek is op die WHR&U-stelsel uitgevoer ten einde die teoretiese model
se resultate te bevestig. Die eksperimentele resultate was dus geneem om die teoretiese
hitteoordragkoëffisiënte aan te pas sodat dit die eksperimente kon simuleer.
Drie energieomsettingstoestelle, naamlik die dubbel funksie absorpsie siklus (DFAC), die
organiese Rankine siklus (ORC) en die Stirling enjin (SE), is as geskikte toestelle uitgewys om
die herwonne hitte op grond van brontemperature tussen 60 ºC en 800 ºC in ʼn bruikbare vorm
om te sit. Hierdie tesis het op vryesuier-SE’s gekonsentreer, met klem op die hitteruiler. Meer
bepaald is die termodinamiese werkverrigting van ʼn SE-hitteruiler ondersoek. Daar is bevind
dat ʼn hitteruiler met ʼn geweefde koperdraadmaas-samestelling oor ʼn betreklik groot gas-totmetaal-
en metaal-tot-vloeistof-oordragoppervlakte beskik. Die verhitter en verkoeler is in ʼn
buis-in-mantel-vorm ontwerp, met die werksvloeistof wat deur tien koperbinnepype vloei en ʼn
koelmiddel deur die mantelbuis. ʼn Saamgevoegde-parameter-model is gebruik om die termodinamiese gedrag van die SEhitteruiler
te beskryf. Ten einde die teoretiese resultate te bevestig, is ʼn eenrigtingvloeiproefondersoek
uitgevoer. Die teoretiese model is aangepas sodat dit die SE-hitteruiler kon
simuleer. Ná die nodige verstellings is daar bevind dat die teoretiese model die proefneming
akkuraat voorspel. Laastens was ʼn dinamiese ontleding van die SE-hitteruiler ook onderneem
om te toon dat, hoewel dit vibreer, die hitteruiler proef samestel inderdaad veilig is.
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Steam jet ejector cooling powered by low grade waste or solar heatMeyer, Adriaan Jacobus 12 1900 (has links)
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2006. / A small scale steam jet ejector experimental setup was designed and manufactured. This ejector setup is of an open loop configuration and the boiler can operate in the temperature range of Tb = 85 °C to 140 °C. The typical evaporator liquid temperatures range from Te = 5 °C t o 10 °C while the typical water cooled condenser presure ranges from Pc = 1 . 70 kPa t o 5. 63 kPa (Tc = 15 °C to 35 °C). The boiler is powered by by two 4kW electric elements, while a 3kW electric element simulates the cooling load in the evaporator. The electric elements are controlled by means of variacs. The function ... / Centre for Renewable and Sustainable Energy Studies
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Die invloed van versteurde inlaatvloeipatrone op aksiaalwaaiersVisser, J. G. J. 12 1900 (has links)
Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. / Thesis (MEng)--Stellenbosch University, 1990. / ENGLISH ABSTRACT: The causes of disturbances in the flow patterns at the
inlets of axial flow fans that are used in mechanical draft
cooling towers and the effects of these disturbances on the
fans, were investigated.
A scale model of a part of a mechanical draft cooling tower
used at a power station was built for the purpose of this
study. In the cooling tower the axial flow fans are
arranged in a matrix configuration. It was noted from the
scale model that the boundary fan was subjected to the
worst inlet disturbances and this had an adverse effect on
the flow it produced. Geometric alterations like the use
of guide vanes and rounded inlets improved the flow through
the boundary fan considerably. It was found that the
performance of the fans is a function of the number of fans
used and the elevation of the fans.
Several tests to determine the performance of the cooling
tower confirmed the results obtained from the experiments
on the scale model. It was also found that the speed and
direction of the prevailing wind have a significant effect
on the cooling tower.
Experiments to investigate the effects of cross-flow at the
inlet of a fan indicated an increased flow through the fan
at low cross-flow velocities and that the flow decreased at
high cross-flow velocities. / AFRIKAANSE OPSOMMING: Die invloed en oorsake van versteurde inlaatvloeipatrone op
aksiaalwaaiers wat in meganiese-trek koeltorings gebruik
word is ondersoek.
'n Skaaimodel van 'n deel van 'n meganiese-trek koeltoring
wat by 'n kragstasie gebruik word is gebou vir die doel van
die studie. Die aksiaalwaaiers wat in die koeltoring
gebruik word is in 'n matrikskonfigurasie gerangskik.
Vanaf die skaalmodel is waargeneem dat die randwaaier die
ergste versteuring by die inlaat ervaar, en gevolglik
minder vloei as die res van die waaiers in die opstelling
lewer. Die vloei deur die randwaaier is verbeter deur
verskillende geometriese wysigings soos leilemme en
inlaatafrondings aan te bring. Daar is gevind dat die
werking van die waaiers in die opstelling 'n funksie van
die hoeveelheid waaiers asook die grondvryhoogte is.
Om die werking van die koeltoring te bepaal is verskillende
toetse daarop uitgevoer. Die resultate het die geldigheid
van die skaalmodel bevestig. Daar is ook gevind dat die
heersende windsterkte en rigting 'n definitiewe invloed op
die koeltoring uitoefen.
Die invloed van 'n dwarsvloei by die inlaat van 'n enkele
waaier is ook eksperimenteel ondersoek. Die resultate toon
dat daar 'n vermeerdering in vloei deur die waaier by lae
dwarsvloeisnelhede plaasvind. Hoë dwarsvloeisnelhede
veroorsaak egter 'n vermindering in vloei.
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Numerical modeling and experimental investigation of the flow and thermal processes in a motor car vehicle underhoodVan Zyl, Josebus Maree 12 1900 (has links)
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2006. / The project aimed at numerically modeling the flow and thermal processes occurring in a Volkswagen Citi Golf Chico underhood using computational fluid dynamics (CFD). The motivation for this investigation was to develop and demonstrate the capability of CFD as an automotive analysis tool. This would allow local automobile analysts and designers enhanced analyses of the thermal and flow conditions occurring in this com-pact environment, leading to improved local vehicles.
A review of relevant literature indicated that the CFD community in South Africa is small with comparison to the international sector. The application of CFD to analyse automo-biles in South Africa is limited and practised by few. This experience requires develop-ment and refinement, such that South Africa may improve vehicles manufacture in the country. The review also indicated that CFD used in the international communities pro-vides good results, promoting simulation-based engineering.
The experimental investigation involved parking a vehicle in the subsonic wind tunnel intake at the Mechanical Engineering Department in Stellenbosch. This tunnel is 3.7 m wide, 4 m long and 2.8 m tall, capable of wind speeds up to 90 m/s. Various equipment including thermocouples, a thermal imager and a hand held hot-wire anemometer pro-vided temperature and velocity measurements within the underhood. A pitot-static probe connected to a pressure transducer measured the wind tunnel velocities.
The numerical investigation started with the creation of a three-dimensional geometry of the underhood from measurements taken of the vehicle. This geometry, created with Solid Edge version 14, formed the domain for automatically generating discretised grids using STAR-Design version 3.2. Subsequently, boundary conditions and numerical models were applied to the grids, which included simplified fan and radiator models. The analysis concluded with results obtained from the numerical CFD simulations, per-formed with STAR-CD version 3.24.
The validity and accuracy of the numerical solutions was verified and quantified with the numerical results. The evaluation consisted of two test cases (wind tunnel speeds of 0 m/s and 5 m/s), each simulated at three different grid resolutions. Each simulation con-tinued until they fully converged to a single solution. The comparison of the three simu-lations from each case indicated that the results were grid independent. The final in-spection of the results in terms of y+ values and boundary conditions indicated that the models implemented were valid.
The comparison of the numerical results for temperatures and fan inlet velocities with the experimentally measured data served as a measure to quantify the applicability of CFD for underhood investigations. The comparison between the two sets of results proved acceptable, with a maximum difference of 10%, indicating that CFD is capable of predicting temperatures and flow fields with reasonable accuracy.
The numerical results indicated that while the vehicle travels at higher velocities, the underhood remains well ventilated. The underhood tends to trap the hot air from the radiator and other heat sources when the vehicle remains stationary, causing the air to heat further. This can be addressed by the installation of vents in the side panels near the top of the underhood environment. This should allow the hot air to escape, possibly resulting in a significant reduction of the underhood temperatures.
Momentum and energy source terms modelled the effects from the fan and radiator. These models worked well for both cases, but improvement is necessary. Special at-tention should be given to the condition where the radiator fan obstructs the flow through the radiator.
A further result of the project was the establishment of a flexible foundation for conduct-ing numerical simulations on automobiles. It allows for the inclusion of additional com-ponents and the implementation of more advanced models for representing effects from various engine components.
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Evaluation and performance prediction of cooling tower spray zonesViljoen, D. J. 12 1900 (has links)
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2006. / Cooling tower spray nozzle performance characteristics such as the water distribution onto the fill material, air side pressure drop, pump head, drop size distribution and heat transfer in the spray zone were investigated experimentally and theoretically. The aim was to evaluate and simulate the performance characteristics of new and existing types of cooling tower spray nozzles with emphasis on the spray zone. Two medium and two low pressure type spray nozzles were tested and the results analysed. Single nozzle water distribution data obtained from tests was used to predict the water distribution obtained from four evenly spaced nozzles by superposition. The results were compared to data obtained from corresponding four nozzle tests. Computer codes and CFD models were developed to predict the drop trajectories, water distribution, total heat transfer and pressure drop for single nozzles and four nozzle grids. This was compared to correlated data found in literature. The performance characteristics expected from an ideal nozzle was discussed and compared to actual nozzle performance characteristics.
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Investigating the influence of fabrication parameters on the diameter and mechanical properties of polysulfone ultrafiltration hollow-fibre membranesRugbani, Ali 12 1900 (has links)
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / ENGLISH ABSTRACT: Polysulfone hollow-fibre membranes were fabricated via the dry-wet solution
spinning technique. The objective was to demonstrate the influence of the various
fabrication parameters on the diameter and mechanical properties of the hollow-fibre
membranes and to optimize the spinning process by controlling these parameters with
a computer control system. The effects of the operation parameters were investigated
using an experimental design based on a fractional factorial method (Taguchi’s design
of experiments). The parameters that were considered are the spinneret size, dope
solution temperature, bore fluid temperature, coagulation bath temperature, dope
extrusion rate, bore flow rate and the take-up speed. A new pilot solution spinning
plant was installed and upgraded, and a computer control system, based on LabView,
was developed to control, monitor and log the experimental data. The diameter of the
hollow-fibres were determined using a scanning electron microscope (SEM) while the
mechanical properties were measured using a tensile tester. The effects of diameter
size and wall thickness of the hollow-fibres on the performance of the membranes
were studied.
The results showed the significance of the fabrication parameters that dominate the
diameter and strength of the hollow-fibres. / AFRIKAANSE OPSOMMING: Polisulfoon holvesel membrane is met ‘n droë-nat oplossingspin proses vervaardig.
Die doel hiermee was om die invloed van verskeie vervaardigingsparameters op die
deursnee en meganiese eienskappe van die holvesel membrane te demonstreer asook
om die spin proses te optimeer deur gerekenariseerde beheer van die aanleg. ‘n
Eksperimentele ontwerp, gebaseer op ‘n gedeeltelike faktoriaal metode (Taguchi se
eksperimentele ontwerp) is gebruik om die invloed van die vervaardigingsparameters
te ondersoek. Die parameters wat oorweeg is, is spindop grootte, materiaal
temperatuur, boorvloeistof temperatuur, stolbad temperatuur, materiaal ekstrusie
tempo and opwen spoed. ‘n Nuwe oplossingspin loodsaanleg was geïnstalleer en
opgegradeer en ‘n rekenaar beheerstelsel, gebaseer op LabView, is ontwikkel om die
aanleg te beheer, moniteer en eksperimentele data te stoor. Die deursnee van die
holvesel is gemeet met ‘n skanderingelektron mikroskoop (SEM) terwyl die
meganiese eienskappe bepaal is met ‘n trektoets apparaat. Die effek van die deursnee
en wanddikte van die holvesels op die werkverrigting van die membrane is ook
bestudeer.
Die resultate toon watter vervaardigingsparameters is beduidend vir die deursnee en
sterkte van die holvesels.
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Numerical design optimisation for the Karoo Array TelescopeJoubert, N. J. D. 03 1900 (has links)
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2009. / Although mass minimisation is an important application within structural
optimisation, other applications include: (1) concept generation, (2) concept
evaluation, (3) design for structural feasibility and (4) data matching. These
applications, except data matching, are discussed and illustrated on a prototype
design of the Karoo Array Telescope (KAT) antenna. The KAT passed through
the design process and a full scale prototype was built, but was found to be too
expensive. A detailed finite element model of the finalised design was considered
as a test bed for reducing costs.
Size-, shape- and topology optimisation are applied to three KAT components,
while considering wind, temperature and gravity loads. Structural- and nonstructural
constraints are introduced. Coupling of the structural optimisation
code with an external analysis program to include non-structural responses and
the parallelisation of the sensitivity calculations are presented.
It is shown that if a finite element model is available, it is generally possible
to apply structural optimisation to improve an existing design. A reduction of
2673 kg steel was accomplished for the existing KAT components. The total cost
saving for the project will be significant, when considering that a large amount
of antennas will be manufactured.
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