Thesis (MScIng)--University of Stellenbosch, 2008. / ENGLISH ABSTRACT: The focus of this project was the application of a passive device in the form of a loop
thermosyphon as a reactor cavity cooling system (RCCS) for a Pebble Bed Modular
Reactor. An extensive literature review showed that loop thermosyphons have been
widely researched, both theoretically and experimentally. In the review attention has
specifically been given to matters such as safety, instability, control and mathematical
modelling.
One of the objectives of the project was to build one of the axially symmetric sections
of Dobson’s (2006) proposed full scale RCCS using a scaled down version consisting
of a single loop heated by a section of the reactor pressure vessel and cooled by a tank
of water. The second objective was to derive a theoretical model that could be used in a
computer code to simulate the experiment. The theory and experiment would then be
compared in order to verify the code.
The mathematical model created used the following three major assumptions: quasistatic
flow, incompressible liquid and vapour and one dimensionality. The conservation
equations in the form of a set of difference equations with the appropriate closure
equations were then solved explicitly. It was found that the theoretical results were
heavily influenced by the surface optical properties as well as the heat transfer
coefficients. The emissivity influenced the transition point from single to two-phase
flow as well as the condenser outlet temperature. The single phase heat transfer
coefficients influenced the condenser outlet temperature significantly while it was
found that for two phase flow the combination of the available boiling and condensation
heat transfer coefficients had only minor effects on the end results.
A stainless steel and aluminium thermosyphon loop was built using water as the
working fluid. A stainless steel heater plate provided the heat input while a 200 L water
tank was the heat sink. Temperature and flow rate measurements were recorded as a
function of time with various heating/cooling transients from start-up to steady state for
three operating modes. The three operating modes were single phase, two-phase and
heat pipe mode. It was found that the theoretical temperatures correspond reasonably well with the
experimental temperatures. The time predicted by the theoretical model to reach the
operating temperature was however somewhat longer than for the experimental. This is
to be expected when considering that there is some uncertainty pertaining to the heat
transfer coefficients as well as surface emissive properties. The correspondence of the
theoretical and experimental fin temperatures was poor due to significant thermal
stratification of the air separating the heater plate and fins. Several shortcomings in the
theoretical model as well as the experimental setup were identified and discussed.
The conclusion was reached that this exploratory study showed that the loop
thermosyphon is a viable option for the RCCS and that the mathematical model is a
viable theoretical simulation tool. Several recommendations were made for further
study to address and overcome the shortcomings identified in the theoretical and
experimental models in order to prove this conclusion. Amongst these is the
determination of better material surface properties and heat transfer coefficients and
improved mass flow rate measurement. Investigating scaling issues, natural convection
outside the loop and updating of the computer program is also recommended. / AFRIKAANSE OPSOMMING: Die fokus van hierdie projek was die toepassing van passiewe apparatuur, in die vorm
van ‘n geslote lus termoheuwel, as ‘n reaktor kamer verkoellings stelsel vir die korrel
bed modulêre reaktor. Die literatuur studie wys dat hierdie tegnologie reeds
breedvoerig ondersoek is teoreties sowel as eksperimenteel. In die literatuur oorsig
word aandag spesifiek gegee aan veiligheid, onstabiliteit, beheer en modelleering.
Een van die doelwitte van die projek was om ‘n klein skaalse model te bou van een van
die aksiaal simmetriese seksies van Dobson (2006) se voorgestelde volskaalse reaktor
kamer verkoellings stelsel. Die model bestaan uit n enkele lus verhit deur ‘n seksie van
die reaktor drukvat en verkoel deur ‘n tenk vol water. Die tweede doelwit was die
afleiding van ‘n teoretiese model wat in ‘n rekenaar program gebruik kan word om die
eksperiment te simuleer. Die teoretiese en eksperimentele data kan dan vergelyk word
om die geldigheid van die program te toets.
Die volgende aanames is gemaak tydens die afleiding van die wiskundige model:
kwasi-statiese vloei, onsamedrukbare vloeistof en gas en een dimensionalitiet. Die
behouds wette is in die vorm van ‘n stel differensie vergelykings met die toepasbare
sluitings vergelykings eksplisiet opgelos. Dit is bevind dat die teoretiese resultate
swaar beinvloed is deur die materiaal oppervlak eienskappe sowel as die
warmteoordrag koëffisiënte. Die emisiviteit beinvloed die oorgangs punt van enkel na
twee fase vloei sowel as die kondenser uitlaat temperatuur. Die enkel fase
warmteoordrag koëffisiënt het n beduidende invloed op die kondenser uitlaat
temperatuur terwyl dit voorkom asof die spesifieke kombinasie van die koking en
kondensasie warmteoordrag koëffisiënte minimale invloed op die resultate het in die
twee fase gebied.
Vlekvrye staal en aluminium is gebruik om die lus te bou met water as die verkoelings
middel. Warmte is toegevoeg tot die stelsel deur ’n vlekvrye staal verhittings plaat
terwyl ‘n 200 L water tenk die warmte onttrek het. Temperatuur en massa vloei tempo
is aangeteken as ‘n funksie van tyd vir verskeie verhitting/verkoellings oorgangs
gedragte vanaf begin tot bestendige toestand vir drie bedryfs modusse. Die drie bedryfs
modusse was enkel fase, twee fase en hitte pyp modus. Dit is bevind dat die teoretiese temperature redelik goed ooreengekom het met die
eksperimentele waardes. Die tyd wat dit neem om by die bedryfs temperatuur te kom
soos voorspel deur die teorie is egter langer as wat in die eksperiment gevind is. Dit is
te verstane wanneer die onsekerheid in die warmteoordrag koëffisiënte en materiaal
oppervlak eienskappe in ag geneem word. Die fin temperature het ‘n swakker
ooreenkoms getoon as gevolg van beduidende termiese stratifikasie van die lug tussen
die fin en verhittings plaat. Verskeie tekortkominge in die teoretiese model en
eksperimentele opstelling is geïdentifiseer en bespreek.
Die gevolgtrekking is gemaak dat die ondersoek bewys dat geslote lus termoheuwels ‘n
lewensvatbare opsie is vir ‘n reaktor kamer verkoellings stelsel en dat die wiskundige
model lewensvatbaar is vir teoretiese simulasie. Verskeie aanbevelings word egter
gemaak om die tekortkominge in die teoretiese en eksperimentele modelle aan te spreek
om so doende die gevolgtrekking te staaf. Dit word aanbeveel dat beter waardes vir die
materiaal oppervlak eienskappe en warmteoordrag koëffisiënte gevind word en
verbeterde massa vloei meetings gedoen word. Dit word verder aanbeveel om
skaleering asook natuurlike konveksie buite die lus te ondersoek en om die rekenaar
program by te werk.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/20858 |
| Date | 03 1900 |
| Creators | Ruppersberg, Johannes Coenraad |
| Contributors | Dobson, R. T., Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xiv, 1 v. (various foliations) : ill. |
| Rights | Stellenbosch University |
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