doctoral thesis
Rok Krpan (Author), Ivo Kljenak (Mentor)

Abstract

During a severe accident in a light water reactor nuclear power plant, hydrogen may be generated, and its combustion could threaten the integrity of the nuclear power plant containment, which could lead to release of radioactive material into the environment. The study of hydrogen distribution in the containment is important to predict the occurrence of regions with flammable mixture, in order to effectively install hydrogen mitigation systems. A theoretical model to describe the homogenization of the layered atmosphere due to the vertical injection in an enclosure was developed. In linear two-equation eddy viscosity turbulence models, constant values of turbulent Schmidt and Prandtl numbers are usually used. It was shown that constant values fail to reproduce some phenomena observed in experiments. For the purposes of this work, firstly, a local Froude number is introduced, which can be directly used in computational fluid dynamics calculations. Secondly, a model that prescribes turbulent Schmidt and Prandtl numbers dynamically, which uses local Froude number and vertical velocity, is proposed. Non-essential parts of the experimental facilities, which do not contribute much to the mixing process, were identified according to experimental results. Accordingly, axisymmetric two-dimensional and three-dimensional numerical models of cylindrical vessels were developed, and the effects of numerical domain reduction were studied. Since OpenFOAM is a multipurpose computational fluid dynamics code, it was first adapted to simulate the considered cases. An additional term accounting for enthalpy diffusion due to compositional changes was implemented into the energy equation, and a term describing the molecular diffusion had to be added to the convection-diffusion equation for mean mass fraction. The developed physical model for simulation of mixing in nonhomogeneous atmosphere in large enclosures was verified and validated on fourteen different experiments performed in three experimental facilities. The comparison with experimental results showed that the selected modelling approach is able to correctly predict the helium distribution and temperature values. Furthermore, with the proposed model the successful modelling of breaking-up of atmosphere stratification, induced by vertical injection, was extended to isothermal conditions with high mass inflow.

Keywords

nuclear safety;computational fluid dynamics;turbulence modelling;atmosphere homogenization;vertical injection;turbulent Schmidt number;turbulent Praldtl number;

Data

Language: English
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UL FMF - Faculty of Mathematics and Physics
Publisher: [R. Krpan]
UDC: 532.5
COBISS: 133677315 Link will open in a new window
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Downloads: 4
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Other data

Secondary language: Slovenian
Secondary title: Modeliranje nehomogene atmosfere v zadrževalnem hramu jedrske elektrarne
Secondary abstract: Med težko nesrečo v lahkovodnem jedrskem reaktorju lahko pride do nastanka vodika, njegovo zgorevanje pa lahko ogrozi celovitost zadrževalnega hrama jedrske elektrarne, kar bi lahko vodilo v izpust radioaktivnih snovi v okolje. Študija porazdelitve vodika v zadrževalnem hramu je pomembna za napovedovanje območij z vnetljivo mešanico. Razvit je bil teoretični model za opis homogenizacije razslojene atmosfere zaradi navpičnega vbrizgavanja v eksperimentalni napravi zadrževalnega hrama. V turbulentnih modelih, ki temeljijo na modeliranju vrtinčne viskoznosti se običajno uporabljajo konstantne vrednosti turbulentnega Schmidtovega in Prandtlovega števila. Pokazano je, da konstantne vrednosti teh števil ne morejo pravilno reproducirati nekaterih fizikalnih pojavov opaženih v eksperimentih. Za namene tega dela je najprej definirano lokalno Froudovo število, ki se lahko neposredno uporablja v programih za računsko dinamiko tekočin. Poleg tega je predlagan model za dinamično predpisovanje turbulentnega Schmidtovega in Prandtlovega števila, ki uporablja lokalno Froudovo število in navpično hitrost. Nepomembni deli eksperimentalnih naprav, ki ne prispevajo veliko k procesu mešanja, so bili identificirani s pomočjo eksperimentalnih rezultatov in rezultatov simulacij. V skladu z ugotovitvami so bili razviti poenostavljeni osno simetrični dvodimenzionalni in tridimenzionalni numerični modeli cilindričnih posod ter proučeni učinki redukcije numerične domene. Ker je OpenFOAM večnamenski program za računsko dinamiko tekočin, je bil najprej prilagojen za modeliranje obravnavanih primerov. V energijsko enačbo je bilo potrebno implementirati dodatni izraz, ki upošteva difuzijo entalpije zaradi sprememb sestave, konvekcijsko-difuzijski enačbi za masni delež plina pa je bilo potrebno dodati člen, ki popisuje molekularno difuzijo. Razviti fizikalni model za simulacijo mešanja v nehomogeni atmosferi je bil preverjen na štirinajstih eksperimentih izvedenih v treh različnih eksperimentalnih napravah. Primerjava z eksperimentalnimi rezultati je pokazala, da razviti fizikalni model pravilno napove porazdelitev helija in temperaturo znotraj zaprte posode. S predlaganim modelom za dinamično predpisovanje turbulentnih števil je bilo uspešno modeliranje erozije razslojene atmosfere z navpičnim vpihovanjem razširjeno na izotermne pogoje z visokimi hitrostmi vpihovanja, pri katerih pride do nastanka Kelvin-Helmholtzeve nestabilnosti.
Secondary keywords: jedrska varnost;računska dinamika tekočin;modeliranje turbulence;homogenizacija atmosfere;navpično vbrizgavanje;turbulentno Schmidtovo število;turbulentno Prandtlovo število;
Type (COBISS): Doctoral dissertation
Study programme: 0
Embargo end date (OpenAIRE): 1970-01-01
Thesis comment: Univ. v Ljubljani, Fak. za matematiko in fiziko, Oddelek za fiziko
Pages: XV, 141 str.
ID: 17369239