Numerical modeling of non-condensable gases transport in two-phase flow

Murad, Afeef (author), Kljenak, Ivo (mentor), Patel, Giteshkumar (co-mentor)

The presence of non-condensable gases at the gas-liquid interface significantly decreases the heat and mass transfer, and disturbs the fluid flow. Thus, modeling the transport of non-condensable gases in two-phase flow is of great importance in nuclear reactor safety. Non-condensable gases transport in two-phase flow is considered as a multi-component flow, where the non-condensable gases are assumed to be homogeneously mixed with steam in the gas phase. Modeling of non-condensable gases transport has been already done by Computational Fluid Dynamics, system thermal-hydraulics codes, and numerical simulations. This work is dedicated to the Apros thermal-hydraulics system code. Some stability and convergence issues can show up when modeling non-condensable gases in the the current version of the code Apros 6; for example, when the gas phase consists of a high mass fraction of non-condensable gas with a very low mass fraction of steam. In the present work, a new procedure is proposed to integrate the non-condensable gases transport equation solution in the two-fluid six equations solution algorithm to develop a new thermal-hydraulic solver. A new pressure equation is derived based on considering the non-condensable gas transport equation solution from the beginning of the numerical algorithm formulation. This is expected to improve the stability and convergence by ma- king the non-condensable gas transport equation solution more tightly coupled with the pressure equation. The oscillating U-tube numerical benchmark test case was simulated to verify and validate the new implementation of the new pressure equation. The results of the oscillating U-tube show good agreement with the analytical solution when using a dense nodalization. The second test performed was the simulation of experiments on con- densation in the presence of non-condensable gases in the separate test facility CONAN. Experiments were simulated with a constant heat transfer coefficient and a heat transfer coefficient, obtained from the Uchida correlation. The simulation results were quantita- tively reasonable. Moreover, the results showed qualitatively the same behavior as the experimental data. In addition, a hypothetical case with a varying velocity behavior was also simulated (with a constant heat transfer coefficient).

• Language: English
• Keywords: Apros, non-condensable gases, two-phase flow, thermal hydraulics system codes.
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2021
• PID: 20.500.12556/RUL-134748
• COBISS.SI-ID: 90436355
• Publication date in RUL: 29.01.2022

Secondary language

• Language: Slovenian
• Title: Numerično modeliranje transporta nekondenzibilnih plinov v dvofaznem toku

Abstract

Prisotnost nekondenzibilnih plinov na vmesni površini plin-kapljevina znatno zmanjšuje prenos toplote in snovi ter vpliva na tok tekočine. Zato je modeliranje transporta nekondenzibilnih plinov v dvofaznem toku zelo pomembno pri varnosti jedrskih reaktorjev. Transport nekondenzibilnih plinov v dvofaznem toku se obravnava kot večkomponentni tok, kjer se predpostavlja, da so nekondenzibilni plini homogeno pomešani z vodno paro v plinasti fazi. Modeliranje transporta nekondenzibilnih plinov je bilo že opravljeno v računski dinamiki tekočin, sistemskih termo-hidravličnih programih in drugih numeričnih simulacijah. To delo je namenjeno obravnavi termo-hidravličnega sistemskega programa Apros. Pri modeliranju nekondenzibilnih plinov v sedanji verziji programa Apros 6 se lahko pojavijo nekateri problemi stabilnosti in konvergence; kot primer, ko je plinasta faza sestavljena iz velikega masnega deleža nekondenzibilneg plina in nizkega masnega deleža vodne pare. V tem delu je predlagan novi postopek za vključitev enačb transporta nekondenzibilnih plinov v reševanje šest-enačbnega dvo-fluidnega modela dvofaznega toka s ciljem razvoja novega termo-hidravličnega algoritma za reševanje. Nova tlačna enačba je izpeljana na podlagi obravnave nekondenzibilnega plina od samega začetka formulacije numeričnega algoritma. Pričakujemo, da bo to izboljšalo stabilnost in konvergenco s tem, ko bodo enačbe transporta nekondenzibilnih plinov bolj tesno sklopljene s tlačno enačbo. Za preverjanje in validacijo novega algoritma reševanja je bil simuliran testni primer nihajoče U-cevi. Rezultati se pri uporabi goste nodalizacije dobro ujemajo z analitično rešitvijo. Drugi preizkus je bil simulacija eksperimentov kondenzacije v prisotnosti nekondenzibilnih plinov, ki so bili izvedeni v separatni eksperimentalni napravi CONAN. Pri simulacijah sta bila uporabljena dva različna pristopa: konstanten koeficient prenosa toplote in koeficient prenosa toplote, dobljen s korelacijo Uchida. Dobljeni rezultati simulacij so bili kvantitativno smiselni. Rezultati so pokazali tudi kvalitativno enako vedenje kot eksperimentalni rezultati. Poleg tega je bil simuliran še hipotetični primer s spreminjajočim obnašanjem hitrosti, s predpisanim konstantnim koeficientom prenosa toplote.

• Keywords: Apros, nekondenzibilni plini, dvofazni tok, termo-hidravlični sistemski programi.