A Generic Non-Miscible Liquid-Gas Medium Model in Modelica with Analysis of Incompressibility Assumptions

Abstract

Motivated initially by the specific needs of Small ModularReactors (SMR) transient analysis, including Molten SaltReactors (MSR) draining or leak scenarios, or SodiumNatural Circulation Reactor (SNCR) scenarios, this paperpresents the development of a generic two-substanceliquid-gas medium and an approach used for a liquid Sodiummedium. Such media have a low isothermal compressibility,which is a key issue for the numerical robustness of thesimulation. These media are designed for compatibility withstandard libraries like Modelica.Fluid. We detail theirformulation, capabilities, and limitations. A significantportion of the paper is then dedicated to investigating acrucial aspect arising in such models: the representationof the liquid phase’s compressibility. We compare differentapproaches, analyzing their impact on the resultingequation system structure and numerical robustness usingthe OpenModelica debugger. This analysis demonstrates thetrade-offs involved and provides insights into selectingappropriate liquid models for dynamic simulations involvingnon-miscible flows.