Impact of thermal equilibriumassumption on modelling of coreuncovery in FIX-II experimentby APROS

Abstract: When simulating a thermohydraulic system, the general assumption isthat the conservation equations for mass, energy and momentum apply.And in such systems both liquid and gas exist, in this project liquidwater and steam. This report examines two different approaches tosimulate a thermohydraulic system. One is called 6-equation model,where separate conservation equations exist for both gas and liquid.The other is known as 3-equation model, where the conservationequations describe a mix of the two states. This examination is doneby modeling an experiment known as FIX-II in the software APROS, whichis capable of using both the 3- and 6-equation model. This reportshows that in several cases, both equation models give quite goodresults. However, whereas the 3-equation model in some cases, mainlypressure, were able to produce results more in line with experimentaldata, it struggled when it comes to counter current flow. Countercurrent flow, or CCF, means that gas and liquid flow in oppositedirections, something that the 3-equation model is unable to handle.For experiment 3051 this leads to a break flow in the 3-equation modelthat significantly deviates from experimental data. Generally, it isconcluded that the 3-equation model needs to be used with cautionbecause of its inability to deal with CCF. Besides CCF, there is alsoa significant difference between the two equation models in how heatflow is calculated. This difference in heat flow is the main reasonfor differences in pressure between the two equation models, which inturn caused the difference in break flow.