Handledare på Vattenfall: Milan Tesinsky.
When simulating a thermohydraulic system, the general assumption is that the conservation equations for mass, energy and momentum apply. And in such systems both liquid and gas exist, in this project liquid water and steam. This report examines two different approaches to simulate 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 conservation equations describe a mix of the two states. This examination is done by modeling an experiment known as FIX-II in the software APROS, which is capable of using both the 3- and 6-equation model. This report shows that in several cases, both equation models give quite good results. However, whereas the 3-equation model in some cases, mainly pressure, were able to produce results more in line with experimental data, it struggled when it comes to counter current flow. Counter current flow, or CCF, means that gas and liquid flow in opposite directions, something that the 3-equation model is unable to handle. For experiment 3051 this leads to a break flow in the 3-equation model that significantly deviates from experimental data. Generally, it is concluded that the 3-equation model needs to be used with caution because of its inability to deal with CCF. Besides CCF, there is also a significant difference between the two equation models in how heat flow is calculated. This difference in heat flow is the main reason for differences in pressure between the two equation models, which in turn caused the difference in break flow.