In the present study, a Eulerian-Eulerian two-fluid model is developed to analyze the flow boiling phenomena under near-atmospheric pressure conditions. The required constitutive correlations for the two-fluid model are provided as flow regime dependent within the algebraic interfacial area density framework. The two-fluid model developed with Rensselaer Polytechnic Institute (RPI) wall heat flux partitioning is used to analyze the subcooled nucleate boiling of water at low pressure in three vertical annulus channels of different heated lengths over a wide range of inlet mass flux, wall heat flux, and inlet subcooling conditions.

The subcooled water enters the heated annulus channel from the bottom end and is heated to near-saturation temperature. Upon reaching the saturation temperature, the wall boiling generates dispersed vapor bubbles near the heated wall. Farther along the heated length, larger bubbles can be formed by coalescence and evaporation, and the bubbles move on to the channel core region with increased vapor fraction so the flow regime changes from bubbly to transition regime. Farther along, it may turn to an annular flow regime. The benchmark experimental cases chosen are used to validate the model capability in predicting the bubbly flow and transition flow regime (slug flow regime) characteristics with the proposed methodology. Further, the low-pressure boiling model developed is successfully extended to predict the liquid sodium boiling in flow channels similar to sodium-cooled fast reactor fuel subchannels using suitable interfacial correlations.