EVALUATION AND ANALYSIS OF ACTIVE NUCLEATION SITE DENSITY MODELS IN BOILING

Computational Fluid Dynamic (CFD) models employed to simulate thermohydraulic flows where boiling is occurring, require the knowledge of several parameters that are crucial for the flow prediction. It has been shown in several publications that the active nucleation site density (ANSD) is one of the key parameters that has strong influence on the simulation accuracy. This work which aims to evaluate the main used ANSD models, takes place within a larger project frame dedicated to improving the prediction of subcooled boiling using both measurements and CFD simulations. Existing empirical and semi-empirical ANSD correlations are generally formulated and validated solely against a restricted range of experimental conditions. This limits their validity domain and justifies their re-evaluation undertaken in this publication. 3D CFD simulations are performed to investigate subcooled flow boiling heat transfer and evaluate ANSD models by comparing the flow results to our own published measurements in Kromer 2015 and Kromer et al. 2016. The selected measurements consist of temperature values at three axial positions situated on the heated wall center-line of a vertical mini-channel water model (400mm x 100mm x 3mm) at atmospheric pressure and heated from one side. The reviewed ANSD models are implemented in the open-source finite volume CFD code OpenFOAM in order to benchmark them versus temperature measurements and identify the parameters that are affecting most the accuracy of the predictions. The results show that the axial wall temperature is underestimated by all the tested models. However, the models based on a mechanistic or experimental approach, Benjamin & Balakrishnan 1997 and Hibiki & Ishii 2003, that take into account the heated wall surface and material characteristics e.g. density, thermal conductivity, specific heat and roughness, provide much better results with a temperature difference of 4°C compared to more than 10°C for the othersDigiBoil – Development of computational fluid dynamic boiling heat transfer models with applicability to high heat flu