Assessment of Components of the Subcooled Boiling Model for CFD Simulations

This paper describes the details of validation of heat and mass transfer models used for subcooled boiling simulation with a CFD two-fluid model. This research was focused on assessment of the wall heat flux partitioning model using the state-of-the-art multidimensional experimental data available in the literature. Various constitutive relations used to close the vapor generation rate at the heated wall were studied and the best suited combination of these was obtained. The current study was restricted to vertical flows through pipe and annulus geometries. Three data sets from the literature were considered: first with R12 at about 26 bar pressure, second with water at atmospheric pressure and third with R113 at 2.69 bar pressure. In these data sets, the bubble diameter distribution across the ducts was measured. Bubble diameter estimation brings in the largest uncertainty in the two-fluid model predictions and hence using the data with known bubble sizes allowed to focus on assessment of other parameters which use constitutive relations to model vapor generation rate, e.g. bubble nucleation site density and bubble departure frequency at the wall. The simulations were carried out using the CFD code CFX-12. The R12 data used here corresponds to fluid-vapor density ratio which is equivalent to that of water-steam at 150 bar. Therefore the density ratio varies over two orders of magnitude. The surface tension also varies over a wide range from 0.0017 to 0.057 N/m. The ratio of the flow channel hydraulic diameter to the bubble diameter in these simulations varied between 4 and 40. The two-fluid model was modified, for cases involving bubble sizes too large to be represented using the continuum assumption, in order to obtain satisfactory results. Hence, the model that has been developed for this study is applicable for a wide range of physical conditions and bubble sizes.