Numerical modelling of subcooled boiling flow based on mechanistic approach: A validation study using wet steam (IAPWS) as working fluid properties

Due to the safety conditions for operating the nuclear reactors, a number of researches have attempted to gain more knowledge and to understand the boiling flow phenomena. In this research, the wall boiling models, based on the mechanistic approach, were improved into ANSYS CFX 14.5 for studying the sub-cooled boiling flow. Basically, these constituted models are required for predicting the main parameters at the heated wall boundary, which include (i) nucleation site density, (ii) bubble departure diameter, and (iii) bubble departure frequency. Currently, the wall heat flux partitioning closures have been modified to consider an influence of bubble sliding along the wall before the lift-off, which usually happens in the flow boiling. For the simulation, it was performed based on the Two-fluid model, together with the k-e turbulent model. Also, the properties of Wet Steam (IAPWS) at considered temperature and pressure operations were adopted as the working fluid conditions. The available experimental data, which observed the boiling flow at the low pressure, were chosen. The results showed that the void fraction, vapor velocity, liquid velocity, and Sauter Mean Diameter (SMD) from the predictions were found to be in fair agreement with the experiments. Thus, the current mechanistic models are necessary to develop further to obtain more accurate prediction of this flow. According to the experimental works, the mechanisms, such as a merging of bubbles during sliding, a shrinking of bubbles during the condensation, will be considered for the code development in the future work.