MODELING OF DEPARTURE FROM NUCLEATE BOILING

Abstract

Modeling of departure from nucleate boiling (DNB) essentially requires predicting key parameters like bubble departure diameter and nucleation site density. Based on experimental evidence, different hypotheses of DNB occurrence have been envisaged. Among them, the bubble crowding model performs well under high subcooling conditions. Validations of the DNB model are required under a wide range of operating and geometrical conditions. With the scarcity of experimental data for bubble departure diameter and nucleation site densities at high pressure conditions it is difficult to validate the models. It is observed that the computational Quid dynamics model's performance is good compared to mechanistic models, especially for high pressure conditions. The DNB models are still evolving and are less matured as compared to the dry-out modeling of annular flow regime, which has a definite flow pattern and can be mechanistically treated with significantly less uncertainty. On the contrary, DNB involves agitated bubbles, and bubble dynamics are very complex, hence it is a challenging task to mechanistically treat the same. This chapter provides the state-of-the-art approach for predicting the DNB and describes the gray areas requiring further effort for understanding the physics of the same.