Mechanistic Modeling of Departure from Nucleate Boiling in Subcooled Flow Boiling at Near-Critical Pressure Conditions

The Departure from Nucleate Boiling (DNB) leads to a drastic deterioration in heat transfer and thus limits the cooling capability through boiling processes in technical applications. Supercritical power plants, such as SCWRs, inherently operate without a two-phase flow and therefore without the risk of boiling crises during standard operating conditions. Yet, transient scenarios such as startups, shutdowns, or accidents can cause a transition into high subcritical conditions, potentially leading to the occurrence of DNB. Since the immediate and significant increase in wall temperature can lead to destruction of the reactor material, an accurate DNB prediction is highly relevant for a safe design and operation. In the pressure range up to reduced pressures of 0.7, the prediction of boiling crises has been investigated intensively. At higher pressures, however, only few models describe the Critical Heat Flux (CHF) accurately. In this study, a mechanistic model that is based on the modeling of subcooled nucleate boiling is implemented numerically by using established heat partitioning approaches. Further focus is put on the implementation of different sub models and boiling closures (e.g., nucleation site density, bubble departure diameter, etc.). The modifications are investigated and discussed regarding their performance in the near critical pressure region with reduced pressures pr > 0.7. The analysis compares the modeling results with experimental CHF data from literature within a broad parameter and pressure range.     «

The Departure from Nucleate Boiling (DNB) leads to a drastic deterioration in heat transfer and thus limits the cooling capability through boiling processes in technical applications. Supercritical power plants, such as SCWRs, inherently operate without a two-phase flow and therefore without the risk of boiling crises during standard operating conditions. Yet, transient scenarios such as startups, shutdowns, or accidents can cause a transition into high subcritical conditions, potentially leadin...     »