Critical vapor volume fraction for the prediction of departure from nucleate boiling in Eulerian multiphase flow model framework

This study introduces a generalized criterion for the prediction of critical heat flux (CHF) in an Eulerian multiphase flow model framework. In principle, the criterion is chosen as the vapor volume fraction adjacent to the heated surface that reflects the near-wall bubble-crowding mechanism. In the literature, different correlations for this critical vapor volume fraction (CVF) were used to detect the CHF occurrence, but the validity of these criteria is specific to the limited range of operating conditions considered. The values of CVF that dictate the onset of CHF are essentially governed by the operating conditions. Therefore, the present study aims at developing a new and generalized criterion for CVF as a function of three dimensionless parameters, viz., density ratio (ρl/ρv), Reynolds number (Re), and Jakob number (Ja), accounting for pressure (p), inlet mass flux (G), and subcooling (ΔTsub), respectively. The CHF predictions were validated against the experimental data of CHF in vertical tubes using both the widely known fixed value (αv,cr=0.83) and the developed correlation. It was observed that the fixed CVF criterion results in an over-estimation of CHF at very high pressure conditions, demonstrating the need for a generic model that is sensitive to operating conditions. Whereas, the developed CVF correlation significantly improved the error in CHF predictions from 67.29% to 18.48%. To further establish the reliability of the developed CVF correlation, the validation studies were extended to various vertical tube diameters and annular test sections. The predictions confirm that the developed correlation accurately captures the CHF occurrence corresponding to different experimental datasets across a wide range of operating conditions.