Developing a semi-mechanistic correlation for the onset of significant void in subcooled flow boiling

A new correlation was developed for the point of the onset of significant void (OSV) or the net vapor generation (NVG) in subcooled flow boiling. The correlation was developed based on a mechanistic model that assumes that the OSV occurs when the void fraction in the bubble layer formed near the heated wall reaches the critical value to cause global bubble coalescence. To achieve a good agreement with the experimental data reported in the literature, the model constant associated with the effect of the micro-convection induced by bubble growth was expressed as a function of several dimensionless numbers. For 232 experimental data points for the thermodynamic equilibrium quality at the point of OSV (POSV) reported in 22 experimental datasets, the mean absolute error was 47.8% for the original mechanistic model and it was reduced to 21.6% after the above-mentioned modification. The semi-mechanistic correlation developed in this work would hence be useful in technical applications for which the accurate prediction of the POSV is required, as in the nuclear reactor safety analysis.