Interpreting Thermally and Hydrodynamically Controlled Regions in Subcooled Flow Boiling

The simple dimensionless correlation developed by Saha and Zuber is known as one of the most accurate correlations in estimating the point of net vapor generation (NVG) in subcooled flow boiling. In this correlation, the Nusselt and Stanton numbers are supposed to be constant in the low and high Peclet number ranges, respectively. Based on qualitative discussions, these ranges were called the thermally and hydrodynamically controlled regions, respectively. This work theoretically investigated why the important dimensionless number switches at a Peclet number of 70 000, based on the hypothesis that a sudden decrease in the condensation rate caused by large bubble formation triggers the NVG. It was shown that the ratio of the heat flux spent for vaporization to the total wall heat flux is the parameter of primary importance in determining the NVG point. The vaporization heat flux ratio remains constant in the low Peclet number range, while it decreases with an increase in the Peclet number in the high Peclet number range mainly due to convective heat transfer enhancement. From this perspective, the low and high Peclet number ranges would be referred to as the nucleate boiling heat transfer region and the convective heat transfer region, respectively.