Approach for calculating subcooled boiling characteristics

With postulated-accident analyses in nuclear reactors becoming increasingly important, it has become necessary to enhance their understanding of transition boiling under forced convective conditions over a wise range of mass flows and subcoolings. Although, recently developed thermal-hydraulic (T/H) codes included transition boiling correlations, the transition boiling phenomenon has not been well predicted. This was basically because these correlations were based on steady-state data and have a limited range of applicability. Numerous correlations have been reported for the prediction of void fraction in subcooled flow boiling. In this paper, an analytical model has been developed to overcome the deficiencies of the transition boiling correlations existing in T/H codes. The model determined the subcooled boiling characteristics under forced convective conditions over a wide range of mass flows and subcoolings. The model considered two regions of subcooled boiling. In the former region, local boiling occurred with bubbles not detaching from the heated surface. Local boiling took place in the latter region with bubbles detaching and flowing with the liquid.