Heat transfer in transition domain on film boiling in case of forced convection

The design of reactor safety installations is based on the assumption, that nucleate boiling changes immediately to film boiling or vice versa, if hypothetical emergencies like LOCA (loss of coolant accident) or ATWS (anticipated transient without scram) occur. Between nucleate and film boiling, however, a 'transition boiling' region exists with better heat transfer conditions than in the film boiling region. The report presents experimental and theoretical studies on transition boiling. Such data are suitable to improve computer codes which simulate LOCA and ATWS emergencies and prove the effectiveness of safety installations. The experiments are carried out under steady-state thermal and hydrodynamic conditions, to promote a better understanding of the basic physical mechanisms of heat transfer in the transition region. Measurements with temperature-controlled heat transfer surfaces are required to permit stabilized evaporation in the transition region. A vertical tube of 14 mm inner diameter is used as test section, to simulate approximately the size of flow channels in BWR and PWR reactors. The experiments are carried out with refrigerant R114. Methods for scaling of the results to transition boiling heat transfer of water are presented. (orig.)