CHF-Transition Boiling

This chapter deals with topics on critical heat flux (CHF) point and transition boiling in aspects of heat-transfer mechanisms based on both model simulations and measurements of each authors' research in recent decades. First three sections report heat-transfer modeling on CHF based on measurements and visualization results; i.e., macrolayer dryout model (Section 3.1), microlayer evaporation model (Section 3.2), and contact-line-length density model (Section 3.3). Section 3.4 demonstrates unique surface structures for enhancement of CHF. The following three sections cover other aspects of CHF and transition boiling mechanisms. Section 3.5 reports unified explanations for heater size dependence of CHF. Section 3.6 shows unique characteristics of transition boiling, i.e., negative differential resistance, and focuses on uniformity of surface temperature and heat flux during transition boiling. Section 3.7 discusses liquid-solid contact fraction in transition boiling region and derives correlation for transition boiling curve. The last four sections deal with CHF in flow boiling conditions. Section 3.8 discusses CHF triggering mechanisms and prediction methods in subcooled flow boiling. Section 3.9 shows influence of flow oscillation on convective boiling phenomena. Section 3.10 reports relation between appearance of dry patches and CHF in forced flow boiling. The last section 3.11 focuses on boiling transition phenomena and CHF for boiling water reactor (BWR) fuel assembly.