Abstract
The direct transition boiling process from non-boiling to film boiling at critical heat flux (CHF) by exponentially increasing heat input, Q0exp(t/τ), was investigated in a pool of FC-72. Investigations were made on a 1.0 mm diameter gold horizontal cylinder heater under a wide range of system pressures for saturated condition. Direct transition predominantly occurs from heat conduction process in the non-boiling regime by rapid increasing heat input and then followed by incipient boiling and CHF simultaneously. However, during quasi-steady-state heat transfer, it was observed that the direct transition also occurs from single phase natural convection at around atmospheric pressure and by an extent of pre-pressure, which was given to the cylinder surface at atmospheric pressure. Direct transition phenomena were confirmed to exist due to the explosive-like heterogeneous spontaneous nucleation (HSN) in originally flooded cavities on surface. The predictions of direct transition phenomena were also derived from typical incipient boiling superheat and CHF.
