During reactivity initiated accidents (RIA) in a core of a nuclear reactor, a power excursion occurs on some fuel rods. The consequent rapid boiling is a matter of study by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) for the nuclear power plants safety evaluation, because of the risk for rod's clad failure. In order to better understand the influence of power excursions and to characterize the phases of the rapid boiling phenomenon, an experimental set-up has been built at the Institut de Mécanique des Fluides de Toulouse (IMFT). Subcooled HFE-7000 flows vertically upward in a semi annulus test section. The inner half cylinder is made of a stainless steel foil, heated by Joule effect. Its temperature is measured by an infrared camera filming the backside of the foil, coupled with a high speed camera for the visualization of the flow topology. The main objective is to study the transient boiling phenomenon over a wall simulating a fuel rod while mastering its heating rate, the bulk flow subcooling and velocity, and characterizing precisely the heat transfer and the boiling incipience.
During a typical test that models RIA power excursion, there are different heat transfer regimes, which are all transient: forced convection, nucleate boiling and film boiling. Transient heat transfer are enhanced, respectively deteriorated, during stages of increasing, respectively decreasing, wall temperature, with respect to similar wall temperature in steady state heat transfer. A model for the onset of boiling has been developed to take account of the dependence on the heating rate. The increase of the heat flux with the heating rate in the nucleate boiling is still not completely understood. In this study, we combine the wall temperature and heat flux measurements with flow visualizations to estimate the thickness of the bubbly layer. The study of the transient nucleate boiling is made with several heating profiles such as power pulses, triangles and transients between stationary states to shed light on the boiling mechanisms.