of rapid transient boiling is an important issue in the nuclear safety. Such a phenomenon
may occur in the case of a RIA (Reactivity Initiated Accident) in the core of a
nuclear reactor power plant, where a power excursion can trigger the formation
of a vapour film around the fuel rod, leading to an important rise of the rod
temperature and a risk of failure.
studies in reactor conditions provided transient boiling curves but the
modelling lacks of reliability. In collaboration with the IRSN (Institut de
Radioprotection et de Sûreté Nucléaire) an experiment model was built at the
Institute of Fluid Mechanics of Toulouse. It generates the flow of a
refrigerant, HFE7000, in a semi-annular section channel, whose inner wall is
made of a metal foil rapidly heated by Joule effect, simulating the heating of
a fuel rod.
thermography is used to measure the temperature of the metal foil, painted with
a black paint to increase its emissivity, causing also an increase of the wall
thermal resistance. The measurement accuracy of the interest temperature has
been optimized according to the paint thickness and a correction on the energy
balance takes account this parameter. These measurements are coupled with a
high-speed camera that allows visualizing the boiling regimes and get bubble
sizes using image processing algorithms.
flux-temperature diagram, the heat transfers are represented both for steady
and transient regimes. Each boiling regime is then reviewed : convection, onset
of nucleate boiling, nucleate boiling, boiling crisis, film boiling and
regimes are correctly modelled by usual correlations. Transient convection is
characterized over the whole wall and its evolution is closed to the
quasi-steady solution. It is shown that heat transfer during the transition to
nucleate boiling are strongly related to the formation of a large vapour phase
that spreads on the wall. A local study of this propagation is then necessary.
In order to simulate and control transient temperature during nucleate boiling,
a P.I.D. is implemented to impose a steady or ramps temperature (from 5 to 500
K.s−1). The results in nucleate boiling make it possible to recover the results
of the literature in both steady and transient conditions. The experiment
allows to study the heat transfer when a vapour film is formed and insulates
the wall. The film boiling regime during heating or the cooling of the wall can
thus be stabilized for several seconds with this system. The conditions for
triggering of film boiling are thus characterized, as its spread dynamic and
its transfers once established.
Finally, the implementation of the physical
characteristics of our experience in IRSN’s SCANAIR code allows us to begin to
calculate and compare our experimental results with numerical simulations.
Unsteady conduction calculations are applied to the measured temperature to analyse
our results during the convection regime and after the onset of boiling.