This work is motivated by
the need to better understand the phenomena occurring while some water
is injected into a heated porous debris bed. This reflooding operation
is a part of the planned mitigation procedure used during a Loss Of
Coolant Accident (LOCA) that may occur into a nuclear power plant and
results into a severe core damage. Our experimental study aims to
characterize the boiling crisis that can happen in a boiling flow taking
place within a heatgenerating model porous medium. The test section is a
two-dimensional model porous medium, composed of an array of 276
cylinders placed between two ceramic plates spaced from one another by 3
mm, one of which is transparent and allows visualizations of the flow.
The 2 mm diameter cylinders are Pt100 resistance temperature detectors
that perform a dual function: they act as heating elements (heated by
Joule effect) and are also used as temperature probes. A fluid loop
allows controlling the liquid injection flow rate, its inlet temperature
as well as its pressure. The test section is held vertically, the
liquid injected from bottom at a temperature close to the saturation
temperature.
In a first series of experiments, the thermal power applied
to a bundle of heating cylinders is progressively increased until a dry
zone is detected in the porous medium. Two kinds of phenomenology are
observed during these “dryout experiments”. First, at low liquid
injection rate (4 kg.m^-2.s^-1 maximum mass flux), reaching the dryout
power results into a liquid front receding down to the upper limit of
the heated zone, while downstream the heated zone, the porous medium is
vapour-saturated. Second, at higher flow rate, the boiling crisis
happens at the surface of a single heating element, resulting in a local
film boiling, whereas a two-phase flow still go through the whole test
section. High-speed visualizations allow characterizing the flow
regimes.
Other experiments focus on determining the local critical heat
flux on a given cylinder, for different upstream flow configurations.
The inlet liquid flow rate is fixed. A thermal power is uniformly
applied to a line of heating cylinders, upstream the cylinder under
study. Results show that the local critical heat flux decreases as the
power applied to the heated line increases. The distance from the
cylinder under study to the heated line seems not to have a significant
effect on the critical heat flux. Visualizations are used to
characterize the two-phase flow upstream the heated line, aiming at
expressing the critical heat flux as a function of the hydrodynamic
parameters (saturations, phase velocities).
The image analysis is
particularly challenging. In order to calibrate the image processing
parameters, we use a second model porous medium with the same geometry
as the heat generating one, but where an isothermal two-phase flow is
obtained by injecting gas into the liquid flow rather than generated by
boiling. The gas injection flow rate is controlled and measured.
Isothermal two-phase flow visualizations provide a reference case and
are compared to flow boiling visualizations.