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 (IMFT). 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. Infrared thermography is
used to measure the temperature of the metal foil. These measurements are
coupled with a high-speed camera that allows visualizing the boiling regimes.
Several
video animations (© IRSN) of the boiling regimes have been realized using the
synchronized pictures of infra-red and high-speed cameras.
Captions
Heating rate and
high-speed visualization
Visualization
of a half-cylinder heated by Joule effect thanks to an exponential power signal
(фgen = ф0.exp(t/τ)) for three
different characteristic periods (τ 1 = 70ms, τ 2 = 20ms et τ 3 = 5ms). All the boiling regimes are observed, from
convection to film boiling.
For the
largest period (τ 1= 70ms),heating is
sufficiently slow for boiling regimes that set up on the wall to be close to a
quasi-steady state case. When the period decreases, boiling regimes have no
time to set up till film boiling spreads over the wall. In the latter case, the
wall is insulated from the bulk liquid flow, leading to the burnout of the
metal.
Synchronized analysis at
the onset of nucleate boiling
Visualization
of synchronized pictures from infrared thermography and high-speed video together
with time evolution of wall temperature and heat flux along the centerline of
the heated wall during a test leading to the onset of nucleate boiling.
Evidence of a vapor pocket that induces rapid variation of the heat transfer
intensity between wall and flow.
Statistical dispersion of
temperature and heat flux during onset of nucleate boiling (ONB)
Illustration
of the probability density function of wall temperature field as measured by
infrared thermography with maximum, minimum and standard deviation values. The
time evolution of local temperature at different wall locations are also shown.
At the onset of nucleate boiling, large spatial variations of temperature along
the wall implies that an average of temperature over the whole length of the
wall is meaningless.
Synchronized video of
thermography and high-speed camera during film boiling
Visualization
and coupling of infrared and high-speed cameras with time evolution of wall
temperature and heat flux at the center of the heated wall during a test leading
to film boiling. Evidence of a vapor
film spreading over the wall and inducing degraded heat transfer between wall
and fluid.
