IRSN, Institut de radioprotection et de sûreté nucléaire

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Study on transient boiling in the context of RIA accident

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.

 Experimental device

 Heating rate and high-speed visualization

 Synchronized analysis at the onset of nucleate boiling

 Statistical dispersion of temperature during ONB

 Statistical dispersion of heat flux during ONB

 Synchronized video during film boiling


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.
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