<p>When the dry region at high temperature is cooled down by liquid, the dry region is transformed into liquid region as the temperature shifts in a short time. Vigorous boiling occurs during the transition of temperature. Especially, it is called rewetting phenomenon that liquid contacts dry region on high temperature in the first time. In nuclear plants, it is essential that this phenomenon is recognized for nuclear power plant safety. Under severe accidents, liquid film formed by emergency core cooling system (ECCS) removes the decay heat of fuel rods. To operate this system, the duration of which liquid completely covered the fuel rod should be clarified. Previous research shows how calculates the cooling time by falling liquid film to assume the boundary condition (= heat transfer distribution). However, those heat transfer distributions are not experimental data but postulated one to lead the same rewetting velocity given from experimental results. This is because the temperature is measured by thermocouples which are not enough to take temperature distribution. In this study, two kinds of experiments to recognize the rewetting phenomenon done. First, the high-temperature wall is cooled down by the falling liquid film which is along the wall. During the falling liquid film, the temperature distribution is measured by an infrared thermal camera. The results of temperature distributions bring heat transfer distributions. Based on this experimental heat transfer distribution, the correlation of heat transfer distribution is invented about experimental parameters. Second, similar experiments are conducted using infrared transparent silicon wafers as the heat transfer surface. The infrared camera takes temperature distribution from the backside means the opposite side of the liquid film. High-speed camera takes liquid film condition from the front side means liquid film side. Both cameras are synchronized. This experimental set up realized to observe both heat transfer distribution and liquid film condition. From experimental result shows that the precursory cooling region is the region that is wet or dry region due to boiling.<br>
&nbsp;In summary, heat transfer distribution through rewetting phenomenon clarified on experimental correlation and understand the thermal-hydraulics phenomenon.</p>

Microscopic Heat Transfer Characteristics During Cooling of High Temperature Surface by a Falling Liquid Film

Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE28/volume/85246

2021 28th International Conference on Nuclear Engineering (ICONE28)

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