Microscopic heat transfer characteristics during cooling of high temperature surface by a falling liquid film

When the dry region on the high-temperature wall is cooled down by liquid, the dry region is transformed into wet region as the temperature shifts in a short time. Vigorous boiling occurs during the temperature transition. Especially, it is called the rewetting phenomenon that liquid contacts dry region on the high-temperature wall in the first time. In nuclear plants, it is essential for nuclear power plant safety to understand this phenomenon. Under severe accidents, a liquid film formed by an emergency core cooling system (ECCS) removes the decay heat of fuel rods. To determine the start time of ECCS, the time required for the liquid film to cover the fuel rods needs to be clarified. Working time is calculated by using rewetting velocity which means the boundary movement speed between liquid and dry region. Previous research reported rewetting velocity was calculated by heat conduction equation using the boundary condition (= heat transfer coefficient (HTC) distribution). However, proposed HTC distributions were not defined directly from experimental data but defined to match experimental results. This is because the temperature was measured by thermocouples which were not enough to obtain temperature distribution. In this study, two kinds of experiments were done to define HTC distribution from experimental results and to elucidate the rewetting phenomenon. First, as liquid film fell along high temperature wall, the transition of temperature distribution of the wall was measured by an infrared camera. Copper plate was used to heat transfer surface. From the results of temperature distribution, HTC distribution was calculated. And then, four critical parameters that consisted of HTC distribution were investigated against experimental parameters. Second, a similar experiment was conducted using silicon wafer which is transparent for infrared rays as the heat transfer surface to observe the rewetting phenomenon, especially the precursory cooling region. The precursory cooling region is considered as one of the important regions to calculate exact rewetting velocity. An infrared camera and high speed camera was used in this experiment. An infrared camera took temperature distribution from the backside which is the opposite side of the liquid film. A high-speed camera took liquid film condition from the front side. Since both cameras were synchronized, this experimental apparatus was able to observe both HTC distribution and liquid film condition at the same time. As a result of observation, the precursory cooling region is the area where drying and wetting are repeated by boiling.