This study is to investigate an effect of natural convection or natural circulation on a transport process by molecular diffusion in a stratified fluid layer consisting of two component gases. There are many experiments and analysis regarding natural convection in a vertical slot or natural circulation in a circular tube. However, there are few studies on natural convection or circulation with molecular diffusion in the stratified fluid layer consisting of two component gases. It is confirmed that these phenomena appear when the depressurization accident occurs in the very high temperature reactor (VHTR). Therefore it is important to evaluate the transport and mixing processes of two or more component gases during the depressurization accident of the VHTR. The experiment has been performed regarding the combined phenomena of molecular diffusion and natural convection or natural circulation in a parallel vertical slot filled with two component gases. The one vertical slot consists of the heated wall and the other side cooled wall. The other one consists of both the cooled wall. The dimension of heated wall is 500mm×200mm and thickness is 3mm. The width of the slot is 20mm and the aspect ratio of the slot is 25. Combination of nitrogen/argon is used as the two component gas system. The density change of the gas mixture and the gas temperature distribution in the slots were obtained. The mixing process of the heavier gas from the bottom side of the slot filled with the lighter gas was discussed in this paper.

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