Experimental investigation of natural convection and gas mixing behaviors driven by outer surface cooling with and without density stratification consisting of an air-helium gas mixture in a large-scale enclosed vessel

•We investigated the natural convection in the presence of density stratification.•The CIGMA facility in JAEA was used to perform eight experiments, in which we focused on the relative location between the cooled region and stratification.•The stratification behavior was classified into two types, dissolution and breakup.•The experimental result was compared with the previous data based on the interaction Froude number. This paper describes an experimental investigation of natural convection driven by outer surface cooling in the presence of density stratification consisting of an air and helium (as mimic gas of hydrogen) gas mixture in an enclosed vessel. The unique cooling system of the Containment InteGral effects Measurement Apparatus (CIGMA) is used, and findings reveal that the cooling location relative to the stratification plays an important role in determining the interaction behavior of the heat and mass transfer in the enclosed vessel. When the cooling region is narrower than the stratification thickness, the density-stratified region expands to the lower part while decreasing in concentration (stratification dissolution). When the cooling region is wider than the stratification thickness, the stratification is gradually eroded from the bottom with decreasing layer thickness (stratification breakup). This knowledge is useful for understanding the interaction behavior of heat and mass transfer during severe accidents in nuclear power plants.