Multidimensional measurement of air–water two-phase flow in particulate bed using refractive-index-matching method and wire-mesh sensor

Abstract

In severe accidents in light-water reactors, the core melt, which falls to the bottom of the containment vessel, is expected to be cooled in the retained water. The particulate debris is generated after the molten material falls from the pressure vessel. For example, the cooling characteristics remain to be further elucidated in a system where particulate debris and structure material coexist and the gas-liquid two-phase flow inside particulate debris. This study investigated air–water two-phase flow inside a particulate bed and near a structure wall using a high-speed camera with a refractive index matching the transparent bed with purified water and a wire-mesh sensor (WMS). Particles with diameters of ø 3, 5, and 10 mm were used and subjected to air–water two-phase flow tests with a superficial gas velocity of 4.0–2.0×103 mm/s and superficial liquid velocity of 0.5–75.3 mm/s. Visualization results of bubble behavior near the wall and inside the bed showed the advection of bubbles from within the bed to the vicinity of the structure wall. This finding explains the higher void fraction measured in the vicinity of the structure wall using the WMS. Analysis of the void fraction distribution based on the theoretical models indicated that the higher void fraction near the structure wall observed in the case with larger particle size may be attributed to the slip velocity between the gas and liquid phases in the particulate bed. The developed techniques and insights gained in this study contribute to a detailed understanding of the thermal-hydraulic phenomena that accompany two-phase flows in particulate debris.

Keywords

• particulate debris
• particulate bed
• air–water two-phase flow
• void fraction
• wire-mesh sensor (wms)
• refractive index matching (rim)