Experimental investigation of gas-phase migration behaviors in a tight lattice bundle under two-phase non-equilibrium flow

Abstract

During pressurized water reactor (PWR) operations, the two-phase flow may occur in local subchannels, such as the reactivity-insertion accident. The gas phase will migrate laterally to the surrounding subchannel, whose flow characteristics are still not clear but are very important for safety analysis. This paper conducted an experimental investigation on the gas-phase migration across the gap under two-phase non-equilibrium flow inside a vertical coupled subchannel tight lattice (P/D = 1.06). The subchannel injected with air is called the donor channel, and the other is called the receptor channel. 36 flow conditions were obtained (0.07 m·s−1 < 〈jg〉 < 1.02 m·s−1 and 0.93 m·s−1 < 〈jl〉 < 1.86 m·s−1), containing bubbly, cap-bubbly, and slug flow. In each case, four flow positions were measured (Z/Dh = 28.95, 57.90, 86.86, and 115.81). Results show the tight lattice bundles strongly restrict the gas-phase migrations across the gap. For bubbly flow, bubbles cross the gap in the modality of bubble clusters. The poor gas-phase migration was attributed to the bubbles' phase-interface tension and the lift force's action in the receptor channel. For cap-bubbly flow, the gas phase migrates across the gap by the coalescence and expansion of cap-shaped bubbles. The structure of the cap-shaped bubbles in the receptor channel is stable and does not break into small bubbles. Thus, the flow is still the cap-bubbly flow in the receptor channel as though the gas content is low. For slug flow, the gas-phase migrations are stronger than that of the bubbly and the cap-bubbly flows. As the flow fully develops, Taylor bubbles reach distribution symmetrically in both subchannels at Z/Dh = 115.81. Characteristics analyses indicate the two-phase non-equilibrium flow of this study promotes the bubbles’ aggregation under cap-bubbly flow. It will reduce void fraction under the two-phase non-equilibrium flow compared with that under the two-phase equilibrium flow for the same two-phase flux. Thus, bubble-size factors were recommended for consideration in the development of the two-phase flow model.