Periodic large-scale structural characteristics of two-phase flow in tight lattice bundles

Abstract

This paper investigated the two-phase flow characteristics in a double sub-channel tight lattice (P/D = 1.06). Compared to the prototype, the test channel was magnified by 2.7 times. A self-developed 16 × 32 wire-mesh sensor was utilized to measure the phase distribution at two fully developed positions of the test channel (Z/Dh = 86.86, 115.8). The flow conditions are 0.036 m/s < jg < 1.04 m/s and 0.93 m/s < jl <1.86 m/s at Z/Dh = 115.8, including the bubbly flow (B), the cap-bubbly flow (CB), and the slug flow (S). Combined with the pseudo-side view of the flow pattern and the power spectrum, the coherent structure of the gas phase under the bubbly flow and the cap-bubbly flow was confirmed. For the bubbly flow, the coherent structure is in the form of bubble clusters consisting of discrete bubbles. These bubble clusters are staggered on both sides of the gap but do not traverse across the gap, which is speculated to be caused by the large-scale vortex of the liquid flow field. For the cap-bubbly flow, the coherent structure is in the form of cap-shaped bubbles. In this study, the cap-shaped bubbles are confined in the center of the subchannels and staggered in adjacent subchannels, which is attributed to the compact arrangement of rod bundles and the inter-subchannel liquid cross-flow caused by the cap-shaped bubbles. The characteristic parameters, including the pulsation frequency f, convection velocity v, wavelength λ, and amplitude A were defined to characterize the coherent structure. Data analysis results show that the coherent structure frequency under the bubbly flow increases with the increase of the liquid-phase velocity and the coherent structure frequency under the cap-bubbly flow increases with the increase of the gas-phase velocity. In addition, the coherent structure is suppressed under three flow conditions, including the bubbly flow with low liquid velocity (jl < 1.3 m/s), the B-CB transition, and the slug flow.