Experimental investigation on local flow structures of upward cap-bubbly flows in a vertical large-size square channel

Gas-liquid two-phase flows are typical in engineering systems involving heat and mass transfers. The crosssectional geometries and sizes of these flow channels vary, exerting a notable impact on the thermofluid behavior. To develop thermofluid models, it is imperative to obtain local measurements of two-phase flows and gain insights into their characteristics from these measurements. Numerous experimental studies have investigated the local flow characteristics in circular pipes and rectangular channels; however, few studies on square channels have been conducted, particularly for flow regimes beyond bubbly flows, such as cap -bubbly flows. Given the importance of two-phase flows in large square channels for advanced nuclear reactors, such as the economic simplified boiling water reactor (ESBWR), we experimented with upward cap -bubbly flows in a large square channel. Detailed cross-sectional distributions of the void fractions, axial gas velocities, and interfacial area concentrations for two bubble -size groups were obtained at three axial locations using local measurements of a four -sensor optical probe. Based on the database, the cap -bubbly flow characteristics were understood, including flow development in a large square channel. In addition, the existing drift -flux correlations for large circular pipes predicted the measured void fraction with an accuracy of approximately +/- 10 %, whereas the existing interfacial area concentration correlations reasonably correlated with the measured interfacial area concentration with an accuracy of approximately +/- 30 %. Furthermore, the database was used to calculate the covariances of the void fractions. The correlations for large circular pipes significantly underestimated the void fraction covariance of the larger bubble group, probably because of the peculiar void fraction distribution of the group in a large square channel. The other covariances were well predicted.