Effects of pipe size on horizontal two-phase flow: Flow regimes, pressure drop, two-phase flow parameters, and drift-flux analysis

This paper performs experimental studies to investigate the effects of pipe size on horizontal air-water two-phase flow in a wide range of flow configurations. Some of the major characteristic two-phase flow phenomena of interest include flow regime transitions, two-phase frictional pressure drop, local interfacial structures, and drift-flux analysis. Based on the experimental results obtained in two pipe sizes, the flow regime maps and the correlations for two-phase frictional pressure drop employed in conventional nuclear reactor system analysis codes (TRACE and RELAP) are evaluated. It is found that the flow regime maps in the codes incorrectly predict plug and slug flows as bubbly flow. Effects of pipe size on flow regime transitions are observed, while these effects are not reflected on the maps employed in the codes. The frictional pressure drop analysis suggests that the closure relations used in both TRACE and RELAP can be improved for horizontal two-phase flow. The local time-averaged two-phase flow parameters acquired by a four-sensor conductivity probe including void fraction, interfacial area concentration, bubble velocity, and bubble Sauter-mean diameter provide quantitative descriptions for the interfacial structures in different pipe sizes. It is found that increasing the pipe size tends to cause bubbles to be more concentrated near the top wall in bubbly flow. As a result, the bubble layer occupies a smaller portion of the flow area as pipe diameter increases. Based on the experimental database, drift-flux analysis using both the 〈〈vg〉〉 vs. 〈 j〉 and 〈α〉 vs. 〈β〉 methods is performed. The effect of pipe size on drift-flux analysis is found to be negligible. The global slip is found to be less than one, indicating that the gas phase moves slower than the liquid phase in horizontal bubbly flow. This analysis also provides a predictive method to estimate the void-weighted bubble velocity and void fraction, which can be predicted with an average percent difference of ±3.3% and ±3.1%, respectively.