Experimental investigation of the spatial evolution of local flow parameters in vertical LBE–N2 bubbly flow

To support closure-model development for thermal-hydraulic analysis of SGTR/HXTR-related two-phase flow in lead-cooled fast reactors, a separate-effect experimental study was conducted on lead–bismuth eutectic (LBE)–nitrogen bubbly flow in a vertical circular pipe using a loop-type high-temperature liquid-metal two-phase-flow facility. The test section was a vertical pipe with an inner diameter of 50 mm and a length of 2000 mm. A single-sensor conductivity probe and four-sensor conductivity probes were employed for different measurement objectives. The single-sensor probe was used mainly to examine local void-fraction trends in the upper developed section, whereas the four-sensor probes were used to obtain local void fraction, bubble frequency, bubble velocity, equivalent bubble radius, and interfacial area concentration (IAC) at two representative axial sections and multiple radial positions.The measurements indicate that the void fraction exhibits clear spatial evolution along both the radial and axial directions. The radial void-fraction profile gradually changes from a relatively flat distribution in the near-inlet region to a center-peaked distribution at the upper measurement section. With increasing superficial gas velocity, both the local void fraction and the three-point area-weighted estimate of the cross-sectional void fraction increase. At relatively high superficial liquid velocity, the estimated cross-sectional void fraction tends to decrease because of the shortened gas residence time. Bubble velocity and equivalent bubble radius generally decrease along the flow direction, while the IAC varies in close correspondence with bubble frequency.These results provide a local-parameter database for the validation and improvement of interphase-interaction and interfacial-area closure models in LBE two-phase-flow simulations. The present study focuses on dispersed gas transport and bubbly-flow evolution in a high-density liquid-metal system, and should therefore be interpreted as a separate-effect bubbly-flow study relevant to SGTR/HXTR analysis rather than as a full reproduction of prototypical tube-rupture transients involving high-pressure steam release, condensation, flashing, and thermal shock.