Numerical study on the local aggregation of helium bubbles in liquid lithium and its thermal analysis

Liquid lithium is widely regarded as an optimal cooling medium for space nuclear reactors due to its exceptional heat transfer properties and low density. However, the helium bubbles generated by liquid lithium under space irradiation pose significant hazards to the safe and stable operation of nuclear reactions. In this study, the localized accumulation of helium bubbles in liquid lithium is investigated using a two-phase flow turbulence model. The effects of helium bubble distribution and inlet velocities on various parameters in the pipeline are focused on. A non-isothermal model for bubble flow is developed to examine the influence of gas-liquid mixture concentrations on overall heat transfer performance under low concentration conditions. Agglomerated bubbles with radii between 5 μm and 150 μm are classified into three categories based on local concentrations: circular (≤20.37%), irregular elongated (up to 30.44%), and banded (up to 36.31%). Interconnected banded bubbles can be up to 8 times larger than irregularly elongated ones, impacting physical properties and wall temperature disturbance in the pipeline. Elevated inlet velocity initiates the occurrence of bubble impact and fragmentation. However, high flow rates near the wall do not diminish wall temperature disturbance. Mixed flows with bubbles scales <15 μm show no significant impact on overall heat transfer up to 1% concentration. This study reveals the effects of bubble number and distribution, providing insights for manipulating bubble structure and guiding localized and comprehensive thermal analyses.