Sub-channel thermal hydraulics is a specialized field within thermal hydraulics that analyzes fluid flow and heat transfer phenomena within interconnected flow regions, known as sub-channels, defined within complex geometries such as nuclear fuel bundles or heat exchanger arrays. This research area employs computational or experimental methods to investigate the detailed distribution of local parameters like velocity, temperature, pressure, and void fraction, which is critical for predicting system performance, assessing safety margins, and preventing critical heat flux conditions.