Autonomous multi-object tracking coupled with infrared thermography for two-phase flow analysis in pulsating heat pipes under variable gravity

A Multi-Object Tracking (MOT) algorithm is introduced for analysing two-phase slug–plug flows through the simultaneous measurement of bubble velocity and infrared-based liquid temperature. Vapour bubbles are detected through the infrared intensity contrast arising from the distinct emissivity and transmissivity of the liquid and vapour phases, enabling robust dichotomic phase segmentation. The tracking procedure builds on a nearest-neighbour approach, augmented with a custom weighted function designed to ensure reliable bubble pairing across consecutive frames. The approach is demonstrated on a flexible polymeric flat Pulsating Heat Pipe (PHP) filled with FC-72 and tested under microgravity and hypergravity conditions during the 77th ESA Parabolic Flight Campaign. More than 5800 tracking events were processed, providing detailed velocity profile and liquid temperature trends in the adiabatic section, with a minimum detectable velocity of 4 mm/s within a range of ±200 mm/s. The method offers a quantitative characterization of slug–plug flow dynamics together with a combined velocity–temperature analysis under variable gravity, establishing a robust pathway for automated diagnostics of two-phase flow in thermographic experiments.