Visualization of condensation fractionation in ultra-thin flat heat pipe with binary mixed fluid

An ultra-thin flat heat pipe with a sintered wick thickness of 0.5 mm and an overall thickness of 1.3 mm was designed. Visualization experiments were conducted to investigate the thermal performance and flow behavior of ethanol-water binary mixtures. The work aims to elucidate the condensation fractionation phenomenon and its impact on thermal resistance. The results show that binary mixtures known to exhibit excellent performance in conventional heat pipes undergo condensation fractionation in ultra-thin flat heat pipes. The different condensation points of the vapor components cause extensive condensation of water-rich vapor near the evaporation section. This fractionation intensifies with increasing heat load, leading to incomplete fluid circulation and significant heat transfer deterioration. As a result, thermal resistance remains persistently high. Furthermore, the mixture with a high ethanol mole fraction (90 mol%) is proposed as the optimal working fluid. It effectively mitigates condensation fractionation and achieves enhanced thermal performance.