Start-up characteristics of a large evaporator flat loop heat pipe with a biomimetic structure wick under gravity-assisted conditions

Loop heat pipes are highly efficient passive two-phase heat transfer devices, offering great potential for advanced electric vehicle battery thermal management applications. This study experimentally investigates the start-up characteristics of a triply periodic minimal surface (TPMS) biomimetic structure wick-based flat loop heat pipe (FLHP) with double-sided heating capability under gravity-assisted conditions. The effects of heat flux, heating mode, coolant flow rate ( q _cool), and coolant temperature ( T _cool) on start-up performance were examined. A back-propagation neural network (BPNN)-Sobol sensitivity analysis method was employed to quantify the interaction between these parameters. Experimental results show that as the heat flux gradually increased, the start-up time of FLHP based on TPMS core was shortened by an average of 70.01%, accompanied by suppressed temperature oscillations and a synergistically coupled rise in internal pressure and evaporator temperature. The double-side heating mode improves thermal homogeneity, reducing the start-up time, temperature, and pressure by an average of 5.04%, 3.43%, and 14.48%, respectively. The positive effect of increasing q _cool weakens with increasing heat flux. Although a high T _cool makes it difficult for the FLHP to start-up at low heat fluxes, the combined effect of the TPMS wick structure and the gravity-assisted orientation ensures the circulation of the working fluid. Sensitivity analysis identifies heat flux as the dominant factor for start-up time, and T _cool has the greatest impact on start-up temperature and pressure. A strong interaction appears between heat flux and T _cool on start-up time, and between q _cool and T _cool on start-up pressure, which shows that coordinated optimization of the heating and cooling parameters is crucial for achieving a robust start-up performance. This study may provide guidance for start-up performance optimization of the FLHP in battery thermal management applications.