Experimental investigation on thermal performance of coaxial annular heat pipes under different filling ratios

Thermal protection of infrared sensors in confined high-temperature environments requires compact passive devices capable of maintaining stable two-phase heat transfer under non-uniform heating and variable orientation. In this study, coaxial annular heat pipes (CAHPs) with acetone charges of 1.5, 4.0, 5.0, and 6.0 mL, corresponding to wick-based filling ratios of 136.4%, 363.6%, 454.5%, and 545.5% (FR136, FR364, FR455, and FR546), were experimentally investigated. The effects of the filling ratio on start-up behavior, equivalent thermal resistance, outer-wall circumferential temperature uniformity, and inclination sensitivity were evaluated. The results show that the filling ratio controls the balance between liquid supply and vapor transport. At 15 W, FR364 achieved the lowest equivalent thermal resistance of 0.49 K/W, the shortest start-up time of 209 s, and the minimum evaporator outer-wall circumferential temperature difference of 7.47 K, indicating the best overall vapor–liquid matching among the tested cases. Heating position further affected the thermal performance: bottom heating produced the lowest thermal resistance, whereas circumferential heating promoted a rapid and relatively uniform temperature rise. Positive inclination reduced thermal resistance, while negative inclination deteriorated heat-transfer performance. FR364 exhibited the smallest thermal-resistance variation with inclination, indicating the best orientation tolerance. These results provide guidance for CAHP-based thermal protection of infrared sensors in confined high-temperature environments.