A new type ultra-thin flat heat pipe (UTFHP) with a total thickness of 1.30 mm was prepared by using multiscale composite structures composed of porous layer (PL) and porous wire (PW) as wicks. After chemical modification treatment

the nanostructures modified the surface of the wick

which had super-hydrophilic properties. The thermal performance of UTFHP was investigated with deionized water used as the working fluid. The effects of the nanostructures

filling ratios and inclination angles on thermal properties of UTFHP were analyzed at different heating powers. The results show that the nanostructures can greatly increase the critical heat flux (CHF) and reduce the total thermal resistance of UTFHP when the filling ratio is 25%. Compared with the sample without nano

the CHF is increased by 255% and the minimum total thermal resistance is reduced by 43.2% at the horizontal angle. In addition

when the filling ratio is low

the nanostructures can reduce the thermal resistance of condensation in the entire heating power range but increase the thermal resistance of the evaporator because of greater flow resistance at low heating power. But the nanostructures inhibit the heat transfer performance of UTFHP when the filling ratio is relatively high. The inclination angles effectively influence the heat transfer characteristics. When the evaporation section is located directly below the condensation section

the thermal performance of UTFHP is optimal. The unmodified and modified heat pipes both have great heat transfer performance

the maximum heating power is 83.7 and 44.3 W respectively.