Experimental Optimization of Flat-Plate Heat Pipes Using Biphilic and Graded-Pore Wicks with Nanofluids under Varying Orientations

Passive two-phase spreaders enable heat flux management by redistributing energy through evaporation and condensation without external pumping. Flat-plate heat pipes are used in electronics, yet orientation and interfacial wetting limit their reliable design. An angle-resolved dataset that couples engineered inner surfaces with controlled nanofluid charging under a single protocol is missing. This study evaluates biphilic and graded-pore wicks with 0.5 vol% nanofluids in a copper flat-plate heat pipe across multiple inclinations. Three inner surfaces were fabricated (baseline sinter, biphilic evaporator, graded-pore wick), and five fluids were prepared (DI water, Al 2 O 3 , GNP, CuO, and TiO 2 ) by screening and filtration. Calibrated thermometry with condenser calorimetry was used to fit quadratic RSM models with desirability optimization. Thermal resistance was reduced by biphilic wetting control; at matched loads, the mid-power reduction was 18–26% relative to the baseline. The maximum heat transport is enhanced by graded permeability, resulting in approximately a 40% increase in capacity. Multiresponse optimization is well defined; desirability peaked at 0.975 near 60–61% fill and 20 W, where the thermal resistance was 0.1716 K/W and the axial temperature drop was 2.98 K. These outcomes support the protocol-driven selection of the surface architecture and charging for orientation-variable thermal hardware. Future work will focus on long-duration cycling and expanded nanoparticle chemistries, with the same quality controls.