Optimization of Fill Ratio and Angle Improved Energy Efficiency in Two‐Phase Copper Heat Pipes

ABSTRACT High heat‐flux dissipation in compact low‐voltage direct current (LVDC) power electronics is limited by temperature gradients that elevate thermomechanical stress, and two‐phase copper heat pipes provide passive heat transport. Nanofluid working fluids can strengthen phase‐change heat transfer when stable dispersions are maintained in sealed devices. An optimization that couples nanoparticle concentration, heat input, inclination, and filling ratio for copper heat pipes has not been reported. The objective of this study is to identify operating conditions that minimize thermal resistance (TR) and maximize the heat‐transfer coefficient for copper heat pipes charged with deionized water or Al 2 O 3 , CuO, or Ag nanofluids. Nanofluids were prepared via a two‐step route, screened for zeta potential over 30 days, and charged into copper heat pipes with axial thermocouples. Experiments were conducted within the defined factor bounds, and a rotatable CCD–RSM framework was used to fit quadratic models and optimize desirability. Thermal performance, supporting energy efficiency in LVDC hardware, is maximized at 1.98 vol%, 99.47 W, 89.44°, and 74.74% fill, at which a TR of 0.0736 K/W and a heat‐transfer coefficient of 1710 W/(m 2 ·K) were predicted, and this operating point provides a target for LVDC implementation. A pronounced interior optimum in filling ratio is identified, and heat input dominates the linear response while two‐factor interactions remain nonsignificant within the explored region. These model‐derived set points enable heat‐pipe integration into LVDC modules with predictable thermal margins. Long‐duration aging, corrosion‐compatibility screening, and independent verification at the predicted optimum are targeted to extend the validated operating window.