Thermal conductivity of water, ethanol, methanol and dielectric FC-72 in oscillating heat pipe

• OHP fabricated and tested with a true adiabatic section made of glass tubes. • Condenser heat output is measured and compared with evaporator heat input. • Equivalent thermal conductivities of four fluids estimated from experimental data. • Maximum fluid thermal conductivity ranges from 3337 W/mK to 67049 W/mK. • Fluid thermal conductivity in passive OHP exceeds that of any known solid. Oscillating heat pipes (OHPs) have attracted significant interest as passive thermal management devices due to their high heat transport capability, ease of manufacturing, and design flexibility. Despite decades of research, capturing the oscillatory behavior of the working fluid into a single thermal fluid property remains largely unexplored. This challenge arises because OHPs are typically made entirely of thermally conductive materials, allowing heat to transfer from the evaporator to the condenser not only through the working fluid but also via conduction in the solid material. To address these gaps, a tubular OHP with a true adiabatic section made of glass tubes was fabricated, ensuring that heat transfer occurs solely through the oscillatory motion of the working fluid. Both the heat input at the evaporator and the heat output at the condenser were measured to quantify the heat transferred across the OHP. The device was tested with four different working fluids – DI water, ethanol, methanol, and FC-72 – at a constant filling ratio. Due to the presence of the true adiabatic section, two distinct equivalent thermal conductivities, which capture the oscillatory heat transfer behavior of the working fluid, were estimated from experimental data. The first was calculated across the entire OHP using the temperature difference between the evaporator and condenser, while the second was determined solely across the adiabatic section using local temperature measurements. Remarkably high maximum thermal conductivities were attained for the four fluids, ranging from 3336.79 W/mK to 6452.34 W/mK for the full OHP, and from 8573.51 W/mK to 67048.62 W/mK for the adiabatic section. All these values exceed the conductivity of any known solid at room temperature, including graphene and diamond. Furthermore, the thermal conductivity of all working fluids used in this study exceeded that of copper and silver at heat inputs as low as 50 W, highlighting the potential of OHPs for efficient heat dissipation in various applications.