Investigation of flow and thermal performance of water-based nanofluid in gravity heat pipe

• Performance of gravity heat pipe using various aqueous nanofluids is compared. • GO nanofluid reduces thermal resistance by 5.2 % and enhance startup speed. • Optimum heating power maximizes convective heat transfer coefficient. • Suitable nanofluid enable stable heat-flow fields, especially for higher power. This study presents a numerical investigation into the flow and thermal performance of gravity heat pipes, employing the Volume of Fluid (VOF) multiphase flow model in conjunction with a User-Defined Function (UDF) to simulate evaporation and condensation processes. The results demonstrate that the graphene oxide nanofluid heat pipe achieves a 33 % faster startup, forming a stable condensate film in 2 s compared to 3 s for pure water, and exhibits superior thermal performance. When the heating power increases from 10 W to 40 W, the total thermal resistance decreases by approximately 42 % for the pure water heat pipe, but only by 5.2 % for the graphene oxide (GO) nanofluid pipe, indicating its more stable performance across a power range. At 40 W, the GO nanofluid (0.2 wt%) enhances the equivalent convective heat transfer coefficient by 5 % and reduces the total thermal resistance by up to 5.2 % compared to pure water. Most notably, at the optimal power of 50 W, the GO nanofluid achieves a maximum reduction in thermal resistance of 7.8 % and an enhancement in the convective heat transfer coefficient of 4.5 %, while maintaining a more stable flow field, thereby extending the operational limit beyond 50 W.