The effect of heat pipe internal fluid properties on enhancing heat transfer under forced convection flows

A numerical and experimental study into the effect of heat pipe internal fluid properties on heat transfer through cooling a high-temperature convection airstream was carried out. Using pure water inside the heat pipe as the baseline working fluid, the influence of varying liquid density, thermal conductivity, dynamic viscosity and specific heat capacity on convective heat transfer and Prandtl No. was investigated. Numerical analysis for predicting air temperatures was carried out using Computational Fluid Dynamics (CFD). The physical domain comprised of 19 cylindrical heat pipes arranged in a staggered grid subjected to an inlet velocity and source temperature of 2.3m/s and 314K. The temperature profiles obtained from the CFD model using water as the working fluid were experimentally validated using wind tunnel testing. The results showed a pre-cooling potential of approximately 2K or 1kW under the forced convection flow. Good correlation was observed between the CFD and experimental techniques with a mean temperature drop variation of 0.6K or 20%. The findings revealed that the specific heat capacity has the most significant impact on enhancing heat transfer through convection as an increase of 39% was obtained when the specific heat capacity was increased from 1,000J/kgK to 6,000J/kgK. Conversely, the study showed that the least dominant parameter in augmenting heat transfer was the liquid dynamic viscosity as an increase of 16% was calculated when varied between 10-5 Pas and 10-1 Pas. The work highlighted the operative range where convective heat transfer of air passing over the bank of heat pipes can be influenced by the internal fluid within the heat pipe.