Experimental investigation and semi-empirical correlations for a pulsating heat pipe filling with hybrid nanofluids applied for low-grade energy recovery

Pulsating heat pipe (PHP) is a high-efficiency thermal transportation technology being utilized in the low-grade energy fields. In current work, the novel-hybrid working fluid - graphene oxide nanoplatelets (GO nanofluid) and surfactant solution were jointly utilized to reduce flow resistance and enhance overall thermal performance of PHPs. Sodium Dodecyl Sulfate (SDS) was employed as a surfactant to prepare the water-based surfactant solutions of 500 PPM. The charge ratio was maintained at 60%, vertical bottom heating position of PHP was applied with the power range of 10-100W. Our experimental results illustrated that the oscillation frequency of temperature fluctuation was higher than that with charged GO-SDS hybrid nanofluid in comparison with DI water and pure GO nanofluid. Additionally, maximum average improvement rate of thermal performance 8.55% was successfully achieved in 0.03 wt% GO-SDS hybrid nanofluid than that with pure GO-nanofluid. Furthermore, multiphase fluid flow inside PHP has been comprehensively analyzed, and its flow regimes such as formation of foam and slug, coalescence and breakup of bubbles, and annular flow were observed. On site visualization results illustrated that a fast nucleation of vapor bubbles could be promoted in the evaporator section due to lower surface tension. Specially, a series of bubbles were expected to form cluster for GO-SDS hybrid nanofluid, which further enhanced the oscillation circulation inside PHP. This experimental research and its results could contribute to the optimization and application of PHP.