Effect of graphene/hydrofluoroether (HFE-7100) nanofluids on start-up and thermal characteristics of pulsating heat pipe

As a passive cooling device, pulsating heat pipes (PHPs) are frequently utilized in high power electronic cooling. A typical PHP made up of a copper capillary tube with internal diameter of 2.0 mm, and eight turns in each of the evaporator and condenser sections is used in the present study. A comprehensive understanding of the thermophysical properties of working fluid is crucial to evaluate their effect on start-up and thermal characteristics of PHP. In this study PHP is operated in the vertical heater down position with varying particle loadings of graphene (0.05, 0.1 and 0.5 mass%) in HFE-7100 as the working medium. The start-up behavior of PHP with HFE-7100-based graphene nanofluids under different particle concentrations, heat input (10-30 W) and filling ratios (30%, 50% and 70%) has been reported. The response of thermal resistance to varying heat loads ranging from 10 to 120 W at different concentrations of nanofluids with 50% filling ratio is observed. The results demonstrate that an optimal concentration of nanoparticles needs to be dispersed in the base fluid for improved start-up and heat transfer capability of PHP. A best start-up performance of PHP with graphene concentration of (0.05-0.1 mass%) at a filling ratio of 50% is observed. It was identified that the presence of the graphene nanoparticles contributes to the promotion of heat transfer mechanisms due to its superior thermal conductivity (5000 Wm(-1) K-1) and intensification in micro-scale phenomena such as Brownian motion. In addition maximum heat transfer enhancement efficiency at a power input of 30 W with filling ratio of 50% may reach up to 33% making PHPs of such configurations a viable option for high-heat-flow thermal control applications.