Performance of two-phase loop thermosiphon with graphene nanofluid

In this research, thermal resistance of the modified inflated aluminium plate that features a closed thermosiphon under natural convection was tested. HFE-7000 dielectric fluid and graphene nanofluid were used as the working fluids for different filling ratios (30%, 50%, and 70%), graphene concentrations (0.3 wt%, 0.5 wt%, 1 wt%), and supplied powers (30 W, 60 W, 90 W). The effect of uniform and non-uniform heat source on the thermal resistance are examined and a separate flow visualization is also conducted to understand the boiling phenomenon. It is found that a 70% filling ratio offers the best heat transfer performance. At a supplied power of 90 W and filing ratio of 70%, the thermal resistance is reduced by around 4% relative to the filling ratio of 30% for the same supplied power of 90 W. In terms of input power, the decrease in thermal resistance was found as the power increases. The uniform heat source has a better heat transfer performance than the non-uniform one. From the visualization experiment, appreciable graphene was entrained from evaporator to condenser due to boiling especially at a higher concentration of 1%, causing the blockage and raising the thermal resistance. Therefore, the best concentration of graphene should be kept to below 0.5 wt% in which the size of the bubbles was sufficiently large enough to facilitate latent heat transfer effectively for supplied powers of 60 and 90 W.