TRANSPORT AND FLOW CHARACTERISTICS OF GRAPHENE-DOPED NANOFLUIDS AT MODERATE TEMPERATURES IN DOUBLE-PIPE HEAT EXCHANGERS

The objective of this study is to assess the thermal and hydraulic characteristics of water-based graphene-doped nanofluids experimentally as well as analytically in counter-flow concentric tube heat exchangers for the purpose of evaluating their viability as compared to pure water and other water-based solutions such as methanol and propylene glycol. A concentric pipe system (copper-PVC) was configured such that graphene-doped primary fluid flowed in the inner tube (copper), while a secondary flow of deionized water was introduced in the annulus. The nanofluids in volume fractions of 0.001, 0.005, 0.01, and 0.015 of graphene nanoplatelets were tested using a base fluid of deionized water at varying temperatures of 30 degrees C, 45 degrees C, 60 degrees C, and 75 degrees C, with the secondary fluid set to a constant temperature of 4 degrees C lower. The increase in the volume fraction of the graphene nanoplatelets showed an appreciable corresponding increase in the thermal conductivity of the nanofluid. However, the flow regime was adversely affected as it transitioned from turbulent to laminar owing to increased viscosity of the nanofluid. This flow transition resulted in a corresponding decrease in the convective heat transfer coefficient. The research results have implications on the assessment and understanding of heat transfer characteristics of graphene based nanofluids in various heat transfer processes. The results show that in the turbulent flow regime the incremental improvements in the thermal conductivity of the nanofluid are far outweighed by disproportionally large increases in pumping power requirements. However, it is observed that appropriately doped graphene nanofluids appear to have advantages over propylene glycol in the laminar flow regime and may be considered as a replacement.