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This research investigates the thermal performance of an annular passage using eco-friendly nanofluids as substitutes for conventional working fluids. Specifically, the study focuses on the utilization of covalently functionalized Graphene Nanoplatelets (GNPs) dispersed in an aqueous medium, with Gallic acid as the functionalizing agent. Nanofluids were synthesized at different weight concentrations (0.1 wt%, 0.05 wt%, and 0.025 wt%) and exhibited an excellent long-term colloidal stability for 39 days, with a relative concentration of 94.4% at 0.025 wt%. Thermophysical property evaluations revealed significant enhancements of 18.25% and 8.23% in thermal conductivity and viscosity, respectively at 0.1 wt%. The nanofluids exhibited average enhancements of convection heat transfer coefficient evaluations indicating improved thermal performance for GaGNPs/DW nanofluids, with the highest average enhancement of approximately 12.11% achieved at 0.1 wt%. To investigate the impact of different annular geometries, a numerical study (CFD) was conducted. Various inner rod shapes, including circular, rectangular, square, triangular, and pentagonal, were considered. The pentagonal cross-section exhibited the highest average increase in convection heat transfer coefficient at 13.9%, followed by rectangular and triangular shapes. In a numerical case study with different thermal boundary conditions, the circular inner rod shape demonstrated the highest thermal performance at 0.1 wt%.
AbdRabbuh et al. (Fri,) studied this question.