The current study deals with a numerical simulation of heat transfer inside enhanced tubes having the coaxial double-twisted tape in the presence of an eco-friendly nanofluid containing graphene nanoplatelets (GNPs). The results unveil that when the particle concentration (ϕ) escalates, the thermal entropy generation rate (S˙h) tends to decline, whereas the heat transfer coefficient enhances. It was found that around 21.6% reduction in the S˙h is attained by a decrease of the twisted ratio (ω) from 3.5 to 2.5 at the concentration of 0.075% and Reynolds number of 10,000. Adding more GNPs to the water yields an increment in either the pressure drop or the frictional entropy generation. The findings refer to this issue that a multi-criterion optimization to find the best optimum state is required. A genetic algorithm-based optimization with the entropy generation rate, heat transfer, and pressure loss results in the optimum values at Re = 16,995.4, ϕ = 0.093%, and ω = 3.41.
