Heat transfer characteristics of deionized water-based graphene nanofluids in helical coiled heat exchanger for waste heat recovery of combustion stack gas

Heat transfer phenomena of a fully developed laminar flow of deionized water-based graphene nanofluids (DI-water/GNPs) in vertical helical coils for heat recovery of combustion stack gas was carried out with different helical coil dimensions. The GNPs particle concentrations were 0.05 and 0.08% by weight and the calculation of experimental heat transfer data was based on countercurrent flow LMTD method under condition of constant wall heat flux. With higher thermal conductivity and lower specific heat capacity, the graphene nanofluids provided better heat transfer performance compared to pure DI-water. The increase of the thermal conductivity was found to be 13.36% at particle fraction of 0.05% by weight which resulted in the increase of heat transfer coefficient by 21-25% compared to that of the DI-water. The better heat transfer characteristics could be obtained at more particle concentration. The effect of coil dimensions on heat transfer phenomena of helical coils had been discussed and a new correlation of heat transfer data for the nanofluids was created. The overall heat transfer coefficient between the hot exhaust gas and the heat transfer fluid in the helical coil was also considered.