Abstract
The integration of nanotechnology offers a promising pathway to improve legacy heat-exchange systems by addressing the fundamental limitation of conventional fluids: their poor thermal conductivity. This experimental work examines the thermal and hydraulic behaviour of Copper-Water (Cu-W) and Graphene-Water (Gr-W) nanofluids in a horizontal tube-in-tube heat exchanger operated in the transitional turbulent range at a Reynolds number of about 4000. The nanofluids, stabilized using Sodium Lauryl Sulfate (SLS), were tested over a nanoparticle mass concentration range of 0-2.5 % and hot-fluid inlet temperatures between 60°C to 90 °C. The results show that increasing both nanoparticle concentration and operating temperature raises the overall heat transfer coefficient, U, and effectiveness, while Gr-W consistently delivers better thermal performance than Cu-W under identical test conditions. The thermal gains are accompanied by distinct hydraulic behaviour: Gr-W shows a monotonic increase in friction factor due to its higher effective viscosity, whereas Cu-W can exhibit a modest reduction in friction factor under the tested conditions, linked to the combined influence of density and viscosity on the Reynolds number at fixed volumetric flow rates. For both fluids, the counter-flow configuration outperforms parallel flow by maintaining a higher logarithmic mean temperature difference. Across all tested conditions, the PEC remains above unity, showing that the heat-transfer gain exceeds the corresponding pumping-power penalty. At 2.5 % mass concentration, the maximum PEC values of approximately 1.32 for Cu-W and 1.36 for Gr-W correspond to net efficiency gains of 32-36 % relative to the base fluid. Considering the measurement uncertainty, Gr-W nanofluid appears to be the more promising working fluid.
Keywords: Copper-Water (Cu-W), Graphene-Water (Gr-W), Heat Transfer Enhancement, Nanofluids, Performance Evaluation Criterion (PEC).