Investigation of heat transfer performance and friction factor of a counter-flow double-pipe heat exchanger using nitrogen-doped, graphene-based nanofluids

Nitrogen-doped graphene (NDG) nanofluids are prepared using a two-step method in an aqueous solution of 0.025 wt.% Triton X-100 as a surfactant with various nanosheets at several concentrations (0.01, 0.02, 0.04, 0.06 wt.%). This paper reports results of experiments on thermal conductivity, specific heat capacity, and viscosity of the NDG nanofluids, as well as their convective heat transfer behavior flowing in a double-pipe heat exchanger. To assess the thermal properties, we used various water-based nanofluids as coolants to analyze the total heat transfer coefficient, convective heat transfer coefficient, the percentage of wall temperature reduction, pressure drop, and pumping power in a counter-flow double-pipe heat exchanger. A novel MATLAB code carried out the calculations for Reynolds numbers between 5000 and 15,000 (turbulent flow) and nanosheet weight percentages between 0.00% and 0.06%. An increase in Reynolds number or the percentage of nanomaterial could perhaps enhance the heat transfer of the working fluid. As an example, using 0.06 wt.% nanomaterial in the base fluid led to 15.86% enhancement of the convective heat transfer coefficient in comparison with water. Nonetheless, the penalty in terms of the rise in the pumping power was rather small. For a particular material, increasing Reynolds number or nanomaterial weight percentage would augment pumping power. Power consumption, heat removal, and heat transfer rate were greater for nanofluids than for water in all investigated cases, for a particular pumping power. The average increase in heat transfer coefficient was nearly 16.2%. As a result, choosing NDG/water as the working fluid can improve the performance of double-pipe heat exchangers.