Experimental investigation of a biomass-derived nanofluid with enhanced thermal conductivity as a green, sustainable heat-transfer medium and qualitative comparison via mathematical modelling.

In this study, bio-based carbon nanospheres (CNSs) were synthesized from lignocellulosic-rich groundnut skin ( Arachis hypogaea ) and tested for their practical application in nanofluids (NFs) for enhanced heat transfer. The CNSs were characterized using various techniques, including FESEM, EDS, XRD, Raman spectroscopy, zeta potential analysis, and FTIR. Thermal conductivity (TC) and viscosity measurements were conducted using transient plane source (TPS) technique with a Hot Disk thermal analyser and discovery hybrid rheometer, respectively. The nanoparticles (NPs) were dispersed in two base fluids: ethylene glycol (EG) and a 60 : 40 mixture of deionized water (DI) and EG. Optimization studies were performed by varying the stirring and measurement times to improve TC values. The results showed that when a power source of 40 mW was applied at a high concentration of nanoparticles ( i.e. , 0.1 wt%), there was a 91.9% increment in thermal conductivity (TC) compared to the base fluid EG. DI-EG-based nanofluids (NFs) exhibited enhancements of up to 45% compared to the base fluid DI-EG (60 : 40), with a heating power of 80 mW and concentration of 0.1 wt%. These results demonstrated significant TC improvements with NP incorporation. Further experiments were performed by varying the temperature in the range of 30-80 °C with readings taken for every 10 °C increase, which showed a direct relation with the TC values. At 80 °C, EG-based NFs showed increments of 77%, 111.49%, 139.67% and 175% at 0.01, 0.02, 0.05 and 0.1 wt% concentrations of NPs, respectively. It was also found that with the increase in the concentration of NPs, viscosity increased, whereas an increase in the temperature led to a decrease in viscosity. The CNS nanofluid exhibited a Newtonian behaviour with the nanoparticle concentration and temperature, resulting in an approximately 114% enhancement compared to the base fluid when the concentration of CNSs was 0.1 wt% at 30 °C but decreased by up to 18% when the temperature was increased to 90 °C. Using appropriate mathematical models for assessing thermophysical quantities, it was discovered that the model values and experimental values correspond reasonably well. Our method thus validates our experimental results and deepens the understanding of the mechanisms behind enhancing thermal conductivity in biomass-derived nanofluids. In summary, our work advances sustainable nanomaterial synthesis, providing a new solution for boosting thermal conductivity while maintaining environmental integrity, thereby inspiring further research and innovation in this field.