Unlocking efficiency: experimental and theoretical insights into biomass-derived carbon nanofluids with enhanced thermal conductivity†

The study presents an experimental investigation, supported by theoretical analysis, into the effects of nanoparticle (NPs) concentration, particle size, and shape on the thermal conductivity (TC) of carbon nanosphere (CNS)-based nanofluids (NF). CNS was synthesized from garlic peels (Allium sativum) via pyrolysis at varying temperatures and characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and other techniques. The NFs were prepared using a two-step method with different CNS concentrations in propylene glycol (PG) and deionized water (DI)/PG mixtures. Particle size distribution and colloidal stability were evaluated using dynamic light scattering (DLS) and zeta potential analysis. The TC of the NFs was measured across various temperatures, revealing a significant dependency on both particle size and concentration. All NFs exhibited enhanced thermal conductivity to the base fluid (BF), with increases of 52.60%, 101.28%, 108.51%, 114.60%, and 122.64% at 80 °C for CNS synthesized at 500 °C (AS500), 600 °C (AS600), 700 °C (AS700), 800 °C (AS800), and 900 °C (AS900), respectively. Rheological analysis showed a linear increase in dynamic viscosity (V) with rising CNS concentration within the dilute limits (0.01 to 0.1 wt%) and a strong correlation between particle size and thermal conductivity enhancement. These findings emphasize the critical role of CNS particle size in optimizing thermal performance, with potential applications in heat transfer systems. The study culminates with an exercise aimed towards presenting thermal conductivity and dynamic viscosity as surface plots. These plots provide behavioral trends for understanding the dependence of TC and V on nanoparticle size and temperature.