Rheological behavior and stability of mono and hybrid carbon nanotube—Titanium dioxide nanofluids in ethylene glycol and water basefluids

Nanofluids have gained significant attention for their potential to enhance heat transfer processes and reduce energy consumption in thermal systems. This study examines the rheological behavior and stability of hybrid nanofluids composed of Multi-Walled Carbon Nanotubes (MWCNT) and Titanium Dioxide (TiO 2 ) nanoparticles, dispersed in ethylene glycol (EG) and distilled water (DW). Nanofluids were prepared using a two-step method with a low volume concentration (0.005 %) and various MWCNT:TiO 2 ratios of (0:1), (0.25:0.75), (0.75:0.25), and (1:0). Stability was assessed using zeta potential and photocapturing techniques, revealing that TiO 2 in DW had the highest stability ( mV) and TiO 2 in EG had the ( mV). Rheological behavior varied between the nanofluids, with EG-based nanofluids exhibiting shear-thickening behavior and DW-based nanofluids showing mixed behavior, transitioning from shear-thickening to shear-thinning under different conditions. Temperature increases led to a reduction in viscosity for all nanofluids, with MWCNT in EG causing the most significant viscosity decrease (2.06 % lower than pure EG at 20 C), while TiO 2 in DW resulted in the highest viscosity increase (5.9 % higher than pure DW at 20 C). Additionally, a comparison was made between the experimental viscosity data and the literature values, which validated the findings of this work. The newly developed rheological model, which predicts the viscosity behavior of nanofluids, demonstrated excellent predictive capability and physical consistency, with an R 2 value of 0.9720. These hybrid nanofluids show promise for heat exchanger applications, enhancing thermal performance and reducing energy consumption.