The study investigates the thermal conductivity of nanoparticle-fluid mixtures, specifically Al2O3 and CuO nanoparticles dispersed in water, vacuum pump fluid, engine oil, and ethylene glycol. The steady-state parallel-plate method is used to measure the thermal conductivities, which show that the mixtures have higher thermal conductivities than the base fluids. Theoretical models for effective thermal conductivity are compared with experimental data, revealing that these models significantly underestimate the measured values, indicating their limitations in describing nanoparticle-fluid mixtures. The enhancement in thermal conductivity is attributed to microscopic particle motion and particle structure, such as chain formation. The viscosity of the mixtures is also studied, and it is found that the viscosity increases with the addition of nanoparticles. The findings suggest that further research is needed to fully understand the mechanisms behind the enhanced thermal conductivity and to develop more accurate models for nanoparticle-fluid mixtures.The study investigates the thermal conductivity of nanoparticle-fluid mixtures, specifically Al2O3 and CuO nanoparticles dispersed in water, vacuum pump fluid, engine oil, and ethylene glycol. The steady-state parallel-plate method is used to measure the thermal conductivities, which show that the mixtures have higher thermal conductivities than the base fluids. Theoretical models for effective thermal conductivity are compared with experimental data, revealing that these models significantly underestimate the measured values, indicating their limitations in describing nanoparticle-fluid mixtures. The enhancement in thermal conductivity is attributed to microscopic particle motion and particle structure, such as chain formation. The viscosity of the mixtures is also studied, and it is found that the viscosity increases with the addition of nanoparticles. The findings suggest that further research is needed to fully understand the mechanisms behind the enhanced thermal conductivity and to develop more accurate models for nanoparticle-fluid mixtures.