Effective Thermal Conductivity of 1-Pentanol-Based Nanofluids Containing Oblate Silicon Dioxide Nanoparticles

This work investigates the effect of the shape of oblate particles on the effective thermal conductivity λ_eff of nanofluids. Spherical silicon dioxide (SiO₂) nanoparticles with a mean diameter of 176 nm were plastically deformed in a stirred media mill to obtain non-spherical oblate particles of nearly cylindrical shape without significantly changing the volume of the individual particles. This allowed to study 1-pentanol-based nanofluids with varying particle sphericity ψ of 1.00, 0.50, or 0.45 at particle volume fractions φ_p up to 0.16, which exceeds the value range by a factor of two compared to previous studies on nanofluids with non-spherical particles. Measurements of λ_eff and of the thermal conductivity of the base fluid 1-pentanol, λ_bf, were performed at temperatures T from (298.15 to 358.15) K at ambient pressure using a steady-state guarded parallel-plate instrument (GPPI) with an expanded (coverage factor k = 2) uncertainty between (2.0 and 2.2) %. The experimental results indicate that the varying shape of the particles with about ten times higher thermal conductivity than λ_bf does not have a significant effect on λ_eff within the experimental uncertainties over the investigated ranges of φ_p and T. This behavior is not reflected by the semi-empirical Hamilton-Crosser (HC) model that predicts an increase in the thermal conductivity ratio λ_eff·λ_bf⁻¹ with decreasing ψ. A better representation is given by the correct formulation of the effective medium theory via the model of Nan et al., which can be applied to dispersions with completely misoriented ellipsoidal particles. This is apparently related to the counterbalance of the enhancing effect of non-spherical particles on λ_eff due to their orientation-averaged extended heat conduction paths along the heat flux compared to spherical particles and the reducing effect caused by a Kapitza resistance at the particle/liquid interface, which increases with decreasing ψ for a given φ_p.