Effective Thermal Conductivity of Nanofluids: Measurement and Prediction

Abstract In the present study, the effective thermal conductivity of nanoparticle dispersions, so-called nanofluids, is investigated experimentally and theoretically. For probing the influence of the nanoparticles on the effective thermal conductivity of dispersions with water as liquid continuous phase, nearly spherical and monodisperse titanium dioxide (TiO2), silicon dioxide (SiO2), and polystyrene (PS) nanoparticles with strongly varying thermal conductivities were used as model systems. For the measurement of the effective thermal conductivity of the nanofluids with particle volume fractions up to 0.31, a steady-state guarded parallel-plate instrument was applied successfully at temperatures between (298 and 323) K. For the same systems, dynamic light scattering (DLS) was used to analyze the collective translational diffusion, which provided information on the dispersion stability and the distribution of the particle size as essential factors for the effective thermal conductivity. The measurement results for the effective thermal conductivity show no temperature dependency and only a moderate change as a function of particle volume fraction, which is positive or negative for particles with larger or smaller thermal conductivities than the base fluid. Based on these findings, our theoretical model for the effective thermal conductivity originally developed for nanofluids containing fully dispersed particles of large thermal conductivities was revisited and also applied for a reliable prediction in the case of particles of relatively low thermal conductivities

Effective Thermal Conductivity of Nanofluids Containing Silicon Dioxide or Zirconium Dioxide Nanoparticles Dispersed in a Mixture of Water and Glycerol

The present study investigates the effective thermal conductivity of nanofluids containing crystalline or amorphous silicon dioxide (SiO2), or zirconium dioxide (ZrO2) nanoparticles dispersed in a mixture of water and glycerol with a mass ratio of 60:40. Such fluids are relevant as potential cutting fluids in tribology and feature a broad distribution of irregularly shaped non-spherical particles of dimensions on the order of (100 to 200) nm that were produced by comminution of larger particles or particle aggregates. A new steady-state guarded parallel-plate instrument was applied for the absolute measurement of the effective thermal conductivity of the nanofluids with an expanded uncertainty (coverage factor k = 2) of 3% for temperatures from (293 to 353) K and particle volume fractions up to 0.1. For a constant volume fraction of 0.03 for the three particle types, the measured thermal-conductivity ratios, i.e. the effective thermal conductivity of the nanofluids relative to the thermal conductivity of the base fluid, are less than 1.05 and not affected by temperature. In the case of the nanofluids with crystalline SiO2, with increasing particle volume fraction from 0.03 to 0.10 the thermal-conductivity ratios increase up to values of about 1.18 for all temperatures. A comparison of the measurement results with the Hamilton-Crosser model and an analytical resistance model for the effective thermal conductivity of nanofluids shows that the former one allows for better predictions for the present nanofluids with a relatively large viscosity. In this context, it could be shown that detailed knowledge about the sphericity and thermal conductivity of the dispersed nanoparticles is required for the modeling approaches.Open Access funding enabled and organized by Projekt DEAL.Friedrich-Alexander-Universität Erlangen-Nürnberg (1041

Effect of the degree of hydrogenation on the viscosity, surface tension, and density of the liquid organic hydrogen carrier system based on diphenylmethane

For the efficient design of hydrogenation and dehydrogenation processes, a comprehensivedatabase for the viscosity, surface tension, and density of mixtures of thediphenylmethane-based liquid organic hydrogen carrier system and the pure intermediatecyclohexylphenylmethane measured by complementary optical and conventionalmethods and calculated by molecular dynamics simulations at process-relevant temperaturesup to 623 K is presented. The simulations employ self-developed force fieldsincluding a new one for cyclohexylphenylmethane and reveal surface enrichment andorientation effects influencing the surface tension. Relatively simple correlation and predictionapproaches yield accurate representations as function of temperature and degreeof hydrogenation (DoH) of the mixtures with average absolute relative deviations (AARD) of0.07% for the density and 2.9% for the surface tension. Application of the extended hardspheretheory considering the presented accurate density data allows capturing thehighly nonlinear DoH-dependent behavior of the dynamic viscosity with an AARD of 2.9%.© 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved