Effective thermal conductivity of dispersions with a liquid continuous phase

• Applicant: Professor Dr.-Ing. Andreas Paul Fröba
• Subject Area: Technical Thermodynamics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 463473804

Dispersions with a liquid continuous phase have been in the focus of scientific investigations for more than two decades, motivated primarily by the often-reported disproportionate increase in the effective thermal conductivity leff of nanofluids (NFs) compared to the thermal conductivity of the base fluid. Yet, research over the last ten years, including the work carried out within the first funding period (FP1) for NFs and microemulsions (MEs), has shown that these systems do not exhibit any unusual characteristics. By mastering accurate measurement methods, it was shown that experimental data for leff are well represented by the effective medium theory developed by Maxwell. The investigations from FP1 made it possible to analyze the behavior of leff of NFs and MEs as a function of material and fluid properties as well as of particle concentration and morphology. Inspired by still open and new questions, the main objective of this renewal proposal is a deepened characterization of leff of dispersions with a liquid continuous phase, some of which show an increased complexity compared to the systems investigated in FP1 and in the literature. Corresponding answers are to be found through the study of NFs, which allow a greater variation of the thermal-conductivity contrast and/or the anisotropy with regard to the shape or volume of the dispersed phase. To transfer the prediction methods tested in FP1 for dispersions with rather small leff to those with significantly larger values, liquid metal alloys with dispersed solid nanoparticles (NPs) will be investigated. This allows the contrast to be varied from 100 to 0.06, with large deviations from 1 also serving to analyze the influence of shape anisotropy on leff. Here, volume- and interface-related contributions of non-spherical solid single-phase NPs to the behavior of leff are elaborated by extending the studies started in FP1 to oblate and prolate NPs of different sphericity. A further step addresses the volume anisotropy of dispersed phases consisting of two-phase NPs. Here, analogies between leff of such multiphase dispersions with two-phase NPs based on solid or liquid cores as well as relationships to the values for the corresponding subsystems are to be worked out. To answer the scientific questions of the renewal proposal, a reliable database for leff will be established using an existing measurement apparatus. Dynamic light scattering is used to determine the morphology of the dispersed phase and the dispersion stability. Based on the measurement results, structure-property relationships will be identified and used to test and further improve available prediction methods, including their transfer to dispersions with two-phase NPs.

• DFG Programme: Research Grants