Effective Thermal Conductivity of Dispersions with a Continuous Liquid Phase Relevant for Energy and Process Technology

Description

Thermal conductivity (lambda) is fundamental to optimizing industrial heat transfer. Liquid dispersions, such as nanofluids and microemulsions, have gained significant attention for their enhanced thermal properties, offering potential for increased efficiency, reduced operational costs, and mitigated climate impact. This thesis provides a systematic experimental investigation into the mechanisms governing the effective thermal conductivity (lambda eff) of these multiphase systems. This research examines the influence of particle volume fraction, temperature, and base fluid properties, alongside particle-specific effects including Brownian motion, morphology, and solid-liquid interfacial resistance (Kapitza resistance). Systems studied include various nanofluids (PMMA, PS, SiO2, TiO2, CuO) and non-ionic microemulsions. Experiments were conducted at ambient pressure (0.1 MPa) across a temperature range of (283.15 to 358.15) K. A core contribution of this work is the development of a guarded parallel plate instrument (GPPI) for the absolute determination of the thermal conductivity. The GPPI proved versatile for measuring liquids, solids, and gases with high accuracy, achieving expanded uncertainties between (2 and 3)%. Furthermore, this work proposes a new model based on the geometric mean for the prediction of lambda eff. By incorporating particle diameter and Kapitza resistance, the proposed model demonstrates superior predictive capability across diverse nanofluid systems. This dissertation represents one of the most comprehensive experimental studies to date, providing a reliable database and a deeper fundamental understanding of the mechanisms driving heat conduction in complex dispersions.