Effective thermal conductivity of dispersions with a liquid continuous phase

具有液体连续相的分散体是由液体分散介质和胶体颗粒分散相组成的非均相体系。这些颗粒可以是纳米流体中的固体颗粒、液体颗粒(即形成乳状液的液滴)或泡沫中存在的气泡形式的气态颗粒。表征分散体的一个关键性质是它们的有效导热系数(等)。许多文献报道，通过在液体中添加少量的固体纳米颗粒，纳米流体的ETC相对于基础流体可以显著增加，而其他研究没有观察到任何显著的增强。直到今天，关于影响纳米流体中有效导热的相关机制的争论仍在继续，其中考虑了与颗粒有关的布朗运动、界面分层和聚集或这些影响的组合。这反映在纳米流体ETC的相关建模方法中，预测值往往有很大的变化。目前关于分散体ETC的争论也与实验方法的可靠性有关。本项目的主要目标是促进对具有液体连续相的分散体的ETC的基本了解。为此，应对相关影响进行系统调查。这里的重点在于分散相和连续相的导热系数、分散颗粒的形态和颗粒聚集。此外，应该得出关于界面热阻和颗粒的布朗运动对ETC的作用的结论。为了解决这些影响，需要用稳态保护平行板装置从理论上和实验上研究纳米级固体或液体颗粒在具有不同物理化学特性的液体连续相中的分散。与经过严格评估的文献数据一起，实验结果应该成为研究上述对此类系统ETC的影响的可靠数据库。它们还应该有助于回答这样一个问题，即对含有固体颗粒的分散体所获得的结果是否可以适用于含有液体颗粒的分散体。对于作为ETC基本参数的分散体的颗粒大小和形状的表征，应通过动态光散射来获取有关颗粒的平移和旋转扩散率的信息。基于实验结果、理论考虑和现有的建模方法，应发展一种适用于液体连续相分散体系ETC的通用预测方法。

Dispersions with a liquid continuous phase are heterogeneous systems which consist of a liquid dispersion medium and a dispersed phase of colloidal particles. These particles can be solid particles as given in nanofluids, liquid particles, i.e. droplets forming emulsions, or gaseous particles in the form of bubbles which are present in foams. A key property characterizing dispersions is their effective thermal conductivity (ETC). Many literature studies report that by adding a small amount of solid nanoparticles to liquids, the ETC of nanofluids can be increased extraordinarily relative to the base fluid, while others do not observe any significant enhancement. Until today, debate has continued on the relevant mechanisms affecting the effective thermal conduction in nanofluids where Brownian motion, interfacial layering, and aggregation related to the particles or a combination of such effects are considered. This is reflected in related modeling approaches for the ETC of nanofluids providing often strong variations in the predicted values. The ongoing controversy about the ETC of dispersions is also connected to the reliability of the experimental methods. The main objective of the present project is to contribute to a fundamental understanding of the ETC of dispersions with a liquid continuous phase. For that, the relevant influences should be investigated systematically. Here, the focus lies on the thermal conductivities of the dispersed and the continuous phase, the morphology of the dispersed particles, and particle aggregation. Furthermore, conclusions about the roles of an interfacial thermal resistance and of Brownian motion of the particles regarding the ETC should be drawn. For addressing these effects, selected dispersions of solid or liquid particles on the nanometer scale in a liquid continuous phase with different physical and chemical characteristics should be investigated theoretically and experimentally by using a steady-state guarded parallel-plate instrument. Together with critically evaluated literature data, the experimental results should serve as a reliable database for studying the aforementioned influences on the ETC of such systems. They should also help to answer the question whether the findings obtained for dispersions with solid particles are transferable to those with liquid particles. For the characterization of the dispersions with respect to particle size and shape as essential parameters for the ETC, information about the translational and rotational diffusivity of the particles should be accessed by dynamic light scattering. Based on the experimental results, theoretical considerations, and exisiting modelling approaches, a generalized prediction method for the ETC of dispersions with a liquid continuous phase should be developed.