Development of the shadowgraph method for the accurate determination of diffusivities of fluid mixtures under high pressures and high temperatures

在拟议的第二个供资期内，应进一步发展阴影法，主要侧重于准确和同时测定二元混合物的菲克扩散系数D11和热扩散系数a。为此，现有的热扩散池应优化实验策略之前，开发，以克服目前有限的适用性的技术，以特定的特性的混合物。计划的研究活动取决于第一个供资期的结果。在此，已经设计、构建并证明了适用于高温高压的热扩散池，其允许建立非常稳定的温度梯度，从而基于Soret效应产生稳定的浓度梯度。利用新开发的光学装置，可以高分辨率地分析非平衡波动的阴影。这些波动的动力学可以用与多种输运性质相关的守恒定律来描述。它可以证明的第一次，D11，a，运动粘度和Soret系数ST可以同时确定的阴影法，但与特定的混合物和热力学状态的限制。对于具有负ST的系统，平流发生在流体层内，该流体层的厚度迄今为止一直保持较大，以确保获得运动粘度和ST。由于可以表明，只有小强度的平流仍然允许获得所需的特性，应在第二个供资期内实现抑制平流的措施，以将阴影法的适用性扩展到几乎任意的混合物。除了不断进一步开发数据评估策略外，这些措施还包括灵活减少流体层厚度以及可选的温度梯度反转。流体层厚度的减小意味着对于ST < 0的混合物，运动粘度和ST将不可及。这是合理的，在任何情况下，大的不确定性，而平流的最小化预计将明显增加的系统的数量，D11和一个可以测量的明显较小的不确定性。对所选系统的研究应表明，流体层厚度的减少，这也减少了信号统计，如何可以通过温度梯度的增加来补偿。后者的增加是有益的信号强度和特别重要的混合物具有小幅度的ST和小的纯组分的折射率差。在拟议项目的最后一部分，系统选择的二元烃混合物的研究应验证所取得的进展，并有助于更好地理解结构与性能的关系，在分子扩散的背景下。

In the proposed second funding period, the shadowgraph method should be further developed with main focus on the accurate and simultaneous determination of the Fick diffusion coefficient D11 and the thermal diffusivity a of binary mixtures. For this, the available thermodiffusion cell should be optimized before experimental strategies are developed to overcome the currently restricted applicability of the technique to mixtures with specific characteristics. The planned research activities rely on the results of the first funding period. Here, a thermodiffusion cell suitable for high temperatures and high pressures has been designed, built up and proven to allow the establishment of very stable temperature gradients that result in stable concentration gradients based on the Soret effect. With a newly developed optical setup, the shadows of non-equilibrium fluctuations can be analyzed with high resolution. The dynamics of these fluctuations can be described by conservation laws that are connected with multiple transport properties. It could be demonstrated for the first time that D11, a, the kinematic viscosity and the Soret coefficient ST can be determined simultaneously by the shadowgraph method, yet with restrictions to specific mixtures and thermodynamic states. For systems with negative ST, advection occurs within the fluid layer whose thickness has been kept large so far to ensure access to kinematic viscosity und ST. As it could been shown that only a small intensity of advection still allows access to the desired properties, measures to suppress advection should be realized in the second funding period to extend the applicability of the shadowgraph method to virtually arbitrary mixtures. In addition to a continuous further development of the data evaluation strategies, the measures include a flexible reduction of the fluid layer thickness as well as an optional reversal of the temperature gradient. The reduction of the fluid layer thickness implies that kinematic viscosity and ST will not be accessible for mixtures with ST < 0. This is justified by their, in any case, large uncertainties while the minimization of advection is expected to clearly increase the number of systems for which D11 and a can be measured with distinctly smaller uncertainties. The investigation of selected systems should show how the reduction of the fluid layer thickness, which also reduces the signal statistics, can be compensated by the increase of the temperature gradient. An increase of the latter is beneficial for the signal intensity and of particular importance for mixtures with small magnitude of ST and small refractive index difference of the pure components. In the last part of the proposed project, the study of systematically selected binary hydrocarbon mixtures should validate the achieved developments and contribute to an improved understanding of structure-property relationships in context with molecular diffusion.