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间，应进一步发展阴影图法，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，在开发实验策略之前，应优化可用的热扩散池，以克服目前该技术对具有特定特性的混合物的限制适用性。计划的研究活动依赖于第一个资助期的结果。在这里，设计了一种适用于高温和高压的热扩散池，并证明它可以建立非常稳定的温度梯度，从而基于索雷特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涨落的阴影。这些涨落的动力学可以用与多种输运性质相关的守恒定律来描述。首次证明了用阴影图法可以同时测定D11、α、运动粘度和Soret系数ST，但对特定混合物和热力学状态有限制。对于ST为负的系统，平流发生在流体层内，到目前为止，流体层的厚度一直保持较大，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能获得所需的性质，因此应在第二个供资期间采取抑制平流的措施，以便将阴影图法的适用范围扩大到几乎任意的混合物。除了不断进一步发展数据评估战略外，这些措施还包括灵活地减少流体层厚度以及可选地反转温度梯度。流体层厚度的减少意味着运动粘度和ST对于ST&lt；0的混合物是不可获得的。这是合理的，因为它们在任何情况下都有很大的不确定性，而平流的最小化预计将明显增加可以用明显较小的不确定度测量d11和a的系统的数量。对选定系统的研究应该表明，如何通过增加温度梯度来补偿流体层厚度的减少，这也减少了信号统计。后者的增大有利于信号强度的提高，对于ST值较小、纯组分折射率差较小的混合物尤为重要。在拟议项目的最后部分，对系统选择的二元碳氢化合物混合物的研究应证实所取得的进展，并有助于更好地理解分子扩散背景下的结构-性质关系。

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.