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

• Applicant: Professor Dr.-Ing. Andreas Paul Fröba
• Subject Area: Technical Thermodynamics
• Term: from 2017 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 379151958

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.

• DFG Programme: Research Grants
• International Connection: France
• Co-Investigator: Dr.-Ing. Michael Heinrich Rausch
• Cooperation Partner: Dr. Cédric Giraudet