Understanding transport and mixing in stratified saline systems is critical for predicting the behavior of brines in natural aquifers, industrial reservoirs, and engineered disposal sites. These multi-ion solutions often exhibit complex instabilities driven by differential diffusion and compositional gradients. The onset and morphology of such convective mixing remain poorly predicted. We investigated double-diffusive (DD) and diffusion-layer convection (DLC) in superimposed aqueous solutions of the salts typical of saline aquifers, sodium chloride (NaCl) and sodium sulphate (Na 2 SO 4 ). The study combines thermodynamic modeling, optical interferometry experiments, and nonlinear numerical simulations to explore convective instabilities in a ternary system. Our findings reveal a rich variety of convective scenarios depending on salt configuration and concentration ratios. When the faster-diffusing NaCl was placed above Na 2 SO 4 , diffusion-layer convection occurred with a delayed and asymmetric onset of instability, an experimentally demonstrated feature not reported previously. In contrast, when the positions were reversed, the system developed double-diffusive fingers that grew slowly due to the small density ratio. These fingers exhibited an unusual morphology, consisting of extremely fine, vertically textured structures that gradually merged away from the interface. This formed a large area of diffuse mixing and suppression of coherent convective structures. In all cases, classical stability criteria failed to fully predict the onset and nature of convection. Instead, we identified the initial position of the system on the stability map, as determined by the full diffusion matrix, as a critical factor.

Abstract The presence of thermocapillary (Marangoni) convection in microgravity may help to enhance the heat transfer rate of phase change materials (PCMs) in space applications. We present a three-dimensional numerical investigation of the nonlinear dynamics of a melting PCM placed in a cylindrical container filled with n-octadecane and surrounded by passive air. The heat exchange between the PCM and ambient air is characterized in terms of the Biot number, when the air temperature has a linear profile. The effect of thermocapillary convection on heat transfer and the topology of the melting front is studied by varying the applied temperature difference between the circular supports and the heat transfer through the interface. The evolution of Marangoni convection during the PCM melting leads to the appearance of hydrothermal instabilities. A new mathematical approach for the nonlinear analysis of emerging hydrothermal waves (HTWs) is suggested. Being applied for the first time to the examination of PCMs, this procedure allows us to explore the nature of the coupling between HTWs and heat gain/loss through the interface, and how it changes over time. We observe a variety of dynamics, including standing and travelling waves, and determine their dominant and secondary azimuthal wavenumbers. Coexistence of multiple travelling waves with different wavenumbers, rotating in the same or opposite directions, is among the most fascinating observations.

In a ternary mixture with the Soret effect, the interplay between cross-diffusion, thermodiffusion, and convection can lead to rich and complex dynamics including spatial patterns and oscillations. We present an experimental and three-dimensional numerical study of dynamic regimes in the toluene-methanol-cyclohexane ternary mixture with the Soret effect in the geometry of a thermogravitational column. An important feature of the system is that for the first component, toluene, the Soret and thermodiffusion coefficients have opposite signs, which triggers the oscillatory instability. Our experiments and numerical analysis show that the primary long-wave instability manifests itself in the form of a standing wave, and the secondary one emerges in the form of a swinging pattern. The computational model provides insight into the role of cross-diffusion coefficient D12 in the emergence and development of oscillatory instability. This study demonstrates that the long-wave oscillatory instability in transverse direction occurs only within a limited range of the D12 values and outside of this range it decays to a stationary pattern of either Turing-like or monotonic instability.

We show that a subtle coupling between the thermal and solutal gradients driven by the thermodiffusion, cross-diffusion, and buoyancy force can lead to oscillatory dynamic behavior of a ternary mixture in the thermo-gravitational column. The potentially unstable stratification results from the interplay of mass fluxes of the two heaviest components where the leading role belongs to one of them. Our experiments and numerical analysis evidence not only the presence of oscillatory instability, but demonstrate the emergence of the secondary instability in the form of swinging pattern in addition to the large-scale standing wave. We suggest that the region, where oscillatory instability occurs, is characterized by opposite signs of Soret S'T1 and thermodiffusion D'T1 coefficients due to large cross-diffusion contribution.

This manuscript summarizes the educational and scientific outcome of the Research-based learning activities performed in the bachelor’s, master’s, and doctorate programmes in aerospace engineering at the Technical University of Madrid. The activities are related to the line of research in Phase Change Materials in microgravity developed at the Spanish User Support and Operations Centre. The principal scientific results obtained during these years are outlined, drawing particular attention to those related to the “Thermocapillary Effects in Phase Change Materials in Microgravity” experiment and the “Effect of Marangoni convection on heat transfer in Phase Change Materials” project. The outcomes of this research are discussed from an educational perspective. Since 2016, we observe an increased interest from students to participate in research activities, which has had direct positive impact on the production of scientific results

Abstract In the present work, an interferometric unsteady analysis of the thermogravitational technique was for the first time attempted in a paralelepipedic microcolumn using binary mixtures with negative Soret coefficients. In particular, water/ethanol and toluene/methanol were considered, as they have significantly different thermophysical properties and relaxation times. Experiments were run with different temperature gradients in order to understand their impact on the stability of separation. Experimental results were compared with theoretical ones, predicted by Fury-Jones-Onsager theory, and by OpenFOAM 3D numerical simulations. Correlations between the separation and the flow in the third dimension perpendicular to the thermal gradient of the thermogravitational microcolumn were analysed. Numerical simulations were also conducted in traditional cylindrical columns in order to compare the results with those previously reported. In these cases, the impact on separation stability was correlated with the azimuthal component of velocity. Thus, in both configurations, the disturbing convective current, always generated in the direction perpendicular to the thermal gradient applied, was shown to be vital for flow stability analysis.

Abstract In the present work, interferometric unsteady state analysis of the thermogravitational technique is for the first time attempted in a microcolumn. In this way, three transport coefficients, diffusion, thermodiffusion and Soret coefficients can be obtained in a single experiment. Two different models — one from the Furry-Jones-Onsager theory and the other with the ‘forgotten effect’ included — have been tested for six different binary mixtures. These mixtures are the three binary pairs of the Benchmark of Fontainebleau— tetrahydronaphthalene-isobutylbenzene (THN-IBB), tetrahydronaphthalene-dodecane (THN-nC12) and isobutylbenzene-dodecane (IBB-nC12)—toluene-methanol dimethyl sulfoxide-water (DMSO-H20) and dimethyl sulfoxide-deuterated water (DMSO-D20). The first three benchmark binary mixtures are used to validate the inteferometric technique and also to establish when it is reasonable to treat a mixture as ideal, without considering the forgotten effect. After that, the three remaining mixtures are deeply analysed. In the end, the isotopic effect in the Soret coefficient will be highlighted through the DMSO-H20 and DMSO-D20 mixtures.