We present here an extensive analysis of the free surface dynamics driven by the thermocapillary effect in half-filled elliptical containers in microgravity. Depending on the cell ellipticity δ , which selects the preferred static equilibrium via surface energy, and on the applied thermal forcing Δ T , interesting dynamics are found. Simulations show that the steady, thermally-driven position of the interface — perpendicular to Δ T — undergoes a pitchfork bifurcation at a critical δ _cr that breaks the vertical reflection symmetry of the system. These results are supported by (leading order) estimates of the opposing thermocapillary and surface tension forces, predicting the linear dependence of δ _cr on Δ T . Finally, the free surface relaxation after switching off the thermal control is explored. As a whole, the present analysis indicates that one can combine thermocapillary flows and an adequate cell design to manipulate and control fluids in microgravity, with potential in a wide variety of applications.

Both Singular Value Decomposition (SVD) and Artificial Neural Networks (ANNs) can be powerful tools for image processing. Here they are applied in the context of the “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” (MarPCM) microgravity experiment [Porter et al. (Acta Astronautica 210, 212–223, 2023)], which investigates the use of thermocapillary (Marangoni) convection to expedite melting of organic Phase Change Materials (PCMs) in cuboidal and cylindrical domains. The processing of the cylindrical “melting bridge” experimental images is particularly challenging due to the converging lens effect caused by the curved interface and the refractive index of the liquid PCM. A combination of SVD and ANNs is used to propose an algorithm to process these images. The network is trained on a set of synthetic images of the melting bridge, generated via ray-tracing [Martinez et al. (Advances in Space Research 72, 1915–1928, 2023)] then projected onto the eigenmodes associated with the largest singular values of the image database, which includes snapshots of the melting process in all representative cases. Two optimal algorithm architectures are described, characterized by the number of SVD modes considered in the projection and the hyperparameters of the ANN. The performance of the algorithm is analyzed in terms of its ability to associate images with the correct liquid fraction. The processing strategy is tested by applying it to images obtained from ground experiments using the scientific prototype of the MarPCM cuboidal cell.

The melting of a phase change material (PCM) in a cuboidal domain under microgravity conditions is investigated numerically. The upper surface of the PCM is free (in contact with air, for example) and variations in its surface tension drive thermocapillary convection in the liquid phase, which significantly enhances heat transfer and accelerates melting. Furthermore, the change in liquid fraction during melting is associated with transitions among various modes of thermocapillary dynamics, including an oscillatory instability to hydrothermal waves. While the characteristics of PCM melting and thermocapillary dynamics have previously been investigated in this system using a two-dimensional model, the current work examines the important question of transverse dynamics and their effect on the melting process. Careful quantitative comparisons are made between the three- and two-dimensional models in terms of melting times, solid/liquid interface evolution, thermal fields, and spectrograms. The results show that transverse modes are often, but not always, reflection symmetric about the midplane and that their influence on melting and PCM performance is relatively minor in most cases. Thus, two-dimensional models may be used to reduce computational costs while still providing a reasonable approximation of the melting process for high Prandtl number materials, especially when compared to the midplane of the full cuboidal domain.

Three strategies for enhancing the melting rate of phase change materials (PCMs) are analyzed numerically: natural convection, thermocapillary convection, and variations in container geometry motivated by the natural shape of the advancing solid/liquid front. An enthalpy-porosity formulation of the Navier-Stokes equations is used to model the melting process, where the organic PCM n-octadecane is considered as a single phase whose physical properties depend on the local temperature. The phase change is driven by subjecting the material to a constant temperature TH = TM + zT, where TM is the melting temperature, at one of its lateral boundaries of length lH; the remaining boundaries are assumed adiabatic. Melting dynamics are described for both individual and combined enhancement strategies, using as reference the melting process in a rectangular container purely driven by conduction. The efficacy of each strategy is compared using the time tau required to melt 90% of the given PCM volume V; the inverse ratio of this time to that of the reference case, G = tau 0/tau, defines the enhancement factor. Comparisons are made for small and large values of zT and lH. Increasing zT drives stronger convective flow and increases the associated enhancement factor. The isothermal length lH has opposite effects on each type of convection, favoring the enhancement of natural convection or thermocapillary flows at large and small values, with rates of G is an element of (5, 12) and G is an element of (12, 33), respectively. The optimal container geometry is characterized by a type of aspect ratio P = lc/hcthat compares its characteristic length and height. For pure conduction or with natural convection, P similar to 0.5, while with thermocapillary convection, P similar to 2.5 - 5.

The potential for sloshing reduction of passive, moving baffles with translational motion constrained by springs is investigated numerically in partially filled containers. Simulations are based on the level-set formulation, considering water as the liquid under study and a baffle made of aluminum; this selection allows for direct validation of the model against experimental and analytical data. Depending on the filling ratio V∈(0,1), which simply measures the volume of liquid relative to the total container size, one can find an optimal spring stiffness K(V) that minimizes the kinetic energy and decay time of the sloshing response when subjected to a pulse-like acceleration. Results show that moving baffles significantly reduce sloshing compared to their fixed counterparts, with decreases up to 84.6% in decay time, τd. Frequency analyses for different V reveal one or two resonances in the range of 0.1–1.5 Hz whose amplitudes are directly influenced (lowered) by K, analogous to the behavior of Tuned Mass Dampers. For potential applications, the selection of K should look for an adequate sloshing response over the entire range of V, and account for both quasi-static and harmonic excitation, conveniently weighted in accord with the expected operational loads.

Fluid manipulation and control is crucial for space exploration. Motivated by the “Thermocapillary-based control of a free surface in microgravity" (ThermoSlosh) experiment (Salgado Sánchez et al. in Acta Astronautica 205:57–67, 2023), we conduct here a detailed numerical analysis of interfacial dynamics in a two-dimensional cylindrical cell, half-filled with different silicone oils or a fluorinert, and subjected to thermal forcing and vibrations. The effect on the free surface dynamics of the applied temperature difference, vibrational amplitude, fluid viscosity, and contact angle is analyzed; both static and dynamic contact angle models are considered. Results strongly suggest that thermocapillary flows can be used to control the interface orientation within the cell, while supplemental vibrations can be added to increase the system responsiveness. This control can be further improved by using classical proportional-integral-derivative feedback to adjust the cell boundary temperatures in real-time. The proportional and derivative gains of the controller can be selected to optimize the stabilization time and/or energy cost, while the integral contribution is effective in reducing the steady-state error. Overall, the present analysis highlights the potential of using the thermocapillary effect for fluid management in reduced gravity, and evaluates different types of experimental tests that can be executed in the frame of the ThermoSlosh microgravity project.

Here, we investigate the performance of phase-change materials (PCMs) in the passive thermal control of space habitats. PCMs are able to absorb and release large amounts energy in the form of latent heat during their (typically, solid-to-liquid) phase transition, which makes them an ideal choice for passive temperature control. In this study, a conceptual design of an igloo-shaped habitat is proposed. A scaled model for laboratory experiments is manufactured via 3D printing, using tap water as the PCM. The setup is used to conduct experiments and analyze PCM performance, based on temperature measurements inside and outside the habitat. Results demonstrate the effectiveness of PCMs in increasing thermal inertia and stabilizing the habitat interior temperature around the melting temperature, confirming that PCMs can be a suitable alternative for passive thermal control. The present study holds significant interest for the future of space exploration, with the emerging need to design habitats that are capable of accommodating astronauts.

We numerically investigate the melting dynamics of n-octadecane in a rectangular container that is subjected to a constant lateral flux and is open to air in microgravity. As the melting progresses, the temperature gradient existing at the open boundary drives thermocapillary flow within the liquid phase and modifies the phase change dynamics. We conduct a parametric study varying key dimensionless parameters — the Marangoni and Stefan numbers, and the aspect ratio — and compare the associated temporal dynamics with those observed in the (so-called) reference case, where heat is transferred solely by conduction, and with those driven by isothermal boundary conditions [1], [2]. Compared to the isothermal case, where the overall effect of thermocapillary flows is to accelerate melting by a factor up to 20, the analogous contribution with lateral flux is about one order of magnitude smaller, with values up to 2.8. Finally, we perform spectral analyses of the oscillatory standing wave and hydrothermal traveling wave modes that eventually appear during melting for large applied flux. The critical Marangoni number (Macr) is estimated considering the characteristic dependence ∝(Ma−Macr)0.25 of the (so-called) oscillatory mode contribution.

In this work, we analyze the thermocapillary-enhanced melting of n-octadecane driven by a constant heat flux, applied at the free surface, in microgravity. The material is enclosed in an open rectangular container of dimensions 2L x H, and its solid-to-liquid transition is described using an enthalpy-porosity formulation of the Navier-Stokes equations, assuming laminar and incompressible flow. We study the influence of key governing parameters, including the effect of the heated length Ĩphi is an element of (0, 1], the applied flux phi ⢚ is an element of (0, 8], and the container aspect ratio r is an element of [1.5, 22.8]. Heat transport is analyzed by comparing the thermocapillary-enhanced process with that driven solely by conduction, and quantified by the enhancement ratio G, which simply compares melting times in each scenario. We find that G increases with phi ⢚ and r, and is maximum at an optimal heated length Ĩphi similar or equal to 0.5. Compared to previous works on the melting of n-octadecane in microgravity, the associated enhancement G is more moderate in this system, and oscillatory thermocapillary convection is not observed over the range of parameters explored.

We experimentally confirm the efficacy of submerged oscillating baffles for sloshing mitigation in a partially filled open container of water. As with the more typical applications of tuned mass dampers, when the natural frequency of the baffle is close to that of the first sloshing mode, the main resonance peak is split into two peaks of lower amplitude. The reduction in resonant sloshing amplitude is superior to that of a fixed baffle.

We numerically investigate the effect on sloshing in microgravity of systems with passive, moving baffles whose motion is restricted by linear springs. The liquid is assumed to have physical properties similar to those of 5cSt silicone oil while the baffle is made of aluminium. Two kinds of numerical models are developed depending on whether the allowed baffle motion is translational or rotational. Simulations show that moving baffles significantly improve sloshing mitigation compared to fixed baffles, with decreases of up to 48% in the decay time and 23% in the fluid kinetic energy following excitation by a certain pulse-like acceleration. In all configurations considered, there exists an optimal spring stiffness that minimises the kinetic energy or decay time. Frequency analysis reveals two peaks in the range of 0.1–2Hz whose amplitudes vary as a function of spring stiffness, analogous to the behaviour of Tuned Mass Dampers (TMDs). The effectiveness of the moving baffle system is then assessed using a real microgravity perturbation measured during a reboosting manoeuvre of the International Space Station (ISS) and a significant reduction in decay time relative to the fixed baffle case is found: to 1.3s from 4s.

The present article is intended to study the boundedness of solutions for an unsteady non-Newtonian flow, whose strain–stress relationship is provided by the Sisko fluid model. Such kind of flow can appear in different scenarios, but we make it particular to the field of magnetohydrodynamics, as it is a remarkable area of research in its own right. The ideas exposed in this work can be extended, in a similar manner, to a wide range of applications. To make our fluid further general, we use the Darcy’s law to characterize the porous space in which the fluid is flowing. We develop the boundedness criteria provided that the velocity w and the function g=−∂w/∂r satisfy w,g,∂g/∂r,∂2g/∂r2∈L2(0,T;BMO), where BMO means bounded mean oscillation space. For this purpose, we will consider energy estimates in Sobolev spaces and develop the boundedness criteria for the resulting unsteady parabolic nonlinear equation.

The results of a numerical investigation of the melting of a PCM occupying an axisymmetric volume in the presence of gravity are presented. The PCM is held between two circular supports maintained at different temperatures. The melting process, which is analyzed for n-octadecane, is affected by a combination of thermocapillary and natural convection. If the PCM is heated from above, the convective motion driven by the thermocapillary force is opposed by the buoyant force, which reduces the heat transfer rate. If the PCM is heated from below, natural convection acts in the same sense as thermocapillary convection and the heat transfer rate is increased. The volume 𝒱 of the PCM relative to an ideal cylinder, which selects the shape of the PCM/air interface, is found to play an important role. The overall effect of natural convection on heat transfer is characterized by the ratio of the melting time in microgravity to that of the same system with gravity. This gain factor is greater (less) than unity when heating from below (above) and depends strongly on 𝒱 , particularly for smaller PCM volumes.

A detailed analysis of the design and performance of passive baffles for sloshing reduction in microgravity is presented. Sloshing dynamics are investigated for a rectangular container holding a L×H=30 × 15 mm2 volume of liquid with properties similar to a 5 cSt silicone oil. The system response to a pulse-like perturbation is analyzed in terms of the sloshing frequency ω, decay time τd, and damping ratio ξ=γ/γ2+ω2, characterizing the decay rate γ∝τd−1 relative to ω. We explore first simple rectangular baffles, parameterized in terms of their length and height, orientation, and position of their center, finding that the vertical centered baffle is optimal for its good performance and simplicity. The analysis is further extended to other designs of higher complexity, including multiple-baffle arrangements, cross-shaped baffles, and free surface baffles. Finally, motivated by the recent work of Peromingo et al. [“Sloshing reduction in microgravity: thermocapillary-based control and passive baffles,” Phys. Fluids 35, 102114 (2023)], we also demonstrate the effectiveness of passive baffles combined with active thermocapillary control. As a whole, the present results suggest a maximum achievable sloshing reduction of approximately 90%.

Present as well as future challenges of space exploration point to the need for improved thermal control systems. The “Effect of Marangoni Convection on Heat Transfer in Phase Change Materials” experiment, which is approved by ESA for execution on board the International Space Station, aims to contribute directly to current knowledge and basic understanding of heat and mass transport in phase change materials (PCMs) that incorporate a free surface in reduced gravity. The experiment will apply fixed temperatures to opposite ends of PCM samples held in cuboidal and cylindrical containers in order to drive controlled melting and solidification cycles that will be observed by means of optical cameras. The recorded images will be complemented by thermal measurements at key positions along the samples, which will allow different thermocapillary flow regimes to be distinguished according to their temporal dynamics. It is anticipated that thermal Marangoni (thermocapillary) convection will increase the heat transfer rate in these PCM devices by a significant factor (on the order of two or more) compared to melting governed by thermal diffusion (conduction). If the PCM designs prove robust, the experiment results can be expected to lead to substantial improvements in future designs for passive PCM applications in space missions.

This paper presents a numerical analysis of the melting dynamics of n-octadecane in microgravity. The phase change material (PCM) is held in a rectangular container of aspect ratio Γ=L/H=1.5, and the upper boundary is open to a layer of air, which both exchanges heat with the PCM and generates thermocapillary convection in the liquid phase via the Marangoni effect. This study extends the analysis conducted by Martinez et al. [“Effect of surface heat exchange on phase change materials melting with thermocapillary flow in microgravity,” Phys. Fluids 33, 083611 (2021)] in which the air temperature was assumed to vary linearly between the temperatures applied at the lateral walls. Two different scenarios are analyzed here. In the first case, the air temperature is assumed to be homogeneous and equal to the mean value of the temperatures applied at the lateral walls throughout the melting process. In the second case, the air temperature is similarly taken to be constant but with a value of 23 °C, which is representative of a laboratory environment, including many microgravity platforms. The investigation reveals the effect of key dimensionless parameters, including the Marangoni number (Ma), which quantifies the heat transport due to the thermocapillary flow, and the Biot number (Bi), which characterizes the heat exchanged across the PCM/air interface. In contrast to previous analyses of pattern selection, only oscillatory standing (pulsating) waves are observed under these boundary conditions. The results in each case are presented via stability maps in terms of Bi and Ma.

Present and future challenges of space exploration require better and improved strategies for fluid control and management. The "Thermocapillary-based control of a free surface in microgravity" (ThermoSlosh) experiment aims to contribute directly to current knowledge and basic understanding of fluid phenomena in reduced gravity, in particular, to study the effectiveness of thermal forcing for fluid control in weightlessness and applications. The experiment proposes to analyze the dynamics of a free surface in a cylindrical cell, half filled with 5 cSt silicone oil, subjected to controlled temperatures and accelerations. Simulations suggest that the thermocapillary effect can be used in microgravity to control the orientation of the free surface within the cell. The response of the free surface to the applied thermal gradient is characterized using the rise time, the stabilization time, and the overshoot; these representative quantities further help evaluating the effectiveness of the strategy. The use of supplemental vibrations is shown to improve the overall performance of the thermal control. Finally, among various potential applications, the ability to control sloshing motion during the real microgravity scenario of an ISS reboosting maneuver is assessed. ThermoSlosh was recently presented to the International Space Science and Scientific Payload competition, and is part of the selected proposals for the competition final.

We explore the preliminary design of a space habitat thermally controlled using phase change materials (PCMs). The PCM is used to maintain a suitable, habitable temperature inside the habitat by isolating it from the external solar radiation. The system is studied numerically considering only diffusive heat transport (conduction), a scenario with practical application to microgravity or reduced gravity environments. The system dynamics are explored for a wide range of governing parameters, including the length of the PCM cell L, the thermo-optical properties—absorptivity α and emissivity ε—at the external boundary of the habitat wall exposed to solar radiation, the eclipse (illumination) fraction τe (τi) of the solar cycle, and the PCM used. We find that the thermo-optical properties at the external radiated boundary, characterized by the absorptivity–emissivity ratio (α/ε), play a key role in the system response and largely define the optimal design of the habitat. This optimum balances the heat absorbed and released by the PCM during repeated illumination and eclipse cycles.

A ray-tracing algorithm is developed to obtain synthetic images for tracking the melting of the phase change material (PCM) noctadecane in a liquid bridge configuration. The translucent nature of the solid n-octadecane is modelled through a sigmoid function which provides a distance-dependent illumination intensity throughout the solid. The code is validated against two types of experiments: front shapes made of 3D-printed PLA and cast front shapes of solid n-octadecane. Comparison with these experiments allows the numerical constants involved in the model to be tuned to appropriate values. For application to the MarPCM microgravity experiment (Porter et al., 2023), the current study provides the expected images of the solid/liquid front obtained by the optical diagnosis module for three different values of the dimensionless volume (defined with respect to a perfect cylinder), V = 0:9, 1, and 1.1, and two applied temperature differences, DT = 5 and 30 K. The solid/liquid front shapes are obtained numerically from a two-dimensional axisymmetric code as in Varas et al. (2021). The results reveal a strong dependence of image quality on the liquid fraction L. Visual inspection leads to the conclusion that, below L  60%, an accurate determination of the front location may be difficult, particularly for the relatively shallow layer of liquid near the thermocapillary interface.& COPY; 2023 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).