Introduction

The behavior of liquids during transport and use is a subject of fundamental interest in numerous applications. Industrial and energy networks require natural gas transport in ships, for example, while the space industry must manage propellant in spacecraft, launchers, and rockets. One of the primary concerns in such applications is liquid sloshing, a type of motion that can appear in any system with a partially filled container as a result of transient or low-frequency excitations.

Here, we summarize recent microgravity results on sloshing mitigation in microgravity, including the use of fixed baffles (Peromingo et al., 2023a), thermocapillary flows (Peromingo et al., 2023b) and a novel type of moving spring-mass baffles (Gligor et al., 2024).


Method

We consider an open rectangular container, holding L x H = 30 x 15 mm² of liquid in weightless conditions. The container is subjected to a pulse-like perturbation that excites sloshing. A fixed baffle, or a movable one attached by a spring, is placed within the liquid bulk to reduce sloshing. The motion of the free surface is tracked using standard level-set moving mesh schemes.

Results are obtained for a liquid with properties similar to 5 cSt silicone oil and with a static contact angle of 70°. The dependence of thermo-physical properties on temperature is neglected except for surface tension, which is the driving mechanism of thermocapillary flow. The problem is simplified by considering only 2D dynamics since previous 2D models of interfacial dynamics showed excellent agreement with microgravity experiments on immiscible liquids (Salgado Sánchez et al., 2019).


Results

A summary of the obtained results is provided in the figure, which illustrates different baffle designs of increasing complexity (left panel) and their associated performance (right panel). Square markers denote the sloshing frequency and decay time relative to a no-baffle container and the associated performance envelope is shown (in selected cases) when combined with thermocapillary control (solid lines and circular markers) and when converted into spring-mass elements (shaded regions). Overall, the results indicate a potential reduction in decay time by up to 90%.


Conclusions

Several sloshing mitigation strategies are explored. The effectiveness of fixed baffles is evident, as is the trade-off between design simplicity and performance. Combining these with thermocapillary control or substituting for a spring-mass baffle system allows an additional 50-60% performance improvement, for a total sloshing reduction of approximately 90%.

Experiments conducted on board the International Space Station in the framework of the Particle Vibration project (T-PAOLA) confirmed the existence of a new class of solid-particle attractors, which had been previously predicted through numerical and theoretical analysis in 2014. These experiments and the underpinning theory were originally conceived for the case of cubic cavities with a uniformly heated and uniformly cooled wall subjected to vibrations perpendicular to the resulting temperature gradient. In this study, an attempt is made to extend such a framework by considering a new thermal configuration, where the corners of the cavity are differentially heated rather than entire walls. The ensuing novelty being the multiple directions in which local temperature gradients can be oriented (involving directions both parallel or perpendicular to the walls and “diagonal” directions).
In view of the preparation of a future space experiment, the problem is approached numerically, through the solution of the governing (Eulerian) equations for mass, momentum and energy of the fluid phase and the (Lagrangian) equations accounting for the motion of each separate solid (spherical) particle immersed in the fluid. The two sets of equations are properly coupled by allowing the liquid to exert an influence on the solid particles (one-way coupled approach).
Several influential parameters are considered. More specifically, the Prandtl number, particle Stoke’s number and particle-fluid density ratio (ξ) are switched between two values; Pr = 6.11 and 18.69 (of water and ethanol at ambient temperature, respectively), St= 4x10-⁶ and 5x10-⁶, and ξ= 1.65 and 2 whereas the vibrational Rayleigh number and angular frequency of vibrations are varied through the ranges 10⁴≤Raω≤10⁵ and 10³≤Ω≤10⁴ respectively.
It is shown that in the considered space of parameters, new phenomena show up, which differ from those observed in the space experiments due to the particle structure morphology and the presence of a new category of formations. These do not display an external particle-dense boundary such as that produced in the case for which a unidirectional temperature gradient is considered; rather they resemble those observed in an earlier numerical study pertaining to the same line of inquiry (Phys. Fluids, 2023, 35(10), 103316) where the otherwise uniformly heated and cooled opposing walls were perturbed with a central temperature spot of variable size. Notably, the emerging particle structures in the corner heated cavity generally consist of a central columnar structure with two surrounding sets of dense particle structures, each including four distinct conical surfaces

Introduction
Integral to all phases of NASA’s projected planetary expeditions is affordable and reliable cryogenic fluid storage for use in propellant or life support systems [1]. It is greatly advantageous to develop innovative vent-less pressure control designs based on cooling/mixing of the bulk tank fluid to allow storage of the cryogenic fluid with zero or reduced boil-off. The presence of noncondensable gases can interfere with condensation at the interface impacting tank pressure control during subcooled jet mixing, especially, in microgravity.
The Zero-Boil-Off Tank (ZBOT) Experiments are a series of small-scale experiments aboard the International Space Station (ISS) that use a transparent volatile simulant fluid in a transparent sealed tank to delineate various fundamental fluid flow, heat, and mass transport, and phase change phenomena associated with storage tank pressurization and pressure control in microgravity [2, 3]. The ZBOT-1 experiment was performed on the ISS in the 2017-2018 timeframe and collected data to validate a state-of-the-art CFD model for tank pressurization and pressure control for a pure system. The ZBOT-NC Experiment is the second experiment, in the series, to be performed on the ISS in 2025. Its goal is to investigate the effects of noncondensable gases on interfacial evaporation and condensation during self -pressurization and jet-mixing pressure control in microgravity for a two-component system.

Materials & Methods
In this work, we will describe the detailed features of the ZBOT-NC experimental hardware and Diagnostics that include a nonintrusive Quantum Dot Thermometry (QDT) technique for whole field temperature measurement. All microgravity pressurization and pressure control tests will be performed for both the pure and the two-component systems with Xenon and Neon as the two noncondensable gases spanning the small and the large molecular weights and sizes. The two-phase CFD model that is developed as part of the project will be also presented and discussed.

Results
Ground-based pressurization and jet mixing experiments and CFD simulation results are compared to each other to validate both the fidelity of the CFD model predictions and the accuracy of the QDT measurements (Fig 1). Model simulations for noncondensable gas effects will also be compared against large Cryogenic LH2-GHe experiments to indicate the noncondensable gas effects on tank pressure control during jet mixing in 1G (Fig 2). Finally, CFD results will be presented to predict the effects of the noncondensable gas during subcooled jet mixing in the ZBOT-NC tank in advance of the microgravity experiment in 2015 (Fig 3).

Introduction
Before filling a propellant tank on the ground or in Space, the transfer line between the donor and receiver tanks must be cooled down preferably by sacrificing a minimum amount of the cryogenic fluid. The cryogenic line chill-down process involves a transition between different flow boiling regimes, namely, film boiling, transition film boiling, and nucleate boiling which are complex and may be quite gravity-dependent [1]. Capturing these boiling phenomena and predicting the transition between them in a CFD framework is new and challenging both for 1g and microgravity applications.
Materials & Methods
The present work addresses this challenge by employing a two-phase Eulerian approach for a homogeneous mixture together with the Lee phase change model to capture the film boiling regimes of the chill-down process using ANSYS Fluent®. The nucleate boiling regime is predicted by incorporating an in-house developed sub-grid model that accounts for bubble nucleation, bubble growth, bubble departure diameter, and their shedding frequency. The sub-grid model is implemented into Fluent via a user-defined function for the wall-fluid heat flux calculations. The mathematical formulation and numerical implementation of the CFD model are described in detail. The coupled CFD-Subgrid model is validated against published experimental data for liquid nitrogen chill-down of a heated stainless-steel pipe in 1g [2].
Results
Numerical simulation results show good agreements between the CFD predictions of the wall temperature evolution, rewetting temperature, and transition between film and nucleate boiling, with the experimental measurements published by Darr et al [2] for several different LN2 flowrates in the vertical pipe orientation. The CFD predictions for the wall temperature distribution indicate rapid quenching of the wall at two upstream and downstream temperature sensor locations with excellent agreement with the experimental measurement as shown in Fig 1. The only tuning parameter in the CFD model is the Lee mass transfer coefficient. The CFD Model predicts the Liedenfrost rewetting temperature also in close agreement with the experiment. This marks a transition from the stable to the transitionary film boiling regimes. The CFD-predicted boiling curve for the downstream sensor location is also compared against its experimental counterpart in Fig. 2 and indicates that the model is able to predict all the key temperature and heat flux parameters during the transitions from stable film boiling to nucleate boiling regimes in close agreement with the experiment. A sequence of predicted volume fraction and temperature contours depicting these transitions is shown in Fig 3.

Introduction

Describing the evolution of aerosols is an important challenge in climatology. Several questions remain unanswered so far, particularly concerning cloud microphysics. Since 2018, CNES has supported the development of a new instrument that enables aerosol production under controlled conditions. This work presents the latest results of experiments conducted in reduced gravity conditions using parabolic flights. It focuses on the problem of experimental repeatability and on the droplet evaporation dynamics at different relative humidities.

Experiment

Aerosols under consideration are comparable to clouds and are composed of micrometric water droplets (with an initial diameter of 5 µm) evolving in air. They are investigated with optical microtomography far from the edges of the experimental cell. During the microgravity phase, droplets move very little, resulting in a high degree of coherence between successive tomographic images. A 3D reconstruction is therefore possible, enabling the droplets to be tracked over time [1].

The experiments were conducted with varying relative humidity (H) ranging from 65% and 100%. A temperature-controlled cold point was used to monitor the humidity. When the conditions were unsaturated, the droplets evaporated. The reproducibility of this process was studied during the parabolic flight campaigns, performed in March and October 2023. The figure displays the evaporation time (t_evap) as a function of H for the 119 experiments carried out. The data was acquired through direct visual inspection of the tomographic videos.

Result

The data follow the trend t_evap=k/(100-H(%)) where k is a fitting parameter (k=78 s) and seem therefore qualitatively in good agreement with diffusive evaporation theory [2]. The analysis of the tomographic videos using dedicated image processing software is still ongoing to minimize their variability.

Conclusion

These experiments demonstrate the potential for studying the kinetics of water droplet aerosols evaporation under microgravity conditions. Forthcoming work will focus on experimentally describing the nucleation processes involved in the formation of aerosols, not only for air alone but also for air/heptane mixtures.

This study investigates the thermal properties of a deployable Pulsating Heat Pipe (PHP) during the 83rd ESA Parabolic Flight campaign within the "DEPLOY!" student project funded by ESA. The PHP, a passive thermal control device utilizing self-sustained oscillatory fluid motion driven by phase change, is subjected to diverse gravity conditions during parabolic flights. Our focus is on characterizing the thermal behavior of the deployable PHP under static conditions at different folding angles and dynamic conditions during folding/unfolding maneuvers, all while gravity varies.

The device comprises an aluminum capillary tube (ID 1.6 mm) folded into an 11 U-turn serpentine of 800 mm length, featuring an adiabatic section designed as a torsional spring with 3.5 convolutions and a 65 mm coil diameter. The PHP is vacuum-sealed and filled with 40g of HFE-7000 fluid (FR = 70% vol.). In a flight-proof experimental setup, the PHP's mechanical configuration is altered using a remotely controlled stepper motor. Temperature and pressure within the PHP are monitored with 16 thermocouples, two pressure transducers, and an infrared camera.

Through three parabolic flight tests, the PHP's behavior is examined in various static unfolding configurations (0° - 45° - 90° - 135° - 180°) and folding/unfolding dynamics (0° to 180° and 180° to 0°). Different thermal conditions, including heating powers of 34W and 56W and condenser temperatures of 20°C and 25°C, are explored. Results indicate the device reaches a pseudo steady state within 15 to 25 minutes, maintaining stable operation in all tested configurations with occasional local stop-overs during microgravity.

Crucially, the device sustains functionality throughout unfolding configuration variations, exhibiting different parameter values. Gravity variations influence tube temperature and fluid pressure, yet neither hypergravity nor microgravity intervals disrupt the device's operational regime. These findings underscore the potential of the deployable PHP design for future utilization in space thermal control applications.

Human deep space exploration will rely on efficient and sustainable life support systems for the production of oxygen and other chemicals as well as the recycling of carbon dioxide. Photoelectrochemical (PEC) devices are investigated for the light-assisted production of hydrogen and carbon-based fuels from CO2 within the green energy transition on Earth. Similarly to natural photosynthesis, they only require water and solar energy for the process and release oxygen as a by-product. Their monolithic, compact design comprising integrated semiconductor-electrocatalyst systems for light absorption, charge separation and catalysis as well as their sole reliance on solar energy makes them attractive for applications in space, where they can directly convert solar into chemical energy without requiring additional accessories.

Here, we will highlight our recent experiments with PEC devices in microgravity environments realised for 9.3 s at the Bremen Drop Tower and links results regarding device efficiencies to gas bubble management and optoelectronic simulations. We will discuss obstacles such as the limiting solar irradiance on Mars as well as the reduced gravitation on the Martian and lunar surface for the application of PEC and other electrochemical devices in these environments and point to practical, sustainable solutions how to overcome them.