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• Bubble terminal velocity increases with the gravity level, although bubbles are smaller at higher values of gravity. • The bouncing process has been modelled by a damped oscillator, in which the free surface acts as an elastic membrane. • The frequency of bouncing increases with gravity, in accordance with the theoretical model. • Viscosity is not the main responsible for damping in the bouncing process. Bubble detachment, rise, and bouncing upon impact with a free surface is studied experimentally in variable gravity conditions. Previous investigations focused on the effects of fluid properties such as viscosity or surface tension on the rise and bouncing dynamics. Gravity force is a crucial factor in the detachment, rise and bouncing processes. However, the effect of different gravity levels has never been studied experimentally. In this paper we analyze the role of gravity in the detachment, rise velocity and bouncing motion of millimetric bubbles colliding with a free surface. Single air bubbles in ethanol are detached from a nozzle by the buoyancy force. After reaching a terminal velocity, the rising bubble interacts with the free surface in a bouncing process prior to coalescence. The equivalent bubble diameter at detachment decreases as the gravity level increases, in agreement with the theoretical prediction. An expression for the terminal velocity as a function of gravity is proposed. The terminal velocity is found to increase with the gravity level, although bubbles are smaller at higher values of gravity. The bouncing process has been modelled by a damped oscillator, in which the free surface acts as an elastic membrane. An expression for the frequency of bouncing as a function of gravity has been obtained, showing a good agreement with the experimental results. The motion of the bubble during the bouncing process can be approximated by an underdamped oscillator even if viscosity is negligible. Therefore, viscosity is not the main responsible for damping, which is probably due to energy transfer from the bubble to the fluid in the form of vortex and surface waves generation. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics of two impinging bubble jets have been experimentally studied in microgravity conditions. The experimental setup, designed to be used in a drop tower, allows to change the gas and liquid flow rates for the jet generation, and the separation distance between the jets. A slug flow with millimetric air bubbles is created in a T-junction and injected through a nozzle in a water tank. The formation and dynamics of the generated bubble jets have been recorded by means of a high-speed camera. The effects of the momentum flux and the separation distance on the two-phase flow have been investigated. When jets do not interact, they show a conical shape with an opening angle which decreases from to as the momentum flux is increased from to . Bubble velocities in the direction of injection decrease from to in the most extreme case, as the bubble center position reaches the impingement region. Interacting bubble jets form a cross-like shape as the jets evolve in time. Bubble mean diameter decreases from cm to cm, as the momentum flux increases from to . Coalescence events, occurring mainly near the nozzles and in the impingement region, are responsible for the widening of the tail in the bubble size distribution. The global jet behavior and the individual bubble dynamics present several differences in microgravity compared to previous observations carried out on ground. [Copyright &y& Elsevier]

Abstract: The rise, bouncing and coalescence processes of millimetric gas bubbles impacting at a free surface were studied experimentally. Single air bubbles were released from a syringe into a methacrylate tank filled with ethanol. The position and shape of the bubbles were measured from the images recorded by means of a high-speed camera. Bubbles with equivalent diameters mm rise until they touch the free surface and coalesce with it immediately. In contrast, bubbles with equivalent diameters mm bounce repeatedly before the coalescence with the free surface occurs. We present results on the bubble terminal velocities, drag forces, and shape of the bubble during its steady rise before coalescence takes place. The oscillatory behavior of the bubble shape after the collision is described, and the position and velocities of bubbles during the bouncing process are analyzed. The motion of the bubble after the first bounce is found to be very similar to that of a damped oscillator. Bouncing time increases with bubble size, and so does the height of the bounce. [Copyright &y& Elsevier]

Abstract: The structure of two colliding water jets containing small gas bubbles is studied experimentally. The effects of the separation distance between jets, as well as the orientation angle, on the spatial distribution of bubbles have been considered. Results on the global structure of the final jet and bubble properties have been obtained using a high-speed video camera, and measurements of the positions of coalescence events are presented. Jets are introduced through inclined pipes (with a diameter of 0.7mm) into a large water tank to avoid wall effects. Inclination angle has been changed from 0° to 45° with respect to the horizontal, resulting in a 0° up to 90° impact angle between jets. Generation of bubbles is controlled by a T-junction device where a regular slug-flow is created prior to injection. Bubble sizes have been measured, and a mean diameter of around 1mm has been obtained using high values of the liquid flow rate. In the studied range of separation distances between the bubbly jets, a more homogeneous dispersion of bubbles is created as the distance between jets is decreased and the momentum flux of each jet is increased. Higher numbers of coalescences are observed when using smaller distance between jets, and the obtained measurements revealed that the number of bubble coalescence events is reduced significantly using high values of liquid flow rates. [Copyright &y& Elsevier]

• Acoustic actuation is a feasible approach for the thermal management of boil-off in cryogenic propellants or electronics cooling in space. • Acoustic actuation in microgravity can generate a heat flux larger than the heat flux in a gravity scenario without actuation. • A larger heat flux is generated in microgravity with acoustic actuation than without it. • Heat transfer enhancement increases with acoustic amplitude. A critical issue in future long-term space exploration missions is the storage and management of cryogenic propellants. Storage tanks are exposed to extreme temperatures and radiation, which can generate hot spots on the tank internal walls even with an efficient temperature insulation. Counteract the effects of the sources of heat on the propellant is a necessity as vapor bubbles may be created, which can become dangerous for some engine components apart from decreasing the amount of available propellant. We present an experimental study carried out in terrestrial gravity and in microgravity conditions on the effects of acoustic actuation in the heat transfer between a two-dimensional heating element and a liquid in a nucleate boiling regime. Two configurations of the heater orientation with respect to the direction of propagation of the acoustic wave and several acoustic frequencies were considered. Heater surface temperature and heat flux were measured in all the performed tests. Acoustic actuation in microgravity increased the heat flux from the heater to the liquid an 8.6% in comparison with the gravity scenario without actuation, and an 8.4% in comparison with a microgravity scenario without actuation. The heat transfer enhancement is larger at frequencies providing higher acoustic amplitudes, which are the frequencies close to the nominal frequency of the piezoelectric acoustic actuator. The influence of the material of the heater substrate on the acoustic field plays a role in the behavior of temperature and heat flux. The results of this work show that acoustic actuation is a feasible approach for the thermal management of boil-off in cryogenic propellants. [ABSTRACT FROM AUTHOR]

Mass gauging of fluids in space systems (such as propellant tanks in upper stages and satellites, or life support systems) is a challenging task due to the absence of buoyancy. The vast number of possible liquid–gas configurations makes some current mass gauging ground techniques unreliable. The Spectral Mass Gauging technique has been recently presented as a promising approach for mass gauging in microgravity conditions given its independence on the liquid configuration. In this technique, the volume of liquid in a tank is determined from the spectral analysis of the frequency response of the tank walls to an acoustic excitation. We present an experimental study of the technique in terrestrial gravity and microgravity. Experiments in weightlessness were carried out in 15 drops at the drop tower of the Center of Applied Space Technology and Microgravity (ZARM) in Germany. We provide a criterion for the spectral analysis based on some characteristics of the Fast Fourier Transform of the acquired data, namely the Root Mean Square and the prominence of the spectral peaks. This criterion is used to study the effects of the acoustic actuation characteristics and the location of the data acquisition devices on the application of the acoustic technique in the considered scenarios. Our analysis also shows an independence of the technique on the gravity level, which confirms the feasibility of its use in space. The influence of the tank size on the applicability of this technique is discussed too. • Experimental setup and procedure to test the Spectral Mass Gauging technique in a drop tower. • Definition of a criterion for mode identification in the spectral analysis of data. • Solenoid actuation is more appropriate than the tested PZT actuation for the SMG technique in the studied scenarios. • Analysis of the SMG technique at different gravity levels showed no significant differences. [ABSTRACT FROM AUTHOR]

A long-duration microgravity environment has numerous detrimental effects on the human physiology. The most obvious solution for this problem related to long duration space exploration missions is to remedy the lack of gravity. This could be done using short arm human centrifuges but they do not seem to be sufficiently effective, perhaps because of the short duration exposure of this countermeasure and/or the huge body gravity gradient. New views have to be investigated, such as to see if a (very) long-arm rotating system generating a continuous 1 g or partial gravity field might resolve this issue. Besides the expected benefits regarding astronauts' microgravity pathologies, additionally the spacecraft itself, its on-board (sub-)systems and procedures might benefit from a rotating configuration. In this paper we address very briefly the medical issues, but the work is mainly focused on the advantages regarding engineering, operations, life support, safety and budget of having a constantly rotating spacecraft first in Low Earth Orbit and later for long duration missions to Mars. A large rotating spacecraft is feasible and affordable to build, operate and maintain. It has advantages for governmental and commercial use but also in light of the expected increase in space tourism. It will also save crew time and billions of dollars now being spent to counteract the effects of microgravity. A continuous rotating spacecraft providing Artificial Gravity will: • Improve crew health and wellbeing. • Improve mission safety and success. • Reduce costs significantly. • Simplify numerous operations in flight. • Provides the use of 1 g verified systems. [ABSTRACT FROM AUTHOR]

Phase separation in multiphase flows in space systems is a challenging task due to the absence of buoyancy. Several phase separation approaches have been presented in the last years given the important role that systems containing multiphase flows will play in future space missions. We present a review of these techniques and a trade-off analysis for their application in life support systems and, in particular, in the MELiSSA (Micro Ecological Life Support System Alternative) photosynthesis bioreactor. The candidate approaches are evaluated, both quantitatively and qualitatively, according to the defined requirements and criteria. The outcome of the trade-off analysis shows passive static and acoustic techniques as the most recommended methods to carry out phase separation in the considered bioreactor. • Review of phase separation techniques for lifesupport systems in space. • Definition of the trade-off methodology (criteria, scores). • Trade-off analysis of phase separation techniques for a MELiSSA photobioreactor. • Discussion and selection of the most appropriate techniques. [ABSTRACT FROM AUTHOR]

Abstract: One of the biggest challenges of the exploration of the Moon is the survival of the crew and the lunar assets during the lunar night. The environmental conditions on the lunar surface and its cycle, with long periods of darkness, make any long mission in need of specific amounts of heat and electricity to be successful. We have analyzed two different systems to produce heat and electricity on the Moon's surface. The first system consists of Thermal Wadis, sources of thermal power that can be used to supply heat to protect the exploration systems from the extreme cold during periods of darkness. Previous results showed that Wadis can supply enough heat to keep lunar devices such as rovers above their minimum operating temperature (approximately 243K). The second system studied here is the Thermal Energy Storage (TES), which is able to run a heat engine during the lunar night to produce electricity. When the Sun is shining on the Moon's surface, the system can run the engine directly using the solar power and simultaneously heat a thermal mass. This thermal mass is used as a high temperature source to run the heat engine during the night. We present analytical and numerical calculations for the determination of an appropriate thermal mass for the TES system. [Copyright &y& Elsevier]

Fifty years after the first human step on the Moon, many challenges for its exploration have yet to be overcome. Among them, the survival of the crew and/or lunar assets during the lunar night is mandatory for long duration missions. The environmental conditions of the lunar surface and its day-night cycle, with long periods of darkness, make the provision of energy a critical challenge. Several approaches have recently been considered to store and provide energy in the surface of the Moon by means of ISRU (In-Situ Resource Utilisation). We present a trade-off analysis of the options identified for an ISRU-based system to store heat and generate electricity for lunar missions with both robotic and human activities. A critical review of the energy requirements for a mission scenario consisting of long duration stays on the lunar surface has been carried out. Technologies potentially suitable for system components have been identified. These technologies are related to solar energy collection, heat transport, heat storage, heat-to-electricity conversion, and heat rejection. The outcome of the trade-off analysis provides a selection of the most suitable technologies to use in an ISRU-based heat storage and electricity generation system. • Review of power requirements for a lunar habitat. • Definition of a trade-off methodology. • Trade-off analysis for each subsystem and for the full ISRU-based power system. • Selection of the most suitable set of components for the power system. [ABSTRACT FROM AUTHOR]

• Numerical investigation of bubble trapping by an acoustic wave at levitation positions in different scenarios. • Levitation position determined by bubble, liquid and acoustic wave characteristics; bubble initial position; gravity level. • Analytical criterion based on balance between average acoustic and buoyancy force to compute levitation position. • Bubble management by means of acoustics is of particular interest for space applications. Multiphase flows management is a major challenge in many space applications given the different gravity levels involved. While many of the numerical investigations of liquid-gas phenomena deal with the radial bubble behavior and thus the heat exchange, only a few studies have been conducted on the translational motion of bubbles. We present a numerical investigation of the translational motion of gas bubbles immersed in a liquid that is subject to an acoustic wave at different gravity levels. In the computation, the equations for radial oscillation and translational motion are solved simultaneously. The dynamics of bubbles at different gravity levels (from microgravity to hypergravity) are discussed. Bubbles can be trapped by the acoustic wave at levitation positions in different scenarios. The dependence of the levitation position on the initial bubble position at different pressure amplitudes has been computed, giving rise to the bubble being directed to different nodes of the acoustic wave. The bubble radius also determines if and where the bubble levitates. We propose an analytical criterion for the capture of bubbles in a levitation position in terms of a new dimensionless parameter. The criterion is based on the balance between the average acoustic force and the buoyancy force. With the proposed criterion, the position of bubble levitation can be calculated analytically for any scenario. [ABSTRACT FROM AUTHOR]

Human, tele-operated rovers, and surface infrastructures are now being actively considered for lunar polar exploration. Current approaches to energy provision consider, among others, hybrid direct energy/chemical technologies, such as solar photovoltaic arrays, batteries, and regenerative fuel cells. Due to the long period of darkness on the Moon and the challenges this poses to the aforementioned conventional energy generation and storage technologies, there is a need to assess the potential of In-Situ Resources Utilization (ISRU) methods to enable or supplement long duration missions. We present a computational model (MATLAB) of a Thermal Energy Storage (TES) system coupled to drive a heat engine (Thermoelectric Generator) to produce electricity. The TES medium designed is based off processed lunar regolith, an abundant material present on the surface of the Moon. The architecture has been optimized to provide a minimum electrical power of 36 W per unit after 66 h of polar night, but the modular nature of the model allows other ranges of parameter to be simulated. A trade-off between this ISRU-based concept and conventional approaches for energy production and storage was performed and ranked TES and thermoelectricity generation as the least appropriate option. This result is valuable in a period of enthusiasm towards ISRU. It shows that processes exploiting extraterrestrial materials instead of Earth supplies are not systematically attractive. Despite the non-favorable performances for the proposed concept, some perspectives for the TES system are given as well as potential model improvements such as the need to assess the use of a Stirling heat engine. • ISRU approaches are not systematically preferable to Earth supplied infrastructures. • Thermoelectricity generation from lunar thermal energy storage is not attractive. • A 200-kg TES/TEG system using lunar regolith could produce 36 W during a polar night. [ABSTRACT FROM AUTHOR]