Low Gravity Conditions Research Articles

Effect of electric field on bubble dynamics in channel flow boiling using lattice Boltzmann method

The application of an electric field in flow boiling has been proven to effectively enhance heat transfer and reduce pressure drop instability. This study aims to elucidate the mechanism of the impact of electric fields on flow boiling bubble dynamics through the pseudo-potential lattice Boltzmann method (LBM). The interplay between the externally imposed electric field incoming velocity in different gravity conditions examined. These factors can regulate flow boiling heat transfer in horizontal channel. The results demonstrate a competitive relationship between electric field and gravity and between incoming velocity and gravity. Therefore, under higher gravity condition, an electric field is less effective to enhance flow boiling heat transfer than in low gravity condition and vice versa. Additionally, there exists a synergistic relationship between incoming velocity and the electric field that mitigates their competition. Moreover, when considering multipoint nucleation processes, applying an electric field can attenuates bubble–bubble interactions and inhibit large bubble formation so as to accelerates bubble condensation in supercooled flows and enhance boiling heat transfer. This work provides comprehensive physical insights into the mechanism of electric field to enhance the heat transfer in flow boiling, which is instructive for the development of electrohydrodynamic technique in flow boiling enhancement.

Probing the melting dynamics in a phase change Rayleigh–Bénard system under low gravity conditions

The present work aims to investigate dynamic characteristics of Rayleigh–Bénard convection during the solid–liquid phase change process under the influence of variable low-gravity conditions. Low-gravity conditions are crucial to gain physical insights into the complex convection dynamics relevant in terrestrial and space environments that feature diverse gravitational fields. The melting dynamics of a paraffin-based phase change material with Prandtl number Pr ≈ 71 in a square rigid walled enclosure is analyzed by subjecting it to a fixed temperature difference resulting in a Stefan number Ste ≈ 0.33. The enthalpy porosity method is employed to simulate the solid-liquid phase change process in Rayleigh number range 105<Ra <107 to take wide range of gravitational acceleration (0.0g, 0.1g, 0.2g, 0.4g, 0.6g, 0.8g, and g) into account. The simulation results allow detailed insights on the buoyancy-driven convective flow phenomena in a phase-change Rayleigh–Bénard system. Various characteristic flow regimes, the criteria for the onset of convective instabilities, and scaling analysis are presented under varied gravitational acceleration. The findings reveal that the critical Rayleigh number (Racr) for the onset of convection is found to vary between 1351.5 and 1810.5 under the gravity range investigated in contrast to the Racr ≈ 1707.7 for classical Rayleigh-Bénard system under the standard gravity. Furthermore, the scale analysis based on the numerical results shows that the relationship between effective Nusselt number (Nue) and effective Rayleigh number (Rae) follows a power law behavior Nue = 0.27Rae0.25 under each gravity condition, similar to the classical Rayleigh–Bénard system.

Role of gravity magnitude on flowability and powder spreading in the powder bed fusion additive manufacturing process: Towards additive manufacturing in space

Understanding powder spreading under low gravity conditions is essential for optimizing final products using additive manufacturing in space. In this study, we investigated the role of gravity on flowability and spreading mechanisms through combined experimental and discrete element method (DEM) studies. Three powders with different theoretical densities were used to reenact low compressive conditions resembling those in a low-gravity environment. The influence of low compressive conditions on flowability and spreading behavior was examined using the Hall flowmeter, rotating drum, and spreading experiments. In the experimental result, the static flowability was primarily affected by the presence of elongated particles rather than the compressive conditions. The dynamic AoR of TD_4 powder increased compared to that of TD_8 powder, despite the presence of spherical particles with a smooth surface finish. A DEM simulation study was conducted using TD_8 powder to investigate the impact of different gravity levels on dynamic flowability. The DEM studies revealed that the dynamic flowability under rotation was decreased under low gravity owing to the promoted cohesive interactions. The powder spreading experiment was performed using the three powders with different theoretical densities. The in-situ observation with particle image velocimetry analysis revealed that kinetic energy dissipation in the spreading process was accelerated in the TD_8 powder pile, despite its high interparticle friction and cohesive force. The powder spreading simulation was conducted using TD_8 powder to clarify the effect of low gravity on the powder spreading process. In TD_8 powder under 1 G, particle supply was facilitated by a synergistic effect of free-falling and deposited particles. However, increased cohesive interactions under 0.5 G and 0.16 G restricted particle supply via free-falling, consequently reducing the powder bed density by about 2 % and 6.2 %, respectively. These findings prove that the cohesive force predominantly controls dynamic flowability and powder bed quality in the spreading process under low gravity conditions.

Water infiltration rate in fine glass beads under micro- and partial gravities

The applicability of the theory of water infiltration in porous media under low gravities is controversial. We evaluated the hypothesis that infiltration under low gravities can be simulated from horizontal infiltration experiments under 1G. Parabolic flights provided low gravity conditions, assuming in the space station (µG), on the moon (0.19 G), and Mars (0.38 G). We measured infiltration rates in uniform particle-size porous media. No significant differences existed between infiltration rates under low gravities and ground-based predictions, i.e. the 1G condition. The air was entrapped little in this study, which may have prevented the infiltration rate reduction reported in previous studies. We concluded that the current infiltration theory adequately simulated water infiltration in fine uniform porous media under low gravity conditions.

Investigating combined effects of varying gravity and heat flux direction on the melting dynamics of phase change material in space

The present work investigates the combined effect of varying gravity and heat flux direction with respect to gravity on the melting dynamics of Phase Change Material. Similar conditions are relevant to applications in space, at different space infrastructures, such as orbiting satellites, as well as various extraterrestrial surface assets, landers, and rovers. The numerical simulations are performed to study the melting dynamics of a paraffin-based phase change material with Prandtl number Pr ≈ 71 and Stefan number Ste ≈ 0.33 inside a differentially heated square enclosure. The mathematical model employs a control volume-based enthalpy porosity approach to simulate the melting process inside enclosure. The direction of the incoming heat flux relative to the gravity vector is defined in terms of an orientation angle, which is varying circularly with a step of 45°, whereas the gravity level is ranging from the terrestrial surface value g to 0.2g to analyze the melting process over a wide range of Rayleigh number 100 ≤ Ra ≤ 107. The study provides a detailed insight into the attributes of heat transfer, flow dynamics, and energy storage, along with a quantitative analysis of the transition between various melting regimes and temporal fluctuations in the performance parameters. The findings demonstrate that the mutual orientation between the directions of incoming heat flux and gravity, as well as the value of the latter, significantly affect features of the convective motion in the liquid phase, as well as the entire thermally driven heat transfer within the domain. In particular, for the oppositely directed gravity and heat flux, the melted fluid closely resembles Rayleigh-Bénard convection with the presence of multicellular flow structures, while at other orientation angles, except for a co-directed gravity and heat flux case, a circular convective motion of the melted fluid takes place. The results of numerical simulations reveal declining melting rates as the mutual orientation of gravity and heat flux changes from opposite to co-directed and vice versa. The low gravity conditions delay the onset of convection-driven melting, reducing the melting rate significantly.

Effects of vibration on granular chute flow under low-gravity conditions

The essence of most geological hazards and industrial processes is gravity-driven flows of dense particles. In this research, we use the discrete element method to simulate the particle chute flow, and discuss the effect of vibration acceleration on the granular flow under different gravity conditions. The results indicate that vibration has a greater impact on the particle flow state under low-gravity environments and low inclination angles. The granular temperature (T) and inertia number (I) increase with increasing vibration acceleration. Vibration enhances the flowability of particles by changing the stress state and reducing the volume fraction of the granular system. This work reveals the rheological state of vibration-triggered particle flow on an inclined surface under different gravity environments.

A micromechanical model for estimating the shear modulus and damping ratio of loose sands under low stresses. Application to a Mars regolith simulant

The dynamic properties of loose sands under low stresses have been poorly investigated because of the higher order of magnitude of stress levels in terrestrial geotechnical structures. However, low densities and low stresses prevail in the sandy surface deposits of some other rocky planets, making low stress conditions relevant for extra-terrestrial soil mechanics. This is the case of Mars, on the surface of which a seismometer has been placed during the InSight mission. In this context, a dynamic shear rheometer was used to measure the shear modulus and damping ratio of a Martian regolith simulant under very low stresses to improve the interpretation of the InSight dataset on surface materials. This paper also revisits the grain contact stiffness and the overall modulus of a random packing of identical spheres, based on the Hertz-Mindlin contact theory. A micromechanical model accounting for the effects of both grain roughness and slipping in the soil degradation curve is proposed. The results of the model show a good agreement with experimental data, capturing the non-linear transition from low to high-shear strains. The model hence provides a new framework for a better understanding of the behaviour of granular materials in low gravity (extra-terrestrial) conditions.

Large Carbonaceous Chondrite Parent Bodies Favored by Abundance–Volatility Modeling: A Possible Chemical Signature of Pebble Accretion

Primitive meteorite groups such as the Vigarano, Mighei, and Karoonda carbonaceous chondrites have enigmatic patterns of elemental abundances, with moderately volatile elements—those that transition from vapor to condensate between ∼400 and ∼900 K—defining plateaus of subequal abundances despite a wide range in volatility. In detail, each group defines a plateau with distinctive nonmonotonic “chemical fingerprints” that have been attributed to combinations of mixing, vaporization/condensation, and fluid-mediated metasomatism—but the extent to which these processes can reproduce the observed variability has not been quantified. Starting with primitive Ivuna chondrite, a two-stage, two-component equilibrium condensation–vaporization model—with gravity implemented as Jeans escape—can explain large-scale plateaus in these chondrite groups, as well as more complex, nonmonotonic small-scale variations. For all three chondritic meteorite groups, models favor earlier high-temperature fractionation under low-gravity conditions followed by a low-temperature fractionation event that took place on a protoplanet at least as large as Ceres. The second fractionation event may represent the fractionation of incoming materials to the planetesimal during protracted pebble accretion. Models with only thermally driven volatile loss, gravity, and mixing can explain more than 80% of the observed compositional variability in these meteorite groups. In our five-parameter model, using only five randomly selected elements yields uselessly large ranges of planet sizes and temperatures, ranges that converge with increasing numbers of elements. These results suggest that even simple models are prone to generating inaccurate conclusions when constrained by too few observations, a fault likely held by more complex models as well.

Optimization of thermochemical hydrolysis of potassium hydroxide-delignified Cocoa (Theobroma cacao L) pod husks under low combined severity factors (CSF) conditions

This study identified the optimal conditions for acid hydrolysis of cocoa pod husks (CPH) pretreated and then hydrolyzed under combined low gravity conditions. Pre-treatment was carried out using 1% potassium hydroxide (KOH), using a biomass to solvent ratio of 9.143% (w/v), for 4 h at 30 °C, then the mixture is autoclaved for 15 min at 128 °C. Lignocellulosic analyzes were carried out on raw and pretreated husk. In order to determine the structural changes operated on the pretreated pod, Fourier transform infrared (FTIR) analyzes were also carried out. These analyzes confirmed a 93.87% removal of lignin after delignification. Furthermore, scanning electron microscopy/energy dispersive X‐ray spectrometry (SEM/EDX) revealed several changes within the pretreated matrix; showing a dominant presence of the elements carbon, oxygen and potassium, associated with an intensification of the peaks of magnesium and potassium. Acid hydrolysis on the pretreated pod was optimized under low combined severities factors (CSF) conditions between 1.11 and 2.45, using response surface methodology. A three-variable Box-Behnken design (autoclaving time (X1), biomass to solvent ratio (X2), and sulfuric acid (H2SO4) concentration (X3)) was used. The quadratic model with an R2 of 0.98 proved to be the most suitable, giving the following optimal hydrolysis conditions: 26.95 min of autoclave, biomass to solvent ratio of 9.53% (w/v) and 4.92% (w/v) H2SO4.18.77 g/L of reducing sugars was obtained for a CSF of 2.06. In addition, a marked decrease in the content of phenolic compounds was observed, shifting from 19.45 g/L in the raw delignified sample to 3.11 g/L in the delignified sample after reduction the extractable compounds. Thus, it is proved in this study that optimization of the acid hydrolysis process of CPH, after delignification with KOH, can improve the acid hydrolysis of polysaccharides, under conditions of low combined severity factors.

Accounting for the instability of a liquid flow front through a porous medium under conditions of low gravity and in the presence of chemical interactions between the phases

Safe operation of orbital platforms relies heavily on stability of fluid flows in microgravity, which are strongly affected by capillary forces. The paper considers multiphase fluid seepage through a porous medium. Questions of instability of the interface between liquids are discussed. The effect of capillary instability is shown, which was revealed during the processing of experimental data on the imbibition of a porous structure in microgravity conditions during parabolic flights. A mathematical model is proposed to describe the process of multiple imbibition of a porous medium under conditions of variable gravity. The model takes into account hysteresis, various models of capillary pressure are used during imbibition and drainage. The paper also considers a method for modeling the instability by transforming the constitutive equations. The method allows modeling additional blurring of the imbibition front. The results of modeling based on this method for capillary imbibition of a porous medium under microgravity conditions, as well as for the case when chemical interactions occur between flowing fluids, are considered.

Glycome profiling and immunohistochemistry uncover changes in cell walls of Arabidopsis thaliana roots during spaceflight

A large and diverse library of glycan-directed monoclonal antibodies (mAbs) was used to determine if plant cell walls are modified by low-gravity conditions encountered during spaceflight. This method called glycome profiling (glycomics) revealed global differences in non-cellulosic cell wall epitopes in Arabidopsis thaliana root extracts recovered from RNA purification columns between seedlings grown on the International Space Station-based Vegetable Production System and paired ground (1-g) controls. Immunohistochemistry on 11-day-old seedling primary root sections showed that ten of twenty-two mAbs that exhibited spaceflight-induced increases in binding through glycomics, labeled space-grown roots more intensely than those from the ground. The ten mAbs recognized xyloglucan, xylan, and arabinogalactan epitopes. Notably, three xylem-enriched unsubstituted xylan backbone epitopes were more intensely labeled in space-grown roots than in ground-grown roots, suggesting that the spaceflight environment accelerated root secondary cell wall formation. This study highlights the feasibility of glycomics for high-throughput evaluation of cell wall glycans using only root high alkaline extracts from RNA purification columns, and subsequent validation of these results by immunohistochemistry. This approach will benefit plant space biological studies because it extends the analyses possible from the limited amounts of samples returned from spaceflight and help uncover microgravity-induced tissue-specific changes in plant cell walls.

Granular flow experiment using artificial gravity generator at International Space Station

Studying the gravity-dependent characteristics of regolith, fine-grained granular media covering extra-terrestrial bodies is essential for the reliable design and analysis of landers and rovers for space exploration. In this study, we propose an experimental approach to examine a granular flow under stable artificial gravity conditions for a long duration generated by a centrifuge at the International Space Station. We also perform a discrete element simulation of the granular flow in both artificial and natural gravity environments. The simulation results verify that the granular flows in artificial and natural gravity are consistent. Further, regression analysis of the experimental results reveals that the mass flow rate of granular flow quantitatively follows a well-known physics-based law with some deviations under low-gravity conditions, implying that the bulk density of the granular media decreases with gravity. This insight also indicates that the bulk density considered in simulation studies of space probes under low-gravity conditions needs to be tuned for their reliable design and analysis.

Effect of composition on nucleation and growth of equiaxed dendrites in the transparent alloy Neopentylglycol-(d)Camphor during solidification in microgravity conditions

We investigate the effect of composition of hypoeutectic Neopentylglycol-(D)Camphor (NPG-DC) alloy on nucleation and microstructure kinetics during alloy solidification in low gravity conditions without convection and buoyancy effects. Two alloys with compositions of 0.2 and 0.3 wt-frac. DC have been solidified in a small thermal gradient to obtain equiaxed dendritic microstructures with different crystallographic growth orientation. In-situ optical and thermal characterization in the transparent material provided evolution of equiaxed dendrite density, nucleation undercooling, dendrite tip velocity and kinetic law. Significant differences in the results for the two compositions were found, which are discussed in relation to temperature and concentration dependent solid–liquid interfacial properties. For NPG-0.2 wt-frac. DC equiaxed dendrites with 〈100〉 crystallographic growth orientation exist which tend to grow faster and fill the liquid volume quicker compared to the NPG-0.3 wt-frac. DC alloy. This behavior hinders nucleation of further dendrites and decreases their final number. In contrast, for NPG-0.3 wt-frac. DC equiaxed dendrites with 〈111〉 crystallographic growth orientation nucleate at higher undercooling and the kinetic law indicates smaller dendrite tip velocities correlated to a larger number of dendrites.

Electrowetting-assisted pool boiling heat transfer characteristics under low gravity conditions

In pool boiling under low gravity conditions, the vapor bubbles remain attached to the heating surface, which deteriorates the heat transfer rate. The present work demonstrates the integration of the electrowetting (EW) technique to resolve the bubble departure problem in pool boiling at low gravity conditions. A phase-field-based numerical method with a dynamic contact angle model has been implemented in COMSOL Multiphysics (V5.3) to investigate the problem. The bubble dynamics in the electrowetting integrated pool boiling for low gravity conditions are analyzed at various frequencies for different voltage waveforms, i.e., square, sinusoidal, and triangle. The EW technique increases the departure frequency of the bubble and the heat transfer rate compared to the conventional pool boiling. The sinusoidal voltage waveform shows the best performance among different voltage waveforms, producing 119.3% higher bubble departure frequency and 74.3% heat transfer enhancement than the conventional pool boiling.

Conceptual design of a dual drum-controlled space molten salt reactor (D2-SMSR): Neutron physics and thermal hydraulics

Space nuclear reactors are becoming popular in deep space exploration owing to their advantages of high-power density and stability. Following the fourth-generation nuclear reactor technology, a conceptual design of the dual drum-controlled space molten salt reactor (D2-SMSR) is proposed. The reactor concept uses molten salt as fuel and heat pipes for cooling. A new reactivity control strategy that combines control drums and safety drums was adopted. Critical physical characteristics such as neutron energy spectrum, neutron flux distribution, power distribution and burnup depth were calculated. Flow and heat transfer characteristics such as natural convection, velocity and temperature distribution of the D2-SMSR under low gravity conditions were analyzed. The reactivity control effect of the dual-drums strategy was evaluated. Results showed that the D2-SMSR with a fast spectrum could operate for 10 years at the full power of 40 kWth. The D2-SMSR has a high heat transfer coefficient between molten salt and heat pipe, which means that the core has a good heat-exchange performance. The new reactivity control strategy can achieve shutdown with one safety drum or three control drums, ensuring high-security standards. The present study can provide a theoretical reference for the design of space nuclear reactors.

Stochastic parametric excitation of convective heat transfer.

We study the parametric excitation of the free thermal convection in a horizontal layer and a rectangular cell by random vertical vibrations. The mathematical formulation we use allows one to explore the cases of heating from below and above and the low-gravity conditions. The excitation threshold of the second moments of the current velocity and the temperature perturbations are derived. The heat flux through the system quantified by the Nusselt number is reported to be related to the second moment of temperature perturbations; therefore, the threshold of the stochastic excitation of second moments gives the threshold for the excitation of the convective heat transfer. Comparison of the stochastic parametric excitation with the effect of high-frequency periodic modulation reveals dramatic dissimilarity between the two. This article is part of the theme issue 'New trends in pattern formation and nonlinear dynamics of extended systems'.

Is frying possible in space?

Dietary nutrition and uptake of earth-like foods are extremely important aspects for the health and performance of astronauts, especially during future planned long-term space missions. Despite the major progress in studying and designing systems for crop cultivation in microgravity conditions in the last years, there hasn’t been equal interest in food preparation processes and cooking. There are several reasons for this but it is chiefly because at present astronauts stay in space for a few months at most, so there is no serious nutritional or psychological need for earth-like food habits. This, however, will change drastically in long-term missions, e.g., to Moon and Mars. French fries are a very popular food commodity across many cultural backgrounds on earth and as such they may be appreciated by long-term space travelers of different origin. The process of frying in hot oil is associated with complex heat and mass transfer along with the growth and detachment of water vapor bubbles. These phenomena are strongly affected by buoyancy and gravitational acceleration making the study of frying at space conditions a challenging task. The present work examines potato frying in hot oil during the short duration low gravity conditions achieved in a Parabolic Flight Campaign organized by the European Space Agency. An innovative device has been constructed, allowing the simultaneous observation of bubbles dynamics above the potato surface and the thermal behavior inside the potato flesh. It is seen that even in the absence of buoyancy i.e., during parabolas, vapor bubbles still detach and depart from the surface of potato permitting hot oil to maintain contact with the potato surface and leading eventually to a fried product. Instantaneous overpressure inside potato pores due to vapor formation upon boiling of potato water is suggested as the mechanism generating the force for bubbles detachment and departure. Moreover, the amount of produced vapor is comparable among the examined values of gravitational acceleration, including the low gravity conditions during parabolas. All in all, the results of the present study provide primary experimental evidence that frying can occur in space.

3D DEM-FEM simulation of the flexible metal wheel–soil interaction in low gravity environments

To investigate the driving performance of the flexible metal wheel of the manned lunar rover in a low gravity environment, we established a 3D discrete element method (DEM)-finite element method (FEM) coupling simulation system. The wheel was modeled by FEM and the soil was modeled by DEM. Then, the direct shear tests were performed to verify the validity of the DEM contact parameters. Finally, the driving performance of the flexible metal wheels with different treads under low gravity conditions was studied, and the velocity field of soil particles under the wheels was observed. The results show that the sinkage of the flexible wheel is almost independent of gravity, while the torque and drawbar pull are significantly affected by gravity. Moreover, by comparing the driving performance of the wheel equipped with treads (WT wheel) and without tread (NT wheel), it is found that the tread have great effect on sinkage, torque, and drawbar pull to wheel weight ratio due to the differences in wheel-soil contact area. Also, through the observation of the particle velocity field under the wheel, it is found that the gravity and load also have significant effects on the movement of soil particles. This study provided a reliable method for studies on the interaction between the flexible wheel and soil in low gravity environments.

Experimental study on downward/opposed flame spread and extinction over electric wires in partial gravity environments

Downward/opposed flame spread over laboratory wire samples under varied gravity conditions were investigated in the range from 0 G to 1 G. Reduced gravity experiments are conducted by parabolic flights of an airplane. Limiting oxygen concentrations (LOCs) and flame spread rates (Vf) are obtained as a function of gravity level, with oxygen concentration, forced flow velocity, and wire characteristics such as insulation thickness and core material as experimental variables. The samples used in this study consist of low-density polyethylene (LDPE) insulation over metallic cores. Copper (Cu) and nickel-chrome (NiCr) were selected as core materials. It is found that the effect of gravity on the insulation flammability varies with the thermal conductivity of the wire core; the LOCs of the Cu sample are less affected by gravity, while those of the NiCr sample decrease with decreasing gravity level. On the other hand, Vf increase monotonically with increasing gravity level in the Cu sample, while Vf of the NiCr sample show a peak value under the low gravity conditions. It is suggested that these differences in the response of LOCs and Vf to the gravity level due to the difference in core materials are controlled by the fuel concentration in the reaction zone, which is a function of Vf. It is also found that the molten LDPE produced during the flame spread process shows unique behaviors depending on the gravity levels and wire characteristics. Some characteristics of the dynamic motion of the molten LDPE during the flame spread process, such as deformation and dripping, are also summarized in this paper. The experimental data obtained in this study provide useful information on the flammability of materials in a partial gravity environment and will serve as a database for fire safety design in future space exploration.

Analysis of the Solidification Process of Metal Alloys Under Microgravity Conditions

The solidification of metal alloys under low gravity conditions mainly deviates in four physical properties compared with samples solidified on earth. The coarseness of the structure and the solidification temperature increase. The latent heat and the solubility of alloying elements in solids are changed. Experimental results are presented and analyzed in order to find a unified theoretical understanding. The differences in the measured quantities between experiments performed in space and on earth were calculated and are presented in this article. Kinetic, thermodynamic, and thermophysical relations were used to analyze those values. The change in the melting point was found to be a result of an increase in the vibrational amplitude of the atoms in the solid. The increase in the latent heat results from an increase in the vibration of atoms in the liquid. The changes in the solubility are a result of an increase in heat of fusion of the alloying element. Furthermore, the increase in the coarseness of the structure is explained as an increase in the atom’s vibration. The analysis highlights that the atom’s vibration increases with decreasing gravity; especially, the amplitude will increase. The explanation for this is given by a decrease in the binding energy between the atoms with decreasing gravity. The decrease in binding energy for liquids is larger than for solids.