Reduced Gravity Conditions Research Articles

Coupled modeling and simulation of tank self-pressurization and thermal stratification

Numerical analysis of thermo-fluid dynamics for cryogenic propellant storage primarily consists of nodal and computational fluid dynamics (CFD) modeling approaches. While nodal approaches prioritize faster computation over fidelity, CFD approaches promise accuracy and fidelity at the expense of significant computational resources. This paper presents an efficient coupling methodology developed to facilitate the simulation of a long-term self-pressurization process of a cryogenic propellant tank as well as associated thermal stratification phenomenon under both normal and reduced gravity conditions. Key highlight of the methodology is the development of a coupling scheme based on a domain decomposition approach that separates the tank into two computational regions at the liquid-vapor interface. SINDA/FLUINT, the nodal code, is utilized to simulate the liquid region, while ANSYS Fluent, the CFD code, handles the vapor region. A data exchange algorithm was developed to compute required boundary conditions for each region using the local thermodynamic properties of assumed infinitely thin interface. The coupling between the nodal and CFD codes was demonstrated by simulating a self-pressurization process in a small size tank using hydrogen as the working fluid. A sensitivity study on the grid sizing and coupling time step was conducted to determine appropriate spatial and ensure a divergent-free explicit data exchange time step size. Using the coupling approach, two numerical case studies were performed to study tank self-pressurization by considering two initial hydrogen fill levels. The effectiveness and efficiency of the coupling methodology was assessed by comparing the temperature and pressure evolution results of the coupled simulation with those obtained solely from the CFD simulation. Results showed that the coupling scheme and data exchange algorithm were successfully implemented, and the coupled simulation agreed well with the CFD simulations. In addition, a stratification number was used to characterize the transient dynamic development of the thermal stratification. Lastly, the computational costs associated with both approaches were compared. Significant reductions in simulation time were achieved for both cases.

Influence of variable gravity on the phase change material for enclosed vertical channels with helical fins

This study numerically investigates the influence of variable gravity on the melting behaviour of a phase change material (PCM- paraffin wax RT27) contained within an enclosed vertical channel under different gravity conditions, specifically those found on the Earth, the Moon, and under the microgravity environments. Furthermore, the impacts of incorporating various number of helical fins within the PCM unit under different gravitational conditions are assessed for the first time. The findings indicate that the melting efficiency of PCM is considerably affected by the reduced gravity conditions, due to changes in the natural convection, velocity, and temperature; the melting time is twice as long under the moon-gravity conditions and sixfold longer under the microgravity conditions compared to the earth-gravity. It is also observed that reduced efficiency can be remedied by the insertion of fins into the PCM unit. The breakdown of the natural convection vortices with different number and orientation of fins expedites the melting process for all gravity conditions thus leading to improved performance in melting and energy storage.

Film flow boiling of a magnetic fluid over a vertical plate exposed to a magnetic field under reduced gravity conditions

In this article, the film flow boiling over a vertical flat plate was studied numerically. The saturated liquid upward flow over a vertical flat plate with a uniform free stream velocity was considered. The Volume of Fluid (VOF) model was used for capturing the liquid-vapor interface. The effects of the wall superheat, the free stream velocity of hydrodynamics, and the heat transfer of the flow film boiling were studied in the absence and presence of a magnetic induction (MI). The results showed increased free stream velocity enhances the heat transfer coefficient (HTC). In addition, applying uniform MI results in a normal force at the liquid-vapor interface. This normal force enhances the local heat transfer coefficient (HTC) during film flow boiling on the heated vertical plate. Also, the percentage of HTC enhancement is higher when the magnetic field strength increases. Moreover, the heat transfer enhancement diminishes as the free stream velocity increases. Furthermore, an increase in the magnetic permeability ratios of the phases contributes to improving the HTC and increases the wall shear stress. The effect of the variation of the acceleration of gravity is also studied, revealing that a decrease in gravity’s acceleration actually slightly enhances HTC.

Intravenous Fluid Resuscitation Capabilities in Simulated Reduced Gravity.

BACKGROUND: Critical care for exploration space missions may require intravenous (IV) fluid resuscitation therapy. Resource constraints may limit availability of standard, Earth-based infusion technologies. The effect of variable acceleration on infusion flow rates using simple fluid resuscitation supplies was investigated.METHODS: Infusions of water or blood analog (40% glycerol) from a 1 L IV bag were performed using pressure bag augmentation at 0, 150, or 300 mmHg. The solution bag rested on an adjustable mount, configured to different heights to simulate relevant gravitational accelerations (1 G, Martian G, lunar G, and 0 G). The bag emptied through an IV line with a 14- or 20-gauge angiocath into a 3-mmHg venous pressure reservoir. Flow rates were measured using an in-line flow probe. Three determinations were made for each test condition.RESULTS: Temporal flow rate data for all test conditions displayed one-phase exponential decay. At 300 mmHg pressurization, maximum infusion rates ranged from 92-222 mL ⋅ min-1 for water and from 21-49 mL ⋅ min-1 for blood analog. All reduced gravity conditions had significantly longer infusion times in comparison to 1 G for both test solutions.DISCUSSION: Reduced acceleration significantly altered flow rates and infusion times for fluid resuscitation. Fluid resuscitation protocols specify a desired volume to infuse for a target time (e.g., 20-30 mL ⋅ min-1 for a 75-kg adult). This data demonstrates that this protocol parameter can be achieved with infusion pressure bag augmentation alone and provides information for the refinement of fluid resuscitation protocols for exploration space missions.Pantalos GM, Heidel JS, Jain IM, Warner SE, Barefoot TL, Baker RO, Hailey M. Intravenous fluid resuscitation capabilities in simulated reduced gravity. Aerosp Med Hum Perform. 2023; 94(8):596-603.

Interfacial area concentration correlation for bubbly flows in small-diameter pipes under normal and reduced gravity conditions

The accurate modeling of interfacial area concentration (IAC) is essential to improve the predictive performance of two-fluid-model-based simulations for state-of-the-art two-phase thermal systems under normal and reduced gravity conditions, such as two-phase heat sinks and space fission power systems. Although numerous empirical and semi-theoretical IAC correlations have been proposed to predict the IAC variations for different two-phase systems, the IAC correlation for small-diameter (hydraulic diameter from 1mm to 10mm) flow channels under normal gravity conditions is still underdeveloped and no IAC correlation has been developed for reduced gravity conditions. The present study collected 456 experimental IAC data measured in bubbly flows in small-diameter pipes under normal and reduced gravity conditions from the open literature and evaluated the predictive performance of 8 representative IAC correlations. The evaluation results illustrated that none of the existing IAC correlations could maintain acceptable predictive ability under both normal and reduced gravity conditions. Therefore, a new IAC correlation applicable to normal and reduced gravity conditions for bubble flows in small-diameter pipes was proposed semi-theoretically by (1) considering the IAC variation caused by the bubble breakup due to turbulent impact and the bubble coalescence mainly from wake entrainment and (2) employing the concept of effective body acceleration to extend the applicability of newly-developed IAC correlation to reduced gravity conditions. The performance of newly-developed IAC correlation has been evaluated against the collected bubbly IAC data taken in small-diameter pipes and the obtained mean relative errors are 0.134, 0.158 and 0.145 for 260 normal gravity data, 196 reduced gravity data and all collected data, respectively.

Influence of Reduced-Gravity Treadmill Running on Sensor-Derived Biomechanics.

Reduced gravity treadmills have become increasingly prevalent in clinical settings. The purpose of this study was to assess the influence of manipulated levels of bodyweight during reduced gravity treadmill running on sensor-derived spatiotemporal, kinematic, and kinetic measures. Reduced gravity conditions would result in significantly altered biomechanical measures compared with 100% gravity conditions, with the most pronounced effects anticipated in the 20% condition. Cross-sectional clinic-based study. A total of 16 runners (8 male [M; age, 28.88 ± 5.69 years; body mass index [BMI], 25.08 ± 3.74 kg/m2], 8 female [F; age, 28.75 ± 5.23 years, BMI, 21.05 ± 3.46 kg/m2]) participated in this study. Participants wore commercially available sensors on their shoelaces and ran in a reduced gravity treadmill at a self-selected pace for 5 minutes each at 100%, 80%, 60%, 40%, and 20% bodyweight in a randomized order. The pace remained constant across all conditions, and rating of perceived exertion (RPE) was obtained following each condition. Step-by-step spatiotemporal, kinematic, and kinetic metrics were extracted to calculate mean outcome measures for each bodyweight condition. Repeated measures analyses of variance were conducted to assess the influence of the different bodyweight reduction levels on RPE and runners' biomechanics. Higher pressure creating lower bodyweight conditions resulted in significantly increased stride length and decreased cadence, contact time, impact g, and RPE, along with a shift toward forefoot strike types compared with higher body weight conditions (P < 0.01). All other outcomes were comparable across conditions. Reduced bodyweight running significantly altered spatiotemporal measures and reduced the vertical component of loading. Our findings offer objective information on expected biomechanical changes across pressure levels that clinicians should consider when incorporating reduced gravity treadmill running into rehabilitation plans.

Influence of Soret effect on the pre-solidification state in the neopentylglycol-(D)camphor system during microgravity experiments

We present results from a combined experimental and numerical approach to estimate the impact of thermodiffusion in the liquid phase of the binary system neopentylglycol-(D)camphor (NPG-DC) during the pre-solidification holding period of experiments on the International Space Station (ISS) in reduced gravity conditions. The Soret coefficient ST, diffusion coefficient D and thermodiffusion coefficient DT have been measured by means of the optical beam deflection technique using mixtures close to the eutectic composition of the alloy (0.547 wt frac. NPG) in the composition range of 0.522–0.581 wt frac. NPG and at different temperatures above their respective liquidus temperatures. Numerically, we implemented a one-dimensional time-dependent thermodiffusion equation to describe the composition evolution during the holding pre-solidification phase in the NPG-DC alloy in a given temperature gradient. The validated model was then applied to the more complex case of spatially changing thermal gradients, which are expected in the experimental facility used onboard the ISS. Numerical simulations estimating a maximum effect of thermodiffusion showed a small but clear effect. Using the previously determined transport coefficients and a holding phase of 10 hours, a change of the NPG concentration of less than 7% of the initial composition was obtained for the given set-up. Even if the real thermodiffusion effect is expected to be smaller, this lateral inhomogeneity in concentration has to be taken into consideration in the microgravity experiments.

Experimental investigation of liquid film thickness and heat transfer during condensation in microgravity

Considering the increasing complexity of future space missions and the growth of heat fluxes to be removed in next generation satellites, thermal control systems will be asked to provide high heat dissipation rates with reduced power input, size and weight. Two-phase systems represent a reliable technical solution to meet all these specifications but their accurate design requires validated tools and a deeper understanding of the effect of gravity on heat transfer. While several experiments and numerical simulations have been conducted to investigate the gravity effect on pool and flow boiling, studies on in-tube condensation under reduced gravity conditions are still limited in the literature. In the present work, the effect of gravity is experimentally investigated during condensation of HFE-7000 inside a 3.38 mm internal diameter channel with mass velocity ranging from 30 kg m−2 s−1 to 50 kg m−2 s−1. Condensation tests were carried out during the 70th ESA (European Space Agency) Parabolic Flight Campaign by simultaneously measuring the liquid film thickness and the heat transfer coefficient inside the channel in both normal gravity and microgravity conditions. The liquid film thickness is determined by coupling a shadowgraph technique with the measurements performed by a chromatic confocal sensor and an interferometer. The reduced gravity condition is responsible for a heat transfer penalization with respect to normal gravity, which is found to be more severe when the mass velocity decreases. The change in the gravity level affects the characteristics of the interfacial waves in terms of frequency, height and velocity. The prediction accuracy of several models for annular flow condensation is assessed against the experimental results taken in microgravity conditions.

Impact of Fuel Type on Toxic Emissions from a Non-premixed Boundary Layer Laminar Flame in Microgravity – A Numerical Study

Examples from non-premixed boundary combustion representative of fire in microgravity environment are presented. A low forced flow of 0.25 m/s is present, which is typical of air circulation speeds in a spacecraft. The numerical simulations have been conducted to evaluate the modes of heat transfer with an emphasis on a discussion about the effects of various fuels on chemical emissions such as soot, CO, CO2 and unburnt hydrocarbons in reduced-gravity conditions. Typically, evaporation temperature is smaller for heptane than dodecane, and thus a lengthening of the heptane flame seems more pronounced as compared to that of dodecane flame with an increase in the visible flame zone by a factor of 2.3 times. A rapid regression rate of heptane due to the enhanced heat feedback contributes to an additional energy by a factor of 80% in heat release rate as compared to dodecane. In all the cases, only about 15% of the heat generated by an exothermic chemical reaction is supplied to the pyrolysis surface, and a large portion of energy is convected with the forward gas flow. The CO molar fraction reaches to a maximum of about 11% for dodecane and 8% for other fuels as ethylene, propane, propylene and heptane. The maximum mean value of CO concentration depends mainly on fuel injection rate, and appears practically insensitive to oxidizer flow velocity. Dodecane and heptane with a longer carbon chain produces more CO and less unburnt hydrocarbons than ethylene, propane and propylene. The existence of large unburnt toxic gas fuels even for a smaller flame size because of the absence of natural convection has important considerations for spacecraft fire safety due to smoke/gas incapacitation.

Effect of sub-atmospheric pressure on the characteristics of concurrent/upward flame spread over a thin solid

The variation of ambient pressure is a potential tool for studying the driving parameters of fire dynamics and heat release in low-pressure environments such as high-altitude locations, aircraft, and spacecraft. The study of upward flame spread over a solid fuel has direct implications on material flammability and fire development, and low pressure environments have recently gained more attention for the possible comparison with the reduced gravity conditions encountered during space missions. In this work, we consider upward spreading flames over thin acrylic sheets in ambient pressures between 30 and 100 kPa. A forced flow velocity of 20 cm/s is added to the naturally-driven buoyant flow, creating a mixed flow field (natural and forced) that varies with pressure. Flame characteristics such as spread rate and standoff distance are measured from the video analysis of the experiments. It is observed that the former decreases with pressure while the latter increases. The larger flame stand-off distance at low pressures partially explains the decrease of the flame spread rate since the convective heat flux from the flame to the solid decreases. Additionally, volumetric concentrations of the combustion products are measured during the experiments. The results show lower O2 consumption and CO2 production rates at lower pressures. Based on these rates, we could calculate the heat release rate from upward spreading flames at low pressure, providing fundamental information for better understanding pressure-gravity correlations. According to the results, the volumetric heat release rate is proportional to pressure, which is consistent with previous studies on pressure modeling of fires. This suggests that chemical kinetics is not a constraint for the conditions tested in this study, which could help make future flammability tests comparable to low gravity ones.

Bioinspired interfacial design for gravity-independent fluid transport control

In nature, numerous creatures have evolved exquisite structures, surface wettabilities, and interfacial interactions to manipulate fluids for survival, such as water collection, water feeding, air capture, air exchange, etc. These unique functionalities have become a constant source of inspiration for human beings to explore versatile applications. Recently, the control of gas or liquid transport has attracted more and more attention owing to the great potentiality in various fields. Particularly, the drive that does not rely on gravity is of great significance for applications under micro- or reduced-gravity conditions. This review gives a brief description of the fluid transport control mechanisms in living organisms and the biomimetic interfacial design to create controllable fluid manipulation devices. The recent progress of gravity-independent liquid transport, underwater gas transport, and multiphase fluids transport are classified and summarized. The crucial influence of geometric structures and interfacial wettabilities on fluid dynamic behaviors and the driven mechanisms behind these behaviors, along with emerging applications, opportunities, and challenges, are presented.

Review of Cryogenic Pool Boiling Critical Heat Flux Databases, Assessment of Models and Correlations, and Development of New Universal Correlations

Despite worldwide interest in a number of applications involving cryogenic fluids that are crucial to future space exploration, there is presently a lack of a large, reliable cryogenic pool boiling critical heat flux (CHF) database that can be used for assessment of accuracy of available predictive tools - model and correlations – or development of new tools. This shortcoming is a primary motivation for the present study, prompting compilation of a new consolidated cryogenic pool boiling CHF database from world literature. The database is used to assess accuracy of previous models and correlations, which are segregated according to ability to predict key operating parameters, such as pressure, surface orientation, and subcooling. A new correlation is constructed which shows very good predictive accuracy, evidenced by a mean absolute error of 16.95%, based on Earth gravity data which comprise a large fraction of the consolidated database. Using a limited subset of datapoints for three cryogens and a reduced gravity range of 0–0.7466, the new correlation is further modified with a reduced gravity multiplier to tackle reduced gravity conditions. The modified correlation has a mean absolute error of 17.47%, slightly higher than for Earth gravity alone. Overall, the new correlations are proven far more accurate than all prior models and correlations and therefore constitute new powerful tools in the quest for constructing universal flow boiling models for design of cryogenic space systems. It is shown CHF is very sensitive to pressure, increasing with increasing pressure up to a maximum before decreasing appreciably toward critical pressure. CHF is also shown to be strongly influenced by surface orientation, being highest for horizontal surfaces and decreasing monotonically with increasing orientation angle, and increasing fairly linearly with increased subcooling.

Underestimation of self-tilt increases in reduced gravity conditions.

During large angles of self-tilt in the roll plane on Earth, measurements of the subjective visual vertical (SVV) in the dark show a bias towards the longitudinal body axis, reflecting a systematic underestimation of self-tilt. This study tested the hypothesis that self-tilt is underestimated in partial gravity conditions, and more so at lower gravity levels. The SVV was measured in parabolic flight at three partial gravity levels: 0.25, 0.50, and 0.75 g. Self-tilt was varied amongst 0, 15, 30, and 45 deg, using a tiltable seat. The participants indicated their SVV by setting a linear array of dots projected inside a head mounted display to the perceived vertical. The angles of participants' body and head roll tilt relative to the gravito-inertial vertical were measured by two separate inertial measurement units. Data on six participants were collected. Per G-level, a regression analysis was performed with SVV setting as dependent variable and head tilt as independent variable. The latter was used instead of chair tilt, because not all the participants' heads were aligned with their bodies. The estimated regression slopes significantly decreased with smaller G-levels, reflecting an increased bias of the SVV towards the longitudinal body axis. On average, the regression slopes were 0.95 (±0.38) at 0.75 g; 0.84 (±0.22) at 0.5 g; and 0.63 (±0.33) at 0.25 g. The results of this study show that reduced gravity conditions lead to increased underestimation of roll self-tilt.

Use of Pressure-Measuring Insoles to Characterize Gait Parameters in Simulated Reduced-Gravity Conditions.

Prior researchers have observed the effect of simulated reduced-gravity exercise. However, the extent to which lower-body positive-pressure treadmill (LBPPT) walking alters kinematic gait characteristics is not well understood. The purpose of the study was to investigate the effect of LBPPT walking on selected gait parameters in simulated reduced-gravity conditions. Twenty-nine college-aged volunteers participated in this cross-sectional study. Participants wore pressure-measuring insoles (Medilogic GmBH, Schönefeld, Germany) and completed three 3.5-min walking trials on the LBPPT (AlterG, Inc., Fremont, CA, USA) at 100% (normal gravity) as well as reduced-gravity conditions of 40% and 20% body weight (BW). The resulting insole data were analyzed to calculate center of pressure (COP) variables: COP path length and width and stance time. The results showed that 100% BW condition was significantly different from both the 40% and 20% BW conditions, p < 0.05. There were no significant differences observed between the 40% and 20% BW conditions for COP path length and width. Conversely, stance time significantly differed between the 40% and 20% BW conditions. The findings of this study may prove beneficial for clinicians as they develop rehabilitation strategies to effectively unload the individual’s body weight to perform safe exercises.

BRAIN IN SPACE. HOW DOES SPACE AFFECT THE HUMAN BRAIN?

Introduction Nowadays more and more people and international companies are interested in Human Spaceflights. Aim and method In this review, the negative effects from space radiation, microgravity and the factor of isolation on the central nervous system will be described in relation to space neuroscience and the relevant studies examined. Results Space radiation can damage neuronal connections with both acute and chronic effects, manifested as altered cognitive function, reduced motor function, and behavioral changes. Moreover, some astronauts report a condition known as Spaceflight Associated Neuro-ocular Syndrome (SANS). The brain scans performed upon those astronauts, who came back from space travel suggest that due to reduced gravity conditions the brain and the fluids in the human body shift upwards, which increases pressure in the skull and may result in opticnerve swelling that causes blurred vision. Another interesting part of space neuroscience is the research of structural neuroplasticity. A study conducted on cosmonauts revealed an increase in the neuronal tissue of sensorimotor structures responsible for movement coordination. In addition to the space radiation and microgravity, long-term confinement also affects the microstructure of the brain white matter, which was proven in the study that used DTI (Diffusion Tensor Imaging). Conclusions To conclude, to continue understanding the risks posed by spaceflight to astronauts’ health research in the field of space neuroscience is important. In addition, the acquired insight could be relevant for terrestrial vestibular patients, patients with neurodegenerative disorders, as well as the elderly population, coping with neurological deficits. Keywords: space medicine, brain, space neuroscience, SANS, space radiation.

Ni self-diffusion in glass forming Pd–Ni–S melts

The Ni self-diffusion in glass forming Pd40Ni40S20, Pd37Ni37S26 and Pd31Ni42S27 melts was probed by incoherent, quasielastic neutron scattering over a temperature range between 773 and 1023 K. The Ni self-diffusion coefficients are on a 10−10 m2 s−1–10−9 m2 s−1 scale and barely change with composition. Each composition exhibits an Arrhenius-type temperature dependence of the Ni self-diffusion coefficients, which results in activation energies ranging from E A = 348 ± 16 meV for Pd40Ni40S20 to E A = 387 ± 6 meV for Pd37Ni37S26. The structural relaxation shows a stretched exponential behavior even far above the liquidus temperatures. In addition, the viscosity of the Pd37Ni37S26 melt was measured under reduced gravity conditions. The diffusion calculated from the viscosity reveals a significant deviation from the measured Ni self-diffusion by a factor between 4 and 8. This may indicate a dynamic decoupling between the atoms within the Pd–Ni–S equilibrium melts.

A Microbial Monitoring System Demonstrated on the International Space Station Provides a Successful Platform for Detection of Targeted Microorganisms.

Closed environments such as the International Space Station (ISS) and spacecraft for other planned interplanetary destinations require sustainable environmental control systems for manned spaceflight and habitation. These systems require monitoring for microbial contaminants and potential pathogens that could foul equipment or affect the health of the crew. Technological advances may help to facilitate this environmental monitoring, but many of the current advances do not function as expected in reduced gravity conditions. The microbial monitoring system (RAZOR® EX) is a compact, semi-quantitative rugged PCR instrument that was successfully tested on the ISS using station potable water. After a series of technical demonstrations between ISS and ground laboratories, it was determined that the instruments functioned comparably and provided a sample to answer flow in approximately 1 hour without enrichment or sample manipulation. Post-flight, additional advancements were accomplished at Kennedy Space Center, Merritt Island, FL, USA, to expand the instrument’s detections of targeted microorganisms of concern such as water, food-borne, and surface microbes including Salmonella enterica serovar Typhimurium, Pseudomonas aeruginosa, Escherichia coli, and Aeromonas hydrophilia. Early detection of contaminants and bio-fouling microbes will increase crew safety and the ability to make appropriate operational decisions to minimize exposure to these contaminants.

Progress in two-phase flow modeling: Interfacial area transport

The interfacial area transport equation (IATE) employs a dynamic approach to predict interfacial area concentration (IAC) in two-phase flows. It eliminates the need for the static flow regime transition criteria and flow regime dependent IAC correlations used in closing the two-fluid model and therefore any potential artificial bifurcation or numerical oscillations stemming from these static correlations. The efforts to develop the IATE are reviewed in this work, focusing on studies within the past decade. The current state-of-the-art in IATE is a two-group interfacial area transport equation that is applicable to predict interfacial area concentration from bubbly to churn-turbulent flows. An extensive experimental database has been established in literature for various two-phase flow conditions. The database is used to develop constitutive models to close the IATE and to validate its performance. These include adiabatic and heated conditions, vertical and horizontal flow orientations, channels with flow restrictions, round, rectangular, annulus, and rod-bundle geometries, and normal-gravity and reduced-gravity conditions. IATE has been implemented into practical system analysis codes such as TRACE and computational fluid dynamics (CFD) codes such as CFX and Fluent to evaluate its performance. In addition, the recent efforts of extending the IATE beyond churn-turbulent flow to churn-annular transition and annular flows are also reviewed. These include the new instrumentation, experimental database, and additional constitutive models to predict annular flows.

Influence of a variable thermal diffusion coefficient on convection of a binary mixture in rectangular cavities

This paper presents the results of numerical simulation of nonlinear convection regimes of a NaCl aqueous solution in square and horizontally elongated rectangular cavities with hard and impermeable boundaries. The vertical boundaries of the cavities are thermally insulated, and the horizontal ones are maintained at different constant temperatures, which corresponds to heating from below. Calculations are carried out within the non-stationary approach using of Boussinesq approximation and taking into account the polynomial temperature dependence of the thermal diffusion coefficient. According to this approximation, at T*≈285.4 K, the thermal diffusion coefficient changes sign, and thus the direction of the concentration gradient also changes. The temperature gradients on the horizontal boundaries are chosen so that the thermal diffusion coefficient changes sign inside the cavity. Other transport coefficients are considered to be constant. Simulations were performed under Earth-gravity and reduced-gravity conditions. Based on the results of simulations, the local and integral characteristics of non-linear regimes were obtained, and the structure of the emerging flow and the distribution of NaCl concentration were determined. It is shown that the temperature dependence of the thermal diffusion coefficient only slightly affect the structure and intensity of the emerging flow but significantly reduces the degree of separation of mixture components. Under the influence of the Earth’s gravitational field, an oscillatory four-vortex flow with reconnection of vortices occurs in the square cavity, and a multi-vortex flow - in the rectangular cavity. The characteristics of the multi-vortex flow vary irregularly. At reduced gravity, a stationary flow is observed in both square and rectangular cavities. Under microgravity conditions, the concentration isolines are “frozen” into the stream function field, i.e., the similarity between the fields is pronounced.

A numerical study of backdraft phenomena under normal and reduced gravity

Backdrafts are violent events that occur when oxygen is suddenly introduced to an oxygen-depleted compartment fire and are primarily driven by the presence of gravity currents. However, with human travel potentially expected beyond the confines of earth gravity, it is not clear how the magnitude of the gravity constant can contribute to the intensity of these events. This issue can be relevant when dealing with fires on space stations or on future bases on the Moon and Mars. In this study, we carry out backdraft simulations in a compartment under different reduced gravity conditions using the NIST Fire Dynamics Simulator (FDS) code. A combustion compartment, initially containing under-ventilated heated methane, is opened to the surroundings using a vertical opening slot. Through the bottom of the opening flows a gravity current of oxygen. When the current reaches the far wall of the combustion compartment, the mixture is ignited, and a violent backdraft event occurs. Measures of heat release, fire development, and pressure rise suggest that the effects of backdraft are highly nonlinear based on the gravity constant. Even small values of the gravity constant (as low as 0.01g) can trigger relatively strong backdrafts.