Microgravity Science and Technology最新文献

Proton Exchange Membrane Fuel Cell (PEMFC) technology has been receiving more attention recently and can play a more expanded role in space missions with low gravity or microgravity. The liquid water generation in the Gas Diffusion Layer (GDL) of a Proton Exchange Membrane Fuel Cell (PEMFC) increases the resistance to oxygen flow toward the catalyst layer. Water flooding inside the GDL can affect the PEMFC performance especially at higher current densities. Therefore, a good understanding of the effect of liquid water amount in the GDL is crucial to water management and, subsequently, to the performance of the fuel cell. The purpose of the present study is to investigate the effect of the microstructure characteristics of the GDL on the water flooding and liquid water distribution inside the GDL. A one-dimensional theoretical model has been developed. Results indicate that the porosity gradient has a significant effect on the liquid water saturation and the performance of the PEM fuel cell.

An integration of both passive and active techniques to enhance the heat exchange has emerged as a promising research area over the past few decades. Our present investigation focuses on the heat exchange due to thermal convection in a square cavity driven by a channel, utilizing ternary hybrid nanofluid. The governing equations were derived from the averaged formulations describing thermal vibrational convection, illustrated using the vorticity of the mean velocity and stream functions relevant to both the mean and fluctuating flows. The influence of vibration on the system is quantified using a dimensionless vibration factor, denoted as Gershuni number (Gs), which is proportional to the ratio of the mean vibrational buoyancy force to the product of momentum and thermal diffusivities. All computations were conducted with fixed values of the Prandtl number (Pr = 6.1) and Reynolds number (Re = 100). The influence of physical parameters, including the Grashof number ((10^3 le Gr le 10^6) ), Gershuni number ((10^3 le Gs le 10^6)), and volume fraction of nanomaterials ((0% le Phi le 4%)), particularly under two scenarios: microgravity ((Gr= 0)) and terrestrial conditions, on the streamlines for both the mean and fluctuating flows, isotherms, and mean Nusselt number are discussed graphically. Numerical results indicate that an increase of Grashof number boosts heat exchange by 250% under buoyancy effects. Elevating nanomaterial volume fractions enhances thermal conductivity, increasing heat exchange by 30%. However, heightened thermal vibration reduces heat exchange.

The study of cell membrane structures under microgravity is crucial for understanding the inherent physiological and adaptive mechanisms relevant to overcoming challenges in human space travel and gaining deeper insight into the membrane-protein interactions at reduced gravity. However, the membrane dynamics under microgravity conditions is not unraveled yet. Moreover, the complexity of cells poses significant challenges when investigating the effects of microgravity on individual components, including cell membranes. Giant Unilamellar Vesicles (GUVs) serve as valuable cell-mimicking models and act as artificial cells, providing insights into the biophysics of membrane architecture. Herein, we have elucidated the membrane dynamics of artificial cells under simulated microgravity conditions. GUVs were synthesized in the size range of 20 ± 2.1 μm and their morphological changes were examined under simulated microgravity conditions using a random positioning machine. We observed that the well-defined spherical GUVs were transfigured and deformed into elongated structures under microgravity conditions. The membrane fluidity of GUVs increased sevenfold under microgravity conditions compared to GUVs under normal gravity conditions at 48 h. It is also noted that there is a reduction in the membrane microviscosity. The study sheds light on the membrane mechanics under microgravity conditions and contributes valuable insights to the broader understanding of membrane responses to microgravity and its implications for space exploration and biomedical applications.

With NASA and other space agencies planning for longer-duration spaceflights, such as missions to Mars, and the rise in space tourism, it is crucial to comprehend the impact of the space environment on human health. However, there is a lack of information on how spaceflight impacts cerebrovascular health. The absence of gravitational force negatively affected various physiological functions in astronauts, especially posing risks to the cerebrovascular system. Exposure to microgravity leads to fluid changes that impact cardiac function, arterial pressure, and cerebrovascular structural changes that may be the cause of cognitive impairment. Numerous experiments have simulated microgravity to study the damage caused by prolonged spaceflight and reported similar findings. Understanding the effect of simulated microgravity on cerebrovascular structure and function has important implications for cerebrovascular health on Earth and in space. Simulated microgravity has been shown to induce endothelial dysfunction, altering nitric oxide (NO) synthesis pathways and increasing oxidative stress. Dysregulation of the Renin-Angiotensin system, NADPH oxidases, K⁺ Channels, and L-type Ca²⁺ Channels contributes to vascular dysfunction, while mitochondrial complexes expression and Ca²⁺ concentration exacerbate oxidative stress. This knowledge is essential for creating effective countermeasures to protect astronaut health during extended space missions. Therapeutic interventions targeting mitochondrial ROS and NADPH oxidases showed promise in mitigating these effects. This review article delves into the significant challenges posed by extended spaceflight, focusing on the cerebrovascular systems. It also provides a comprehensive understanding of molecular mechanisms associated with microgravity-induced cerebrovascular dysfunction and potential therapeutic interventions, paving the way for safer and more effective space travel.

Graphical Abstract

To study the effect of micro-structured surface with wedge-shaped channel on pool boiling heat transfer performance of FC-72, four kinds of mixed wettability surfaces with area ratio of the micro-pillar region to the smooth channel region of approximately 1:1 were fabricated in this study (the surfaces were denoted as the Multi tip surface, Multi star surface, Less tip surface and Less star surface). The experimental results indicated that the CHF increases with the increase of liquid subcooling. The structural surface parameters will affect the bubble dynamics behavior and thus affect CHF. The effect of capillary wick suction on the mixed wetting surface first increases and then decreases. The capillary wick suction plays a significant role in the increase of CHF, and the capillary wick force on the Less tip surface with the best heat transfer performance is the largest. The Zuber model is modified by combining three factors to propose a critical heat flux model suitable for mixed wetting surfaces. With the increase of heat flux, the bubble detachment frequency decreases, the bubble detachment diameter increases and the nucleation site density basically shows exponential growth. Bubbles in the micro-pillar array region will be driven to slip onto the smooth channel due to energy difference and the bubbles in smooth channels will also migrate in the direction of wider smooth channels under the action of Laplace force.

The influence of the forced convection rate on the chemical structure of a polymethyl methacrylate (PMMA) flame in an oxidizer flow under microgravity conditions was studied using numerical modeling. Gas flow around a solid sphere was simulated using the full Navier–Stokes equations for a multicomponent mixture. A multistep chemical kinetic mechanism was considered in the gas phase. The heat transfer and radiation in both the condensed and gas phases were considered in the modeling. On the PMMA surface, the pyrolysis reaction leading to the transformation of fuel from the condensed phase to the gas phase is specified. The forced convection speed varied in the range from 3 to 20 cm/s. Analysis of CO₂ concentration fields near the burning surface under microgravity conditions showed that the maximum CO₂ concentration is observed in the downstream zone. The width of the flame zone and its chemical structure depend on the intensity of forced convection. The width of the flame against the flow decreases, and the maximum CO concentration increases as the forced convection rate increases. Analysis of the rates of fuel consumption reactions showed that at a low convection speed (v_st=3 cm/s), the reaction with the H radical, which has the highest diffusion coefficient, plays a crucial role in MMA oxidation.

For surface tension driven flow, interfacial heat transfer can alter the flow regime and its transition condition. This paper investigates the influence of interfacial heat transfer on critical transition and hydrothermal wave propagation of nanofluid thermocapillary convection for the first time, and three environment temperature conditions is considered, e.g. the cold-end temperature, the average temperature of the hot and cold-end, and a linear temperature distribution. The results indicate that, as nanoparticles volume fraction increases the critical Marangoni number decreases under various ambient temperature conditions, meanwhile, the fundamental frequency of the velocity oscillations exhibits a linear decrease, and the propagation angle and temperature fluctuation range of hydrothermal waves are decreased. Furthermore, for the three ambient temperature scenarios, the linear temperature distribution condition can amplify the propagation angle and temperature fluctuation range of hydrothermal waves. Consequently, the manipulation of both the nanoparticle volume fraction and ambient temperature condition provides a means to control the instability of nanofluid thermocapillary convection.

In this paper, the Electro-elastic instability(EEI) of an Oldroyd-B fluids flow the microchannel with the obstacles and heterogenous surface charged is studied. The changes in fluid flow are presented by considering three different ranges of Weissenberg numbers(Wi), the expansion lengths (textrm{EL}), and the asymmetric potential distributions. Under the combined effects of heterogeneous surface potential and elastic stresses, not only the vortices but also lip vortices are generated near the obstacles. At lower Weissenberg numbers, the stable and symmetric flow field is observed. As Wi increases, it is worth noting that the flow field becomes unstable and chaotic due to the enhanced electro-elastic instability. But the asymmetry of the velocity diminishes as (Wi>10). In addition, the presence of different vortex dynamics is observed as the Wi varies, such as the lip vortices, angular vortices, and oscillating lip vortices. Further, the flow of fluid at different expansion ratios is investigated. With the decrease of expansion lengths (textrm{EL}), the backflow and asymmetry are reduced, the lip vortex disappears and then the angular vortex appears. Finally, by increasing the upper zeta potential ((zeta _{textrm{w}})) of the obstacles, the mixing efficiency is improved. The research results may be helpful to the electrodynamic transport of viscoelastic fluids in porous media and the analysis of micromixers for industrial applications.

Precise formation control is increasingly demanded in high-resolution remote sensing formations, gravitational detection interferometers and distributed space telescopes. One composite formation flying method using disturbance-free payloads was previously proposed to enhance formation accuracy and payload stability. This method divided satellite formation into coarse formation using conventional satellite buses and fine formation using precise payloads. To verify the effectiveness of the proposed formation method and the payload stability performance, this paper develops an experimental system using two air-floating satellite prototypes. First, the experimental design is proposed and the experimental system model is established. Second, the experimental prototype development and system architecture are described in detail. Finally, the composite formation flying effectiveness is further demonstrated by coarse and fine formation control experiments. The experiment results indicate that the composite formation flying method effectively improves the formation accuracy for distributed payloads and isolates microvibrations from satellite buses to enhance payload stability.

Two-phase pumped cooling systems are applied when it is required to maintain a very stable temperature for heat dissipation in a system. A novel additively manufactured evaporator for two-phase thermal control was developed at NASA Jet Propulsion Laboratory (JPL). The Two-Phase Mechanically Pumped Loop (2PMPL) allows to manage the heat transfer with much wider breadth of control authority compared to capillary-based systems, while alleviating the system's sensitivity to pressure drops. The focus of this work is the understanding and capturing the micro-scale evaporation occurring in the porous structure of the evaporator. The Boiling and Phase Change Heat Transfer Laboratory at the University of California, Los Angeles (UCLA) developed an all-encompassing numerical simulation tool to predict the operational thermal behavior of the evaporator considering the effect of the liquid-vapor interface at the wick-to-vapor boundary. The numerical model incorporated the behaviour of the liquid-vapor meniscus at particle level located along the evaporative boundary between the wick structure and the vapor chamber. The numerical model allowed to study the effect of different parameters, such as boundary conditions, geometry, wick and fluid properties. An experimental setup was built at UCLA in order to characterize the heat transfer within an additively manufactured porous sample fabricated at JPL and in particular its evaporative heat load under certain heat inputs. The experimental efforts served as validation for the numerical results and aided in the characterization of the transient phenomena, such as dry-out.