Handling of particulate solids on the International Space Station

We present the principles of a particle-handling system for the International Space Station (ISS) with which experiments with astrophysical and planetological applications will be performed. The principle of dust deagglomeration and dispersion was successfully tested in short-duration microgravity experiments. The flight of the systems on the ISS is planned for 2012.     

Marangoni effects on the boiling of 2-propanol/water mixtures in a confined space

In this study, boiling experiments were conducted with 2-propanol/water mixtures in a confined gap under various gravity levels to examine the Marangoni effects on near-bubble microscale transport. Full boiling curves were obtained and two boiling regimes determined--nucleate and pseudo film boiling. The transition condition and the critical heat flux were also identified. Relative to pool boiling, the gap geometry caused lower CHF values, and deteriorated heat transfer at high superheated temperatures. This influence was particularly significant when greater Marangoni forces were present under reduced gravity conditions. The results of this study demonstrate the complex interaction that these three factors--Marangoni force, gravity level, and gap size--have on heat transfer.     

MICROGRAVITY NUCLEATION OF REFRACTORY MATERIALS: MODELING OF TRANSPORT PROCESSES IN THE NUCLEATION CHAMBER

A two-part mathematical model has been developed to describe the transport processes in a nucleation chamber designed for condensation of refractory vapors in a microgravity environment. The model solves the transient diffusion equations for temperature and concentration fields in cylindrical coordinates using finite differences and the alternating direction implicit method. Vapor supersaturation ratios are then computed from the evolving concentration profiles thus permitting one to estimate the conditions at the location in the chamber where nucleation is observed in experiments.     

Development of a small He II cryostat with optical windows for a microgravity experiment

In order to study film-boiling phenomena in saturated superfluid helium (He IIs) under a microgravity environment, a very compact visualization setup was designed and fabricated at High Energy Accelerator Research Organization (KEK). It consists of a cryostat, a vacuum pump, a high-speed video camera and electrical circuits for measurement. The cryostat in the setup is equipped with optical windows for the visualization of film boiling in He IIs. The setup was tested to verify its thermal and safety performance under a microgravity environment using a 10 m free-drop tower at the Hokkaido Center of the National Institute of Advanced Industrial Science and Technology (AIST). Successful system operation from 1.94 to 2.05 K under microgravity conditions below 1 × 10⁻³ g was confirmed. The design and test results are described in this technical note.     

微重力环境下燃烧科学研究的机遇与进展

微重力环境下，浮力和自然对流对燃烧过程的影响十分微弱，从而能在简化了的条件下研究各种燃烧现象，这对于燃烧机理的研究具有重要意义。通过深入认识微重力环境下的燃烧现象，并提出相应的防、灭火措施，也是载人航天系统的实际需要。本文综述了国外微重力燃烧科学研究的主要方向，探讨了微重力环境为燃烧科学的基础研究所带来的机遇与挑战。     

Alpha: From the International Style to the International Space Station

How has a Modernist predilection for the modular contributed to the design of Space Architecture? Leading space architects Constance Adams and Rod Jones highlight how by embracing a diligent modularity in the early 1980s, NASA initiated a design approach that has enabled replicability, flexibility and technological transparency, and has proved the International Space Station resilient in the face of multiple logistical, financial and political challenges.     

Validation of two-phase CFD models for propellant tank self-pressurization: Crossing fluid types,scales, and gravity levels

This paper examines our computational ability to capture the transport and phase change phenomena that govern cryogenic storage tank pressurization and underscores our strengths and weaknesses in this area in terms of three computational-experimental validation case studies. In the first study, 1g pressurization of a simulant low-boiling point fluid in a small scale transparent tank is considered in the context of the Zero-Boil-Off Tank (ZBOT) Experiment to showcase the relatively strong capability that we have developed in modelling the coupling between the convective transport and stratification in the bulk phases with the interfacial evaporative and condensing heat and mass transfer that ultimately control self-pressurization in the storage tank. Here, we show that computational predictions exhibit excellent temporal and spatial fidelity under the moderate Ra number – high Bo number convective-phase distribution regimes. In the second example, we focus on 1g pressurization and pressure control of the large-scale K-site liquid hydrogen tank experiment where we show that by crossing fluid types and physical scales, we enter into high Bo number – high Ra number flow regimes that challenge our ability to predict turbulent heat and mass transfer and their impact on the tank pressurization correctly, especially, in the vapor domain. In the final example, we examine pressurization results from the small scale simulant fluid Tank Pressure Control Experiment (TCPE) performed in microgravity to underscore the fact that in crossing into a low Ra number – low Bo number regime in microgravity, the temporal evolution of the phase front as affected by the time-dependent residual gravity and impulse accelerations becomes an important consideration. In this case detailed acceleration data are needed to predict the correct rate of tank self-pressurization.     

Bubble formation from a free-standing tube in microgravity

The bubble characteristics and the bubble detachment mechanisms during injection of air from a free-standing capillary tube submerged in water were studied in microgravity. The experiments were conducted in the 2.2-s drop tower at the NASA Glenn Research Center. A tube, 0.51 mm in diameter and 150 mm long, in a rectangular test section ( long) served as the injector. Images of the bubbles during the drops were acquired using a high-speed camera for various gas flow rates. Bubble detachment was observed for all the Weber numbers tested (0.28-31.12). This observation was different from previous studies using plate orifices in microgravity when bubble detachment was observed only for Weber numbers larger than 8. In order to resolve these differences, experiments were carried out using plate orifices. It was found that the bubbles detached from the orifice for all Weber numbers and that the bubbles formed were larger than those formed with the tube injector, particularly at low gas flow rates. The availability of a large area for the bubble to anchor itself and the presence of the chamber underneath the orifice could cause these differences. The effects of the chamber volume on the unsteadiness of bubble formation in plate-orifices at low gas flow rates are discussed.     

Extinction of premixed methane/air flames in microgravity by diluents: Effects of radiation and Lewis number

Laminar burning velocities and flammability limits of premixed methane/air flames in the presence of various diluents were investigated by combined use of experiments and numerical simulations. The experiments used a 1-m free-fall spherical combustion chamber to eliminate the effect of buoyancy, enabling accurate measurements of near-limit burning velocities and flammability limits. Burning velocities were measured for CH₄/air flames with varying concentrations of He, Ar, N₂ and CO₂ at NTP. The limiting concentration of each diluent was measured by systematically varying the composition and ignition energy and finding the limiting condition through successive experiment trials. The corresponding freely-propagating, planar 1-D flames were simulated using PREMIX. The transient spherically-expanding flames were simulated using the 1-D Spherical Flame & Reactor Module of COSILAB considering detailed radiation models. The results show that helium exhibits more complex limit behavior than the other diluents due to the large Lewis number of helium mixtures. The near-limit helium-diluted flames require much higher ignition energy than the other flames. In addition, for the spherically expanding helium-diluted flames studied here (Le > 1), stretch suppresses flame propagation and may cause flame extinction. For the CO₂-diluted flames, the flame speed predicted by the optically-thick model based on the Discrete Transfer Method (DTW) and a modified wide band model has better agreement with measurements in the near-limit region. A significant amount of heat is absorbed by the dilution gas CO₂, resulting in elevation of temperature of the ambient gases. The optically-thick model, however, still overpredicts flame speed, indicating a more sophisticated radiation property model may be needed. Finally, the chemical effect of CO₂ on flame suppression was quantified by a numerical analysis. The results show that the chemical effect of CO₂ is more important than the other diluents due to its active participation in the reaction CO₂ + H = CO + OH, which competes for H radicals with the chain-branching reactions and thus reduces flame speed.     

Predictions of a critical fuel thickness for flame extinction in a quiescent microgravity environment

A simplified analysis and data acquired in the 4.5 s drop tower in MGLAB, Japan in a quiescent oxygen/nitrogen environment are presented for the prediction of the flammability limit in a quiescent microgravity environment. In the experimental matrix the oxygen level and thickness of PMMA are treated as control parameters. Published data from quiescent microgravity experiments on thin ashless filter paper and thick PMMA are also compared with the prediction of the analysis. Based on scale analysis, it is hypothesized that all fuels—from PMMA to cellulose—behave as thermally thin fuels during steady spread of flames in a quiescent environment. An expression for the spread rate that includes radiative effects is proposed for the first time: η₀ ∼ 1/2 + 1/2 , where η₀ is the spread rate non-dimensionalized by its thermal limit and ₀ is the non-dimensional radiation number. For ₀ > 1/4, which in dimensional terms translates to a critical thickness criterion τ > (F²/4)(ρ_gc_g/ρ_sc_s)(λ_g/εσ)[(T_v − T_∞)/(T_v⁴ − T_∞⁴)], flame extinction occurs irrespective of all other environmental conditions. Based on this prediction, an extinction thickness can be calculated even at 100% oxygen level. The experimental data from the MGLAB agree reasonably well with this prediction. Flammability maps with fuel half-thickness and oxygen level as coordinates are developed for PMMA and cellulosic fuels, which are shown to be consistent with the current and published data.