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VI Summary of Recommended Research on Fundamental Phenomena BASIS FOR RECOMMENDATIONS The discussions in the preceding chapters are intended to expose the wide range of questions that exist about the effects of reduced gravity on the roles of the various phenomena operating in HEDS technologies. In deciding which of those phenomena should be recommended subjects of NASA research, the committee considered such factors as the potential of a given phenomenon to affect a wide range of HEDS technologies, the potential importance of the affected technologies, and the potential magnitude of reduced gravity's effect. The research topics were then integrated to formulate specific recommendations. In this chapter, the recommendations for specific fundamental research are listed, and in Chapter VII, more general programmatic recommendations for managing the research are made. Topics are listed by number in the perceived order of importance within each discipline, and a recommendation's overall priority across all disciplines is indicated by the notation "higher," "medium," or "lower." These priorities are assigned with the understanding that setting priorities for research is an uncertain process that risks prejudging the results of the recommended research. The recommendations are based on the supposition that systems and processes important for HEDS must be of assured, predictable, and reliable performance. Before such assurances can be given, a much better understanding of the effects of gravity must be gained through research. Especially, it is clear that systems depending on multiphase phenomena will in turn depend strongly on gravity level, in ways that are not now well understood. It will also be essential for NASA to develop a description, across a wide range of physical phenomena and the corresponding dimensionless param- eters, of the manner in which changes in gravity level produce fundamental changes in system performance. It also seems clear that it will not be possible to arrive at confident design conclusions through a costly process of repeated tests in space of the many important systems and components. Rather, the approach should be to develop physically based computational models that are then fully assessed against relevant microgravity data. These models could be used to optimize design and to direct and support the scale-up of those space experiments used to evaluate subsystems and systems that cannot be tested at full scale in space. Such models would also provide the necessary basis for establishing desired reliability and safety levels for HEDS systems. The foregoing approach obviously depends on a well-planned analytical and experimental research program, carried out on Earth and in space, to quantify the effects of gravity level. Efficient, multivariate experimental test matrices will need to be developed to sample the unknown behavior, so as to model it as efficiently as possible and 179

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180 MICROGRAVITY RESEARCH at reasonable cost. The committee urges that NASA support for, and direction of, such research be continued and intensified, as it is essential to the success of the HEDS enterprise. RECOMMENDED RESEARCH Surface or Interfacial Phenomena In microgravity, surface-tension-related phenomena may dominate fluid behavior. In particular, the handling and storage of cryogenic fluids may entail the use of vanes, wicks, and screens that rely on capillarity to control the location of the fluid. Heat pipes and capillary pumped loops also depend on surface tension effects. Condensation, evaporation, boiling, and sublimation are influenced by both gravity and interracial forces. Marangoni effects lead to convection, which plays an important role in two-phase heat transfer, provides stirring of welding pools, and can be used to promote phase separation. The committee believes that research in the following areas should advance understanding of surface phenomena. Ongoing work on the physical basis of wetting, including the hysteresis effect, the dynamics of wetting. ~7, and the correct description of wetting below the scale of the correlation length of the wetting fluid, should be extended. Empirical and fundamental knowledge of the material combinations and conditions for good wetting and wetting agents is also needed. [Priority Higher] 2. Capillary-driven flows and transport regimes that occur in evaporation and condensation heat transfer need further work, as does the determination of how the flow regime boundaries scale with gravity. Extension of the current work on Marangoni convection is urgently needed, including the complications introduced by the geo- metrical configurations in the various multiphase flows. Dynamical work on the oscillations of liquid drops or bubbles and on the resonances between the applied forces or accelerations (e.g., "jitter) and capillary modes of motion of a mass of liquid in a container, including the so-called sloshing problems and unstable modes, needs to be extended. [Priority Higher] 3. Experimental determination of the parameters that enter into the Marangoni and other relevant dimension- less numbers needs to be greatly extended, including the investigation of tensioactive agents that influence the magnitude and sign of the effect. Thermal and concentration gradients need to be taken into account in assessing the merits of designs where Marangoni flow is possible. [Priority Medium] 4. Classical work on static equilibrium capillary shapes, which minimize the area of the surface of a mass of liquid (in the absence of gravity) subject to boundary conditions of a fixed volume and given contact angles at the perimeter of the surface, needs to be extended. Multiplicity and stability of solutions should be investigated. [Priority Lower] Multiphase Flow and Heat Transfer NASA engineers have often avoided the use of multiphase systems and processes in spacecraft and satellites because they lack a basic understanding of multiphase flow and heat transfer phenomena in reduced-gravity environments. This has prevented the deployment of efficient and high-power-density active systems that other- wise might have been used for NASA's missions. Nevertheless, it appears that the reliable operation of numerous phase-change systems for propulsion, power production, and life support will be required during HEDS long- duration missions. Thus, multiphase flow and heat transfer technology is considered to be a critical technology for HEDS. Indeed, it is expected to be mission-enabling technology that, if properly developed, could lead to revolutionary changes in spacecraft hardware. The proposed research is intended to give NASA the ability to better understand the performance of multiphase systems in space and to provide the basis for the development of accurate computational capabilities. It is recognized that there will be a continuing need for experimental microgravity data and appropriate empirical correlations, since some physical phenomena and HEDS design

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SUMMARY OF RECOMMENDED RESEARCH ON FUNDAMENTAL PHENOMENA 181 issues go beyond our current, and anticipated near-term, computational capabilities. Nevertheless, the primary objective of the proposed research is the development of a reliable, physically based, multidimensional two-fluid model for the computational fluid dynamics (CFD) analysis of multiphase flow and heat transfer phenomena of importance to the HEDS program. To this end, experimental and analytical programs are required that will do the following: Determine the key experimental variables and parameters on which CFD models in microgravity will depend, and conduct appropriate experiments to sample flow behavior efficiently and sufficiently well to identify all important multiphase phenomena that occur in microgravity. [Priority Higher] 2. Perform the experimental and analytical investigations needed to identify the mechanisms governing the effect of gravity on multiphase phenomena, and develop flow-regime-specific models for the various interracial and wall transfers (i.e., mass, momentum, and energy). These models should be formulated so that they can be used in a multidimensional two-fluid model, and they should include surface-tension-induced forces, the various axial and lateral forces on the flowing phases, and flow-regime-specific interracial constitutive laws. This knowl- edge should allow the development of physically based models for the prediction of both adiabatic and diabetic flow regimes and flow regime transition in reduced and microgravity environments. The resultant two-fluid model should be suitable for use in multidimensional CFD solvers, and these computational models should be assessed against phase distribution and separation data taken at various gravity levels. [Priority Higher] 3. Develop flow-regime-specific multidimensional models for multiphase turbulence. These models should also be suitable for use in multidimensional CFD solvers. [Priority Higher] 4. Assess the effect of gravity on forced convective boiling and two-phased forced convective heat transfer and pressure drop. In particular, detailed measurements (i.e., data that can support the development of a multidi- mensional CFD model) are needed for the entire forced convection boiling curve, including the ebullition cycle (i.e., nucleate boiling), critical heat flux (CHF), and transition/film boiling. Active and passive heat transfer enhancement schemes should also be studied. [Priority Higher] 5. Assess the effect of gravity on convective condensation heat transfer. In particular, detailed data that support the development of a multidimensional CFD model, or other models, should be taken in apparatus undergoing direct contact condensation and film-wise condensation. [Priority Medium] 6. Study active and passive single-phase and two-phase heat transfer enhancement and pressure drop reduc- tion schemes in order to increase efficiency and decrease the mass and volume of the equipment. [Priority- Medium] 7. Study the effect of gravity on the flow regimes, thermal limits, and stability of adiabatic two-phase and boiling flows in porous media. Pore-level fluid mechanics needs to be better understood if we are to predict the development of complex flow structures and regimes during boiling heat transfer and CHF in porous media. [Priority Medium] Multiphase System Dynamics Multiphase systems may also exhibit global closed-loop power/propulsion system instabilities, and these instabilities can cause serious operational problems and/or severe damage. The effect of gravity on these instabili- ties is currently not well understood, and the following research is needed if reliable phase change systems are to be developed and used for HEDS missions: Detailed stability data on boiling and condensing systems is required for Earth gravity, gO (baseline), and for fractional, variable, and microgravity environments. These experiments should focus on static and dynamic instabilities in phase-change systems, and the data should be used to assess the predictive capabilities of various analytical models (including two-fluid CFD models) for the linear stability thresholds and the various nonlinear instability phenomena (e.g., limit cycles, chaos) that may occur in multiphase systems. [Priority Higher]

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182 MICROGRAVITY RESEARCH Fire Phenomena Assuring an acceptable level of fire safety in spacecraft and extraterrestrial environments is a critical aspect of the HEDS enterprise. To develop the knowledge needed for design of appropriate fire-detection and fire-suppres- sion systems and for determining suitable fire-safety procedures, further research on combustion in reduced gravity is recommended: The great majority of combustible materials available to fuel an unwanted fire during a HEDS mission are solids. High priority is given to experimental, theoretical, and computational studies of flame spread over surfaces of solid materials in microgravity and fractional gravity. These studies should focus on generic materials, both cellulose and synthetic polymers, and should include ignition requirements, flame-spread rates, flame structure and production of gaseous fuel from solid-fuel pyrolysis. [Priority Higher] 2. Gravity effects in smoldering is another high-priority topic due to its relationship to electrical cable fires, for example. Specific studies for this topic should concern the initiation and termination of smoldering, propaga- tion rates of smoldering fronts, and the production of hazardous or flammable products from smoldering, including conditions for transition from smoldering to flaming combustion. [Priority Higher] 3. Systems for fire suppression operate best by delivering the suppressant to the base of the flame, where the gaseous fuels are generated. The mechanisms of suppressant flow and transport in fire situations at reduced gravity are different from those at normal gravity. There is therefore a need to improve understanding of suppressant transport at reduced gravity for gaseous, liquid, and solid suppressants. Because the carbon dioxide employed in the International Space Station is not likely to be the optimum suppressant for planetary operations, there is also a need for studying possible new suppressants, such as replacements for Halon, for HEDS applica- tions. The need for an understanding of fire suppression is secondary to that for flame spread and smoldering, identified above, in the sense that the combustion process needs to be understood before addressing approaches to suppression. [Priority Medium] 4. Diffusion-flame structure of gaseous, liquid, and especially solid fuels as affected by gravity levels, especially in relationship to the production of soot and toxic products in diffusion flames and the establishment of conditions for and mechanisms of diffusion flame extinction, are areas of research with quite high priority. Knowledge of the hazardous products of fires is extremely important and requires a knowledge of combustion behavior, to be developed in the recommended studies of flame spread and smoldering. Understanding of extinction is needed in connection with fire suppression. [Priority Medium] 5. Combustion topics of importance but of somewhat lower priority to HEDS are the flammability and flame behavior of gaseous combustible mixtures, sprays, and dust clouds, especially the instability and dynamical behavior of such flames. These results would bear on the types of fire histories that may occur and on means for fire detection and suppression. These studies are less relevant to HEDS than those identified above because most of the combustible materials of concern are anticipated to be solids. [Priority Lower] Granular Materials The activities of site preparation, habitat construction, mining activities, and the installation of transportation and energy distribution networks on the Moon and Mars depend on the ability to penetrate and excavate soils using the same types of heavy construction equipment and material manipulation techniques as are used on Earth. Current knowledge of lunar soil behavior indicates that such a transfer of systems and technologies is both impractical and ill-advised: The average properties of regolith are measured by such techniques as the Mohr-Coulomb failure criteria, but less well established are the detailed interactions of granular material as a function of applied stress (frictional or g-level). Research on granular material under applied stress is needed to examine separately the effects of gravity and shearing on granular behavior and, using both modeling and experimental studies, to obtain a detailed description of granular material flow and behavior that accounts for sample history, internal variables, energy

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SUMMARY OF RECOMMENDED RESEARCH ON FUNDAMENTAL PHENOMENA 183 fluctuations between particles, the effects of agitation, and particle size and shape, especially for operation at low pressure. [Priority Higher] 2. In addition to the higher-priority research associated with geotechnical engineering on the Moon and Mars, another important aim of research in this area should be to gain a fundamental understanding of the behavior of dust in spacecraft and extraterrestrial environments. While difficulties are created by the fact that the transport processes associated with the initiation and sustenance of dust storms, though obviously influenced by gravity, are not yet understood, nevertheless, a predictive capability that permits calculation and control of dust transport and deposition will need to be developed. An understanding must be achieved of the cohesion and adhesion mecha- nisms that control dust attachment, where the attraction mechanism appears to be electrostatic. The behavior of dust in the vacuum environment of the Moon is also a serious problem for long-term system operations. [Prior- ity Higher] Solidification and Melting In the processes of welding, liquid-phase sintering, casting, and containerless melting/freezing of special materials, solidification from the liquid plays a central role. Gravity levels affect solidification through their effect on sedimentation of nuclei and convection in the liquid ahead of the advancing interface. These processes in turn determine the microstructure of the freezing solid. When a solid is heated above the melting point, liquid forms at the surface, and in the absence of gravity-induced convection, heat transfer and the propagation of the liquid/solid interface are expected to be greatly modified. Melting has been less studied than solidification but may be important in the operation of heat pipes and other two-phase devices. The predictive capabilities of present theories of solidification are adequate only in relatively simple cases. Therefore, more work in solidification research is needed along the following lines: 1. The effect of gravity on the nucleation of solid from the melt via the distribution of nuclei and bubbles needs to be clarified. More comprehensive computer models are needed of the effect of gravity on the time- dependent evolution of a solidification front and the concomitant microstructure formation (e.g., cells, dendrites, spatial variation of composition), including the effects of crystalline anisotropy, interface kinetics, and convection, as well as suspended particles, and bubble formation. Also, there should be research at the technical level of casting and other practical solidification processes at different gravity levels to determine the effect of gravity on such casting parameters as grain size, porosity, inclusion distributions, and segregation. [Priority Lower] OTHER CONCERNS Reduced-Gravity Countermeasures Because the effects of reduced or variable gravity are generally a troublesome complication of system design for HEDS and have harmful consequences for human health, research should be undertaken on means to counter such effects. Such means would probably be mechanical in nature, involving rotation or vibration, and could range in scale from the whole spacecraft down to small, critical components. If rotation were utilized, then research would be needed to evaluate the collateral Coriolis and gravity-gradient effects on hardware devices (and humans) as they experience solid or fluid motions in an artificial gravity environment. The purpose of such research would be to provide a rational basis for designing booms, tethers, and control methods for maintaining appropriate spacecraft rotation or, alternatively, for the solid or fluid rotation of spacecraft components separately. More generally, applied research looking toward economic and effective artificial gravity should emphasize applications common to both physical and biological systems. Since no practical, large-scale artificial-gravity system has been developed, it is obvious that the many structural and system problems that arise would have to be studied from a design standpoint before feasible concepts and costs could be established. Research on and development of reduced-gravity countermeasures must obviously proceed hand-in-hand with

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184 MICROGRAVITY RESEARCH the microgravity research recommended elsewhere in this report, because microgravity research will establish the target gravity levels desired for various components and systems. In turn, the specific benefits of an artificial gravity system must be understood and weighed against the penalties (e.g., weight and cost) so that design trade- offs can be made. In other words, it is to be expected that artificial gravity will be part of integrated system designs for HEDS. [Priority Higher] Indirect Effects of Reduced Gravity indirect ellects can set design requirements for components that are different from those in Earth gravity. That is, a component may operate according to phenomena unaffected by gravity level but may still behave differently in reduced gravity. Or the component may have been designed to take account of fractional gravity and therefore may be different from the corresponding components designed for Earth gravity. Structural dynamics will affect the performance and integrity of piping and tankage subsystems that have been designed to be light and flexible to take advantage of the lower load-bearing requirements in low gravity. Re- search is needed to relate structural effects to the excitations that can arise from such causes as intermittent multiphase flows. Similarly, structural dynamics will affect the performance and integrity of large-surface struc- tures such as space radiators, solar collectors, antennas, structural panels, tethers for various purposes, and robots of various types. Research is needed to relate the mechanical operations such as positioning of these devices to structural phenomena such as lightly damped vibration and buckling. Seemingly mundane components such as piping, valves, and bearings must be tailored to reduced- and variable-gravity environments. For example, products of wear and decay are presumably less easily managed in microgravity. Such concerns will be among the essential details of a central issue of supreme importance for HEDS, namely the effect of reduced and variable gravity on system reliability and safety. The overall priorities of these topics are difficult to assign, since they would depend on the importance assigned to avoiding the various technical or biological consequences of reduced gravity enumerated in this report.