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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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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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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.
Representative terms from entire chapter:
reduced gravity
