Microgravity Research in Support of Technologies for the Human Exploration and Development of Space and Planetary Bodies (2000) / Chapter Skim
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IV Phenomena of Importance in Reduced Gravity
IV Phenomena of Importance in Reduced Gravity
Pages 111-166
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From page 111... ...
This basic scientific goal is emphasized in the NASA microgravity research program (Woodward, 1998~. Understanding and mitigating the technological consequences of low gravity must be one of NASA's primary research goals.
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From page 112... ...
In some cases, such as the dispersal of particulates or droplets, neutral stability would be desired, and in such cases, reduced gravity could be helpful. Gravity-Density Coupling in Various Basic Processes Various basic flow processes may now be related to buoyancy as expressed by the gravity-density difference product, and typical dimensionless groups may thus be identified.
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From page 113... ...
Therefore, many specific dimensionless groups will be needed to describe complex flow regimes of interest (Ostrach, 1982~. Nevertheless, the crude outline given above shows that, generally, gravity level, g, finds itself in a group that includes density difference and some positive power (n)
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From page 114... ...
Because of their dominance and practical importance in reduced gravity, the following interracial phenomena are discussed in this section: static and dynamic capillary configurations, wetting, and the Marangoni effect (arising from gradients of the surface free energy)
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From page 115... ...
The stability and dynamics of capillary-dominated configurations of liquids are expected to play an important role in the management of liquids and in the boiling/condensation process in heat exchangers under reduced gravity conditions (Westbye et al., 1995~. As mentioned above, they also underlie the operation of heat pipes, capillary pumped loops, microgrooved heat pipes, and veined structures.
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From page 116... ...
Although wetting is a capillary or surface phenomenon not significantly affected by gravity level, it becomes increasingly important when the gravity level is reduced, which makes it one of several phenomena that dominate events under microgravity conditions, and for this reason it is included in this report. Wetting, partial or complete, underlies such important technologies as soldering and welding (Nance and Jones, 1993~; liquid-phase sintering (German et al., 1995~; the operation of wicks in capillary pumped loops (Anatar and Nuotio-Antar, 1993)
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From page 117... ...
Marangoni Effect The Marangoni effect (Hondros, 1998; Antar and Nuotio-Antar, 1993; Legros et al., 1987, 1990; Ostrach, 1982) commonly refers to liquid convection caused by surface tension gradients at the free surface of a liquid or at the interface between two liquids.
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From page 118... ...
Similar phenomena happen with bubbles and can have a strong effect on pool and forced convective boiling heat transfer. Marangoni convection usually dominates gravity-induced convection in weld pools in Earth gravity; it is undiminished in microgravity.
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From page 119... ...
The effects become dominant in reduced gravity, as in the stirring of a weld pool, the migration of liquid in spills, fire control, and two-phase fluid transport, and in capillaryoperated devices such as heat pipes or capillary pumped loops (referred to in the subsection on wetting)
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1995. Liquid-phase sintering under microgravity conditions.
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From page 121... ...
In addition, there is a need to better understand multiphase particulate/fluid systems (e.g., dust transport and deposition) in reduced gravity if extraterrestrial colonies are to be established.
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techniques (Lahey and Drew, 2000~. It appears that multidimensional CFD models of this type can also be developed for microgravity conditions.
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From page 123... ...
This phenomenon is significant because it can lead to flow regime transition, and the thermal-hydraulic characteristics (e.g., pressure drop and heat transfer) of a flowing multiphase mixture are strongly dependent on flow regime and, thus, gravity (Dukler et al., 1988; Jayawardena et al., 1997; Chen et al., 1991; Zaho and Rezkallah, 1993; Bousman et al., 1996; McQuillen et al., 1998~.
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From page 124... ...
Developing models will require a better understanding of multiphase fluid mechanics and of interracial mass, momentum, and energy transfer mechanisms in reduced gravity environments. Proper quantification of these models, through experiments and analysis, should give NASA the ability to reliably calculate phase distribution and separation, as well as phasic mixing, in simple and complex geometries.
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From page 125... ...
It is significant that pronounced phase distribution is seen for forced flow conditions, since only multiphase flows of this type are suitable for microgravity conditions (that is, as discussed previously, natural circulation will not be effective in microgravity environments)
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From page 126... ...
26 MICROGRAVITY RESEARCH 0.250 0.200 u' 0.150 0.1 00 0.050 Two-fluid model · ~ ~ ~ v L Serizawa's data (radial) ALAA u'L Serizawa's data (axial)
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- ~ lo lo by 1.6 1.4 1.2 _% ~ 1.0 Q 0.8 u, 0.6 u) `~ 0.4 0.2 O- ~ -0.2 - _ PHENOMENA OF IMPORTANCE IN RED UCED GRA VITY Axial velocity Liquid temperature 0.6 0.6 0.6 0.6 .
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From page 128... ...
However, the appropriate closure laws will be different from those used for vapor/liquid systems (i.e., particle-to-particle interactions will be very significant) , and appropriate particle transport experiments will be needed to establish flow behavior in reduced gravity and for atmospheric conditions relevant to the extraterrestrial site.
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From page 129... ...
Research Issues In summary, experimental and analytical research focused on measuring and modeling flow-regime-specific multiphase turbulent phenomena in reduced-gravity environments is required if reliable predictions are to be made of the multiphase flow and heat transfer phenomena expected to occur in many of the power, propulsion, and life support systems and subsystems to be used for HEDS missions. Multiphase Systems Dynamics Multiphase flow also exhibits important global phenomena, which can affect system/subsystem operation and performance.
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From page 130... ...
. However, these fixes can cause a large penalty in terms of increased system pressure drop and reduced boiling heat transfer effectiveness.
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From page 131... ...
This is particularly true for multiphase flows, where inherent unsteadiness in some flow regimes (e.g., slug flow) may create a forcing function for structural vibrations.
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From page 132... ...
It appears that an experimental and analytical research program focused on, at a minimum, Ledinegg (i.e., excursive) pressure drop and density-wave instabilities is needed if reliable multiphase systems are to be used on HEDS missions.
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PHENOMENA OF IMPORTANCE IN RED UCED GRA VITY 133 by.
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From page 134... ...
is an example of fluid flow in porous media associated with plant cultivation for life support in fractional gravity or microgravity environments (Eckart, 1996~. Two-phase devices such as heat pipes and capillary pumped loops have become key elements in the thermal control systems of space platforms.
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From page 135... ...
Research Issues In summary, there are many fundamental research issues for single, multiphase, and multicomponent flows and transport in porous media that are not understood and have not been studied under fractional or microgravity conditions. These issues include the following: (1)
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From page 136... ...
1998. Capillary effects and multiphase flow in porous media.
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From page 137... ...
, including power generation and storage, propulsion, life support, thermal management, and in situ resource utilization. Because fluid flow and heat transfer are affected by reduced gravity or microgravity, they represent critical processes in efficient and reliable active power generation technologies.
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From page 138... ...
, and a detailed account of the gravity effects on fluid flows, including identification of relevant scaling parameters, has been provided by Ostrach (1982~. Within the last decade, the Committee on Microgravity Research has also reviewed the status of microgravity research (NRC, 1992, 1995~; the review need not be repeated here.
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From page 139... ...
However, the existing scaling relations have not been carefully compared against experimental data obtained under reduced gravity conditions. The scaling with gravity of thermal phenomena, i.e., the identification of boundaries at which the physics of phenomena changes with the gravity level, has been recommended by a recent NASA workshop.
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From page 140... ...
For example, thin evaporating liquid films produce high heat transfer rates and are used in heat pipes, sweat coolers, grooved evaporators, and other enhanced heattransfer-surface devices that depend on the formation of thin-film regions owing to capillary action (Bankoff, 1990; Faghri and Khrustalev, 1997~. As a basic physical process, the evaporation of a thin liquid layer plays a key role in heat transfer.
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From page 141... ...
In the past, almost all investigators concentrated on the convective instability in the liquid layer rather than heat transfer in the layer and evaporation from the free surface. Evaporation and latent heat are expected to play important roles in the onset of convection, and the convection itself in thin liquid layers will be influenced by surface tension under microgravity conditions.
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From page 142... ...
The magnitude and direction of the gravitational acceleration with respect to the heater surface influence the fluid dynamics and thermal boundary layers and, consequently, the bubble trajectory. Pool boiling heat transfer results obtained under reduced gravity conditions have been contradictory (Lee et al., 1997; Sitter et al., 1998~.
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From page 143... ...
In recent experiments steady-state pool boiling of R- 113 has been achieved, and a pool boiling curve has been generated (Lee et al., 1997, 1999~. Analysis of the transient data has revealed that steady-state nucleate boiling heat transfer under microgravity conditions is enhanced relative to that in Earth gravity, whereas the CHF is considerably reduced.
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From page 144... ...
The physics of bubble growth and detachment, bubble merger (coalescence) at and away from the heated surface, and vapor removal and the contributions of various mechanisms to the total heat transfer rate for pool and forced flow boiling under reduced gravity conditions are not fully understood.
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From page 145... ...
For example, the very important problem of forced-flow nucleate boiling under reduced gravity has received only very limited research attention, and there is no known international effort under way to develop mechanistic models for nucleate boiling or to obtain CHF data under low-velocity conditions in microgravity. Fundamental research needs are identified for developing a basic understanding of the mechanisms responsible for heat transfer and vapor removal from the vessel wall.
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From page 146... ...
heat transfer is less than in Earth gravity and pressure drop greater. Research Issues for Flow Boiling Many questions are still wide open, and to resolve them would require both experimental and theoretical work on forced boiling heat transfer under microgravity conditions.
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From page 147... ...
Fundamental studies of condensation phenomena under reduced and microgravity conditions need to be undertaken. In parallel with experiments, detailed theoretical analyses should be carried out to develop an understanding of the fundamental fluid physics responsible for condensate film growth, film instability, and the resulting interfacial motion under reduced gravity, and the corresponding implications for forced-flow condensation heat transfer.
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From page 148... ...
The measured heat transfer coefficients at reduced gravity were found to be lower than predicted by normal-gravity correlations. For annular flow the pressure drop data agree well with the well-known Lockhart-Martinelli correlation, whereas for the slug flow regime the data do not correlate well either with the Lockhart-Martinelli or the homogeneous flow correlations.
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From page 149... ...
For the bubbly flow regime, no heat transfer measurements have been made under reduced or microgravity conditions. In such a flow regime, the capillary-induced migration of the bubbles in the presence of temperature gradients perpendicular to the walls of the conduit may have a significant effect on the phase distribution and convective heat transfer.
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From page 150... ...
Phase-Change Heat Transfer in Porous Media Capillary and porous structures are used widely in two-phase devices such as heat pipes, capillary pumped loops, and loop heat pipes to provide liquid transport and enhanced heat transfer during evaporation and condensation in spacecraft fluid and thermal management systems, but neither boiling nor condensation in porous structures has been studied under reduced-gravity or microgravity conditions (Khrustalev and Faghri, 1997~. For example, capillary heterogeneity, induced by variation in permeability, has application in heat pipes operating in a microgravity environment.
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From page 151... ...
Research Issues Among the phenomena relevant to many HEDS mission-enabling technologies are the forced and surfacetension-gradient-driven single- and two-phase flows, evaporation, condensation, and boiling heat transfer that occur in porous media under reduced-gravity or microgravity conditions. Fluid thermal behavior in capillary porous structures at high heat fluxes under microgravity conditions needs to be understood; several areas have been identified for research, including the following: (1)
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From page 152... ...
1995. Spray cooling characteristics under reduced gravity.
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From page 153... ...
1990. Pool boiling in a reduced gravity field.
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From page 154... ...
occurs include latent heat-of-fusion thermal energy storage systems and off-design freezing of liquid lines, space radiators, and heat pipes. In addition to the obvious cases of traditional casting methods and unidirectional solidification (Larson and Pirich, 1982; Coriell and McFadden, 1990)
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From page 155... ...
To make the problem complete, boundary conditions for the temperature and composition fields on the moving interface must be specified. These are often taken to be given by the phase diagram adjusted for the effects of capillarity (i.e., surface tension and local curvature)
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From page 156... ...
P 149 in Materials Processing in the Reduced Gravity Environment of Space: Proceedings of the Materials Research Society (MRS)
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From page 157... ...
Near-limit premixed flames at reduced gravity exhibit unusual behavior not observed at normal gravity. For Lewis numbers less than unity, spherically expanding flames propagate and then extinguish.
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From page 158... ...
At normal gravity, the soot mantle is swept away from the lower, windward side of a droplet and consumed in the upper reaches of the flame plume above the droplet. Cloud Combustion Arrays or clouds of droplets or combustible particles may exhibit different flame propagation characteristics at normal and reduced gravity.
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From page 159... ...
For shallow pools, in which buoyancy would be absent even at normal gravity, when the flame spreads at a uniform rate, normal and reduced gravity give the same result. Numerical modeling predicts that pulsation will occur in microgravity with forced convection (Schiller and Sirignano, 1996)
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From page 160... ...
Further work on materials and environment selection is needed, but the emphasis should be on robust fire detection and suppression in reduced gravity. As discussed previously, a number of areas of reduced-gravity combustion research are particularly relevant to improving fire detection and suppression in a reduced gravity environment, including flammability and flame behavior (such as flame instabilities and dynamics under different gravity conditions)
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From page 161... ...
After mixing, density differences would rapidly separate such a solution into layers on Earth, whereas in reduced gravity the dispersed droplets, with their greater surface area, could remain suspended for a longer period of time, allowing reactants in the two phases to interact at a higher rate. Surface tension would still be present, however, as a driving force for coalescence of the phases and thus a reduction in the surface area of reaction.
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From page 162... ...
1995. Microgravity Research Opportunities for the 1990s.
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From page 163... ...
The behavior of dust in reduced gravity is also an important consideration. While the composition, particle shape, and atmospheric loading of Mars dust is neither fully known nor understood at this time, their potential impact on future HEDS missions must be considered.
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From page 164... ...
The strong dependence of the dynamic behavior on grain packing density gives new insight into the properties of granular materials. When granular materials are agitated in Earth gravity, size segregation of grains can occur by several mechanisms, causing preferential filling of the space beneath large particles by smaller particles (Jenkins and Louge, 1997~.
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From page 165... ...
The modeling of granular materials under applied stress is important to a number of HEDS activities, such as construction, surface transport, and materials processing at low pressure. Studies that separate or counter the effects of gravity while examining the effects of shearing on granular behavior in three dimensions would help in understanding the phase transition observed at a critical packing density of granular material.
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