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II
Brief Descriptions of Phenomena
Important in Reduced Gravity
Chapter III surveys a set of HEDS-enabling technologies for the purpose of identifying critical underlying
physical phenomena that are affected by, or that play a dominant role in, reduced gravity. These phenomena, and
the research needed to extend our knowledge of them, are discussed in Chapter IV. The purpose of Chapter II is
to briefly introduce these phenomena, along with associated concepts such as scaling, so that reference to them in
the following chapters will be intelligible to a wider range of readers.
Because of the equivalence between force and mass times acceleration, many of the dimensionless groups
used to generalize design data may be expressed as force ratios that contain gravitational acceleration. The
consequences of local variations in Earth's gravitational acceleration can be accounted for easily through the
scaling that is inherent in the relationships among the various dimensionless groups, and it is expected that for the
gravitational accelerations that exist on the surfaces of the Moon and Mars, such known scaling laws are adequate
for describing a number of phenomena. (For this reason most HEDS technology problems that need to be
addressed by fundamental research on basic transport phenomena are likely to occur at near zero gravity rather
than at fractional gravity levels.) As the gravitational level decreases below that of Earth, the Moon, and Mars, as
may obtain in the space environment, the magnitudes of the forces associated with differences in density may
become small compared with the forces associated with other phenomena, which then may become significant and
even dominant. In some cases, the emergence of other phenomena is predictable as gravitational levels are
reduced, while in other cases, the influence of competing phenomena may be unexpected, and the engineering
systems needed for successful HEDS developments may then be compromised. There are a number of phenomena
that may play a significant role in such systems.
Interfacial phenomena refer to effects caused by the presence of an interface or bounding surface between two
different thermodynamic phases, i.e., a solid, liquid, or gas. The defining characteristic of an interface is the work
required to create a unit area of the interface, which is referred to as the free energy per unit area or (for a liquid)
the surface tension of the interface. Many of the static forms assumed by liquids on a small scale reflect the
tendency of the multiphase system to minimize the total interracial free energy, which in simple cases is equivalent
to minimizing the total interracial area. This accounts for the spherical shape of isolated droplets and bubbles and
other more complicated configurations of liquids in contact with a solid of prescribed shape, as occurs in the
storage of liquid fuels. These shapes, both static and dynamic (e.g., ripples on a liquid surface), are discussed in
Section IV.B. In addition, surface tension regulates wetting phenomena, as illustrated by the flow of solder on a
work piece. Also, variations in surface tension over a liquid surface, arising from temperature or composition
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18
MICROGRAVITY RESEARCH
gradients, drive special flows referred to as Marangoni flows. Surface tension is not affected by gravity level, but
surface tension as a driving force for the configuration and motion of liquids becomes increasingly important as
the gravity level is reduced. For this reason, understanding and utilizing interracial phenomena are key to the
control and management of liquids in reduced gravity.
Multiphase flow, described in Section IV.C, refers to the flow of more than one fluid phase in a system of
pipes, pumps, and phase-change components; an example is the flow of vapor and liquid in a power-generating
system. In Earth gravity, phases tend to stratify depending on their density, with the denser phase flowing or
collecting in the bottom of a pipe or component, whereas in microgravity there is no driving force for stratification.
Various flow regimes occur corresponding to different configurations of the two phases, including bubbles of gas
in a liquid, droplets of liquid in a gas, and the annular flow of a film of liquid along the walls of a conduit with gas
and entrained liquid droplets flowing in a central core. Flow-regime-specific predictive models of multiphase flow
are essential for the design of multiphase systems for operation in microgravity, including power-generating
systems that depend on the flow of more than one phase, as in the case of a Rankine cycle power plant (typified by
a steam power plant on Earth). Rankine-cycle-based systems are attractive because of the efficient heat transfer
due to phase change during evaporation and condensation.
Flow in porous media is exemplified by the movement of liquids in the wicks used in heat pipes and other
thermal management devices based on capillarity, the flow of fluids in packed and fluidized beds used in chemical
reactors, and the flow of nutrients and gases in soils used for plant cultivation in space. Gravity, pressure
gradients, and capillarity (i.e., surface tension forces) are the primary driving forces for flow, and gravity becomes
less important as it is reduced. The behavior of the flow under these conditions is poorly understood, as discussed
in Section IV.C.
Heat transfer refers to the flux of heat across a boundary or interface by conduction, convection, or radiation
and is central to the operation of power and propulsion systems and to the management of the energy budget in a
spacecraft. Of these processes, convection (with or without phase change) driven by thermally induced density
differences is strongly dependent on gravity level and hence is the focus of Section IV.D. In this case, the
transferred heat is conveyed to (or from) the bounding surface from (or to) another part of the system by a moving
fluid or fluids. If the fluid is single-phase, as in the case of a Brayton cycle, then thermally induced buoyancy
convection is diminished in reduced gravity and absent in microgravity. If the fluid is two-phase, as in the case of
a Rankine cycle, then all the issues of multiphase flow and heat transfer referred to above are present. In addition,
the process of heat transfer at the walls of a condenser or evaporator, where the phase change occurs, is critically
affected by gravity level. In microgravity, surface tension or capillary forces play a dominant role in the degree
and dynamics of the wetting of the walls by the liquid phase.
In systems using multiphase flow, system effects on a global scale may emerge from the interaction among the
components of the network. In particular, the phase or time lags in feedback loops can cause potentially dangerous
instabilities that are only revealed by analysis of the system as a whole. These effects are discussed in Section
IV.C.
Solidification of a melt occurs in such processes as casting, welding, and liquid-phase sintering. It is affected
by gravity level, as discussed in Section IV.E, because of buoyancy-induced convection in the liquid phase that, in
turn, affects the distribution of solutes, foreign particles, and bubbles in the liquid as it freezes. This in turn affects
the grain structure, porosity, and impurity distribution (i.e., the microstructure) of the solidified or cast material.
The microstructure and properties of a material solidified in microgravity differ markedly from those of a material
solidified in Earth gravity (Curreri and Stefanescu, 1988~.
Combustion, either intentional in power-generating and propulsion systems or unintentional in accidental
fires, is influenced by gravity level because of the large density differences between the ambient gas and the hot
product gases. Flame structure is drastically altered from the upward convective form in Earth gravity to a
quiescent spherical form controlled by diffusion in microgravity. The effects of gravity on such combustion
characteristics as flammability, smoldering, and flame spread are discussed in Section IV.F.
Granular mechanics includes such topics as the angle of repose of piles of granular material, the response of
granular media and soils to loads and to digging, and the flow of granular materials in chutes and hoppers. Gravity
affects the degree of compaction of soils and provides the driving force for flow. In addition, the behavior of
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Representative terms from entire chapter:
gravity level
