Scientific and Technical Aerospace Reports Volume 39 April 6, 2001

current is caused by the impurity content (O2, 0.107%) in CO2 gas source. In region II, the cell’s current stays at a nearly constant
level although the CO2 electrode potential increases in magnitude. The anodic activation overpotential at the oxygen side remains
unchanged, while the cathodic overpotential increased sharply. At this point, all O2 taking part in electrochemical reaction comes
from O2 impurity of CO2. There is no CO2 dissociating into CO and O. Because of limited O2 source, the cell is in a mass limited
flow condition. All the increase in applied potential is utilized at the cathode, while the activation overpotential on anode side
remains unchanged. The overpotential at the cathode side is still not sufficient to provide the free energy required to crack CO2.
In region III, the cell’s current increases with applied voltage, after the CO2 electrode voltage reaches a certain threshold value.
The activation overpotential at CO2 side drops sharply, while activation overpotential on the anode side increases. The magnitude
of the threshold is a function of temperature. It can be determined by the CO2 energy of formation and is termed the Nernst potential.
In region III the activation overpotentials fit the Tafel plot well. The data on exchange current density and charge transfer
coefficients obtained from the Tafel plots can be used to fit the well known Butler Volmer equation. SOECs have been used in
a CO2 environment to produce O2. The current-voltage characteristics and overpotentials associated with the process have been
measured. Three distinct regions are observed, and been analyzed.
Author (revised)
Solid Oxide Fuel Cells; Ceramics; Electric Potential; Electrolytic Cells; Electrolytes; Ionic Mobility; Ion Motion
20010024909 Johns Hopkins Univ., Dept. of Mechanical Engineering, Baltimore, MD USA
Production and Removal of Gas Bubbles in Microgravity
Oguz, H. N., Johns Hopkins Univ., USA; Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference;
December 2000, pp. 462-463; In English; See also 20010024890; No Copyright; Abstract Only; Available from CASI only
as part of the entire parent document
This study focuses on two topics relevant in many microgravity applications: formation of gas bubbles from orifices in tubes
and removal of individual bubbles from liquids. Bubble injection from an axisymmetric slot is investigated experimentally and
numerically. Three dimensional boundary integral simulations of bubble injection from multiple holes along the axial direction
of a tube have also been carried out. Several difficulties encountered in the numerical modeling of these flows have been overcome
and a stable method has been developed. The main difficulty is the tendency of a growing bubble to touch nearby solid surfaces.
An artificial force is introduced to keep bubbles away from the walls and continue the computations without affecting the main
characteristics of the problem. It has been observed that downstream bubbles detach earlier when all bubbles start growing at the
same time. This is expected because the average flow rate seen by downstream bubbles is higher than the ones seen by upstream
bubbles. Bubble shapes obtained from normal gravity experiments are shown in fig. 1. Simulations under similar flow condition
reproduce this flow. A series of experiments have been carried out to investigate gas injection from an axisymmetric slot in a tube
under normal gravity conditions. Three distinct regimes of operation as a function of the gas/liquid flow rate ratios are observed.
At high gas flow rates, the injected gas completely fills the tube and blocks the liquid flow. A Taylor bubble is generated in this
regime. When the liquid flow rate becomes comparable to the gas injection rate, axial symmetry is broken and a regular single
bubble forms. This is identified as the second regime where the liquid flow is not interrupted by the gas. At even higher liquid
flow rates, turbulent conditions and chaotic multi-bubble formations are observed. This is the third regime. Illustrative pictures
for these regimes are shown. An axisymmetric boundary integral code is employed to simulate this flow. Because of the imposed
symmetry the range of validity of the simulations were somewhat limited but the results are useful in understanding the basic
dynamics of the problem. Simulations suggest the existence of a stable liquid column inside the bubble. Assuming that the bubble
front moves with the same velocity as the liquid column, an equilibrium condition that admits one stable solution is derived.
Although there have been many investigations of the formation of bubbles from an underwater orifice, the opposite case, i.e.
removal of bubbles by an orifice, has received very little attention. The dynamics of a bubble entering a capillary tube is studied.
A stream of sparsely spaced bubbles generated from an underwater orifice is positioned just below a capillary tube that is connected
to a vacuum reservoir. Sufficiently small bubbles are captured in the capillary. In some cases, a small bubble detaches from
the back of the main one as a result of severe surface deformation near the entrance of the capillary. Larger bubbles, on the other
hand, break in two while being drawn into the capillary. A boundary integral technique is employed to simulate this process. An
artificial repulsive force is introduced to form a thin layer of water around the bubble in the capillary tube. With this scheme, it
is possible to simulate bubble motion both outside and inside the capillary. Numerical bubble shapes are found to be in very good
agreement with experiment.
Author (revised)
Bubbles; Gas Flow; Gas Injection; Microgravity; Slots; Gas-Liquid Interactions
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