Droplet absorption by a capillary channel for microgravity
phase separation
Kelsey Cline1, David Thiessen2
Seattle Pacific University, Seattle, Washington
2 Dept. of Physics and Astronomy, Washington State University, Pullman, Washington
1
Results
Introduction
Materials and methods
0.45
Fraction of Mass Absorbed
As desired, the helical channel absorbs a large
portion of colliding drops.
Optimum absorption conditions include:
• Zero offset (∆x = 0 mm)
• Low drop velocities (< 1 m/s)
• Lowest stable channel pressure
Drop mass absorbed by channel
Fraction of mass absorbed
The Rankine cycle plays a key role in generating
most of the world’s electrical power. This
technology would have a significant power-toweight ratio advantage over solar cell technology
for power generation on spacecraft, but is not
currently used because of difficulties with phase
separation under microgravity conditions. For
power generation and other life-support
technologies, NASA is interested in developing
efficient methods of liquid-gas phase separation
for long-duration manned missions to the moon or
Mars.
Lower channel pressures increase
drop absorption
0.8
Rr
0.4
0.35
Cc
Reservoir
0.25
0.2
Fig. 10 The channel/reservoir system can be
modeled as an RC circuit. The characteristic time
constant, τ, determines the time to return the
channel to reservoir pressure after a “charge”
(droplet) is applied to capacitor Cc (the channel).
Both the tubing, Rt, and the spring, Rc, resist flow.
0.15
0.1
0.05
0
5mm
0.6
10mm
15mm
Reservoir Height (mm below channel)
0.5
Fig. 6 As the reservoir is lowered and channel
pressure is decreased, more drop mass is absorbed.
However, the channel is more unstable at lower
pressures and therefore more likely to break.
0.3
0.2
0.1
0
Figs. 1 and 2 Absorption mass
was measured as a function of
∆x (left). A stable channel (top).
-4.0
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
4.0
50
40
30
10
As drop speed increases,
absorption decreases
0
0
2
4
6
8
10
12
14
x (mm)
0.5
Fig. 7 (above) A drop’s trajectory before and after
a collision with channel; dark blue points are
experimental data, the light blue line is a fit of a
free-fall trajectory to the data. For constant release
height and offset, the drop’s trajectory is very
consistent. The drop lost most of its kinetic energy
and some of its mass during the collision.
0.4
0.3
0.2
0.1
0
0
50
100
150
200
Channel compliance
measurement
Channel Diameter (mm)
A high-speed video camera and a MATLAB
program were used to analyze the trajectory of a
droplet before and after the collision.
Lastly, a camera with high zoom capability was
used to analyze the shape the channel takes at
different pressures.
3.09
3.07
3.05
3.03
3.01
2.99
References
2.97
2.95
-25
-20
-15
-10
-5
0
5
10
15
Channel pressure (mm H2O)
Fig. 5 This drop, traveling at ~0.9 m/s at
collision, passes completely through the
channel but absorption still occurs, which can
be seen in the change in drop size.
When the drop’s velocity approaches 2 m/s, the
channel completely breaks and becomes unstable
upon impact.
TEMPLATE DESIGN © 2008
www.PosterPresentations.com
When fully developed, this technology will
consist of an array of springs that will achieve
phase separation by absorbing water droplets in
an air flow. Since this technology is still in the
beginning stage, there is extensive research to
be done before we know if it will be useful in
outer space; we should continue to understand
the basics of this system. Hopefully, this
technology will aid in furthering space
exploration and be useful in applications both
on Earth and beyond.
250
Drop Height Above Channel (mm)
Fig. 3 Experimental set-up.
The voltage across Cc decreases exponentially
until Vc = Vr (that is, channel pressure equals
reservoir pressure). `
Conclusions
Channel
20
0.6
τ = ( Rt + Rc )Cc
60
y (mm)
Fig.4 (above) The channel absorbs the greatest drop
mass at zero offset, but significant absorption still
occurs in peripheral regions. Hopefully, this means
that in a large array of helical channels, all drops
will eventually be almost or completely absorbed.
128µ * L
R=
4
π *D
Drop trajectory
70
∆x (mm)
Fraction of mass absorbed
(Average, -2.4<∆x<2.4mm)
A helically-supported capillary channel is being
investigated for capturing drops from a flowing
two-phase mixture. A 1/8” diameter, steel spring
was suspended horizontally and connected at both
ends to a water reservoir that allows a stable
channel of water to be established inside the
spring. When the water reservoir is positioned
below the channel, a colliding droplet will have a
higher pressure than the channel and be sucked in.
The mass fraction of falling droplets absorbed by
the channel was measured as a function of the
offset of the drop trajectory from the axis of the
channel.
Rc
Vr
0.3
0.7
0.4
Channel
McQuillen, John. 2003. Results of the workshop on two-phase
flow, fluid stability and dynamics. NASA/TM 2003-212598.
Oelerich, Jerry J. 2009. Open-channel capillary flow in microscale helical support structures. Master’s thesis, Washington
State University.
Figs. 8 and 9 Photos show the low and high
pressure stability limits. If pressure is decreased
from the lower limit, the channel will break; if
pressure is increased from the upper limit, the
channel will drip. On the graph, green points are
stable and red are unstable.
Thiessen, David B. and Kin F. Man. 2000. Surface tension
measurement. Mechanical variables measurement 12-1 – 1213.