Background
Two phase flow, flow of both a liquid and gas state, is crucial in
countless space applications including power cycles, the storage and
transfer of liquid fuels, cryogens, propellants, life support systems,
and thermal control systems.
However, truly steady and fully
developed flow has not yet been
achieved in low-g experiments due
to limitations in hardware, size,
cost, and time.
Presently, necessary applications
have been either over-designed or
designed to completely avoid twophase flow conditions.
Research
Previous experimentation has been overwhelmingly dominated by
pressure driven flow. Large complex systems have been used to
force a liquid slug or a gas bubble through a long straight tube.
These systems can be large, difficult to control, expensive, complex,
and unable to achieve steady-state conditions.
ValveA
To Syringe
Pump
ValveB
Tube
A
B
To
Syringe
Bubble
Jing-Den Chen, 1986
Fluid Behavior
As a liquid slug passes through a tube, a thin film is deposited along the
inside walls. This film thickness fluctuates depending upon the
gravitational conditions. If correctly predicted and modeled, system
parameters such as pressure drop and heat transfer rate can be calculated
simplifying designs by reducing unnecessary size, weight, and cost.
In reduced gravity, typically
neglected forces become more
dominant and may even
control the flow regime. In the
case of two-phase slug flow,
surface tension becomes a
prevailing force and must be
included in all analysis.
More Fluid Behavior
In a reduced gravity environment, it is difficult to assure of the location
of liquid and gas phases in a two-phase system. Similar to a 1-g
environment, an applied external force can be exploited to collect and
position the liquid into a single continuous slug.
An unusual idea of applying a simple
centrifugal force on a closed circular
loop became the new model for this
experiment. This unique solution
eliminates the control concerns of
pressure driven flow, reduces the
size, weight and complexity of the
experiment, and will ultimately allow
for a steady-state flow field.
1g Design
The simplicity of the reduced gravity design allows for testing and
modeling in a 1g environment. Steady flow is easily achieved by
mounting a tube of fluid on a circular disk. To further predict fluid
behavior in reduced gravity, the disk is tilted at various angles,
therefore lessening the effect of gravity in the direction of flow.
In addition, various tube sizes,
fluid viscosities, and rotational
speeds are examined to
determine the most appropriate
combination for the one chance
reduced gravity experiment.
1g Results
For verification purposes, the 1g data was compared with several
published predictions. The closed circuit loop demonstrated that
steady-state slug flow is easily achieved and well-predicted by
established analyses.
Dimensionless Film Thickness vs. Capillary
Number
3
-2
d/r (x 10 )
2.5
2
1.5
1
Closed Circuit Loop
Chen
Bretherton
0.5
0
0
0.5
1
1.5
2
Ca (x 10-3)
2.5
3
3.5
4
A New Solution
A small disk holding several sealed tubes of a liquid and gas mixture is
mounted above a larger plate. By rotating the large plate, a centrifugal
force is applied to the small disk positioning one continuous slug to the
outside rim. The second smaller disk can then be rotated resulting in
two-phase slug flow.
This simple design allows for
fine control of both rotational
speeds while quickly providing
steady-state, fully-developed
slug flow.
Reduced Gravity Design
The experiment is enclosed in an aluminum frame and undergoes a
rigorous safety evaluation to NASA specifications. Due to the
unusual environment aboard the reduced gravity aircraft, individual
components must be designed to withstand severe loadings, as high
as nine times gravity in certain directions.
Analysis
A remote camera above the
small disk records and
transmits data to an onboard
video display. While the flow
is monitored during the flight,
the images are analyzed at a
later date. The video is
downloaded and the flow
properties scrutinized using
image analysis software.
From these images, film
thicknesses, dynamic contact
angles, deposition rates and
transient and steady state flow
regimes are examined.
References
Balakotaiah, Vemuri and Larry Witte. “Studies on Two-Phase Flows at Normal and
Microgravity Conditions.” 1998/1999. Institute for Space Systems and Operations.
29 June 1999 <http://www.isso.uh.edu/publications/A9798/bala.htm>.
Chen, Jing-Den. “Measuring the Film Thickness Surrounding a Bubble Inside a Capillary.”
Journal of Colloid and Interface Science. 109 (1986): 341-349.
Colin, C., J.Fabre, and A.E. Dukler. “Gas-Liquid Flow at Microgravity Conditions.”
Int. J. Multiphase Flow. 17 (1991): 533-544.
Hallinan, Dr. Kevin, and Jeffery S. Allen and Jack Lekan. “Capillary-Driven Heat Transfer
(CHT) Investigation, MSL-1, STS-83.” 2002. NASA. August, 2002.
< http://microgravity.grc.nasa.gov/expr3/cht.htm>.
Incropera, Frank P., and David P. Dewitt. Fundamentals of Heat and Mass Transfer.
New York: John Wiley & Sons, 2001: 357.
Taylor, G.I. “Deposition of a viscous fluid on the wall of a tube.” Journal of Fluid Mechanics.
10 (1961): 161-165.
Young, Donald F., and Bruce R. Munson and Theodore H. Okiishi. A Brief Introduction to
Fluid Mechanics. New York: John Wiley & Sons, 2001: 304-316.
Special Thanks:
The NASA Reduced Gravity Student Opportunity Program,
the Oregon Space Grant Program, Mark Weislogel, Portland State University’s
College of Engineering and Computer Science, Infinity Images, FMC Allen
Machinery, and Chehalem Machine Works