[ Home ] [ Experiment ] [ Outreach ] [ Team ] [ Photos ] [ Sponsors & Links ]
Fluids In Low G
Perhaps you have pulled out the straw from your favorite fountain
drink to find that while most of the liquid has drained from the
straw onto your lap, a portion of liquid remains at the bottom
of the straw. What is holding it there? Why does this trick work
with one size straw and not another?
If the fluid is stationary there must be a balance of forces.
The force due to gravity most certainly is acting on the fluid
pulling it to the earth. The gravitational force is being balanced
by another force. That force is due to the surface tension at
the liquid/straw interface.
Fluid flows in which the effect of surface tension is significant
are called capillary flows. Generally in normal gravity such flows
are limited to small channels less than a few millimeters in diameter.
The Bond number,
Bo = rgr2/s
is the ratio of the gravitational force and the surface tension
of the liquid, where r = density of
fluid, g = the gravitational acceleration, r = the radius, s = surface tension. When Bo >> 1,
the gravitational force dominates fluid behavior. For Bo<<
1, surface tension plays a significant role in the behavior of
the fluid and in the case of the straw/liquid example, liquid
will remain in the straw. In the absence of gravity Bond numbers
can remain extremely small allowing for the study of surface tension
affects on large diameter geometries. In low-g, a blob of water
left alone in a glass container could climb all over the inner
surface of that container until it found its lowest energy configuration.
For certain geometries this could mean the liquid flows into the
corners and stays there indefinitely. For the fluid in a straw
the balance between gravity and surface tension would no longer
exist and the liquid would be free to climb the walls of the straw.
Background
Two-phase flows, such as the flow of a liquid and its vapor, are critical to many spacecraft fluid systems ( i.e. water processing, liquid propellants, etc.). The specific experiment objectives of the Portland State Team5 is to demonstrate that a certain two-phase flow can be achieved which is steady. The 'steady' nature of the flow allows for precision measurements of key characteristics of the flow such as film thickness and dynamic interface curvature. The measurements of these characteristics unique to the low-g environment can serve as design parameters for future spacecraft systems ( i.e. more efficient heat exchanger designs ).
Our Experiment
While Two-Phase flow cycles are more efficient in the transfer
of heat energy, they have been avoided in low gravity applications
due to the lack of experimental data describing the behavior of
the flow regimes. It was the goal of the Portland State Team to
develop a reliable, inexpensive testing apparatus that would reproduce
a steady slug flow regime that could be easily employed in ground
based micro-gravity test facilities, such as NASA's KC-135.
Concept
The testing apparatus employs the use of four transparent flexible tubes partially filled with a fluid of known properties ( viscosity (µ), surface tension (s), density (?) ). These tubes are made to rotate around two drums. The drums in turn are mounted on a large rotating disk. As the large disk rotates the liquid slugs in the tubes experience a centripetal acceleration. This centripetal acceleration is sufficient enough to drive the fluid motion while maintaining a capillary dominated flow3. As the large disk is rotated the drums are made to rotate dragging the fluid from the outer edge of the drum to the linear portion of the tube path shown in figure1.
1. Aluminum Frame
2. Mounting Plate
3. Motor/Gear Box ( Large Disk )
4. Motor/Gear Box ( Drums )
5. Drum Pack Assembly
6. Counter Weight
7. Digital Video Camera
8. Large Disk Rotational Velocity Display
9. Back Light Switch
10.DV Monitor
11.Speed Controls ( Large Disk, Drum )
12.Power Supply
13.Outreach Experiment Controls
14.Outreach Experiment Housing
Fig. 4 slug traveling through 3/8 in tube
A force balance in the linear path can be obtained between the acceleration force ( Fa ), the viscous dissipation force( Fµ ), and the surface tension force ( Fs ). When these forces balance a steady slug velocity can be calculated.
Balancing forces yields,
From this force balance the governing differential equation describing this flow is,
At steady state the governing differential equation reduces to,
Fig. 5 Dimensionless Film Thickness vs. Capillary Number4
Analysis
With a digital video camera and image analysis software many characteristics of the flow can be studied. Of particular interest to the PSU team was the thickness of the film left behind by a liquid slug as it travels through a tube at steady state. By knowing the initial volume of liquid in a given tube, the captured video images allow for the measurement of the final slug length. The difference of these two volumes is the volume of fluid left behind by the slug. Using this volume difference and the geometry of the tubes the film thickness can be calculated with extreme accuracy. This data can then be compared to theoretical predictions of dimensionless film thickness versus Capillary number, Ca = µV/s where V is the steady state velocity.
References
1 NASA Reduced Gravity Flight Opportunities Program,
http://microgravityuniversity.jsc.nasa.gov/intro.cfm
2 Mark Weislogel Associate Professor Thermal and Fluid Sciences
Group, Portland State University
mmw@cecs.pdx.edu
3 M.M. Weislogal and A.T. Sitorus Steady, Fully Developed Flow:
A New Experimental Approach
For Low-g Multiphase Flow Investigations, AIAA-2004-2644
4 Chen, J.D. (1986) "Measuring Film Thickness Surrounding
a Bubble Inside a Capillary, J. Colloid Int.
Sci., No. 2, 109:341-349.
5 Portland State University Proposal ID 2004-456, http://microgravityuniversity.jsc.nasa.gov/activeteams.cfm,
http://www.me.pdx.edu/~micro-g/