(84a) Rip Currents in Microgravity

Rip
Currents in Microgravity

Thao T.T. Nguyen, Joel L.
Plawsky, and Peter C. Wayner, Jr.

Department of Chemical
and Biological Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA.

 

The Constrained Vapor Bubble (CVB)
heat pipe is a wickless heat pipe that is designed to produce a simple, light,
and reliable heat transfer system that can be used for cooling critical
components of spacecraft. The CVB system consists of a relatively simple setup
- a quartz cuvette with sharp corners partially filled with pure pentane as the
working fluid. A full-scale fluid experiment was conducted on the International
Space Station (ISS) to provide a better understanding of how the microgravity
environment might alter the physical and interfacial forces driving evaporation
and condensation.  Along with temperature and pressure measurements, the Light
Microscopy Module (LMM) was used to determine the two-dimensional thickness
profile of the menisci formed on the wall surfaces as well as at the corners of
the cuvette. Interfacial forces dominate in these extremely small Bond number
systems. The transport processes were found to be complex despite being
conceptually simple.

 

 

Fig. 1. Fluid flow in the interfacial flow
region of the heat pipe.

 

The absence of gravity gives rise to
the effect of the Marangoni force that drives the fluid from the hot region to
the cold region. In the CVB heat pipe, the two opposite, competitive capillary
and Marangoni flows generated a thick drop of liquid on the wall surface called
the “central drop”. This liquid drop creates a third zone in the heat pipe, in
addition to the two traditional zones (evaporation and condensation), called
the “interfacial flow region”. Using the interferometry technique on
10X-magnification images together with different analytical techniques, we resolved
the full picture of how fluid flows in this “interfacial flow region” (Fig. 1).
The two strong capillary and Marangoni flows from the cold and hot ends generated
a high pressure inside the central drop which leads to the formation of a
liquid current in the middle of the wall surface toward the heater end. Along
the edges of the central drop and the liquid current, the liquid flows from the
center to the edges to replenish the liquid lost due to evaporation. Due to the
large temperature gradient, the Marangoni force drives the liquid in the middle
of the wall in the opposite direction of the liquid current generated by the
pressure gradient. Close to the central drop, when the jet flow is still
strong, the “minor” effect of the Marangoni flow creates a “liquid pump” where
the liquid film is thickest (the red circle). In the region near the heater end,
the Marangoni flow dominates. In this region, the Marangoni force drives the
liquid from the heater end to the colder end and from the center of the wall to
the corners of the heat pipe. This creates a thin liquid film across the wall
surface and replenish the flow in the corners going to the central drop.

 

The liquid current on the wall surface
from the central drop to the heater end has a pressure gradient similar to the
pressure gradient driving the liquid from the cold end to the hot end along the
four corners of the heat pipe. This is a very strong current for a micro heat
pipe. This current mimics the “rip current” effect along a shoreline; a strong
and narrow current of water moving perpendicularly away from the shore. This
rip current is also the strongest near the “feeder” and then disperses
sideways. In the same way that the rip current near the shore is fed by the
surface water pushed towards the land by breaking waves along the sides, the
rip current in the CVB heat pipe is fed by the Marangoni and capillary flows pushing
the working fluid from the two sides. A model was developed to further
understand the formation and the fluid mechanics of this rip current in the CVB
heat pipe.

 

This material is based on the work
supported by the National Aeronautics and Space Administration (NASA) under
Grant No. NNX13AQ78G and the National Science Foundation under Grant No.
CBET-1603318.