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Foton spacecraft
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4 January 2006
The Foton-M2 space flight mission, deployed in Low-Earth Orbit
between 31 May and 16 June 2005, has produced some very
interesting results in the area of fluid physics. The ARIEL
experiment, flown in ESA’s FluidPac instrument as part of the
Foton-M2 payload was investigating how the application of an
electric field to a liquid in weightlessness affects the boiling
process.
The test cell of the ARIEL experiment consisted principally of a
chemically stable fluid (FC-72) and an electrical heater. What
the experiment revealed was that when an electric field was
applied to the heating liquid in the cell in weightlessness it
was shown to reduce the amount of vapour bubbles that formed on
the heater surface and was also shown to lead vapour bubbles
away from the heater surface. The implications of this may be
obvious to the fluid physicist, but for the layman it may not be
obvious that these findings could lead to advancements in heat
transfer or cooling systems for use in space.
Boiling is an excellent means of exchanging heat between a
solid and a fluid and this principle is widely used as a method
of cooling apparatus or alternatively used for heating. One can
either make use of pumps to transport heat away from a solid
surface, especially in connection with high heat producing
equipment, or exploit the natural convection currents that occur
in every liquid. Making use of natural convection instead of
pumps can compact the design of such devices, yielding to
lightweight and less power consuming systems.
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Boiling in microgravity without
(left) and with (right) electric field
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In space we are faced with a problem. In
such a weightless environment in a liquid being subjected to a
heated surface, vapour bubbles form on the heated surface as on
Earth, such as on the base of a pan when boiling water. However,
in space there is no effect of buoyancy and gravity-induced
convective currents do not form to promote the removal of vapour
bubbles from the heated surface. Without alternative means such
as pumps, as currently used in space equipment for cooling high
heat producing equipment, this heated gas remains on the heated
surface.
Over time the amount of vapour and size of vapour bubbles
increases and can even join together to create a layer of heated
vapour between the heated surface and the liquid. This greatly
reduces the amount of liquid that comes into contact with the
heated surface, which in turn dramatically degrades the
efficiency of the heat transfer process and can lead to
overheating and damaged equipment.
The results from the ARIEL experiment are therefore a big
step forward in fluid physics, having shown a way to limit the
formation of vapour bubbles on a heated surface and promote the
removal of vapour bubbles from that surface in weightlessness by
applying an electric field. This is the first time that this
kind of experiment has been carried out in weightlessness over
such a large surface with such high heat flux levels.
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Boiling under normal gravity
with no electric field
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“The application of the electric field
proved useful in improving heat transfer performances, though
the quantitative effect on the boiling process has still to be
assessed.” said Antonio Verga, ESA’s Project Manager for
Foton missions. “These results will lead to a better
understanding of the physics of boiling phenomena and to the
assessment of techniques to design innovative heat transfer
apparatus for space applications. At the same time this
technique (acting on bubble size, frequency and motion) proved
to be quite effective for gas management and containment also in
applications other than boiling and condensation”
The ARIEL experiment flown on the Foton-M2 mission was a very
compact experimental apparatus with a mass of only 9kg. This was
housed within the FluidPac payload, an ESA facility specially
designed for carrying out fluid physics experiments in
weightlessness.
The ARIEL experiment included a test cell with a 20x20mm
heater, coated onto a semitransparent Zinc Sulphide window, and
the test fluid, FC-72, a chemically stable liquid currently used
within cooling technology on Earth. It has a similar viscosity
to water, though with a boiling point of 56°C. Five stainless
steel rods connected to a high voltage generator provided the
necessary electrical field. The cell had an additional glass
window to allow side observation of boiling patterns.
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Boiling under normal gravity
(left) and in weightlessness (right) with no electric
field and observed through heater surface
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The experiment was conducted for four
days under excellent weightless conditions and several levels of
liquid temperatures and surface heat fluxes were tested. The
production of vapour in weightlessness revealed, somewhat
unexpectedly, to be much higher than in normal gravity
conditions. According to the images transmitted, this could be
attributed to increased bubble coalescence, which hinders vapour
condensation, i.e. return to the liquid phase.
The application of an electric field confirmed to be very
effective, even at low values of voltage, in reducing bubble
size and thus vapour production and the risk of surface dryout,
as shown in the enclosed images. Finally, it enabled the vapour
bubbles to be pulled away from the heater and into conveyed into
the bulk liquid phase, thus securing the transfer of heat.
ARIEL is
part of a long-term programme of electrically assisted heat
transfer experiments, originally conceived by Prof. Walter
Grassi at the University of Pisa, Italy, more than 15 years ago.
These have utilised a variety of ESA-funded platforms for
weightlessness, which are open to the scientific community.
These include experiments on the ESA-funded Maser 7 and Maser 8
sounding rocket missions in 1996 and 1999, and on the 24th,
29th, and 32nd parabolic flight campaigns in October 1997,
November 2000 and March 2002.
The experiment, lead by Dr. Paolo Di Marco, from the same
University, during the Foton-M2 space flight, is part of an ESA
Microgravity Applications Programme (MAP99-045), aimed at
understanding the physical mechanisms that underlie the boiling
phenomena and at developing advanced heat removal systems.
An overview of the Research Programmes within ESA’s
Directorate of Human Spaceflight, Microgravity and Exploration
Programmes, including the Microgravity Applications Programme,
is available through the following link:
http://www.spaceflight.esa.int/users/index.cfm?act=default.page&level=16&page=resprogs
For more
detailed information, please contact:
Antonio Verga
Foton Project Manager
Directorate of Human Spaceflight, Microgravity and Exploration
Programmes
Noordwijk (The Netherlands)
Tel: +33 1 5369 3098
Fax: +33 1 5369 3141
Dieter Isakeit
Erasmus User Centre and Communication Office
Directorate of Human Spaceflight, Microgravity and Exploration
Programmes
Noordwijk (The Netherlands)
Tel: +31 71 565 5451
Fax: +31 71 565 8008
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