What no bubbles? Results of the ARIEL experiment from Foton-M2 mission

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.

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.

“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.

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