Just add bubbles for cooler future spacecraft
From soft drinks to hot tubs, people add bubbles to liquids for many different reasons. ESA engineers think bubbles produced at the verge of boiling point could help control the temperature of spacecraft in a more efficient and compact way. The main unknown is how bubbles will behave in differing gravities down to weightlessness, so researchers boarded parabolic flight aircraft for testing.
Keeping systems cool in space is a major engineering problem. Both machinery and people produce waste heat as they operate, but in the same way a vacuum flask keeps your coffee hot, the surrounding space environment prevents this heat from dissipating from spacecraft.
The International Space Station (ISS), like many other crewed spacecraft, uses liquid circulated through tubes to absorb heat and pass it to external radiators – known as ‘single phase cooling’.
But ESA engineers are now looking into a more efficient variation, called ‘two phase cooling’ – already widely employed in terrestrial power plants and petrochemical processes.
It is called ‘two phase’ because it employs two separate phases of matter: liquids and gases. Counter-intuitively, the coolant liquid is heated to the verge of boiling point as it flows through the structure being cooled. Contact with waste heat makes the liquid boil into bubbles of vapour. This ‘flow boiling’ approach can transport heat away much more quickly and efficiently.
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“The heightened efficiency of heat transfer in boiling promises more compact and lighter thermal control system devices,” comments Sebastien Vincent-Bonnieu of ESA’s SciSpacE team. “Boiling is a complex phenomenon however, combining heat and mass transfers, hydrodynamics, instabilities and interfacial phenomena.
“Furthermore, gravity affects the fluid dynamics and may lead to unpredictable performances of thermal management systems in microgravity conditions. We therefore decided to perform experiments directly in (near) weightless environments using on parabolic flights aboard the 'Zero-G' Airbus A320.”
Supported by ESA and Airbus Defence and Space, Prof. Catherine Colin's team at the Institute of Fluid Mechanics of Toulouse (IMFT) designed a small compact loop for the study of flow boiling in weightlessness, fitting a circuit with a pump, a flow meter, preheaters, a condenser and associated tubing within the strictly limited volume available.
“This loop is proposed for flight aboard the ISS’s Columbus module in the future,” explains Balazs Toth, ESA payload system engineer.
“To validate the design, a test version has been flown in ESA and CNES parabolic flight campaigns - the former producing bubbly flows in Moon, Mars and terrestrial gravities as well as in microgravity,” comments Olaf Schoele-Schulz, Project Manager from Airbus. “The experimental device proved fully operational, opening the way to follow-up investigations in-orbit.”
Cameras placed within the test device showed the bubbling liquid’s flow was unperturbed even as it moved a short distance downstream from 90° ‘elbow’ bends in the loop.
Boiling point
Microgravity boiling is a topic being probed by multiple European teams, variously performing experimental, theoretical and numerical research, coordinated through the Multiscale Analysis of Boiling Microgravity Application Programme or MAP project, overseen by Prof. Colin.
“Understanding and modelling of the boiling phenomenon requires complementary investigations at different scales: from microscopic scale with isolated bubbles to macroscale with flow boiling in a tube,” adds Julien Sebilleau from IMFT. “The Multiscale Boiling RUBI payload launched to the ISS in July 2019 and operated for a year and a half has been investigating boiling at individual bubble scale.”
“Parabolic flights have meanwhile enabled larger-scale testing, but to go further, longer and better requires prolonged microgravity conditions,” adds Sebastien. “So we are looking forward to flying our flow-boiling experiment aboard the ISS.”
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