Zero gravity: the quest for the perfect bubble
The future of hydroelectric dams is being decided in weightlessness! In a specially equipped Airbus, scientists from EPFL are studying the phenomenon of cavitation, which is responsible for the erosion of turbines.
Tiny bubbles are creating havoc in enormous turbines of several meters diameter. Formed by the acceleration of the blades, they eject matter at supersonic speed and gradually erode other components of the installation. This phenomenon is called cavitation, and is one of the most complex and destructive problems that hydroelectricity engineers are confronted with. To better understand it, scientists from EPFL got on board an Airbus A300. Specially equipped, the aircraft enables weightlessness for a few seconds. Unencumbered by gravity, the scientists strive to model the formation of vapour bubbles. The objective of this research is to contribute to the conception of turbines that are resistant to the bubbles.
“Cavitation remains to be elucidated”, explains Mohamed Farhat, scientist at the Laboratory for Hydraulic Machines at EPFL. “Although we don’t understand exactly why, in some instances this phenomenon doesn’t cause significant erosion, whereas in others it can cause a turbine to lose several tens of kilos of matter each year.” The result? Losses of earnings – sometimes very significant – due to the necessary refurbishment. The problem is sufficiently serious to motivate leading companies in the hydroelectric sector – including Andritz, Alstom ou Voith – to consult EPFL.
Drawn by the turbine blades, water undergoes acceleration and a loss of pressure. Tiny bubbles of vapour, about a micrometer in diameter, are formed, expand, and then implode. The shock wave, and the projected matter, gradually erode other components of the installation. “The surface [of the bubbles] is small, but the energy released is massive. The pressure is then 10,000 times higher than that of the atmosphere. Each bubble that implodes results in damage to a surface of one to two square micrometers”, explains Mohamed Farhat.
Leased by the European Space Agency, the Airbus A300 only flies twice a year. Modified to enable it to perform parabolic trajectories, it allows the occupants – for a few instants – to be free from gravity. Selected for a flight in October, the EPFL scientists succeeded in obtaining almost spherical bubbles, an impossible feat on Earth. “Gravity rather complicates things”, explains Mohamed Farhat. “In eliminating it, it becomes possible to perfect a model that explains cavitation.”
Afterwards, the scientists will be able to reintegrate gravity into their equations. In this way, it will be easier to understand why a particular model of turbine is more affected than another by cavitation, and above all to design them from the outset to limit this phenomenon. These destructive little bubbles haven’t yet finished creating havoc for scientists and engineers, according to Mohamed Farhat. “The problem is sufficiently complex to occupy a whole generation of researchers!”
Research in collaboration with Oxford University and Max Planck Institute
In 2006, Mohamed Farhat’s team had already taken part in a weightless flight in the Airbus A300. The video: