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Ultrasound can reduce air bubbles caused by hydropower

Published on: 27. September 2021

Hydropower plants can cause air bubbles in the river, causing fish to die of “Gas Bubble Disease”, similar to diver’s disease in humans. Researchers have now tested a new method to reduce this problem. 

Ultrasound can reduce air bubbles caused by hydropower

The phenomenon is called gas supersaturation and happens then when high amounts of air get trapped and pressurized inside the hydropower tunnel and is released in the river downstream of the power plant.  This has happened several times in Norway and is lethal to fish. It also impacts surrounding habitats.  

To find this solution the researchers suspected that power ultrasound could be a solution to degas the water before it affects life in the river. The researchers were inspired by other industries where the method is already used in metallurgy and water purification to degas aluminum and to help separate dirt from water. 

Power ultrasound is a low-frequency, high-intensity ultrasound wave. It can initiate so-called “acoustic cavitation”, which means that the air bubbles collapse when the ultrasound waves travel through the water. This enhances the natural degassing process and lowers the air saturation level in the water. 

Laboratory tests 

To test this theory, researchers built a test rig in the Waterpower laboratory at NTNU in Trondheim and did various experiments testing various levels of ultrasound. They have also tested other parameters that affect the degassing process, such as sound intensity, gas saturation level, and flow velocities. 

As part of her Master thesis Vera Gütle, together with Ph.D. Candidate Wolf Ludwig Kuhn developed an experimental procedure to produce and degas gas supersaturated water.  

In the first setup different parameters to increase the amount of total dissolved gas (TDG ) in water were tested. Then, this gas supersaturated water was used to test the degassing effect of power ultrasound in an open flow channel. She evaluated the effect and that the degassing effect depends on the flow velocity, intensity, and the initial TDG. Contrary to intuition the highest level does not lead to the most degassing due to a bubble screen forming around the ultrasonic emitter which dampens the effect.  

 The experimental campaign uncovered some uncertainties in both the test rig design and the sensors used. The test rig is now under revision to cope with the uncertainties, and more reliable sensors are being investigated for use in the next experimental campaign. 

Contact:

Ole Gunnar Dahlhaug, project leader

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