Ultrasonic degassing of total dissolved gas supersaturated, flowing water in a laboratory flume

Abstract

Abstract Artificial total dissolved gas (TDG) supersaturation is a potentially lethal threat to the aquatic ecology downstream of hydropower plants. The severity of this threat dependents on the actual saturation level as well as on the duration of the event. Natural degassing is highly influenced by the river’s morphology and characteristics, and is in most cases insufficiently slow. To protect fish and aquatic invertebrates from gas bubble disease, a direct consequence of TDG supersaturation, the use of ultrasonic degassing to mitigate TDG supersaturation is investigated. In this process, a high-power ultrasonic field is introduced into the water, leading to an immediate reduction of the TDG saturation level. Experiments are conducted at the Waterpower Laboratory at NTNU, Norway. Within an open flume, water is channeled past an ultrasonic transducer, which introduces an acoustic field with a frequency of 20 kHz. The flow rate in the flume is adjustable, and two different flow rates are tested. In addition, the dependency of the degassing process on the acoustic power is investigated. Results indicate a strong dependency on the acoustic power, with the highest tested powers resulting in the largest reduction in TDG saturation level. The flow velocity appears to have a positive effect on the degassing efficiency as well, even though this effect is minor compared to the effect of the acoustic power. A continuous effect of the acoustic degassing in form of a gas bubble cloud leading to increased liquid-gas mass transfer is observed.