Visualization method for volume void fraction measurements in gas-liquid two-phase flows of a water turbine outlet channel

• Main Authors: Klasinc, R, Hocevar, M, Baicar, T, Sirok, B, Doinikov, A A, Dayton, P A
• Format: Conference Proceeding
• Language: English
• Subjects: Air bubbles; Degassing; Hydraulic turbines; Image acquisition; Short circuits; Tailwater; Turbines; Two phase flow; Visualization; Void fraction
• ISSN: 1746-4471; 1743-3533
• DOI: 10.2495/MPF070241

The Pelton turbines are the part of the pump-storage scheme that is working under permanent backpressure conditions. The knowledge of air-absorbing and degassing in the vessel which follows the turbine chamber is important for the safe and economical working of the turbine. In the case of so called hydraulic short circuit the influence of air bubbles for the safe operation of turbines and pumps is very important. To estimate the volume void fraction in two-phase flow, the measurement method was developed, which is based on the computer-aided visualization. Acquisition of flow images by a fast video camera followed by the computer processing of the acquired images enables the determination of the void fraction via the average value of grey level intensity of the acquired flow images. A relationship between the void fracture and the average grey level intensity was obtained through calibration of the measurement system. Simultaneous measurements with the other measurement method revealed suitability and applicability of the visualization method in a real industrial environment such as a tailwater channel of a Pelton turbine. Apart from that, a study revealed that by further processing the images a number of other important information on the flow, such as bubble size and shape, average bubble direction and velocity, can be obtained by the described method.