Abstract
This research proposes an air-injection treatment for the cavitation phenomena in the Kaplan turbine. An unsteady numerical model is created to predict the cavitating flow through a 3-in. axial hydroturbine before and after the air injection. Pressurized air (jet-in-crossflow) injection is set to be through the turbine housing, and the configuration was altered between 1, 2, 4, and 12 circumferential jets to test the effects of air mass flowrate and injection distribution. Interactions between three fluids (liquid water, water vapor, and air) were considered by utilizing the physics models of volume of fluid (VOF) multiphase, cavitation, and large eddy simulation (LES) turbulence. Surface and time-average results are to be compared with a baseline case of pure cavitation. With the vapor volume fraction created on the rotor components during the cavitation, the absolute pressure scenes clarified the connection between the air treatment and cavitation reduction. While rotation causes negative pressure in the system, the injected air is sucked to such low-pressure zones, and consequently, increasing the absolute pressure above the vapor limit reduces the vapor formation. Preliminary outcomes at 1000 rpm show a 55% reduction in the formed vapor on the blades after air injection by a time corresponding to six cycles of the turbine. A corresponding mechanical power of 12% increase was observed. Moreover, the curve fitting of the data shows that the vapor reduction and power regain are in second order correlation with the increased air volume.
