Experimental investigation of air injection in high head Francis turbines

Pressure pulsations caused by unsteady flow can lead to instability during operation. Air injection into the turbine draft tube is a well known method used to mitigate pressure pulsations. Investigation of two methods of air injection into the high head turbines at the La Higuera powerplant were carried out for part load operation. The turbines have a nominal rating of 77 MW each and operate under a 375 m head. The two methods investigated were: air injection through the upper cover; and, air injection through the draft tube wall. Pressure pulsations were measured at the inlet and in the draft tube and the results are presented here.