Runner Core Cavitation Near Full Load In Francis Turbines

摘要

Experimental evidence has shown that spiral-form cavitating vortex ropes may appear at full load of Francis turbines. The pressure fluctuations arising simultaneously with the spiraling vortices may endanger the turbine. These flow instabilities are explained in this paper based on the theory of rotating stall. By P Krishnamachar, Arpad A Fay and Saroj Rangnekar. It is well known that in Francis turbines pressure oscillations appear in the draft tubes at part-load operation. These oscillations may be so high that below a limit value part-load operation is not allowed. To avoid this limitation, or at least to lessen the limit value, continues to be an essential research target even today. In a few cases harmful instabilities were experienced even at full load. This purpose of this paper is to explain full load instabilities. Thoma (1910) and Ackeret (1930) [1] have studied this instability problem of Francis turbines but Deriaz (1960) was the first to give an explanation based on the precession movement of gyroscopes [2]. He also found close correlation between the pressure fluctuations in the draft tube and the rotating cavitation ropes observed downstream of the hub.