*3.5. Flow Patterns in Blade Passage and Draft Tube*

Even if the radial velocity at the runner outlet cannot be exactly monitored like that at the inlet, the outlet backflows can be observed clearly from the flow patterns in the draft-tube and near the blade suction side as seen in Figures 13–16. After the working point enters the S-shaped region, the streamlines in the blade channels are no longer as smooth as before. The main flow will enter the draft-tube along the side wall, or return to the runner from the draft-tube center, due to the changes in rotational speed and discharge [31]. As mentioned before, whether or not the working point enters the RP mode can lead to large differences in flow patterns, which has no exception at the runner outlet. For PT-1 and PT-2 (Figures 13 and 14), although the total flow rate is mainly in the turbine direction, the main stream water flow attacks the blade suction side from the draft-tube center, because of the increase of the pumping effect. Some water jumps into the nearby runner channel, and some go back to the draft-tube. Also, this phenomenon will be very obvious when the minimum discharge condition is approached. But in PT-3 and PT-4 Figures 15 and 16, the working points also enter the RP mode, and the flowing directions reverse to the pump direction. At this time, a part of water flow enters the upstream along the suction surface, and a part escapes to the next blade channel, and a little water returns to the draft-tube.

**Figure 14.** Flow patterns in PT-2. (**a**) *t* = 0.1 s, (**b**) *t* = 7.0 s, and (**c**) *t* = 15.0 s.

(**a**) (**b**) (**c**)

**Figure 15.** Flow patterns in PT-3. (**a**) *t* = 5.0 s, (**b**) *t* = 10.0 s, and (**c**) *t* = 13.0 s.

**Figure 16.** Flow patterns in PT-4. (**a**) *t* = 4.0 s, (**b**) *t* = 5.8 s, and (**c**) *t* = 7.4 s.
