velocity V*. Fig. 15 depicts effect of downstream water level on
turbine performances in V* ¼ 0.93 m/s. The performance degradation in high efficiency range occurs when the downstream water
level Hd becomes 40 cm although in the case of V* ¼ 1.07 m/s the
degradation happens for Hd ¼ 42 cm as shown in Fig. 9. A runner
rotational speed U in V* ¼ 1.07 m/s is faster than that in
V* ¼ 0.93 m/s even if both operations are under a similar point for
speed ratio U/V* which generally decides runner performances. The
interference with water surface by faster U involves the increment
in flow loss. Consequently the degradation of turbine performances
becomes sensitive to decrement of the water level in the case of
larger flow rate.
5. Concluding remarks
The performance of Darrieus-type hydro turbine with the inlet
nozzle and simplified runner casing was examined experimentally.
In addition, effect of downstream water level on turbine performances was investigated in consideration of flow rate variation at
the practical use.
(1) Provided the inlet nozzle is installed to obtain effective
generated torque in the high efficiency blade rotating positions,
it is possible to simplify the runner casing drastically with
keeping the turbine performances higher.
(2) The performance of simplified Darrieus-type hydro turbine is
affected by downstream water level. In the case of higher water
level than the runner height, the operation at the best efficiency point is achieved even if the runner casing is simplified.
On the other hand, when the water level becomes lower than
the runner height, turbine efficiency extremely decrease,
additionally it is impossible to operate at high efficiency point.