Page - 837 - in Book of Full Papers - Symposium Hydro Engineering

Two different gate openings were tested for both BOs: a = 0.05, 0.1 m for the old BO and a = 0.1, 0.2 m for the new BO, respectively. Only one BO was in operation at a time. The reservoir water level varied from 980 to 980.9 m asl. during the test, resulting in an average static pressure head of HE = 84.7 m w.c. for the old BO and HE = 81.6 m w.c for the new BO. An average air temperature Ta of 9°C was used to calculate ρa, resulting in ρa = 1.12 kg/m3 at ~900 m asl. To show the overall trend, the measurements were averaged over 5 s in the following figures. Additionally, the original 10 Hz measurements are shown as thin lines. To calculate characteristic numbers (e.g. β, ζ), the measurement were averaged over the duration of each gate opening. ζ was calculated as ζ = 2Δpa/(ρaUa,ov2) −1, where Δpa is the air pressure drop in the gate chamber Δpa = pa(t=0) − pa(t). The Qw- values for the calculation of β = Qa,o/Qw were provided by the dam operator. 3.3. RESULTS OF BOTTOM OUTLETS MALVAGLIA Table 2 summarizes the measurement results and Fig. 4 shows detailed results for the new BO. If only the new BO is in operation, an equal amount of air is supplied through the air vent and the old BO tunnel (U1 and U2 in Fig. 4a). Ua,o is slightly higher for a = 0.1 m compared to a = 0.2 m. This can partly be explained by the transition from spray flow to free-surface flow for a = 0.2 m (Fig. 5). The sudden rise in Ua,o during the closing of the gate is also caused by a more intense spray formation for a < 0.1 m. For spray flow, i.e. a = 0.1 m, air is flowing out of the tunnel end, whereas for a = 0.2 m air is entering the tunnel from downstream (Fig. 4b). Ua,u is significantly lower than the air-water mixture velocity of ~12 m/s (estimated after [1]). The drop of pa is clearly visible in the gate chamber (P1) and to a lesser extent after half of the tunnel length (P2) (Fig. 4). At P1, more pronounced pressure transients were observed, particularly during gate closure, whereas these transients have already disappeared at P2. No considerable variation in pressure can be observed in the tunnel conjunction (P3), showing that the conjunction is sufficiently aerated from downstream. Table 2 Overview of the measurement results in Malvaglia name Qw [m 3/s] a [m] a/amax [-] Fc [-] Qa,o [m 3/s] Δpa [mbar] ζ [-] β [-] new BO 2.5 0.1 0.08 52 17.7 5.2 2.6 7.1 new BO 5.0 0.2 0.15 36 15.0 3.9 2.8 3.0 new BO 2.5 0.1 0.08 52 17.1 6.3 3.7 6.8 old BO 2.4 0.05 0.03 61 15.2 7.5 5.8 6.3 old BO 5.0 0.1 0.06 43 27.1 23.7 5.8 5.4 old BO 2.4 0.05 0.03 61 11.7 4.5 5.9 4.9 For the old BO no transition to free-surface flow was observed for the larger gate opening a = 0.1 m. On the contrary, the old BO showed an even stronger spray formation for a = 0.1 m (Fig. 5). Consequently Qa,o increased with increasing a for the old BO. 837