# Air Entrainment and Air Demand in the Spillway Tunnel at the Jinping-I Dam
## 3 Results
### Aeration-Related Characteristics of Aeration Devices
#### 3.2.3 Air Concentration  
Air concentration is a direct parameter to evaluate aeration effect of water flow. Eleven measurement points were installed to measure air concentration of water flow at different locations, namely: three downstream of No. 1 to No. 3 aerators, and two downstream of No. 4 aerator (as shown in Figure 3). Field results of air concentration at five different flow rates are listed in Table 2.  
Whether the measurement point is submerged in water or not is related to flow rate. Air concentration of the measurement point can be used to identify its relative position. Generally, air concentration in the air-water interface is 50-60% [32]. If the air concentration of the measurement point is less than 50%, the measurement point is regarded as submerged in water; and if it is greater than 60%, the measurement point is regarded as in cavity or water-free space. As the flow rate increases, water depth in the spillway tunnel increases, then some measurement points on sidewalls may \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Series**} & \multirow{2}{*}{**Q\({}_{w}\) (m\({}^{3}\)/s)**} & \multicolumn{3}{c}{**No. 1 Aerator**} & \multicolumn{3}{c}{**No. 2 Aerator**} & \multicolumn{3}{c}{**No. 3 Aerator**} & \multicolumn{3}{c}{**No. 4 Aerator**} \\  & & \multicolumn{3}{c}{**Downstream**} & \multicolumn{3}{c}{**Downstream**} & \multicolumn{3}{c}{**Downstream**} & \multicolumn{3}{c}{**Downstream**} & \multicolumn{3}{c}{**Downstream**} \\ \cline{3-11}  & & **C1** & **C2** & **C3** & **C4** & **C5** & **C6** & **C7** & **C8** & **C9** & **C10** & **C11** \\ \hline
1 & 297 & 29.5 & 37.7 & 80.82 & 49.5 & 85.2 & 51.7 & 12.1 & 79.3 & 96.9 & 94.8 & 60.1 \\
2 & 791 & 32.4 & 12.4 & 57.9 & 27.99 & 71.4 & 41.3 & 11.7 & 6.2 & 52.3 & 63.8 & 42.4 \\
3 & 1462 & 31.6 & 2.8 & 7.6 & 20.4 & 12.0 & 67.4 & 11.0 & 3.9 & 7.2 & 70.5 & 50.4 \\
4 & 2130 & 29.2 & 1.9 & 3.6 & 20.7 & 1.8 & 68.0 & 9.4 & 3.1 & 5.9 & 84.4 & 48.1 \\
5 & 3200 & 26.5 & 1.5 & 2.9 & 11.7 & 0.70 & 63.7 & 8.5 & 2.6 & 4.0 & 77.5 & 27.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Air concentration (%) along the spillway tunnel at different flow rates. Figure 9: Relationship between cavity sub-pressure (\(\Delta P\)) and the flow rate in spillway tunnel. be gradually submerged in water flow from the water-free space or cavity. Air concentrations of these measurement points correspondingly decrease. According to the above-mentioned criterion, for measurement points located downstream of the No. 1 aerator, C1 and C2 are always submerged in water, and C3 is gradually submerged in water flow as flow rate increases. For measurement points located downstream of the No. 2 aerator, C4 and C5 are gradually submerged in water flow as flow rate increases, and C6 is always upon water flow. For measurement points located downstream of the No. 3 aerator, C7 is always submerged in water, and C8 and C9 are gradually submerged into water from the water-free space. For measurement points located downstream of No. 4 aerator, C10 is always in the cavity considering the relatively high air content, while C11 gradually enters into water from the edge of cavity. Air concentration at the end of protective range of aerator can be used to determine whether the distance between two aerators is reasonable. The air concentration close to the bottom gradually decreases along the flow direction. When it decays to a small value, another aerator should be set up at that position. C2 and C8 were located on the sidewalls at the end of the protective ranges of the No. 1 and No. 3 aerators, respectively, and they were both 1.1 m away the bottom. The value of C2 is less than 3% when the flow rate is greater than 1462 m\({}^{3}\)/s, and the value of C8 is close to 3% when the flow rate is greater than 2130 m\({}^{3}\)/s. According to a study by [PERSON] [9], such low air concentrations perhaps cannot effectively play a protective role, and the arrangement of the aerator in the spillway tunnel seems to be unsatisfactory.