# Air Entrainment and Air Demand in the Spillway Tunnel at the Jinping-I Dam
## 3 Results
### Ventilation-Related Characteristics of the Air Supply System
#### 3.1.1 Average Air Velocity  
In the spillway design, air is sucked into the spillway tunnel through the intake wells, and the sizes of the air intake wells are believed to be sufficiently large to ensure adequate air supply where the air velocity is not too high to cause potential damage to the wells. For each air intake well, two measured cross sections were chosen to measure average air velocity. Eight measurement points in total were installed: three each in the No. 1 and No. 2 air intake wells, respectively, and two in the No. 3 air intake well (as shown in Figure 3). Results of the average air velocities at five different flow rates of the spillway are shown in Figure 5.  
When the flow rate in the spillway tunnel increases from 297 to 3200 m\({}^{3}\)/s, average air velocities of the No. 1 to No. 3 air intake wells increase from 12.3 to 61.1 m/s, 20.1 to 88.6 m/s, and 17.9 to 80.4 m/s, respectively, indicating a clear dependency of air velocity on the flow rate. When the flow rate is less than 2130 m\({}^{3}\)/s, the increasing trends of air velocities of all air intake wells against the flow rate are consistent. However, when the flow rate is greater than 2130 m\({}^{3}\)/s, the trends become inconsistent. With further increasing flow rate, for the No. 1 air intake well the average air velocity slightly decreases, for the No. 2 air intake well it maintains nearly constant, and for the No. 3 air intake well it continues to increase. Restriction from water-free space of spillway tunnel is responsible for the different changing trends. Air is sucked into the spillway tunnel mainly under the action of dragging force imposed by water flow; therefore, greater flow rate generally leads to greater dragging force, then a greater amount of air supply. However, it should be noted that the quantity of air supply is also restricted by cross section size of air intake well and water-free space of the spillway tunnel. When the flow rate is not very large, water depth is relatively shallow. Air quantity through the air intake well is mainly influenced by water flow rate and the air intake well itself, though the influence of the water-free space gradually increases. Thus at flow rates less than 2130 m\({}^{3}\)/s, air quantity increases with  
Figure 5: Relationship between average air velocity (\(V_{\mathrm{a}}\) w) in the air intake wells and the flow rate in spillway tunnel. increasing flow rate, as do the average air velocities in air intake wells. When the water flow rate is large enough, water depth in some section of the spillway tunnel can become very deep, that is to say, the water-free space may become very narrow. Under these conditions, the air quantity may become mainly restricted by the water-free space. Though greater water flow rate could supply greater driving force, the narrower water-free space may limit the increase of air quantity. The outlets of the No. 1 and No. 2 air intake wells are closer to the upstream, water depths in these positions are more sensitive to water flow rate. When the flow rate is greater than 2130 m\({}^{3}\)/s, water depths in these positions become very deep, the narrow water-free space prevents the air quantity from increasing, and the average air velocities in the No. 1 and No. 2 air intake wells do not increase any more. The flow rate has less influence on the water-free space near outlet of the No. 3 air intake well, so the average air velocity in it can continue increasing. The average air velocity in air intake wells increases with increasing flow rate but it may also be affected by water-free space of the spillway tunnel when the flow rate is great enough. The first measured cross section to measure average air velocity in each air intake well is located near the inlet. The air intake well inlets are all set on the hillside. Affected by the topography boundary, vortices may form near the sidewalls. These vortices would result in non-uniform distribution of the air velocity on the cross section near the inlet.