Modelling of air entrainment in dropshafts

The vertical shaft is one of the most important hydraulic structures that is often used in urban drainage systems. The function of the vertical shaft is to transfer water from an arbitrary level to lower levels.The issue of air entering the vertical shaft is very important. The transfer of a significant amount of air into the pressurized tubes can lead to the formation of high-pressure air masses, which gradually grow and may burst into the tubes as they enlarge. (falvey, 1980) which causes damage to the shaft and the pipes that connected to it and also reduces the shaft permeability, which is also undesirable.Due to the lack of attention to the hydraulics of the vertical shaft and the consequences of its incorrect construction, the study on the amount of inlet air seems necessary. In this study, measuring the amount of inlet air at the intensity of different flows is desired. Finding the minimum amount of incoming air can help to better design the projectile shaft. By comparing the behavior of the projectile shaft in different states, the correction efficiency of the shaft can be obtained.

The model is adapted from a laboratory study conducted at the University of Alberta.The vertical shaft under study has a height of 7.72 meters and a diameter of 0.38 meters. The diameter of the inlet pipe of the shaft is half the diameter of its main body (0.19 m) and the diameter of the outlet pipe and its length are equal to 0.38 m and 1.5 m, respectively, and the flow of outlet water is discharged into the open air. the upper part of the shaft is blocked and the ambient air is allowed to enter the shaft only with a circular tube with a diameter of 0.10 m located at 0.20 m above the shaft. An air shaft with a diameter of 0.15 m is located at 0.5 m from the shaft outlet for air circulation, which is connected to the upper part of the shaft.The software used in this research is open foam.In this networking, a total of 121947 elements are used. The KOmegaSST turbulence model is used to solve the current turbulence term. Boundary conditions are defined for velocity (u), pressure (p), fluid type index (α) and turbulence (k, omega, nut) parameters. The initial value of velocity and pressure was assumed to be zero. To consider the initial value of the fluid type index, the number zero is entered so that this shaft is empty of water at first.

as the speed increases, the pressure decreases sharply, causing the pressure inside the shaft to become negative and air to be drawn in from the outside into the shaft.The more intense the water inlet flow, the more air enters the shaft from the surrounding environment. As the dimensionless flow of water inflow increases, the relative demand decreases by 85%.As the inlet current increases, the amount of return air through the air shaft into the vertical shaft increases. As the inlet current intensifies at low current intensities, the amount of air supplied by the air shaft increases, but in higher current intensities this value is almost constant.As the amount of air circulated by this shaft increases, the pressure gradient between this point and the upper part of the shaft is expected to increase. As the water inlet velocity increases, it can be seen that as the inlet flow rate increases more, both the amount of air supplied by the air shaft and the amount of inlet air from outside the shaft do not change significantly, which can create a semi-closed area. At the top of the shaft to allow air to flow and move down.The pressure inside the shaft increases from the bottom to the top and the highest pressure gradient is seen in the middle points, which are called the rainfall area.There is not much difference in the intensity of low inlet currents between the performance of the two types of shafts, but as the amount of inlet water to the shaft increases, the effect of the air shaft on reducing the amount of inlet air becomes apparent.
the pressures inside the shaft are almost equal at low current intensities and are spaced apart at high current intensities.

The amount of inlet air increases with increasing intensity of water inlet flow. By increasing the intensity the amount of air demand decreases.the air shaft performs better at higher current intensities.There is not much difference in the intensity of low inlet currents between the performance of the two types of shafts, but as the amount of inlet water to the shaft increases, the effect of the air shaft on reducing the amount of inlet air becomes apparent. Also, the value of the pressure in the modified shaft is clearly reduced.