Analysis on energy dissipation effect and influencing factors of modified vertical stilling well

The vertical stilling well can effectively eliminate excess energy in long distance pipeline project. The energy dissipation mechanism is based on the impact of jet flow on the bottom of the well, the diffusion of water flow in the well and the friction between water jets. In this study, we compared the energy dissipation effect of 3 different forms of vertical stilling well in order to design an optimized structure for vertical stilling well. Theory analysis and experimental observation were combined. The 3 types of vertical stilling well included traditional type, improved I type and improved II type. The model test of vertical stilling well was set up in water conservancy hall of Shihezi University in Xinjiang. The relevant hydraulic parameters such as inlet and outlet pipeline pressure, the flow and pressure on the bottom of the vertical stilling well were measured. The head loss coefficient and the energy dissipation rate were calculated. The relationship between the structural parameters of multi-hole outlet and the head loss coefficient as well as that between the height of overflow plate and the head loss coefficient were analyzed. The results showed that flow regime of the improved type II vertical stilling well was more complex than the other types, and the collision and mixing between water flows were more sufficient because of the existence of multi-hole outlet and additional overflow plate. The improved II type vertical stilling well had more than 30% energy dissipation rate than the traditional type. If the improved type II vertical stilling was adopted, the flow over weir was free outflow when the flow rate was very small. The head loss coefficient would increase with flow rate until submerging and flowing out. When the relative aperture area remained unchanged, the influence of the aperture diameter on the head loss coefficient of the stilling wells was small, and the relative aperture was better controlled at about 12%. When the aperture distance-to-diameter ratio was small, the head loss coefficient was large. But when the distance-to-diameter ratio was not less than 2.5, the head loss coefficient also remained basically unchanged. Considering the practical engineering application, the distance-to-diameter ratio should be 3. The head loss coefficient decreased slightly with the increase of aperture, and the head loss coefficient of the staggered arrangement was obviously larger than that of the parallel arrangement. When submerging and flowing out the height of overflow plate had negative effects, otherwise, the height of overflow plate had no obvious effect on the head loss coefficient. The pressure around the bottom plate of the improved II type vertical stilling well was slightly higher than that of the traditional stilling well, and the pressure at the center of the well was slightly lower than static pressure of the well, but the pressure on the bottom plate was distributed evenly on the whole, which could prevent the scouring damage of the bottom plate caused by the inlet pipeline flow and the cavitation damage caused by the negative pressure, thus the durability of the stilling well would be improved. Results above suggested that the improved type II vertical stilling well did not only improve the energy dissipation effect, but also effectively prevented the flow from scouring and damaging the bottom plate, which had obvious advantages in structure. This study provides valuable information for the engineering design of modified vertical stilling wells and for solving the problem of energy dissipation in long-distance pipeline water delivery.