Study On Vortex-Flow In Drop Shaft

Using a vortex-flow drop shaft to convey water from a high level to a diversion runnel under the condition of high water-head and large discharge is a new kind of technology in the scheme of reusing a diversion tunnel as a discharge tunnel. In this thesis hydraulic experimental studies and theoretical analyses on the vortex-flow drop shaft were conduced and the following results were accomplished: (1) In the model test some hydraulic characteristics including the shape of air core in the center of the drop shaft, the radial distribution of water flow velocity and the distribution of pressure on the wall of the drop shaft were measured under the condition of high water head above 250m and large discharge above 1400m3/s. (2) A kind of computational method for the spiral water flow in the air core region of drop shaft was discussed and applied and the results agree the experimental data well. (3) The total ratio of energy dissipation in the drop shaft can reach 90% and the energy dissipation in every region was discussed quantitatively. (4) Model tests showed that the perfect depth of stilling basin under the drop shaft should exceed 0.9D(D-diameter of the drop shaft). (5) A kind of method for predicting the amount of entrained air in the water of drop shaft was induced from the experimental data, which can be consulted during the design of air vent pipes on diversion tunnels. (6) Under the conditions of high water head and large discharge, the velocity of water flow at the bottom of air core region of drop shaft is quite higher, therefore, although the pressure is positive along the wall of drop shaft due to the centrifugal force created by swirling water flow, the cavitation number is much lower and cavitation of the drop shaft is an important problem that has been paid much attention. A kind of measurement was developed to measure the air concentration of water and the result showed the possibility of entrained air existing near the wall of drop shaft. That is to say, the wall of drop shaft may be protected by entrained air from cavitation damage to some extent.