Hydraulic Characteristics Of Flow Aeration Type To Prevent Cavitation Erosion

In this paper, the experimental investigation on flow aeration types to prevent cavitation erosion was summarized, and the application and advances in this respect were also reviewed. Based on the emptying culvert's model test for a large hydraulic engineering, the experimental investigation and theoretical study of bucket-drop type was carried out systematically. The main work and achievement are concluded as following: 1. According to the engineering experience and first-step calculation about water surface profile and flow velocity along the tunnel, the model scale and the location of flow aeration type to prevent cavitation erosion had been determined. 2. By comparing the hydraulic characteristics of some flow aeration types to prevent cavitation erosion, a suitable type had been chosen for the free surface flow tunnel with large discharge, low Froude number and slight slope. According to the engineering's special demand itselfly, several bucket-drop shapes were designed and tested in model, thus the best one was picked up finally. 3. The backwater, if existing in the chamber, would obviously affect the hydraulic characters of the aerator. In order to get rid of the backwater in the tunnel and further increase the length of the aerated nappe, the downstream slope of the flow aeration type was steepped in the certain range. On the same time, the incident angle and impact force of the jet flow to the bottom were also calculated. 4. The main factor to affect the capacity of air entrainment is the length of aerated nappe, so it is important to calculate the length. Some methods including empirical relationship and theoretical formula were given. The theoretical formula usually appears in two forms. One originates from the trajectory, the other begins with the fluid element analysis. Finally, the good correspondance between the computed results and the experimental ones shows that the theoretical formula can be widely used in practice. 5. The empirical relationship for quantity of air entrainment was established by fitting the relationship among the length of the aerated nappe, the velocity and the quantity of air entrainment from the modal test data. The quantity of air entrainment is constituted of two parts: one comes from air-water mixture on the bottom of jet flow; the other comes from the strong turbulence due to the impact action. 6. The calculation of protective range against cavitation erosion is a very difficult problem, so the empirical relationship from both the modal test and the prototype observation was put into use in practice. Because the data from the prototype observation are very limited and the modal data cannot completely predict the prototype ones because of the scale effect, the empirical formula has some shortcomings. In this paper, the random walk model—Markov model was used to calculate the aeration concentration distribution along the downstream from the flow aeration type. The computed results fit well with the experimental data, which shows that the mathematical model is a useful method to predict the protective range against cavitation erosion.