birodh manandhar

The tunnel spillway safely discharges the flood water from high rise dam. So, there is high chance of cavitation damages induced by high-speed flow due to high head and large discharge. Different types of aerators are generally introduced to protect the surface from cavitation. In this paper, the pressure distribution, water surface profile, velocity profile and aeration cavity length in free-flow tunnel of Jingping-1 hydropower projects were studied. Two type of aerators namely deflector with offset and only offset were used for the simulation study. The simulations were done by using the realizable k-ε model and volume of fluid method. The real tunnel spillway of Jingping-1 hydropower project contained the aerators by combining the deflector with offset. The results of numerical simulations were compared with experimental result which showed that the aerator with combination of deflector and offset was more reliable than aerator with aerator only offset. There was no chance of cavitation damages inside the spillway of aerator with deflector and offset as there was stable aeration cavity without backwater. There is more chance of cavitation in high velocity and high pressure head in free-flow tunnel spillway. Artificial aerators are generally introduced to reduce cavitation which is very cost-effective and easy measures. Different researches have been done in the study of aeration phenomenon. Chanson [1] explained that when large amount of air is introduced in aeration zone along with air water interfaces of jet and strong, de-aeration occurs at the impact of bottom near nappe impact. Pfisher [2] conducted a hydraulic model test with typical chute aerators with various approach flow features including pre-aeration. The study found that the stream-wise development and the bottom air concentration were affected by pre-aeration in the downstream of the aerators. Chanson [3] studied the performance of a spillway on steep spillway with high velocities. He found that if there were not enough surface aeration or if the tolerance of surfaces finish required to avoid cavitation s were too severe (v>30 m/s), aerators should be introduced at the floor or sometimes on side wall of spillway. Ramón [4] described the most important aspect of the effect that flow aeration has in combating the damages caused by cavitation as well as the natural entry of the air into the water flow and placing of the artificial aeration devices on spillway when natural aeration is insufficient. Chanson [5] established the formula for the calculation of bottom and side cavity length in sudden enlargement and vertical drop aerator. Both experimental and simulation methods were used to study the aeration in the spillways. Shuai li et al [6] studied the diffuse flow downstream of a radial sluice and full section aerator by combining a realizable k-ε turbulent model. Shuai li et al [7] studied the aeration from bottom as well as the side in connective tunnel with high head and large discharge by model test and numerical simulation and obtained the less back water effect in aeration cavity with increasing aeration length. In this paper, the combination of the realizable k-ε model and Volume of fluid (VOF) method was applied to simulate the characteristics of aerated flow of the free-flow tunnel in Jingpin-1 Hydropower Station.