Study On The Atomized Flow In Hydropower Station

The characteristics of atomized flow in hydropower station have been studied in this paper. The atomized flow is a new subject issued in recent twenty years, which means an unnatural rain process and mist flow while water flows through the discharge structure at high-speed. In some hydraulic projects once flood discharges there exists a relatively large area downstream of the dam where violent wind and driving rain occurs, and the sky turns dark, thence normal operation of the hydraulic projects may be heavily affected, and sometimes power failure, building inundation and landslide even occur. The purpose to study the atomized flow is just to prevent its harm. For the dam with water head of 100m scale the atomized flow problem is relatively severe, the planning Xiluodu Hydropower Station and Jingping Hydropower Station are super dam with the dam height of 300m scale, the atomized flow problem would be more severe. Therefore, the investigation of the atomized flow is of great importance in engineering. On the other hand, the water jet in atomized flow is water-air two phase flow, yet in the diffusion region the atomized flow is air-water two phase flow, thence the investigation of atomized flow is also significant in multi-phase flow theory. On the basis of the previous work, a systematic study has been done for the atomized flow problems by theoretical analysis, model test and numerical simulation, and the atomized flow problems in Nuozadu Hydropower Station has also been investigated. The main results got in this paper are as follows:1. People previously only want to know the jumping distance of the water jet owing to application in engineering, the criterion for air entrainment into the water jet has not been taken into account, yet this condition is very important in the study of atomized flow. In this paper the criterion for air entrainment into the water jet was analyzed based on the Kelvin-Helmholtz instability theory, both the critical velocity and the critical Weber number were got. The experimental results for air entrainment into the open channel flow and the instability of the interface of oil and water were used to verify the theoretical results for the critical velocity of air entrainment.2. Based on the statistical theory of turbulent flow an expression for the amount of air entrained into the jet was got, and the experimental results for oil entrained into water and the variation of the jet thickness were used to verify the theoretical results.3. The velocity of jumping jet and the surrounding air in high dam is generally very high, the governing equations for the jumping jet were got based on the multi-phase flow theory in two cases : with air entrainment and without air entrainment. Under the assumption that there exist no shift velocity between water and air and there exist similarity during the development of the jet, the simplified ID and 2D model for the jumping jet were got. Air entrainment and air resistance were considered in the mathematical model. For the ID model, the prototype observation data of six hydropower stations including Zhexi Hydropower Station, Huanren Hydropower Station and Fengman Hydropower Station, was used to test the mathematical model. For the 2D model the experimental results and prototype observation data of Zhexi Hydropower Station, the experimental results of Nuozadu Hydropower Station, and the experimental results of energy dissipation in the open air were used to verify the mathematical model.4. Theoretical analysis and model test were carried for the evolution of the configuration of the jumping jet. For the theoretical results the trajectory and vertical thickness of broad and thin jet were got. In the experiment the overflow spillway of Dongjiang Hydropower Station was used for study background, the main characteristics of the variation of the section topography were got. The experimental results shows that the lateral width of the jumping jet would increase while the jet jumps from the bucket, and based on the vertical variation the jet can be classified into three stages: dense stage, diffusion stage and breaking stage. The relative increase of cross section would be larger than that of wet length, and the higher the upstream water level, the larger the relative increase of cross section and wet length would be.5. The impact of the water drop at the outer edge of the aerated jet against the downstream water surface was analyzed, and the motion of the splash water dropwas investigated with the momentum and energy equation. Furthermore, a new formula for the calculation of the splash length was suggested, which is in good agreement with the data of model test and prototype observation.6. Prototype observation is the very important tool to study atomized flow. The results for the jumping jet and splash were used to correct the original mathematical model of atomized flow, and the prototype observation data of Wujiangdu Hydropower Station, Dongjiang Hydropower Station as well as Xiangkeyan Hydropower Station were used to verify the new mathematical model of atomized flow. The numerical results are all in close agreement with the prototype observation data in rain area and rain intensity distribution. For example, the prototype observation data for rain length in Wujiangdu Hydropower Station is 80-450m, the numerical one is 74.6-431.7m, the predicted rain intensity in 2* and 3# measuring station is also in close agreement with the observed one. For Xiangkeyan Hydropower Station, prototype observation is carried out in 1998 for the atomized flow with seven sluices of uniform opening, the calculated rain area is in close agreement with the observed one.7. Nuozadu Hydropower Station has large installed capacity, large storage and large energy output in the eight cascade hydropower stations of Nanchang River. The new mathematical model for atomized flow were used to study the following problems of this hydropower station: (1) The atomized flow in five operational rules were predicted by the new mathematical model, the extent of rain and the rain intensity distribution were got. The results shows that the region with the rain intensity above drizzling rain in 1044.51 long, 910.95m wide, and the maximum elevation for the lifting of atomized flow is as high as 790.42m. (2) the possible effect of the atomized flow on the transmission system(switch station) , traffic system(downstream bridge, road on the right bank) of this hydropower station were discussed. (3) suggestions were made for the prevention design of downstream bank slope based on the predicted results of atomized flow.