| The problem about pressure fluctuation in draft tube on the part-load conditions is the one crucial factor of hydraulic vibrations affecting stable operation of hydraulic tufbine and power station. In this paepr, steady and unsteady numerical simulations of whole flow passage domain were conducted with 3D N-S equation solver on the basis of the non-staggered grid SIMPLEC algorithm and RNGk - ε turbulence model. The main results in this work are as follows: The steady flow results show that pressure fluctuations of nose of spiral case are anomalous and pressure distributions of spiral case outlet are asymmetric on different openings for the same unit speed. But circumferential pressure distributions are symmetrical at stay vane outlet and guide vane outlet. Circumferential pressure distributions at runner inlet are markedly different from the ones at guide vane inlet as a result of the RSI of runner and guide vane and the difference of blade number. Under the small opening, due to minor guide vane outlet angle, the impinging streams at the head of blade are severe. And there is the flow pattern of cross-flow from hub to shroud at blade inlet. Interblade vortex can also be visible. And there are big back flow regions and cross-flow at blade outlet near crown. There are also serious back flows in the center of conical tube. When the opening becomes bigger, the flow conditions at runner outlet near crown and inlet are improved. The mainstream is located at the center of conical tube. So instability is weakened in conical tube. The unsteady flow results are as follows: when the opening is 12mm, pressure fluctuation atrunner outlet near hub is not obvious and pressure fluctuation amplitudes near midstream and shroud are big. Helical vortex is situated on conical tube on opening 12mm. It will disappear in the elbow tube. Pressure fluctuation of low frequency in draft tube is mainly located at conical and elbow tube and pressure fluctuation amplitudes in conical tube is higher. The frequency of helical vortex is low and is about 0.233 times that of runner frequency. And Frequencies of two points on the same measurement section are almost equivalent. But when the opening is 24mm, there is not back flow region at runner outlet near hub and pressure fluctuation amplitude is smaller. And the ones near midstream and shroud are greatly higher. The vortex whose shape is columnar is situated on conical tube. It will become small helical vortex on account of turn effect of elbow tube wall. In the course of rotation of small helical vortex, the pitch of thread will become bigger. And the small helical vortex will gradually disappear in the elbow tube. Pressure fluctuation in draft tube is mainly located at elbow tube and pressure fluctuation amplitude in conical tube is very low.The flow field in draft tube and runner after injecting air through center hole of shaft was studied by numerical simulation with water-air two phase flow model. The results show that there are some differences about optimal air admission amount at different openings. When the air amount is not enough, the energy of air is lower. The air will fastly been diluted in water in virtue of rotational runner. So litter air can flow into the center of vortex and it can not effect flow flied. When the air amount is excessive, the area of cross section will minish. It may enhance cross-flow of cross section in conical tube.
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