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THE INFLUENCE OF BACK PRESSURE CHANGE ON THE LEAKAGE FLOW CHARACTERISTICS OF THE LAST STAGE BLADE TIP CLEARANCE OF STEAM TURBINE

It is of great significance to fully understand the tip clearance leakage flow of rotor blades for improving the working efficiency of steam turbines. Taking the last stage of low pressure cylinder of a 600MW steam turbine as the research object, the tip clearance leakage flow of rotor blade under different outlet pressure was numerically simulated. The flow field characteristics and the influence of outlet pressure on the leakage flow were analyzed. The results show that there is obvious separation flow on the pressure surface and suction surface. On the pressure surface, with the increase of outlet pressure, the position of separation fluid gradually moves down. The positions of separation fluid in working conditions 1, 2 and 3 are 65%, 50% and 15%, respectively. In case 3, a small reflux vortex appears at the root of the pressure surface. On the suction side, there is an obvious separation vortex in case 1. In case 2 and 3, it can be clearly seen that the lower passage vortex is formed at the blade root. It is opposite to the separation fluid rotation direction, and the influence range increases with the increase of outlet pressure. At the same time, with the increase of outlet pressure, the pressure difference on both sides of the blade decreases, and the blocking effect of leakage flow on the main flow is weakened. When the outlet pressure is 15kPa, there is backflow at the top of rotor blade. It interacts with leakage flow and main flow to further increase the loss. The research has a certain reference value for the optimization of flow structure of steam turbine.