Numerical Simulation On Two-phase Flow In The Turbine With Dehumidification Slots

The last stage blades and stators always works in wet steam condition, thedensity of droplets is much larger than that of steam, while in the flow, there is asmaller velocity of droplets, and the direction of droplets will have a deviation fromthat of the main flow, and hit the blades with a certain angle, which will not onlydelay the rotation of blades, decrease the efficiency, but also have an erosion on theblades surfaces, even affect the safety of set in a serious condition. Therefore,analyzing the flow condition in the passage of last stage, getting the distributionrule of each parameter of droplets in wet steam, applying respect moist removingmethod will have a significant meaning to improve the efficiency and safety ofsteam turbine.Firstly, taking the last stage of a certain condensing steam turbine as studyobject, applying commercial numerical simulation software NUMECA to process afull3D simulation for wet steam flow condition in the passage of cascade for thetwo calculation schemes: prototype stator stage(Old type) and modification statorstage(New type). the calculation result shows that in the last stage of steam turbine,flow parameters change acutely along the height of blade, at the root and top regionof rotor, wet steam flow is not in the designed working condition, there is a largenegative incidence at the inlet of the rotor, which leads to a large deviation awayfrom the match of rotor and stator blade profile, which is much obviously both forthe last stages of New type and Old type.Then, single stator cascade passage mesh is compressed, the calculation resultis compared with that of coarse mesh and the coarse one is modified in order to geta more accurate pressure and humidity distribution condition, regions of differenthumidity are analyzed particularly. The results indicates that large humidity regionof flow in stator cascade centralized occurs at the middle and rear parts of thepassage, humidity region at pressure surface presents “hypsokinesis” bar typedistribution, so if a “hypsokinesis” moist removing slot is set at the upper part ofthis position, the droplets will be condensed, and a good moist removing purposewill be reached.Finally, moist removal slot structure is set for stator, commercial softwareCFX5is applied to process the simulation flow condition in cascade passage, threedifferent diameters of droplets (with diameter10μm,50μm,100μm respectively)distributions are reviewed mainly, and cause of the distributions are analyzed alongwith the effect on the total flow flied condition being acted by the moist removal slot structure. the results shows that the middle and posterior portions on thepressure surface of the last stator stage is the region where condensing dropletsaggraded and moisture film formed, while secondary droplets can be decreased ordiminished when the moist removal slot is set there. Furthermore, the main point ofdesign for moist removal slot is to form negative pressure gradient along the axialline of slot, which can be reached naturally with the help from different expansiondegree of blade height, namely “hypsokinesis” type is applied for the moist removalslot design.