Numerical Analysis On Flow Field Of Steam Turbine’s Last Stage At Small Volumetric Flow

When the last stage blade runs at small volumetric lfow conditions, steam flowsin the channel usually presents as complex three-dimensional flow, In order to ensurethe safety and reliability of the last stage blades at small volumetric flow conditions,it is necessary to study the small volumetric lfow characteristics of the last blade’sflow field in detail by the numeircal simulation. The aim is to find flow separationmechanism, which is of important signiifcance on improving the safety of the unit.In this study, the Gambit sotfware is used to establish the physical model anddivide unstructed grid, then the N-S equations included standard k-e turbulencemodel is adopted by Fluent sotfware platform, and the last stage of a12MW steamturbine twist blade is taken as an example to develop theoretical analysis andnumeircal simulation.Firstly, the simplified model of the last twisted blade is numerically simulated atthe design condition and high back pressure conditions, and the purpose is to studythe inlfuence of back pressure changes on the relative volume flow, the flow line atblade tip and blade root, the incidence angle as well as the out flow angle. The resultsshow that, as the relative volumetric flow rate decreasing, the root of pressure surfaceof the blade is the first to appear flow separation phenomenon; when the volumetricflow reduces to a certain value, the flow near the suction surface root will also fallaway from the cascade. In addition, with the back pressure increasing, the changes ofincidence angle near blade tip and blade root will become much larger.Secondly, the blade is numerically simulated again by changing the inlet steamlfow angle to analyze the inlfuence of inlet steam lfow angle changes on the vorticityof the flow field, streamlines at blade tip and blade root and total pressure losscoeiffcient. The results show that, the large positive incidence angle could easilycause separation flow near suction surface, while the negative incidence angle could easily cause separation How near pressure surface, which brings about greater totalpressure loss than the positive incidence angle.