Unsteady Flow Simulation Through Stator-Rotor Blade Rows in Intermediate-Pressure Steam Turbines With Cutback Blades

Steam turbines in thermal power plants require long-term safety for stable power supply and high power generation efficiency with the save of fossil fuel, whereas the turbine blades are gradually deteriorated during the long-term operation. Because such secular-changed blades decrease the turbine efficiency and reliability, the maintenance, repair, and overhaul (MRO) of the steam turbines is essential. In intermediate pressure steam turbines, stator and rotor blades tend to be eroded by solid particles came from the boiler pipe inner wall. Furthermore, turbine blades are unexpectedly cracked due to metal fatigue or thermal stress deformation. To avoid such accidents, the secular changed or cracked blades should be replaced with new manufactured blades. However, the cost increase is also to be avoided. Electric power companies want to use operating blades as long as possible. As one of the current treatments conducted in the companies, the damaged parts of eroded or cracked blades are cut back during the overhaul. Because the effect of the cut-back blades to flows has not yet been clarified, we numerically investigate the effect on unsteady flows in an intermediate pressure steam turbine in this study.

Our research group at Tohoku University has developed a numerical method for simulating unsteady wet-steam flows through stator and rotor blade rows. This method has been also applied to the simulation of unsteady flows in an intermediate pressure steam turbine. We have already reported the effect of secular-changed blades on unsteady flows in multistage of the intermediate pressure steam turbine. In this study, we consider various lengths of the cut-back at the secular-changed stator blade trailing edge. The shapes of secular-changed blades were measured at the maintenance. The stator-blade trailing edge is assumed to be cut all over the span. The cut trailing edge was rounded to be a circular shape. We simulate unsteady steam flows in three-stage stator and rotor blade rows in an intermediate pressure steam turbine considering the cut-back stator blades. Then, the numerical results obtained by assuming secular-changed blades and cut-back stator blades are compared with each other to evaluate the impact of cut-back blades on unsteady steam flows. In addition, the effect of the cut-back stator blades on the following rotor blades is investigated and the effect on the total turbine efficiency is also predicted.