Results of a Computational and Experimental Study of Wet Steam Flow through the Shroud Seal of a Stage with Long Blades

More accurate evaluation of the energy loss as a consequence of fluid leaks through the clearances between the rotor and stator of turbine machines is a topical problem, which can be solved through more accurately assessing the shroud clearance area by applying a leak flow normal cross section. The article presents the results obtained from measurements of gas dynamic parameters in the low-pressure cylinder (LPC) of a high-capacity steam turbine in the last stage tip zone with improved active protection of the rotor blades against erosion. Flow traversing results have confirmed that, near the stage top at the outlet from the radial clearance between the blade row cylindrical shroud and fins of a three-chamber ledgeless seal (i.e., with the same clearances), the wet steam (two-phase medium) leak direction differs from the axial direction in the turbine and is mainly governed by the working medium swirling in the diaphragm nozzle vane channels and mutually opposite influence of rotation of the shroud and of the main flow from the running cascade channels. The leak flowrate is evaluated by using the flow parameters at the seal outlet and the gas dynamics equation with taking into account the medium compressibility. In estimating the two-phase medium flowrate, the following corrections were calculated: the flowrate coefficient increment caused by the flow dispersity, a correction characterizing the influence of initial wetness and the phase slip ratio. Taking into account the coefficients and corrections is conductive to more accurate description of flow through the seals. The wet steam leak flowrate assessment is approximate in nature because it was carried out proceeding from a simplified physical model describing the two-phase medium outflow. With the jet flowing out at an angle of 35 degrees with respect to the circumferential velocity positive direction, the leak flowrate from the shroud seal at the nominal load expressed in fractions of the wet steam medium flowrate through the LPC last state amounted to (G(shr)) over bar = 0.017. The obtained study results are used in designing the wet steam stages of the LPCs of advanced turbines for thermal and nuclear power plants.