Abstract
As a result of the expansion process within low pressure steam turbines, the last stages typically operate in the two-phase flow regime. Consequently, water droplets are carried with the steam flow leading to the potential risk of water droplet erosion on rotating blades. This operation situation overtime leads to performance degradation, reduction of service lifetime and thus increased product costs. In order to improve reliability and safety of steam turbine modules, special design features can be applied to reduce the erosion risk. One common approach utilizes suction slots on the stationary airfoils of guide vanes, intending to drain the deposited water from the vane surface and consequently diminish the amount of coarse water droplets. This paper presents the development and validation of a model using numerical and experimental methods to determine and assess the performance accuracy as a design and optimization approach for hollow vane suction slots. The analysis focuses on previous investigations for a flat-plate arrangement and is extended to a vane nozzle setup intending to consider surface curvature effects on water rivulets paths. Based on steady-state and transient multiphase Eulerian-Lagrangian framework simulations for different operating conditions, the suction slot performance for both applications is derived by means of collection rate and air leakage considering predominant multiphase flow phenomena (film building, transport, stripping of droplets and wall impingement of droplets). The resulting numerical dataset is used in comparison to available measurement data (air flow and film conditions) and for validation as well as calibration of the prediction accuracy of the simulation model. Results for flat-plate and nozzle setup show that the numerical approach is able to predict the main phenomena of film flow and breakup into droplets. Comparison with test data illustrates that an error of less than 15% is achieved for the slot efficiency based on the water film collection rate and water film properties. Overall, the ability of the modelling approach to predict the performance of suction slots is confirmed. Results thus lead to the conclusion that the numerical model is sufficiently accurate to represent geometric changes relevant during design procedure of hollow guide vane suction slots.
