Effective Formalization of Design Processes as a Key Factor in Achieving Optimal Solutions When Creating the Final Stages of Steam Turbines

Abstract

Based on the existing experience in designing and constructing of the last stage blades of large (critical) length and the analysis of literary sources, the features of the methodology for formalizing the processes of creating such blades, taking into account their specific features (large radial dimensions, suboptimal relative grid steps =0.25–1.0, high static and dynamic loads), are established. A parametric formalization of the main modeling dependencies of the processes on which the creation of rotor blades is based is given: the thermo-gas-dynamic process, blade design and the technological process of manufacturing. The need to create systems (subsystems) for automated design of blades of large length with the presence of a model of the technological process of blade manufacturing in the system is substantiated. It is based on the conclusions that even small deviations from the design option within the tolerance limits during blade manufacturing affect the thermo-gas-dynamic characteristics of the stage, especially when it comes to throat areas. A formalized probabilistic-statistical mathematical model that allows to describe the technological deviations of the blade surfaces taking into account the processing modes used in finish milling with a reliability satisfactory for practical calculations has been developed. This makes it possible to take into account the influence of manufacturing errors and specific features of machine equipment on the blade strength indicators, its gas-dynamic characteristics, and also on the efficiency of the stage operation at the design stage. A two-level approach to the design process, which allows using a two-dimensional model to conduct a directed search for the best solution in an automated mode, analyzing hundreds of options taking into account a wide range of constraints, is proposed. Subsequently, as a result of the blade design and calculation of technological deviations, the option with the best thermo-gas-dynamic characteristics, strength indicators, vibration reliability, and the one taking into account manufacturing errors is selected. At the next level, it can be adjusted using three-dimensional calculation models without losing the indicators of the main selected characteristics. This approach improves the design quality and reduces the time to obtain the best solution.