Transient modelling of whole gas turbine engine: an aero-thermo-mechanical approach

In order to improve gas turbine performances, the operating temperature has been risen significantly over time. The possibility of applying more and more extreme operating conditions is mainly due to an efficient engine cooling. Secondary air system (SAS) design aims at obtaining the maximum efficiency with the minimum demand of mass flow bled from the compressor. Adequate cooling strategies have to be developed in order to guarantee suitable components lifespan and avoid failures. Anyway mass flows and pressure drops inside the secondary air system depend on the fluid-solid heat transfer itself, and in particular on the actual running clearances and gaps determined by the thermal expansion of components according to the current thermo-mechanical loads to which the engine is subjected. Due to changes in power generation market, the relevance of these issues increased considerably for large power generation gas turbines. In recent years their operating conditions have been deeply modified since more frequent and fast startups and shutdowns are required to meet electric load requirements. In order to manage thermal and mechanical stresses encountered in these repeated transient operations, and in order to monitor a number of parameters which should remain inside the pre-established operating ranges, the capability of predicting the thermal state of the whole engine represents a crucial point in the design process. Accurate prediction tools have to consider the strongly coupled phenomena occurring among SAS aerodynamic, metal-fluid heat transfer and deformations of the solid, in order to correctly estimate gaps and develop adequate SAS configurations. According to this, a Whole Engine Modelling (WEM) approach reproducing the entire machine in the real operating conditions is necessary in order to verify secondary air system efficiency, actual clearances, temperature peaks, structural integrity and all related aspects. It is here proposed a numerical procedure, developed in collaboration with Ansaldo Energia, aimed to perform transient thermal modelling calculations of large power generation gas turbines. The aerodynamic solution providing mass flows and pressures, and the thermo-mechanical analysis returning temperatures and material expansion are performed separately. The procedure faces the aero-thermo-mechanical problem with an iterative process with the aim of taking into consideration the mutual interaction of the different solutions, in a robust and modular analysis tool, combining secondary air system, thermal and mechanical analysis. The heat conduction in the solid and the fluid-solid heat transfer is computed by a customized version of the open source FEM solver CalculiX. The secondary air system is modelled by a customized version of the native CalculiX one-dimensional fluid network solver. Correlative and lower order methodologies for the fluid domain solution allows to speed up the design and analysis phase, while the presence of the iterative process allows to take into account the complex aero-thermo-mechanical interactions actually characterizing a real engine. A detailed description of the procedure will be reported with comprehensive discussions about the main fundamental modelling features introduced to cover all the aspects of interest in the simulation of a real machine. In order to assess the physical coherence of these features the procedure has been applied to two different test cases representative of typical real engine configurations, tested in a thermal transient cycle. The first one represents a simplified gas turbine arrangement tested with the aim of a first assessment from the point of view of the thermal loads evaluation. The second one is a portion of a real engine representative geometry, tested for the assessment of the interaction between SAS properties and the geometry deformations.