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In pursuit of flexibility improvements and extension of lifetime, a concept to pre-warm steam turbines using hot air was developed. In order to further optimize the pre-warming operation, an extensive numerical investigation is conducted to determine the time-dependent temperature and stress fields. In this work, the transient thermal and structural analyses of an IP 19-stage steam turbine in pre-warming operation with hot air are presented. Based on the previous investigations, a hybrid (HFEM - numerical FEM and analytical) approach especially developed for this purpose is applied to efficiently calculate the solid body temperatures of a steam turbine in pre-defined pre-warming scenarios. The HFEM model utilizes the Nusselt number correlations to describe the heat transfer between the hot air and the turbine components in the flow channel. These correlations were developed based on unsteady CHT-simulations of multistage turbine models. In addition, most of the thermal energy in turbine pre-warming operation is transferred through vanes and blades. Therefore, the HFEM approach considers the thermal contact resistance (TCR) on the surfaces between vanes/casing and blades/rotor. After the calibration of the HFEM model with experimental data based on measurements of the natural cooling curve, the pre-warming processes for different pre-warming scenarios are simulated. Subsequently, the obtained temperature fields are imported to an FEM model in order to conduct a structural analysis, which, among other variables, includes the values and locations of highest stresses and displacements.

Adaptability of coal-based power generating units to accommodate renewable energy sources is becoming increasingly important. In order to improve flexibility, reduce start-up time and extend the life cycle, General Electric has developed solutions to pre-warm/warm-keep steam turbines using hot air. In this paper two main contributions to optimize the warming arrangements are presented. Firstly, the calibrated model of a 19-stage IP steam turbine is analyzed regarding time-dependent mass flow rates in a pre-warming mode. The influences on the duration time of the process and the thermally induced stress are investigated. This investigation utilizes a detailed 3D hybrid (HFEM-numerical FEM and analytical) model of the turbine including the rotor, inner casing and blading for computationally-efficient determination of transient temperature fields in individual components. The thermal boundary conditions are calculated by means of heat transfer correlations developed for this purpose. Moreover, a separate FEM model allows for the implementation of a structural mechanical analysis. As a result of this investigation, the pre-warming time can be further reduced while simultaneously lowering the thermal load in the components. Secondly, selected pre-warming strategies are compared with the warm-keeping scenarios. This analysis is aimed at a minimum thermal energy use required for a reheating of air in a warming arrangement. Hence, the pre-warming and warm-keeping operating strategies are evaluated with regard to their energy demand before start-up. Thus, based on the duration of standstill, the most energy-efficient turbine warming strategy can be chosen to ensure hot start-up conditions.

In pursuit of flexibility improvements and extension of lifetime, a concept to prewarm steam turbines using hot air was developed. In order to further optimize the prewarming operation, an extensive numerical investigation is conducted to determine the time-dependent temperature and stress fields. In this work, the transient thermal and structural analyses of an IP 19-stage steam turbine in prewarming operation with hot air are presented. Based on the previous investigations, a hybrid finite element method (HFEM—numerical finite element method (FEM) and analytical) approach especially developed for this purpose is applied to efficiently calculate the solid body temperatures of a steam turbine in predefined prewarming scenarios. The HFEM model utilizes the Nusselt number correlations to describe the heat transfer between the hot air and the turbine components in the flow channel. These correlations were developed based on unsteady conjugate heat transfer (CHT) simulations of multistage turbine models. In addition, most of the thermal energy in turbine prewarming operation is transferred through vanes and blades. Therefore, the HFEM approach considers the thermal contact resistance (TCR) on the surfaces between vanes/casing and blades/rotor. After the calibration of the HFEM model with experimental data based on measurements of the natural cooling curve, the prewarming processes for different prewarming scenarios are simulated. Subsequently, the obtained temperature fields are imported to an FEM model in order to conduct a structural analysis, which, among other variables, includes the values and locations of highest stresses and displacements.

This paper presents different modeling approaches on rim seal flow in a 1.5-stage test turbine resulting in the analysis of numerical data in comparison to existing test data. The analysis focuses on a very simple axial sealing gap and a reference operation point which shows significant hot gas ingestion in the experiments. The presented model will be used to gather more information on flow phenomena and their effect on the ingestion and hence contributes to a deeper understanding. In combination with broader research this understanding will help manufacturers to reduce their secondary air mass flow rate and increase gas turbine’s efficiency. Experiments on the hot gas ingestion phenomenon have already been carried out at many international test facilities. As part of a recommissioning of a test rig for investigating hot gas ingestion into the wheel side cavities of axial-flow gas turbines at the RWTH Aachen University, this publication presents the numerical model set up in parallel. In particular, the aim is to investigate the sensitivity of the modeled flow field in the wheel side cavity for changes in the modeling and boundary conditions. In addition to the investigation of the sensitivity, the results are compared with existing measurements in order to be able to classify and evaluate the sensitivities in a specific manner. The investigation starts with a mesh study followed by the analysis of different turbulence models including different inlet turbulence intensities. After analyzing different time steps in transient and steady state simulations, calculations are shown concerning different operation points. For all chosen combinations of settings and boundary conditions the main flow is modeled reasonably well compared to experimental data. Also the operation point dependency of the pressure field is modeled well with simple approaches. For low purge air mass flow rates the flow field in the wheel side chamber, consisting of pressure and sealing gas concentration, is only represented with strong deviations in the one-segment model. These deviations are due to large structures forming in the cavity, which can not be reproduced using one or two pitches of the turbine. Above a certain dimensionless purge air mass flow rate, a transition in the flow field can be observed analyzing the experimental data. Above this flow rate the large structures break down and the sealing effectiveness increases. At these operation points the selected numerical model shows good agreement with experimental data regarding pressure and hot gas concentration. To improve predictions at low purge air flow rate multiple segment models will be used in the future.