Warm-keeping and Pre-warming of a Steam Turbine Using Hot Air

In recent decades, the rising share of renewable energy affects the operational strategy of conventional power plants. In pursuit of flexibility improvements this thesis investigates a warm-keeping/pre-warming concept for a steam turbine utilizing hot air. The work carried out in the present paper can be split into three separate tasks.

Firstly, conducting a numerical analysis of heat transfer and flow field in the turbine channel necessitated novel simulation approaches. Due to the required accuracy of numerical methods, unsteady modeling approaches have to be used. However, as a result of the vastly different timescales of heat transfer mechanisms in solids and fluids, conventional unsteady conjugate heat transfer simulations lead to high computational times. To remedy this problem, suitable calculation methods and tools have been developed and validated based on existing numerical and experimental data.

Secondly, using the results of a wide calculation matrix, the flow patterns in individual turbine operating points were compared to vortex systems described in cited literature and summarized in a so-called blade vortex diagram. Furthermore, convective heat transfer in the turbine channel was investigated to enable the development of analytical correlations and a hybrid FEM (HFEM) approach for the efficient calculation of the solid body temperatures in various pre-warming scenarios.

Thirdly, the HFEM-approach was utilized to conduct thermo-structural analysis of a 19-stage steam turbine for different pre-warming scenarios. After the calibration of the 19-stage HFEM model, different pre-warming processes were simulated. Subsequently, structural analysis was conducted, which, among other variables, includes the values and locations of highest stresses. Finally, to analyze the benefits of the pre-warming concept, a conventional turbine start-up with steam was numerically modeled, validated and compared to the start-up processes after operation with hot air.