ANALYSIS OF TURBULENT EFFECTS IN A LOW-PRESSURE MODEL STEAM TURBINE OPERATING UNDER VARIOUS OPERATING CONDITIONS USING DETACHED EDDY SIMULATION

Steam turbines are sometimes operated at extreme part-load conditions, specifically if the energy balance in power grids featuring a high share of fluctuating energy sources necessitates such operation. At such extreme operating conditions, the last stages of the low-pressure turbine are subject to highly unsteady flow. As the flow is heavily separated, Rotating Instabilities (RI) may establish, among other turbulent effects. These highly unsteady flow structures result in fluctuations of the flow field over a wide range of low frequencies. The simulation of the flow at such extreme operating conditions using CFD is challenging due to its highly turbulent and separated nature. On one hand, the employed flow model must be able to handle turbulent structures with varying size in a computationally efficient manner. On the other hand, the extent of the computational domain must be relevant with respect to the unsteady flow phenomena. The analyzed domain in this study consists of a three-stage low- pressure model steam turbine. To capture the asymmetry, the model spans the full annulus and comprises the non-axisymmetric inlet section, all three stages, the axial-radial diffuser as well as the exhaust hood. Based on the findings of a previous study in which the Improved Delayed Detached Eddy Simulation (iDDES) has been applied to low-volume flow operating conditions [1], the focus of this paper is drawn towards the turbulent structures that establish at extreme flow conditions, and whether they can also be found at increased volume flows. Time-averaged and FFT results of the unsteady pressure signals at different span heights measured during experimental and numerical radial probe traverses indicate a higher unsteadiness during the lowest volume flow condition when compared to higher volume flows. Various turbulent structures have been identified at the hub region, the midspan section and the tip region of the last stage rotor blade that could be linked to the higher flow unsteadiness. Furthermore, the possibility of using URANS simulations for the prediction of such extreme flows has been investigated, leading to the finding that the choice of flow model has an impact on the results. Additionally, it was investigated if the extent of the computational domain could be reduced to save computational costs. It was found that a reduced domain did not yield the low frequencies that are of relevance at such operating conditions.