Numerical Investigation of the 3D Flow Field in a Steam Turbine Axial Exhaust Diffuser: Comparison With Experimental Data and Performance Evaluation

The purpose of this paper is to present a numerical analysis carried out for the performance evaluation of the axial exhaust diffuser of a LP steam turbine. A set of measured data in an actual real scale steam turbine is available for direct comparison. The three dimensional exhaust flow in a LP steam turbine provided with a 48″ LSB is numerically investigated in different real working conditions by means of 3D CFD analysis. A detailed 3D model of the actual geometry is used in order to catch the highly 3D features of the flow field, avoiding the use of numerical periodicity conditions. Boundary conditions are derived both from experimental data and from specific validated 3D simulations of the main flow of the entire LP turbine section from front stages up to the LSN. The comparison with measured data allows to validate the performed CFD simulations and to provide a reliable complete performance curve of the exhaust diffuser geometry coupled with the 48″ LSB design. An important outcome of the work consists also in a generalized method for accurate performance evaluation of axial diffusers.