Basic operational advantages of the Organic Rankine Cycle (ORC) systems and specific issues of turbines working in these systems are discussed. The strategy for CFD simulation of the considered ORC turbine and the main issues of the numerical model are presented. The method of constructing the 3D CAD geometry as well as discretisation of the flow domain are also shown. Main features of partial admission flow in the multi-stage axial turbine are discussed. The influence of partial admission on the working conditions of the subsequent stage supplied at the full circumference is also described.

Partial admission flow in the control stage of a 200MW steam turbine is investigated with the help of a RANS solver with k-ω SST turbulence model in the code Fluent. A 2D model of flow at the mid-span section of the full annulus is assumed. The results exhibit interesting details of the process of expansion in the control stage. Unsteady forces acting on the single rotor blades of the control stage are calculated, and are subject to Fourier analysis. Single blade forces are summed up to obtain the unsteady load at the rotor (forces acting at the rotor disc are neglected due to the assumed 2D model). The calculations take into account pressure pulsations at the entry to the nozzle boxes and rotor blade mistuning / geometrical imperfections.

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Several high vibration amplitude problems have been reported regarding the slender last stage blades of commercial LP steam turbines. This paper presents a numerical study of unsteady forces acting on rotor blades using ANSYS CFX. A 3D transonic viscous flow through the stator and rotor blades with an exhaust hood was modelled. The last stage was modelled as a full blade annulus, so that the axial, radial and circumferential distribution of flow patterns and blade forces could be examined. An unsteady flow analysis was conducted on a typically designed last stage and exhaust diffuser, with measured and calculated downstream static pressure distribution as the outlet boundary condition. The results showed that under off-design conditions, vortices occurred in the last stage and diffuser. Unsteady aerodynamic forces were found at high frequencies (stator passing frequencies) and low frequencies (generated from asymmetric pressure distributions behind the rotor), with the relative dominance of these forces/frequencies shifting as a function of radial span. An FFT analysis was carried out. Three sections were investigated: the hub, midspan and peripheral (tip) section. The steady pressure behind the rotor blade was compared with experimental results in the LP last stage behind the rotor blades and in a specified cross-section of the exhaust hood. The lower frequency unsteady forces had a higher relative contribution towards the tip of the blade.

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