Nucleation and wake-chopping in low pressure steam turbines

06 Oct 2017

While wetness formation in steady flows such as nozzles and cascades is well understood, predicting the polydispersed droplet spectra observed in turbines remains challenging. The characteristics of wetness formation are affected by the expansion rate at the Wilson point. Because the expansion rate varies substantially both axially and circumferentially within steam turbines, the location of the Wilson point within a blade row is a primary factor determining the droplet spectrum and phase change losses. This effect is first investigated using a single streamline with a varying expansion rate, and it is shown that the phase change losses during spontaneous condensation are highest when a large region of high subcooling precedes the Wilson point. The conditions resulting in the highest wetness loss in the nucleation zone do not correspond to those that produce the largest downstream droplets. The effect of nucleation location is then assessed using a non-equilibrium RANS calculation of a realistic low pressure (LP) steam turbine geometry. A quasi-three dimensional (Q3D) flow domain is used to simplify the analysis, which is performed both steadily and unsteadily to isolate the effects of wake-chopping. The inlet temperature is varied to investigate the impact of the Wilson point location on the steady and unsteady wetness loss and droplet spectra. The trends observed in the 1D analysis are repeated in the steady RANS results. The unsteady results show that the Wilson zone is most sensitive to wake-chopping when located near a blade trailing edge and the following inter-row gap. The predicted wetness losses are compared to those predicted by the Baumann rule.