Unsteady Wake-Blade Interaction: A Correlation Between Surface Pressure Fluctuations and Loss Generation

Abstract

This paper presents loss mechanisms associated with unsteady flows in axial turbines due to the interaction of wakes with downstream blade-rows. To evaluate the effect of wake induced loss mechanisms, extensive 3D-URANS computations of an 1.5-stage low pressure turbine rig have been performed. The investigation of the flow field shows how the wake influences well-known effects like the negative-jet effect and resulting flow structures. A detailed view on the impact of these time-dependent flow structures on the fluctuating pressure distribution on the blade is presented by evaluation in both, the time and frequency domains. The effects causing the fluctuations are identified in detail and attributed to their corresponding sources. Unsteady pressure induced by potential effects is distinguished from unsteadiness induced by wake passage. Regarding the latter, it is shown that the amplitude of the pressure fluctuations correlates with the instantaneous state of the wake. In order to describe this behavior, an analytical model is proposed and referred to as the conservation of wake circulation. The evaluation of the entropy generation caused by wake deformation allows the distinction of different loss mechanisms. Finally, an empirical correlation between the amplitude of pressure fluctuation and loss generation is shown.