Numerical investigation of the flow phenomena around a low specific speed Francis turbine's guide vane cascade

Abstract

Guide vanes of Francis turbines convey a significant influence on the flow field at the inlet of the runner. This influence is in the form of pressure pulsation, caused due to rotor-stator-interaction. A guide vane cascade containing a single blade passage was developed to predict the flow field experimentally. Firstly, this paper investigates flow phenomena around the guide vane cascade through computational techniques. A numerical model is prepared with three different turbulence models. The velocity distribution obtained from these models are compared with experimental results at two circumferential midspan locations. Secondly, the influence of increasing the clearance gap on the flow is studied. Such gaps are expected to increase when the flow containing eroding particles passes through the turbine. This paper also shows that the pressure difference between the pressure and the suction side of guide vane influences the leakage flow through the gap. Hence, reduction of the pressure gradient will reduce leakages through clearance gaps, hereby condensing the subsequent effect of pressure pulsations and erosion. This study also shows that the effect of the gap is prominent in the near wall regions which are close to the gap, whereas it dissipates gradually towards the midspan.