Influence of the meridional shape of the nozzle cascade on the NOZZLE cascade flow structure

Abstract

It is known, that last stages of the powerful steam turbine, working in zone of specific volume significant changing, has different blade height of the TE and LE. The wrapping of this cascade are accompanied by additional losses at shroud section of cascade. Numerical methods were used to investigate the effect of meridional shape influence on the secondary flows. The geometrical parameters of nozzle cascade were taken from second stage powerful steam turbine with opening angle about 450. To simplify the research constant height shape of nozzle profile was accepted. The boundary conditions as inlet total pressure, inlet total temperature and outlet static pressure were assigned. Formulation of the optimization task requires a series of additional studies. For this, in addition to the origin nozzle two variants of nozzle cascade with different meridional shroud shape were made. The results of research shown the significant influence of the meridional shape of the inlet cascade part on the secondary flow. Moreover, the improvement of efficiency in additional variant was observed. For future optimization task this variant as initial was accepted. The meridional shape was created with using cubic Bezier curve. The coordinates of reference points for Bezier curve form changing were used. The optimal variant search was occurred with using experiment planning theory method and LPτ sequence. As the result of the optimization a new shroud meridional shape with the losses decrease of 7.5% (in relative values) compared to the original design was obtained. For a detailed analysis of the obtained results the height distribution of losses and streamline visualization in nozzle cascade were listed. It is proved, that optimization of the shroud meridional shape using CFD and experiment planning theory method allows to increasing efficiency of the isolated turbine cascade.