The Effect of Radial Nonuniformity in the Inlet Total Pressure on the Distribution of Losses in a Stator Vane of a Low-Pressure Turbine

Abstract

In designing turbine blade/vane cascades, predicted or experimental distributions of flow parameters, which may differ considerably from the operating conditions of a real turbine, are often used as the boundary conditions. This difference in boundary conditions may lead to inaccuracy in the predicted performance of the entire turbine. In multistage gas turbines, the second stage operates with inlet conditions formed in the cooled and transonic first stage. Therefore, the radial distributions of flow parameters at the inlet to the next stage are considerably nonuniform. This leads to elevated total losses, including secondary losses. The effect of the degree of nonuniformity in the inlet flow parameters on the structure of secondary flows within the stator vane of a low-pressure turbine (LPT) is studied in this paper. In particular, computational and experimental studies have revealed that significant radial nonuniformity of flow parameters (especially of the total pressure) at the inlet to the vane cascade can induce pronounced radial migration of the flow near the convex (suction) surface of the vane cascade in vortex zones at the end-walls of the flow path. In these cases, the application of the standard procedure for averaging flow parameters and processing data from both numerical and experimental studies may yield zones with physically incorrect parameter values depending on the degree of inlet flow nonuniformity at the end regions, where the effect of vortex flows is most pronounced. In particular, narrow regions may appear at the circumference and at the hub where the local total pressure at the outlet exceeds the total pressure at the inlet. This procedure for processing of the calculated data technically results in “negative” values in the radial distributions of the loss coefficient in these areas (“virtual” losses). It has been demonstrated how redesigning of the cascades in the upstream high-pressure turbine (HPT) can reduce the nonuniformity of parameters and increase the efficiency of the LPT.