Calculating Incidence Losses in the Turbine Cascade

Abstract

The flow in turbine cascades with incidences is accompanied by a separated flow around the airfoils, which is difficult to calculate with acceptable accuracy. Therefore, in practice, empirical formulas are used to estimate losses from the incidence. The analysis of these formulas made it possible to identify the geometric and operating parameters of the cascade that have the greatest influence on the losses: the inlet metal angle, thickness of the leading edge of the profile, thickness of the profile, relative pitch, incidence, and exit flow velocity. It is unrealistic to obtain a reliable analytical expression for losses that takes into account the influence of all these parameters. Therefore, the path consisting in creating a program that uses the data of numerous experiments and finds an equation for calculating the losses in a group of cascades close to the specified one in terms of geometric parameters was chosen. Accordingly, a bank of experimental data on losses in a large number of cascades was formed and tested at different incidences and flow velocities. According to the test results, an increase in the flow velocity out of the cascade leads to a decrease in losses from a positive incidence. All cascades are clearly divided into three large groups according to the nature of losses. For the cascades of each group, the ranges of variation of the angles of the flow inlet and outlet and the thickness of the profile and its leading edge are determined. The set of these parameters determines the form of the approximating polynomial for each given cascade. To reflect the effect of the pitch and exit velocity and find the design equation, cascades with narrow deviations of the parameters from the given value are selected from the group, and the unknown polynomial coefficients are calculated using the least squares method. Calculations according to the developed program give smaller deviations from experiments than the known formulas.