Relation of Clocking Effect and Secondary Flow in a 1.5 Stage Axial Turbine

Abstract

A three-dimensional unsteady flow Simulation was conducted to investigate the relation of clocking effect and secondary flow in a 1.5 stage axial turbine. Six relative positions of two stator rows were investigated by positioning the second stator being clocked in a step of 1/6 pitch. The relative efficiency benefit of about 1% was obtained depending on clocking positions. However, internal flows had some different features from the previous study at maximum and minimum efficiency positions, since the first stator wake was mixed Out with the rotor wake before arriving at the leading edge of the second stator at mid-span. Instead of the first stator wake, it was found that the secondary flow plays an important role to the efficiency variation at each clocking position. The time-averaged local efficiency along the span at maximum efficiency was more uniform than that at minimum efficiency. That is, the span-wise efficiency distribution at minimum efficiency has larger values at mid-span but smaller values near the hub and casing compared with the case at maximum efficiency. This spanwise efficiency distribution could be also explained by secondary flow behavior in an axial turbine.