Secondary Flow in Repeating Stages of Axial Turbomachines

Abstract

In a well-designed multi-stage axial flow compressor, the flow settles down to a repeating condition, in which the axial velocity profile does not deteriorate further; it is more or less unchanged between the entry and the exit of a deeply embedded stage. However, experimental data also show that the flow angles repeat, and it is this flow phenomenon that is discussed in the paper. Secondary flow analysis, coupled with empirical data on clearance flows, is used to give a description of the flow in such a repeating stage. The secondary flow at exit from a row involves both the streamwise vorticity generated in that row and the vorticity that exists at entry—the so-called ‘skew’ vorticity due to a non-uniform velocity from a stator being received by a moving rotor (and a similar effect from the rotor to the stator). However, clearance vorticity—shed from the rotor tip (casing) section and the stator root (hub) section—is also present and can be taken into account. Calculations made using the analyses are compared with some limited experimental data drawn from the published literature. Predicted underturning at rotor tip (casing) sections is confirmed by experiments; similarly, predicted underturning at stator tip (casing) sections accords with observations in one compressor but not in another. However, no universal conclusion (on whether underturning or overturning usually occurs) can be drawn for the flow through the rotor and stator root (hub) sections, as either entry or generated secondary vorticity may dominate.