Non-axisymmetric flow field in an axial impulse turbine

Phenomena such as hard landings or geometric flaws can cause non-axisymmetric tip clearance in turbines. Such geometric imperfections induce flow distortions which can, in turn, cause self-excited vibration of the rotor, or rotordynamic instability. Flow field perturbations in a single-stage, unshrouded impulse turbine caused by non-axisymmetric tip clearance have been investigated experimentally. Steady velocity and pressure data have been acquired at the design point with and without static turbine casing offset. Perturbations in tangential velocity and casing wall pressure have been obtained, and rotordynamic forces along and perpendicular to the axis of offset have been inferred. Compared to an unshrouded 20% reaction turbine, the forces due to tangential force asymmetry are much smaller, but the forces due to pressure asymmetry are comparable.