Effect of Geometric Uncertainty on a One Stage Transonic Compressor of an Industrial Gas Turbine

The geometrical uncertainty arising as a consequence of in-service degradation and/or manufacturing tolerance can result in flow asymmetry around the annulus of compressor. This can have an unintended sever effect on the compressor stability and performance. The present paper deals with both in-service degradation (blended blades) as well as manufacturing tolerance (blades with different stagger angle and tip gap) in both inlet guide vane (IGV) and rotor. The blades are blended when the damage occurs at either leading edge due to foreign body impact or near the trailing edge due to sudden surge resulting in blade rubbing against the casing. The present paper considers only the leading edge blended blades at the tip of the blade and has ignored the possibility of a damage occurring in mid span region as well as at the trailing edge. The results in the present paper have been divided into two parts. In the first section, similarities and differences between the blended blade and the blade with different tip gaps have been studied. The second part deals with the effect of stagger angle on the performance of both IGV and rotor.

The study of blended blade was divided into two parts. In the first part, the damage was kept constant in radial direction and was varied in chord-wise direction. In the second part, the damage was kept constant in chord-wise direction while being varied in radial direction. It is seen that the stall margin (SM) is the most affected by variation in chord-wise direction. The radial damage has minimal impact on both the stall margin as well as efficiency. Both, the random blended blades (arising due to different damages across the rotor circumferentially) as well as the random tip gaps (arising due to manufacturing tolerance) mandate a zigzag kind of arrangement (a blade having lowest SM sandwiched between two blades having best SM) to eke out best possible performance. It is seen that such an arrangement results in uniform stall inception throughout the annulus. It is also shown that the worst possible performance is obtained when a sinusoidal arrangement is done. Such an arrangement results in stall cells to grow in one half of the annulus resulting in premature stall.

The stagger angle has been shown to have a detrimental effect on the efficiency of rotor due to change in passage area which results in shock waves moving in opposite direction in adjacent passage. To minimise the effect of change in passage area, it was suggested that the randomly staggered blades in rotor be arranged sinusoidally. This results in best possible efficiency while a zigzag arrangement gives worst possible efficiency. Furthermore, it was also seen that the effect of stagger angle on IGV has zero impact on the overall efficiency of the stage.