EFFECT OF REAL GEOMETRY ON COMPRESSOR PERFORMANCE PREDICTIONS

This paper describes a computational study into the effects of casing distortions, caused by thermal and mechanical loads, on the performance of HP compressors. The unsteady flow solver is 3D, time-accurate and the CFD mesh can move to accommodate moving boundaries. Taking advantage of this feature, the deformed casing geometry is represented, in a novel fashion, as a radial nodal diameter wave in the rotating frame of reference, travelling at the speed of the shaft, but in the opposite direction. Such a model, where the casing rotates and the rotor is stationary, allows a genuinely-unsteady representation of the flow features for non-uniformly distributed tip gaps and does not rely on a quasi-steady assumption. The study shows that different bladerows are affected differently for similar casing deformations, the front bladerows showing a smaller overall effect, as well as smaller sensitivity, to pressure ratio. It is also shown that the worst performing rotor blade is not that associated with the largest tip gap because of a delay which arises from the finite time needed for the flow conditions near the casing to adjust to the changes in tip gap size and due to the additional time needed for perturbations generated near mid-chord to travel upstream and downstream. The findings indicate that the net efficiency deficit cannot be inferred from the study of simpler configurations with all blades having the same large (or small) tip gap. It is shown that bladerows are affected to varying extents and that the bladerow performance is related to the hade of the annulus.