Experimental audit of the mixing-plane approach to turbomachinery analysis and a review of alternative multi-row techniques

Detailed numerical comparisons of pressure and flow angle measurements made in stage three of a four-stage, large scale, low speed, axial compressor are presented. The measurements are in both the rotating and stationary frame and were obtained as part of a BRITE/EURAM collaborative study of cantilevered and shrouded-stator compressor configurations. The numerical analysis is 3D, considers three blade rows simultaneously and incorporates multiple row effects by use of a conservative mixing-plane model allowing circumferential variation at the mixing plane. The paper discusses the early results of a study sponsored by Alstom Gas Turbines to examine steady-state, multiple blade-row modelling techniques. Growth in the endwall flow region due to multi-row effects is revealed from both the numerical and experimental results. The numerical simulation is conducted without altering blade gap spacings to assist numerical stability; the axial gap is increasingly being seen as a critical performance parameter for multiple row analysis. The limitations inherent in an approach using mixing-planes are presented and a review of alternative, more rigourous, treatments of these effects is then discussed. These treatments attempt to retain the unsteady flow structure in a steady-state model by the derivation of so-called deterministic stresses.