Capturing Radial Mixing in Axial Compressors with CFD

Due to the generally high stage and blade count, the current standard industrially adopted to perform numerical simulations on multistage axial compressors is the steady-state analysis based on the Reynolds-averaged Navier-Stokes approach (RANS), where the coupling between adjacent rows is handled by means of mixing planes. In addition to the well-known limitations of a steady-state picture of the flow, namely its inherent inability to capture the potential interaction and the wakes from the upstream blades, there is another flow feature which is lost through a mixing-plane, and which is believed to be a major accountable for the radial mixing: the transport of stream-wise vorticity. Streamwise vorticity arises throughout a compressor for various reasons, mainly associated with secondary and tip-clearance flows. A strong link does exist between the strain field associated with the transported vortices and the mixing augmentation: the strain field increases both the area available for mixing and the local gradients in fluid properties, which provide the driving potential for mixing itself. Especially for the rear stages of a multistage axial compressor, due to high clearances and low aspect ratios, only accounting for the development along the meridional path of secondary and clearance flow structures (e.g. running an unsteady computation) it is possible to properly predict the spanwise mixing. Within this paper, the results of steady and unsteady simulations performed on a heavy-duty multistage axial compressor of Ansaldo Energia fleet are compared with experimental data, showing the importance of modelling the unsteady interactions in capturing the radial mixing. This compressor was selected because of the enhanced radial mixing effect experimentally measured in its high-pressure section.