Investigation of pre-stall mode and pip inception in high-speed compressors through the use of correlation integral

Five high-speed compressor configurations are used to identify, pre-stall p ressure signal activity under clean and distorted inlet conditions, and und er steady injection and controlled injection conditions. Through the use of a nonlinear statistic called correlation integral, variations in the compr essor dynamics are identified fr om the pre-stall pressure activity far bef ore variations (,nodal or pip) are observed visually in the wall static pre ssure measurements. The correlation integral not only discerns changing dyn amics of these compressors prior to stall, but is now? used to measure the strength of the tip flow field for these five high-speed compressors. Resul ts show that correlation integral value changes dramatically when the stall inception is modal; and it changes less severely when the stall inception is through pip disturbances. This algorithm call therefore distinguish from the pre-stall pressure traces when a machine is more likely to stall due t o pips versus modes. When used irt this manner, the correlation integral th us provides a measure of tip flow strength. The algorithm requires no predi sposition about the expected behavior of the data in order to detect changi ng dynamics in the compressor; thus, no pre-filtering is necessary. However , by band-pass filtering the data, one can monitor changing dynamics in the tip flow field for various frequency regimes. An outcome of this is to ass ociate changes in correlation integral value directly with frequency specif ic events occurring in the compressor, i.e., blade length scale events vers us long length scale acoustic events. The correlation integral provides a p otential advantage over linear spectral techniques because a single sensor is used for detection and analysis of the instabilities.