On the mode transition of a double bypass variable cycle compression system

By establishing a three dimensional model of a double bypass variable cycle compression system, the flow patterns and matching characteristics of each working component during mode transition are investigated using numerical simulation. Results show that transition from the single bypass mode to double bypass mode by opening the mode selector valve (MSV) alone would increase the fan operation point while decreasing that of the core driven fan stage (CDFS) and the high pressure compressor (HPC). It would also incur outer bypass flow recirculation and bring radial inflow distortions to the CDFS. Deviations of the fan aerodynamic performance lie mainly in its aft stage, while the HPC first stage undertakes most of the inlet distortion. Reducing the bypass backpressure during mode transition is an effective way to alleviate the outer bypass flow recirculation but would further choke the CDFS. Closing the forward variable area bypass injector (FVABI) could raise the CDFS matching point so as to improve the stator performance without influencing the outer bypass ratio. It is recommended to decrease the bypass backpressure and close FVABI simultaneously in the real transition process.