Integrated flow simulation of the fan and high-pressure compressor stages of a turbofan at windmill

Abstract

The flow in the windmilling regime of a high-bypass-ratio geared turbofan is studied. The nu- merical domain integrates both the low- and high-pressure compression stages, including the bypass flow nozzle. A steady RANS approach with mixing plane interfaces between rows is applied. The results demonstrate the importance of integrating the high-pressure centrifugal impeller in the simulation to properly predict the bypass ratio, which changes from 9 to 45 (in agreement with the experiments) when integrating the compressor. Comparisons with experimental data show that the predicted pressure-ratio profiles are within the measurements uncertainty (0.6 %). The differences observed for the Mach number profiles are below 5 % downstream of the rotor. Then, local analysis of the flow inside the centrifugal compressor is performed, indicating that the stage is in a stirrer mode: the impeller exhibits a positive work input along the whole span, but the total temperature rise is very low, and the losses across the stage are such that that the overall stage pressure ratio is below unity. The radial diffuser is found to be the main source of loss, due to massive upper-side flow separation. Finally, a performance map is computed, and the windmilling operating point of the centrifugal compressor is found to be close to the stability limit of the stage.