Separating Direct and Indirect Turbofan Engine Combustion Noise Using the Correlation Function

摘要

A previous investigation on the presence of direct and indirect combustion noise for a full-scale turbofan enginenusing a far-field microphone at 130u0001nis extended by also examining signals obtained at two additional downstreamndirections using far-fieldmicrophones at 110 and 160u0001n. A generalized cross-correlation function technique is used tonstudy the change in propagation time to the far field of the combined direct and indirect combustion noise signal as ansequence of low-pass filters is applied. The filtering procedure used produces no phase distortion. As the low-pass-nfilter frequency is decreased the travel time increases because the relative amount of direct combustion noise isnreduced. The indirect combustion noise signal travels more slowly because in the combustor entropy fluctuationsnmovewith the flowvelocity, which is slow compared to the local speed of sound. The indirect combustion noise signalntravels at acoustic velocities after reaching the turbine and being converted into an acoustic signal. The directncombustion noise is always propagating at acoustic velocities. The results show that time delays measured at eachnangle are fairly consistent with one another for a relevant range of operating conditions and demonstrate sourcenseparation of a mixture of direct and indirect combustion noise. The results may lead to a better idea about thenacoustics in the combustor and may help develop and validate improved reduced-order physics-based methods fornpredicting turbofan engine core noise.