On the prediction of the effect of interstage liners in turbofan engines

Abstract

The current trends for next generation turbofan engines are towards shorter nacelles and increased distances between the fan and the outlet guide vanes. This leads to an overall reduction in lined surface areas as well as an increase in the relative importance of the interstage liner, which is the liner placed between the rotor blades and the stator vanes. The interstage is different in that the liner is subject to a mean flow with a strong swirl component and shear. This project will contribute to understanding and predicting the effect of the swirl on liner attenuation and consists of 4 steps: To model an eigenvalue problem that includes sheared and swirling mean flows and acoustic absorption, to develop a code based on this eigenvalue problem and to validate it, to compare results from this code with experimental results and to carry out a parametric study to evaluate how the swirling flow affects liner attenuation and optimum impedance. Two models were developed. The first one considers a ducted sheared mean flow and is based on the Pridmore-Brown equation and the second one takes into consideration a mean flow with swirl and shear and is based on the Linearized Euler Equations. For both cases an eigenvalue problem was obtained by applying the normal mode analysis to the governing equations together with the impedance boundary condition. Both models were discretized using a Finite Difference Method. The codes were exhaustively validated against predicted values obtained by other methods for uniform, sheared and swirling mean flows and hard-walled and lined ducts. The swirling mean flow, when present, is a combination of rigid body and vortex swirl. A cross-validation between the Finite difference code based on the Linearized Euler Equation and the JM66 code from Rolls-Royce was carried out for a more realistic case. Axial wavenumbers and pressure and velocity eigenvectors obtained with the JM66 code were compared with the current predictions. A comparison has been conducted of predictions from the current Finite Difference code with measured data at a single frequency for a range of spinning mode numbers. Qualitative agreement is obtained for the measured Power Transmission loss (TL) but the low Mach numbers and modest TL levels meant that the effect of swirl was small nad it was difficult to validate the accuracy at the Finite Difference code specifically for the swirl case. Finally, a parametric study was undertaken for hard-walled and lined ducts for realistic interstage conditions to evaluate the effect sound propagation in swirling flows. This confirmed that the effect of swirl is higher for radial modes near cut-off and tends to vanish for higher radial mode orders. The swirl strongly changes the modal content. When swirl is included, the modal distribution for positive and negative azimuthal mode orders is no longer symmetrical. The higher the swirling flow magnitude, the more the modal content is shifted to negative circumferential mode orders. Co-rotating modes become more cut-on and contra-rotating modes become more cut-off. When acoustic absorptive liners are considered, the swirl changes the liner optimum resistance and reactance and affects the optimum insertion loss. The optimum resistance becomes considerably higher and the change in optimal liner reactance is not as pronounced. The swirling flow also reduces attenuation; the insertion loss is lower when swirl is considered. As a conclusion, swirling flow should be considered when designing liners.

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