Turbofan forced mixer/nozzle temperature and flow field modelling

Abstract

A computational procedure is described for the calculation of the flow and temperature fields in a multilobed turbofan mixer/nozzle combination. The predictions have been obtained using a finite-volume solution procedure for the steady three-dimensional elliptic equations of fluid flow. The procedure allows the calculation of flows within complex geometries using a non-aligned mesh system so that the flow within the lobes themselves may be predicted. Turbulence is modelled using the two-equation k- ε eddy viscosity model. Forced mixer performance is shown to be dominated by a periodic array of axial vorticity cells created by the lobe geometry. The calculation of the large secondary velocities associated with this vorticity within and at exit from the lobes provided the necessary boundary conditions at the lobe exit for the predictions of the flow in the mixing duct region. This removes the dependency of previous calculation methods for mixing ducts on measured secondary velocities at the lobe exit plane. The present work demonstrates the capability of the current method to predict the downstream development of the largescale secondary motions and their strong influence on the temperature signature at the nozzle exit. Results are also presented to illustrate the ability of the method to predict parameters of importance to mixer designers such as mixer total pressure loss and mixer efficiency. The plausibility of the results obtained illustrates the potential of the method to provide a flexible analysis technique for the complete mixer/nozzle system.