The Response to Incident Acoustic Waves of the Flow Field Produced by a Multi-Passage Lean-Burn Aero-Engine Fuel Injector

Abstract

Previous work has shown that compressible unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, with suitable acoustic boundary conditions, are capable of correctly predicting the acoustic impedance of simplified fuel injectors. In this work the method developed is applied to simulating the acoustically forced flow in and downstream of a realistic multipassage fuel injector. The simulations are validated by comparing the impedance of the injector with data obtained experimentally by a multi-microphone technique. Such results can then be used in conjunction with a suitable low-order thermo-acoustic network model to predict the stability of combustors. However the validated simulations can also be used to reveal further details about the effect of acoustic forcing on the flow field. The velocity flow field produced by the injector with and without acoustic forcing is analysed using snapshot POD to determine the large scale energy containing structures within the flow. In the non-acoustically forced simulations it was found that the first four POD modes correspond to two rotating spiral modes, designated as the m = 1 and m = 2 modes with a peak frequency content of 450 Hz for the first mode and 1000 Hz for the second mode corresponding to experimental Hot-Wire measurements made in a separate study. It is hypothesised that these spiral modes will affect the atomisation, evaporation and mixing of the fuel in subsequent planned two-phase simulations. POD analysis of the flow subjected to 300 Hz, 300 Pa acoustic excitation shows that the first four POD modes correspond to similarly shaped spiral modes. The acoustic excitation is responsible for the appearance of 4 POD modes within the injector body that correspond to two push-pull velocity modes with axes of symmetry perpendicular to each other. The acoustic forcing also produces two additional POD modes that most likely represent the non-linear interaction between the push-pull and spiral modes. Further analysis of the fluctuations in pressure, mass flow rate, angular velocity and swirl number, within the passages and at the injector exit plane, show that the fluctuations in pressure and mass flow rate average across the passages while variations in angular velocity and swirl number sum across the passages. The relationship between mass flow rate, angular velocity and swirl number is discussed with reference to general observations of the sensitivity of flames to fluctuations in these quantities.