LES investigation of two frequency effects on acoustically forced premixed flame

Turbulent lean premixed combustion has high potentials for the development of modern low nitrogen oxide gas turbines. However, it renders the combustor prone to combustion instabilities. The non-linear response of turbulent premixed flame to external acoustic forcing plays a dominant role in the development of combustion instability. The present work describes a numerical study investigating the two frequency effects on the non-linear responses and interactions of lean premixed ethylene/air flame externally forced by strong inlet velocity oscillations. The target case is a bluff body stabilized premixed turbulent flame. Large Eddy Simulation (LES) is performed using a low-Mach number solver based on the open source CFD toolbox, OpenFOAM. The lean combustion is modelled using the Partially Stirred Reactor (PaSR) combustion model combining a reduced two-step chemical reaction mechanism. Both the unforced and forced reactive flows with single frequency forcing are simulated in order to validate the computational method. On the basis, the velocity oscillations are introduced at the inlet with two frequencies, namely the primary frequency of f1=160 Hz and the harmonic frequency of f2=320 Hz. The introduction of second harmonic frequency is found to change the heat release rate fluctuation significantly. With two frequency forcing, the amplitudes of heat release responses at the primary frequency are reduced significantly, up to 70% less than those with single frequency forcing. Also the phase values are reduced/increased a lot depending on the level of second harmonic forcing. At the same time, the heat release rate fluctuations are also reduced responding at the harmonic forcing except one case where both the forcing amplitudes of the two frequencies are small. The physical mechanisms are found to be highly related to the vortex flow structures during the acoustic forcing. The central recirculation region and the side recirculation region which generates the flame shear layers in between have different responses to the acoustic forcing depending on the frequencies and amplitudes. This work implies that LES, in this case via OpenFOAM, can be used to study the heat release responses and flame dynamics in complex cases of combustion instability, such as with two frequency forcing, with good accuracy.