FLAME DESCRIBING FUNCTION CALCULATIONS OF A TURBULENT PARTIALLY-PREMIXED FLAME FROM LES

The present work deals determines of the non-linear heat release response to acoustic forcing of a partially-premixed turbulent flame (known as a Flame Describing Function (FDF)) from high-fidelity Large Eddy Simulations (LES). The target case is a bluff body stabilised partially-premixed turbulent flame, for which experimental measurements have been carried out. The simulations are performed using a low-Mach number solver of the open source CFD toolbox, OpenFOAM, with the combustion modelled using the Partially Stirred Reactor (PaSR) combustion model combining a global one-step chemical reaction mechanism. The unforced/forced reactive flows are simulated in order to validate the computational code. The simulations capture the unforced flow fields, the flame dynamics and the response of the flame to harmonic excitation with good accuracy. On this basis, harmonic acoustic forcing is imposed as a hydrodynamic velocity fluctuation at the inlet whose forcing amplitude and frequency can be varied independently. By extracting the gain and phase shift of the heat release rate response to harmonic forcing in velocity, we obtain the full FDF. The nonlinearity of the obtained FDF is clearly observed which is of importance to the appearance of limit cycle phenomena. The work thus confirms that low-Mach number LES, in this case via the open source OpenFOAM, provides a useful tool for characterising the non-linear response of lean partially-premixed turbulent flames to acoustic forcing.