The acoustic response of reacting sprays in cross flow models to forced excitations

Thermo-acoustic instabilities can occur in combustion systems due to unsteady heat release. The derivation of transfer functions is a method to describe the systems dynamics, which is applied to analyse and to control these instabilities. In this work, validated models for spray atomization, evaporation and combustion in OpenFOAM are used to investigate their response to acoustic forcing. Mono-frequency excitations are employed, with a frequency range from 100Hz to 1000Hz in 100 Hz intervals. The fuel considered in this study is hexadecane, for which a reduced kinetic mechanism is available. A primary atomization model, together with a secondary atomization model, is employed to simulate a spray in cross flow. The evaporation model is a zero-dimensional model developed for multicomponent droplets at high ambient pressures and temperatures. The combustion model comprehends a blend of the FGM (Flamelet Generated Manifold) and the SCRS (Simple Chemical Reaction System) models. The acoustic response of each model, in terms of its transfer function, is determined by applying forcing at the inlet boundary. The penetration and global SMD are monitored at 80 mm downstream from the injection point and forcing frequencies are clearly found in the response of both quantities. The rate at which the droplet diameter shrinks is tracked in the droplet vaporization, and a non-linear response is encountered due to its dependence on the relative velocity. Similarly, the heat release is used for 1-D premixed flames, showing forcing frequencies in the model response too.