Acoustic Emission of Thermodiffusive Unstable Premixed Lean Hydrogen-Air Slit Flames

Combustion noise is coupled to fluctuations of the flow field variables. The heat release fluctuations are a direct noise source term caused by perturbations of the flame surface generated by velocity fluctuations, which are generated by the flow field. Flame front perturbations are damped or amplified by intrinsic stability phenomena, like hydrodynamic and preferential diffusion effects.

In this work, two-dimensional lean premixed hydrogen-air slit flames are investigated. The fuel-air-equivalence ratio is varied and the effects on general flame dynamics are investigated. For the various cases, intrinsic instabilities of the non-excited flame lead to strong cellular structures along the flame front, which cause constant pocket shedding and strong pressure fluctuations at the flame tip. Due to Markstein-length related effects, intrinsic differences have been found when compared to more widely investigated methane-air premixed flames. A more detailed investigation of the thermoacoustics by solving the acoustic perturbation equations which contain the input data from the flow field for the source terms enhances the specific understanding of the different acoustic source contributions and the overall acoustic field of hydrogen-air flames compared to methane-air flames.