Hydrogen blended fuels burn with reduced carbon emissions and have been widely studied in recent years. However, addition of hydrogen may cause thermoacoustic instabilities as well as flashback due to increased flame speed. In the present work we aim to identify flashback limits for hydrogen blended natural gas with various quantities of hydrogen (0 – 100 %) in order to restrict the stability predictions in combustion systems to an operating regime with no flashback. To this end, we model the burner as an array of small conical flames whose dynamics is described by the nonlinear G-equation, and their heat release rate expressed as a flame describing function (FDF). Previous studies have shown that the FDF of the hydrogen blended fuels have different characteristics at low and high amplitudes of upstream velocity perturbations. In our study, separate analytical expressions for heat release rate law are obtained for the low and high amplitudes of velocity perturbations. The combustion chamber geometry is approximated by a quarter wave resonator and the stability predictions in terms of thermoacoustic eigenfrequencies are obtained using a tailored Green's function approach. The stability behaviour of the system is presented as a stability map in the amplitude of velocity fluctuation – hydrogen concentration in the fuel plane.
