Dominant chemical source and reaction modes in lean premixed H2/air flames

Chemical Explosive Mode Analysis (CEMA) is used to illustrate how source terms and reactions associated with state variables drive the combustion process across a large database of lean, premixed H2/air flames with detailed chemistry. Based on an eigenanalysis of chemical sources terms, chemical source modes and associated reaction modes are distinguished. The dominant mode amplitude is used to determine whether source terms within the flame structure are promoting or counteracting the dominant chemical source mode. Mode sources are then defined from source terms to further differentiate dominant sources (i.e. species or temperature) that promote or counteract a chemical source mode. Similarly, mode participation distinguishes promoting reactions from counteracting ones. Based on mode sources and mode participation, transition temperatures are introduced to track dominant species and reactions within the flame structure. Concepts are detailed for one reference case (ϕ=0.99 and Tin=302K), which is then applied to a large database of simulated test cases with distributed inlet temperature (300-800 K) and equivalence ratio (0.1-1). In all cases, transitions correlate with the adiabatic flame temperature for a simulation. Results show a perfect collapse of temperatures associated with the definition of the flame sheet in the context of CEMA. Furthermore, transitions within the flame structure show trends that clearly illustrate that flames with similar adiabatic temperatures share a comparable chemical structure.