Aspects of laminar premixed flame extinction limits

Abstract

Numerical methods have been used to examine (1) the combined effects of stretch and upstream heatloss on the properties of a stoichiometric, laminar, pre-mixed methane-air flame, and (2) the combined effects of stretch and an extended, radiative heat loss on the properties of a fuel-lean methane-air flame, equivalence ratio Σ=0.57. Appropriate opposed flow configurations were investigated in both cases. In the first case, fresh gas was discharged towards the stagnation plane from identical, coaxial, plug flow nozzles maintained at constant temperature and separated by a fixed distance. Continuation methods were used in tandem with Newton solutions to study the flame behaviour as the stretch rate (or nozzle inlet velocity) was varied. High and low stretch extinction limits were observed as positions of vertical tangency on the steady-state solution curves of flame property vs stretch rate. The complete series of solutions between these positions formed a limit cycle with both stable and unstable branches. The chemical behaviour at the limits and along the unstable branch is discussed. The effect of radiative losses on the fuel-lean flame was studied in the unburnt-to-unburnt (UTU)configuration as above, but also in the unburnt-to-burnt (UTB) configuration where the ignited unburnt stream is opposed by a stream of its own equilibrium combustion products. If the flame is adiabatic, the product stream is input at the adiabatic equilibrium flame temperature. Such a system shows no abrupt extinction limits. On the other hand, if the radiative “sink” temperature is below some critical value, then, irrespective of the magnitude of the heat loss rate on a volumetric basis, the unburnt-to-burnt system will show abrupt extinctions if the stretch rate is increased sufficiently. Because of extremely slow numerical convergence, the numerical investigation of these limits for the methane-air system is not yet complete. However, such an investigation is in progress, and this combination of stretch and radiative loss effects is clearly of importance in relation to behaviour at composition limits of flammability.