Mode-switching behaviour of preheated and diluted flames in a stagnation burner

Abstract

The combustion mode of preheated and diluted methane/air flames is studied with a one-dimensional (1D) stagnation flame model. The species transport budget is used to determine the combustion mode between premixed flame propagation and autoignition. The autoignition delay time decreases with increasing the dilution level, pressure, oxygen concentration, or adding minor species. The combustion mode can then change from premixed flame propagation to autoignition. This behaviour agrees with previous studies on autoignition. Increasing the inlet velocity has been found to switch the combustion mode from premixed flame propagation to autoignition and back to premixed flame propagation. This double mode switch may be directly related to a competition between autoignition delay time and flame speed of premixed flames as well as features of diluted flames such as the weakened reaction zone.