Lifted reaction zones in premixed turbulent bluff-body stabilized flames

Abstract

Flame liftoff phenomena are well known in turbulent non-premixed flames but less investigated in turbulent premixed flames. Here, in the special case of flames stabilized by the recirculation of burned gases a particular type of lifted reaction zone can be observed which is not only originated simply by the lack of heat, but by some other mechanisms (e.g., ignition delay). In principle, this premixed flame liftoff either might be based on turbulent effects (e.g., local flame extinction due to high turbulent strain rates, which are especially intensive in the shear flow region near the exit) or are considered to be based on chemical ignition delay processes. To investigate these liftoff mechanisms, experiments have been conducted at bluff-body-stabilized premixed methane/air flames, where flow and flame parameters have been varied systematically over a broad range of exit velocities and stoichiometries. While the liftoff height has been measured with laser-induced OH fluorescence, also the local characteristic turbulent flow and temperature field has been measured to allow correlated data determination for the liftoff height. Three different theoretical model approaches are discussed for the prediction of this lifted reaction zones by comparing local flow, turbulence and reaction parameters with the local burning or non-burning status. It is found that for this burner configuration not only one but two different liftoff criteria must be met. For very lean mixtures the chemically dominated ignition delay is found to be the rate-determining step. For other cases, the liftoff height can be determined by a newly described turbulent mixing dominated model. In contrast to this, a dimensionless turbulent strain rate, often described with a Kovasznay or Karlovitz number, is not a suitable criterion here.