Propagation, dynamics and control of laminar premixed flames

Abstract

Current state of knowledge of laminar premixed flame propagation, dynamics and control is reviewed. A distinction is made between planar and multi-dimensional flames. Based on the well-established theory of flame propagation of free adiabatic flames, different mechanisms to control planar flames, mainly based on heat transfer processes in porous media, but also using electrical and gravitational fields, are considered. Flame instabilities, leading to flame pulsations or oscillations, for instance in an acoustic field, are also considered. A simple physical model for the interaction between a premixed flame and a burner is presented to explain the main physical mechanisms of planar instabilities of burner-stabilized flames. Knowledge on flame propagation of planar flames is well-established and physical models to describe the behavior of these flames are mature, although it is expected that more sophisticated methods will be used in future research to control their behavior.Multi-dimensional flames are also well understood, although the state of knowledge is less mature. Models for the interaction of flames with flow distortions, leading to flame stretch and curvature are derived. The influence of such distortions on the burning velocity and flame dynamics in terms of the Markstein number is studied. Weak-stretch models are available but multiple species chemistry and transport effects are less well understood, especially for rich and stoichiometric mixtures. Some ways to extend or improve current models are indicated. Current knowledge on the interaction of premixed (Bunsen-type) flames with flow distortions, such as acoustic waves, is also reviewed. Experimentally observed phenomena can be modeled satisfactory using detailed numerical models. However, for this research area, it is clear that current physical models or analytical descriptions are less mature because the origin of some phenomena is still largely unknown. Extensions of current (mostly G-equation like) models to account for flame–flow interaction, flame–flame interaction and flame–burner interaction are needed. To conclude: a multiple set of measures to adapt the flames to the combustion system of interest is at hand and different ways to influence the flame by heat transfer, acoustic waves and other less often used control mechanisms like electric fields are available. This is needed to modify and control premixed laminar flames in practical devices to guarantee safe and proper operation.