Passive control of combustion instabilities involving premixed flames anchored on perforated plates
Résumé
This study deals with passive control of acoustic-flame instabilities in a multipoint injection configura- tion where a large number of small conical flames are anchored on a perforated plate. This configuration is close to that found in practical drying or heating systems. It features unstable combustion regimes resulting from a coupling between the unconfined reaction layer and resonant eigenmodes of the upstream manifold. Systematic experiments carried out by varying the geometric characteristics of the injection plate and the size of the upstream duct indicate that the unstable regimes are influenced by these parameters and that oscillations appear at the eigenfrequencies of the upstream cavity. A mode hopping phenomenon is also observed as the length of this element is varied over a wide range of values. Flame visualizations during the cycle and analysis of the pressure and heat release signals indicate that a large amount of flame surface rapidly disappears at one instant giving rise to a strong source of sound. An acoustic power balance anal- ysis is carried out to determine conditions leading to instability. The analytical model is based on previous results concerning combustion noise, flame transfer functions and perforated plates acoustics. This yields two criteria for the onset of self-induced oscillations which are then used to explore passive control meth- ods. It is found that theoretical predictions agree well with experiments and that the extent of the region of instability closely follows that determined analytically.