Experimental investigation of mechanisms of sound production by partially premixed flames

International audienceCombustion noise from partially premixed flames is less documented than that of well premixed systems. Fundamental analysis are required to determine the main mechanisms of sound generation resulting from combustion of non perfectly mixed reactants. To this purpose an experimental study is conducted to investigate sound production during the interaction of a vortex train impinging on the periphery of a flat laminar methane-air flame in a stagnation flow. In response to these flow disturbances the flame enrolls around vortices and gases from the surrounding are entrained in the combustion region. Sound production is characterized for different compositions of the surrounding gases. Three cases are explored under the same mean flow and perturbations conditions using nitrogen, air and a diluted methane-air mixture for the surrounding flow. Time traces of radiated sound pressure and CH* chemiluminescence emission signals during these interactions are characterized and compared together with detailed imaging of the flame and flow motions. It is shown that classical combustion noise relations based on CH* emission used to analyze sound production from perfectly premixed flames are still valid in these partially premixed systems for the operating conditions explored. The main mechanisms of sound production are identified in the three cases and correspond to entrainment of hot combustion products burning in the fresh mixture leading to a positive pressure peak and destruction of flame elements by vortex interaction leading to a negative pressure peak. The strength of these mechanisms depends on the surrounding gases composition. Entrainment of air leads to slightly louder noise levels than entrainment of premixed reactants because flame destruction takes place on a shorter period. It can be inferred that the dominant fundamental mechanisms of sound production in premixed and partially premixed flames are essentially the same and may be analyzed using simple diagnostic tools