Self-excited oscillations in combustion chambers with premixed flames and several frequencies

Experimental results of self-excited oscillations in combustion chambers, using a simple laboratory burner with flames of small extension, are presented for situations when more than one frequency is excited and, owing to the unsteady sound field, phase shifts vary continually between sound pressure and sound particle velocity. Here, in sequence phase positions in which the sound particle velocity precedes the sound pressure interchange with those in which the velocity lags behind the pressure. In these cases the question arises as to how the flame adapts itself the pressure oscillations which change with time. It is the purpose of this work to reach conclusions regarding this by experiments with simultaneous recording of oscillations in the sound pressure, sound particle velocity, and combustion.The multiplicity of ways of maintaining self-excited oscillations when several frequencies are excited should be regarded in the light of the difficulty of avoiding such oscillations in technical systems.