The 1999 Lanchester lecture - Vortices, sound and flames - a damaging combination

The interaction between vortices, sound and combustion can lead to self-exc ited oscillations of such large amplitudes that structural damage is done. These occur because any small unsteadiness in the rate of combustion is a s ource of sound, generating pressure and velocity fluctuations. However, the velocity fluctuations perturb the flame, thereby altering the instantaneou s rate of heat release. Instability is then possible because while acoustic waves perturb the combustion, the unsteady combustion generates yet more s ound! Combustion oscillations can occur in afterburners and at idle in conv entional aeroengine combustors. Lean premixed, prevapourised technology has tremendous potential to reduce NOx emissions, but is proving highly suscep tible to self-excited oscillations. An overview of the physics of the inter action between vortices, sound and flames is presented, and illustrated by examples of instability in generic premixed ducted flames and in aeroengine combustors. The potential for both passive and active control is discussed .