Question

What are the acoustic instabilities? How can they be avoided? (Rocket Propulsion Elements 9th ed, Sutton)

Answer 1

Combustion instabilities are physical phenomena occurring in a reacting flow in which some perturbations, even very small ones, grow and then become large enough to alter the features of the flow in some particular way.

The simplest example of a thermoacoustic combustion instability is perhaps that happening in a horizontal Rijke tube ( also thermoacoustics): Consider the flow through a horizontal tube open at both ends, in which a flat flame sits at a distance of one-quarter the tube length from the leftmost end. In a similar way to an organ pipe, acoustic waves travel up and down the tube producing a particular pattern of standing waves. Such a pattern also forms in actual combustors, but takes a more complex form.The acoustic waves perturb the flame. In turn, the flame affects the acoustics. This feedback between the acoustic waves in the combustor and the heat-release fluctuations from the flame is a hallmark of thermoacoustic combustion instabilities.

This graphical representation of a hypothetical combustor allows to group three methods to prevent combustion instabilities: increase the losses; reduce the gains; or move the combustor's peak response away from the region where gains exceed losses.