Reheat buzz: an acoustically coupled combustion instability. Part 1. Experiment

Abstract

Reheat buzz is a combustion instability which occurs in the afterburners of jet aeroengines. Similar oscillations have been observed on a laboratory rig in which a confined flame is stabilized in the wake of a conical gutter. The source of energy for the instability is the unsteady heat release as the flame responds to velocity fluctuations in the approach flow. The amount of energy fed into the instability is determined primarily by the phase relationship between the unsteady heat release rate and pressure fluctuations. Two types of relationship between pressure and heat release have been observed. In the first kind, perturbations in heat release rate convect downstream from the lip of the flame stabilizer at the axial velocity of the cold reactants. In the second type, which occurs downstream of the first, the phase of the heat release rate at the buzz frequency is constant and close to the phase of the unsteady pressure. The characteristics of the resulting instability depend on which of these occupies the larger portion of the duct. Consequently two types of instability exist and the transition between them occurs sharply, with little change in mean flow conditions. The transition is associated with an abrupt change in buzz amplitude and frequency as well as with a change in the shape of the modal distributions.