Influence of system parameters on the dynamic behaviour of an LPM combustor: Bifurcation analysis through CFD simulations

Lean premixed (LPM) combustion allows lowering flame temperatures, thus reducing thermal NOx production. Unfortunately, at low equivalence ratios a loss of combustor stability may arise, leading to spontaneous oscillations of pressure and temperature. These oscillations must be controlled and limited, as they reduce engine performance and endanger structural integrity. In this paper, unsteady-RANS simulations are performed of a lab-scale LPM combustor in order to address the effects of geometric (length of the inlet duct) and operating (inlet gas velocity, temperature and fuel equivalence ratio) parameters on the occurrence of oscillating behaviours and on their properties (amplitude and frequency). The results obtained are analysed in the framework of the bifurcation theory. Self-excited oscillations are found originating from two different instability mechanisms identified by applying a time lag re-formulation of Rayleigh's criterion: feed mixture instability and thermo-kinetic instability owing to heat losses. It is observed that these mechanisms exhibit opposite trends of amplitude and frequency as functions of the parameters investigated. In addition, it is found that, at different initial conditions (and constant parameters), different regimes (steady and dynamic) establish, thus suggesting the presence of state multiplicity.