Ignition delay time modulation as a contribution to thermo-Acoustic instability in sequential combustion

Pressure pulsations due to combustion instabilities have been encountered in a premixed sequential gas turbine combustor. Measured noise spectra display one or several distinct peaks at Strouhal numbers significantly larger than unity. Height and location of the peaks depend in a sensitve manner on fuel type and/or operating conditions. The paper identifies a possible mechanism of the observed combustion instability and presents a mathematical model of acoustic self-excitation. The mechanism of self-excitation comprises interactions between the acoustic field in the fuel injector / burner with the ignition delay time of the fuel-Air mixture and the heat release intensity: pressure drop in the fuel injector nozzle changes with variations of the acoustic pressure in the burner, variations of pressure drop and air flow velocity modulate the fuel concentration, acoustic perturbations in the pre-flame region influence the delay time for self-ignition and consequently lead to fluctuations of flame velocity and-position. fluctuations of flame velocity influence the refracation of acoustic waves at the flame front. fuel inhomogeneities modulate the heat release rate and consequently the rate of volume production by the flame. Based on this structure of a self-excitation mechanism, an analytical model has been developed and used to compute eigenfrequencies and growth rates of instabilities. Some characteristics of the suggested self-excitated instabilities as they are predicted by the model match well with empirical information.