Ignition Delay Time Modulation as a Contribution to Thermo-Acoustic Instability in Sequential Combustion

Abstract

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.