The main aims of this research are, at first, combustion instability study based on equivalence ratio oscillation, and, secondly, investigation various frequency modes of combustion instability, taking combustion chamber geometry into account. Considering the configuration of the simulated combustion chamber, excitation probability of the longitudinal modes is higher than that of transversal modes. The reason of this fact is that the resonance frequency values of the longitudinal modes are less than those of transversal modes. So, the most important frequency mode, during combustion instability, is the first longitudinal mode. In this paper thermo-acoustic instability model is utilized for pre-mixed gas turbines combustion chamber, founded on equivalence ratio oscillation. For this purpose Lieuwen method is developed in order to attain the phase difference between pressure and heat release oscillations. Results concluded from combustion instability simulation for the first longitudinal mode, considering its importance, are compared with experimental data and good agreement is observed.
