A model gas turbine swirl combustor is investigated experimentally for the combustion instability. The combustor is characterized for acoustic excitations over wide range of variations of flow and geometric parameters. LPG is used as fuel. The fuel injection location is varied at and upstream the swirler plane. The combustor acoustic characteristics are observed over wide range global equivalence ratios altered by varying the air and fuel flow rates. The acoustic measurements along the combustor revealed that, shifting fuel injection upstream the swirler plane induces much higher amplitudes than that are obtained by injecting the fuel at swirler plane oscillations. The slopes of Helmholtz-Reynolds number lines and Strouhal- Reynolds number lines are used to explain the mixed modes. The mixed modes are seen influencing the onset of combustion instability. The maximum amplitudes are experienced by injecting the fuel at an optimum distance upstream the swirler plane. To study the mechanism of combustion instability, OH* PLIF experiments are performed. The phase locked imaging showed the evolved flame structure under the influence of the high amplitude acoustics.