The presented study is on a laboratory scaled industrial gas turbine combustor of intensity 25MW/m3 atm, where an open loop active control technique is investigated. Combustion noise is classified as direct and in-direct combustion noise. The present study is focused on the investigation of direct combustion noise. It occurs when the volume of the gas fluctuates due to the fluctuations in heat release rate, caused perhaps due to flow turbulence. This results in sound waves, which propagate outside the boundary of the flame. The radiated acoustic waves are reflected from the boundaries of the combustion chamber, perturbing the fuel flow rate and hence the spray characteristics. This eventually leads to perturbation in the heat release rate and thus a feedback loop is established. At certain conditions, if the unsteady heat release rate drives the acoustic oscillations, satisfying Rayleigh criterion, pressure oscillations grow leading to discrete tonal sound and this phenomena is termed as combustion instability.
Experiments are performed in a scaled down swirl stabilized liquid fueled gas turbine combustor, where a new scheme for open-loop control of combustion noise using periodic fuel injection is employed without drastically altering the combustor design or forfeiting its performance. Fuel is modulated in the frequency range of 0.6 to 5 Hz with various duty cycles [25–75%] using square wave. Fuel modulation is achieved by passing fuel through a direct current (DC) powered solenoid valve, which is being controlled using a custom-made circuit. The modulated fuel enters the combustor through an air-blast atomizer and is metered through a turbine flow meter.
The main objective of this paper is to investigate the potential of active control to reduce combustion noise in laboratory scaled gas turbine combustor. Pressure transducer is used to capture the sound pressure level inside the combustor. A reduction in overall sound pressure level of 14dB is achieved by modulating fuel with 50% duty cycle at 1.5Hz.