Combustion Oscillations in Burners With Fuel Spray Atomisers

Abstract

Most types of combustion-driven devices experience combustion instabilities. For aero-engine combustors, the frequency of this oscillation is typically in the range 60–120Hz and is commonly called ‘rumble’. The rumble oscillations involve coupling between the air and fuel supplies and unsteady flow in the combustor. Essentially pressure fluctuations alter the inlet fuel and air, thereby changing the rate of combustion, which at certain frequencies further enhances the pressure perturbation and so leads to self-excited oscillations. The large residence time of the liquid fuel droplets, at idle and sub-idle conditions, means that liquid and gaseous phases must both be considered. In the present work, we use a numerical model to investigate forced unsteady combustion due to specified time-dependent variations in the fuel and air supplies. Harmonic variations in inlet air and fuel flows have been considered and the resulting unsteady combustion calculated. The influence of droplet size distribution has also been investigated. The calculations provide insight into understanding the interaction between atomization, unsteady combustion and flow oscillations.