Study of Flame Transfer Function With Three Dimensional Calculations

Combustors with fuel-spray atomisers are particularly susceptible to a low-frequency oscillation at idle and sub-idle conditions. For aero-engine combustors, the frequency of this oscillation is typically in the range 70–120Hz and is commonly called ‘rumble’. The mechanism involves interaction between the plenum around the burner and the combustion chamber. In our previous work, the CFD calculation has been conducted in an idealised 2D axisymmetric annular combustor to calculate unsteady combustion flow at idle conditions. In this work, in order to investigate the effects of asymmetrical geometry and flow distributions on the transfer functions of flame and shape factors, the CFD code has been extended to fully three-dimensional geometries. The results are compared with those from 2D calculations. Though the differences of the distribution local flow properties are evident, the integrated results for the 3D flow are broadly similar to those obtained in 2D. One substantial difference arises due to the more accurate modelling of the downstream contraction near the combustor exit, which is treated as a smooth contraction in our 3D calculations and as an abrupt change in the simplified 2D geometry. The gradual downstream contraction not only accelerates the fluid near the combustor exit but also unifies the flow properties. As the consequence, we can see that, near the exit, the phase of the flame transfer function increases rapidly, and the shape factors tend toward unity. This work is a further development of our systematic investigation into the ‘rumble’ phenomenon, and gives encouragement that much of the essential physics can be captured in a quasi-one-dimensional model.