On the Generation of Entropy Waves in the Dilution Zone of a Rich-Quench-Lean Combustion Chamber

Entropy waves are responsible for the generation of indirect combustion noise and risk of enhancing combustion instabilities. Their prediction and modeling, especially in non-premixed flames, are challenging, which gives the necessity of experimental studies. In this work, the entropy waves generated by temperature perturbations in the dilution zone of an academic Rich-Quench-Lean (RQL) combustor are investigated. The temperature gradients are visualized downstream of the flame with Background-Oriented Schlieren (BOS). This work reports on a novel post-processing of BOS images to resolve entropy waves downstream of a confined flame. The post-processing strategy enables the two-dimensional spatial representation of entropy waves. For this purpose, temperature gradients detected in BOS are solved for two-dimensional temperature fields with the Finite Difference Method. Boundary conditions were set by the Neumann type and complemented with information about the phase-averaged velocity. The obtained entropy waves are attributed to velocity perturbations coming either from the primary zone or the dilution air. It is found that the excitation from the dilution air produces significant asymmetric temperature inhomogeneities. A low-pass filter behavior is found for the excitation from the primary zone, which is similar to the behavior of the fluctuating heat release rate. The experimental investigation is extended to a variation of the equivalence ratio in the secondary zone for the excitation from the primary zone. The obtained magnitudes of the temperature perturbations in the dilution zone scale with its thermal power in the low-frequency regime.