Combustion Instability and Flame Structure of Turbulent Swirl-Stabilized Liquid-Fueled Combustion

The present paper reports flame structure and combustion instability characteristics in a turbulent liquid-fueled swirl-stabilized lean direct fuel injection combustor. Because of the complexities in droplet size and distribution, evaporation, droplets vapor air entrainment and mixing, and droplets-flame-turbulence interactions, the flame structure is remarkably different from lean premixed gas-fueled combustion. With the preheat temperature above 423K, heat release is completed within a compact doughnut ring, about 12 nun downstream of the dump plane; with decreasing equivalence ratios at the approach of lean blowout, the doughnut ring shrinks in diameter and gradually converges to the axis; after that, the heat release zone becomes elongated and tilted off the axis. Chemiluminescence only provides qualitative information of the heat release rate, suggesting the limitations of global chemiluminescence measurements for spray combustion. Phase-locked intensified charge coupled device imaging of CH* chemiluminescence shows that during combustion instability there are mainly variations in the chemiluminescence intensity rather than in the spatial distribution of heat release. Depending on the working conditions, the one-wave mode, the half-wave mode, or both of the combustion chambers can be excited. Although the combustion chamber is highly blocked at both ends, no pressure antinodes are found at the chamber exit and the chamber inlet. Simultaneous excitation of the one-wave mode and the half-wave mode is intrinsically unsteady and unstable, which can be attributed to the nonlinear response of the reacting swirling shear layer to acoustic oscillations. Both the amplitude and the frequency of thermoacoustic oscillations are constantly time-varying. With decreasing oscillation intensity, the limit-cycle oscillator becomes increasingly vulnerable to external disturbances, as indicated by a larger neighborhood of the state trajectory.