Flame stabilization in gas-fueled inverse diffusion flames and liquid-fueled swirling combustion

Abstract

Three parts are illustrated in the present work on the non-premixed flames, which includes near-field flow stability of buoyant methane/air inverse diffusion flames, wall effects on the stability of buoyant inverse diffusion flames of methane and the influence of centerbody air injection on liquid combustion in a model gas turbine combustor. In the first part, experiment and simulation were performed to investigate buoyant methane/air inverse diffusion flames, with emphasis on the near-field flow dynamics under non-reacting and reacting conditions. In the non-reacting flow condition, the initial shear flow and the buoyancy effect induce opposite-direction vortices, which interact with each other and cause flow instability similar to the mechanism forming the von Karman vortex street. The instability is greatly intensified at around unity Richardson number, when the two vortices are comparably strong. In the reacting inverse diffusion flame, the density gradient is reversed due to chemical heat release and so is the buoyancy-induced vortex that it has the same direction with the vortex of the initial shear flow. As a result, the buoyancy-induced vorticity generation would facilitate the growth of the initial shear layer, thus the near-field flow remains stable. However, the growing shear flow would eventually lead to the development of the Kelvin-Helmholtz instability in the far field. In the second part, experimental and numerical studies were performed to examine the stability of inverse diffusion flames (IDFs) with the focus on the boundary wall effects. A regime diagram for flame stability was obtained based on the visual characteristics of flames and verified by the simulated flow fields. The boundary wall effect was identified and investigated by simulating the IDFs with different outer burner diameters. It was found that the wall-bounded induced shear layers (WISLs) either reduce the flow instability by vorticity diffusion or enhance it by vorticity convection. Proposed as a way to control the IDF stability, an additional shear layer with opposite vorticity sign to the main shear layer was created in the fuel inflow to emulate the wall effects on the IDFs. This non-uniform fuel inflow was found to suppress the flame instability by restraining the development of the main shear layers through vorticity diffusion. In the liquid gas turbine non-premixed combustion, a portion of centerbody air is injected directly into the central recirculation zone. The experimental and numerical results show that an enlarged recirculation zone with higher temperature can be formed in the swirl combustor with the centerbody air injection, which therefore may hold potentials in enhancing flame stabilization and combustion efficiency In addition, the centerbody air injection pushes the recirculation zone to the farther downstream of the fuel injector and hence can reduce the exposure of the fuel injector to the high-temperature combustion zone. Since the centerbody air injection was found to only slightly modify the effective swirl number, the modified velocity and temperature characteristics are mainly due to the changes of the shape and location of the central recirculation zone and the local stoichiometry in the vicinity of fuel injection. A parametric study for various inlet air velocities and excess air factors further substantiates the effectiveness of the centerbody air injection on improving the combustion performance although the extent of the improvement relies on other factors that therefore merits future studies for optimization design.