V-GUTTER STABILIZED TURBULENT PREMIXED FLAME AND LEAN BLOWOUT

摘要

Three-dimensional non-reacting and reacting flows past a V-gutter are modeled using FLUENT. Incompressible Large-Eddy Simulations with Dynamic Subgrid Kinetic Energy Model, C-progress variable equation and Zimont turbulent flame speed closure are used. Grid sensitivity study is conducted and the numerical results are validated with results reported in the literature. The non-reacting flow conditions show that the flow separates at the trailing edges. This leads to two shear layers shedding counter-rotating vortices that frequently interact with each other due to the V-gutter opening. Vortex stretching is greater than vortex diffusion in the near-field wake and flow non-uniformity is nearly negligible for the temporal and spatial spanwise vorticity rate of change (DWε/Dt). The reacting flow is established by igniting the stoichiometric flow (Φ=1.0). A propane-air flame attached to the trailing edges of the flameholder is obtained. The flame sheet is thin and smooth near the flameholder and thick and wrinkled further downstream the shear layers. Reaction zones from the shear layer flame sheet are transported towards the transverse centerplane near the minimum flow velocity creating a concave-like region. This region exhibits greatest turbulent flame speed, and thermal expansion increases the flow velocity downstream the concave-like flame region. Baroclinic torque is the largest source for DWε/Dt. Nevertheless, flow non- uniformity is enhanced due to thermal expansion and can act as a source and sink for(D(Wε)/Dt. Reducing Φ to 0.8 causes the flame to nearly blowout, but the hot spot inside the flameholder re-ignites the flame. The flame tries to propagate downstream, but the von Karman street extinguishes the flame in downstream areas. The flame is now nearly perpendicular to the mainstream, but it is still attached to the flameholder trailing edges. The wake shrinks and thermal expansion no longer occurs downstream the flame. The baroclinic torque and prevailing Strouhal number (St) are reduced, while drag (C_D) increases and stretching becomes the major source for (DWε/Dt). At 0=0.6 the flame blows out globally and the wake further shrinks until it matches the same profile of that of the non-reacting condition. Now, stretching is practically the only contributor to (D(Wε)/Dt) in the near wake. C_D increases and St further decreases, respectively, reaching values closer to that of the non-reacting condition. Based in this numerical study it is inferred that highly statically stable flameholders would be characterized by increased principal St, baroclinic torque, shear layer length, and wake length, while reducing C_D and vortex stretching.