Numerical analysis of fuel-air mixing in a two-dimensional trapped vortex combustor

The concept of trapped vortex combustion is emerging as a viable method of burning fuel effectively in aero-gas turbine engines. In a two-dimensional trapped vortex combustor (TVC), fuel-air mixing is characterized by using the parameter intensity of segregation (I_s). When fuel and oxidizer are present (with some macro-mixing already having taken place), the parameter quantifies the extent of local micro-mixing within the domain. The regions of maximum fuel-oxidizer mixedness are identified to be the vortex edge and dump shear layer of the cavity. These two regions are observed to be zones of very good fuel-air mixing with higher levels of turbulence. Increasing aspect ratio (L/D) and the mainstream velocity (V_ms) increases mainstream air entrainment into cavity, resulting in improved fuel-air distribution and mixing. However, an optimal L/D=1.2 and V_ms=40 m/s results in stable vortex being trapped with better fuel distribution throughout the cavity. While non-optimal L/D and V_ms may result in improved mainstream air entrainment, it is found to disrupt the trapped vortex, which is not conducive for combustion. Hence, for a given L/D, there is an optimal momentum ratio between the cavity injections and mainstream flow, which results in a single large vortex being trapped that can roll up the fuel-air injected and distribute them across the cavity, required for stable combustion in a TVC.