Large eddy simulations of fuel-air mixing in a trapped vortex combustor-effect of cavity length to depth ratio

Turbulent mixing in a trapped vortex combustor (TVC) is investigated using large eddy simulation (LES) coupled with filtered mass density function (FMDF). The impact of cavity length-to-depth ratio (L/D) as a crucial geometrical parameter on fuel-air mixing quality is evaluated in non-reacting flow conditions. The vortical structure analysis along with various quantitative measures such as mean cavity and near stoichiometric equivalence ratios, global fuel distribution and mixing efficiency curves are invoked to compare different L/D ratios. The predicted results show that increasing L/D ratio from 0.60 to 0.85 improves mixing quality within the cavity due to expanding the main vortex. Further increment of L/D ratio to 0.93 temporarily impairs the mixing quality, whereas more increasing this ratio to 1.00 leads to resumption of mixing quality improvement. Nevertheless, L/D=0.85 provides the best mixing curves and fuel distribution about mean cavity and near stoichiometric equivalence ratios, and consequently the best mixing quality. These findings are commensurate with expectations based on the vortical flow structures inside the cavity.