Models for Large-Eddy Simulation of reheat combustion

Two-phase reactive turbulent Large-Eddy Simulations (LES) of the single flameholder postcombustion test rig of Georgia Tech (Cross et al., 2011) are presented and compared to experimental measurements with and without combustion. Postcombustion differs from usual turbulent flames in many ways: the inlet oxidizer stream is hot ( K), the flow speeds and Reynolds numbers are high (U , Re ), the fuel injection systems are specific and only found in reheat chambers. As a result, LES models that were developed and calibrated for conventional turbulent flames, such as those found in primary chambers of gas turbines are not adapted to reheat combustion. This study presents a collection of models specifically developed for reheat flows. The chemical scheme and the turbulent combustion model are changed to take high temperature and vitiated flow conditions into account. The Lagrangian injection methodology for the liquid jets in crossflow is modified to account for the unusually strong crossflow conditions. A modified version of the Droplet Deformation and Breakup (DDB) drag model (Ibrahim et al., 1993) is implemented to improve the drag force prediction through drop deformation. As a result, the LES shows good agreement with the experimental results both for liquid and gaseous phases, with and without combustion. Based on such results, more details about the combustion regimes and dynamics are extracted. Right downstream of the injection zone, at the bluff body trailing edge, rich premixed flames are observed along with pockets of diffusion flames in the bluff body recirculation zone, due to the highly three dimensional nature of the flow. Further downstream however, combustion takes place mainly in two diffusion flame sheets in the shear layers between the evaporated fuel, and the two vitiated air streams.