Simulation of Fuel/Air Premixing Under the Effects of Acoustic Forcing

To achieve ultra-low NOx emissions in practical lean premixed combustors, a uniform spatial and temporal fuel/air distribution is required. In addition the response of the fuel/air premixer to acoustic perturbations plays a central role in the coupling of the combustion and acoustic processes that potentially lead to thermoacoustic instabilities. In this paper, results from the simulation of a fuel/air premixer with and without acoustic forcing are reported. An unsteady Reynolds-Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) code is used in the simulations and the computed results are compared with data obtained using a synchronous PLIF technique. Planar phase-locked images of the fuel/air distribution during the forcing cycle are measured from which the planar averaged (bulk) fuel/air ratio “signal” is obtained. The fuel/air ratio to velocity transfer function (time delay and magnitude) at a specific forcing frequency is computed corresponding to a given jet-to-air momentum flux ratio. The comparisons between the computed and measured time delays are quite good, while the magnitude comparisons are currently inconclusive.