LES-based prediction of technically premixed flame dynamics and comparison with perfectly premixed mode

Abstract

The present study combines Large Eddy Simulation (LES) with System Identification (SI) to determine the Flame Transfer Functions (FTFs) of technically premixed flames that respond to fluctuations of upstream velocity as well as equivalence ratio. Two variants to obtain the corresponding FTFs from numerically determined time series data are reported and compared with the experimental results. The experiment does not measure heat release rate directly but instead the CH* chemiluminescence. This is insufficient for FTF identification of technically premixed flames but can be used for the validation of the simulation. We implemented a CH* post-processor in the simulation and validated with the experiment. After validation, the simulation is used to identify the contributions of velocity and equivalence ratio to the FTF of technically premixed flame dynamics. We propose and compare two approaches for the identification of FTFs. The direct approach via multiple-input single-output system identification requires one simulation with simultaneous excitation of fuel and air inlets and carefully chosen input signals. The second approach reconstructs the FTF decomposition from two separate simulations, one perfectly premixed and one technically premixed, with reduced requirements on signal quality. We compare both approaches and discuss the FTFs of perfectly and technically premixed flames. Overall, the LES/SI approach proved to be flexible and reliable for technically premixed flames.