Spatio-temporal evolution of harmonic disturbances on laminar, non-premixed flames: Measurements and analysis

This paper analyzes the dynamics of harmonically forced, non-premixed flames, both experimentally and computationally. Prior theory has made a number of predictions on how wrinkles on non-premixed flamelets are excited by flow disturbances, convect axially, and evolve in disturbance magnitude, including setting up interference patterns in wrinkle disturbance magnitude. The objective of this study was to obtain measurements from forced flames to determine if these features are present, and to compare the gain/phase of these wrinkles with predictions using the measured velocity field as inputs. High speed PIV data was taken on a coflowing methane-air diffusion flame, equipped with speakers for harmonic forcing, over a variety of flow velocities, forcing frequencies, and forcing amplitudes. These measured velocity fields were used as inputs to a Z-equation solver, and the resulting space-time dynamics of iso-Z surfaces were extracted from the Z field solutions. Both experimental and numerical results show that flame wrinkles propagate axially at the mean flow velocity, a result consistent with previous analytical findings. These wrinkles start with near zero magnitude at the fuel tube lip and grow with downstream distance, until peaking at some axial location. Further downstream, the wrinkle magnitude modulates, indicative of interference effects which have been previously predicted in analytical studies.