Mixing characteristics in strongly forced non-premixed methane jet flames

Abstract

Previous and current work show that flame length and soot luminosity of laminar and turbulent non-premixed jet flames can be significantly reduced by high-frequency, high-amplitude forcing of the fuel flow rate. The current work focuses on understanding the physical mechanisms responsible for these changes to the flow/flame structure for the case where the forcing is sufficiently strong that significant reverse flow exists at the exit of the fuel delivery tube. The high-amplitude forcing is achieved by pulsing the flow at the organ-pipe resonant frequency of the fuel delivery tube. Quantitative mixture fraction imaging in non-reacting jets indicates that the strongly pulsed jets exhibit dramatically enhanced mixing as compared to unpulsed jets. In the pulsed jets, the mean mixture fraction falls below 0.08 in as little as 3 D downstream of the nozzle exit and no pure jet fluid exists outside of the nozzle due to in-tube mixing. Simultaneous OH and acetone PLIF performed in methane jet flames shows that the OH zones are much broader and more wrinkled in the resonantly pulsed jet as compared to the unpulsed jet, and are greatly contorted by the vortical structures. A unique feature of the pulsed flames is that the reaction zones appear to close downstream of a vortical structure, just a few diameters downstream of the nozzle exit, in a region where the mixture fraction imaging (for non-reacting flows) shows reduced mixture fraction. Furthermore, the flame anchors on the outer-upstream edge of the vortical structures where the fuel mixture fraction is reduced due to enhanced entrainment. The significant in-tube premixing and the enhanced entrainment appear to be the predominant mechanisms that cause the reduction in length and luminosity of high-amplitude, high-frequency pulsed non-premixed jet flames.