Characterization of strongly forced non-premixed methane jet flames
Previous and current work show that ﬂame length and soot luminosity of laminar and turbulent nonpremixed 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 fuel at the organ-pipe resonance frequency of the fuel delivery tube. Quantitative mixture fraction imaging in nonreacting 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 diameters downstream of the nozzle exit and no pure jet ﬂuid exists outside of the nozzle due to in-tube mixing. Simultaneous OH and acetone planar laser induced vi ﬂuorescence (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 nonpremixed jet ﬂames.