Combustion and flame position impacts on shear layer dynamics in a reacting jet in cross-flow

Abstract

This study experimentally investigates reacting jets in cross-flow (RJICF), considering flame/shear layer offset, momentum flux ratio ( $J$ ) and density ratio ( $S$ ) effects. These results demonstrate that non-reacting JICF and RJICF can exhibit very similar or completely different dynamics and controlling physics, depending upon streamwise and radial flame location. Consistent with prior measurements of Getsinger et al. (Exp. Fluids, vol. 53 (3), 2012, pp. 783–801), spatial amplification rates of shear layer vortex (SLV) structures increased with decreasing $S$ for non-reacting cases. Similar $S$ dependencies exist for reacting cases in which the flame lay outside the jet shear layer, whose flow topology is also quite similar to non-reacting cases, albeit with reduced SLV growth rates. However, although the reacting cases have lower growth rates, these SLV structures ultimately reach approximately the same peak strength as in the non-reacting cases. Finally, the SLV decay rate in both non-reacting and reacting cases was found to similarly scale inversely with the initial SLV growth rate. As such, primarily inertial mechanisms govern SLV growth and decay for reacting cases where the flame lies outside the shear layer. In contrast, very different behaviour is exhibited by reacting cases where the flame lies inside the shear layer, where the locally increased viscosity exerts significant influences. In these reacting cases, SLV roll-up is completely suppressed and the entire jet column undulates over a long length scale relative to the jet diameter. As such, the relative roles of inertial and viscous mechanisms in controlling combustion influences on the SLV dynamics, changes markedly with shear layer–flame offset.

Near-field evolution and scaling of shear layer instabilities in a reacting jet in crossflow

“…The R2 cases show a characteristically different value, suggesting that since the roll-up of vortices in the near field is suppressed, the fluctuations due to the jet core undulations (also noted in Nair et al. 2022) are likely a different instability mechanism. Finally, the transition from amplifier to self-excited behaviour was mapped across all cases, demonstrating that the NR and R1 cases largely exhibited the pathway to self-excited behaviour observed in low-density jets.…”

“…Hajesfandiari & Forliti (2014) noted a significantly increased instability growth rate when the flame was moved towards the high-velocity stream, analogous to the flame being moved 'inside' the shear layer in a jet diffusion flame. But experimental evidence from Füri et al (2002) performed on co-flowing jets, and even the results presented in Nair et al (2022), demonstrate clearly additional effects in play -in fact, these studies observed the opposite behaviour. For the case where the flame was moved inside the shear layer, increased local viscosity likely contributes to the stabilization of the jet shear layer.…”

“…Consequently, this study utilized a much faster sampling rate (40 kHz) and a smaller interrogation region than used in Nair et al. (2022), spatially spanning in the plane of symmetry in the near field of the jet (figure 2). Illumination was provided via a pulsed Nd:YAG laser (Continuum Mesa particle image velocimetry, PIV) operating at 40 kHz with pulse width 150 ns.…”

“…The vortex identification technique discussed in Nair et al. (2022) is utilized to identify shear layer vortices in the jet near field. This technique employs topological segmentation (Bremer et al.…”

“…Indeed, for atmospheric pressure configurations, most RJICF experiments with attached flames will fall in this regime (Nair et al. 2022), just by virtue of the need to keep the flame from blowing off. As such, additional Reynolds number effects, in addition to inviscid, inertial effects, can lead to differences in RJICF and JICF dynamics – as we show in this study, this occurs for cases where the flame lies inside the shear layer.…”