Frequency regimes and stroke-length collapse in a forced reacting jet in crossflow

The jet in crossflow (JICF), or transverse jet, is a canonical element of fluid-mixing systems, and combustion systems rely on it for the transport of cooling air, fuel, and for fuel–air mixing. Many emerging low-emissions combustors adopt the JICF for axial fuel staging, in which a fuel-bearing jet is injected into a hot, oxygenated crossflow, where it autoignites to establish a reacting jet in crossflow (RJICF) whose behavior and dynamics differ from those of its non-reacting counterpart. In a combustor this reacting jet may be continually excited by background acoustic oscillations. Alternatively, acoustic forcing may be applied deliberately as a means to control the flame. This work experimentally measures how the mean flame structure and chemistry respond when a RJICF is forced through its plenum, over a range of frequencies ( St ≈ 0.2 to 1.8) and amplitudes ( u j ′ / U j uⱼ/Uⱼ up to 8) at a fixed operating point ( J = 4, ϕ j = 1.5). The flame was imaged with line-of-sight OH* chemiluminescence and its global chemistry tracked with the OH*/CH* chemiluminescence ratio. The results show a forced response that is both strong and organized, with a frequency (Strouhal) threshold near St ≈ 0.4 that divides it into two regimes. Below the threshold, forcing alters the jet’s penetration and trajectory while changing the flame’s length, liftoff, and chemistry only weakly. Above the Strouhal threshold, the flame shortens dramatically, lifts from an attached configuration to a detached configuration, and burns leaner; this is accomplished with little modification to its trajectory. Within this high-frequency regime, the structural and chemical responses collapse onto a single parameter, the stroke length L / D . Forcing therefore acts as a single-knob control over the flame state by shortening, lifting, and leaning the flame in a direction which is potentially favorable for improving emissions and combustor performance.