The aim of this paper is to analyze the self-ignition of a jet flame in hot vitiated cross flow using Large Eddy Simulation with analytically reduced chemistry. A rich premixed ethylene-air mixture ($$\phi = 1.2$$ ϕ = 1.2 ) at 300 K is injected into a hot vitiated crossflow at 1500 K. The simulated reacting flow steady-state was validated against experiments in previous publications and the focus of the present work is the transient self-ignition of the jet. It is shown that spontaneous ignition occurs at very lean mixture fractions in the form of reacting patches in the windward jet mixing layer. These patches grow, laterally wrap the jet and extend into the recirculation region. Chemical explosive mode analysis is performed to identify the chemically active regions that are precursors of the patches undergoing spontaneous ignition. It is shown that the self-ignition occurs at very lean fuel concentrations regions, which are leaner and hotter than the most reactive mixture fraction of the jet and crossflow. This is explained by the fact that the scalar dissipation is significantly lower in these very lean regions. Ultimately, the peak heat release moves toward the richer regions and an autoignition cascade governs the steady state flame anchoring.
The effect of the regime of Nanosecond Repetitively Pulsed Discharges (NRPDs) on ignition and stabilization of a natural-gas/hydrogen/air flame in the sequential stage of a lab-scale atmospheric pressure sequential combustor is investigated experimentally. Electrical parameters of the NRPDs are characterized by measuring voltage, current, and deposited energy. Fast Gas Heating (FGH) of the nanosecond discharges is measured in a single pulse regime and validated by means of 0D kinetic modelling. It was found that conventional scheme for energy release from internal degrees of freedom adequately describes the dynamics of fast gas heating in vitiated hot environment diluted with air. Short-gated ICCD imaging and spatially-resolved emission spectroscopy are used to identify the coupling between the NRPDs and the vitiated hot flow. The effectiveness of the NRPDs actuation is assessed through the OH* chemiluminescence images of the sequential flame. The distance of the center of gravity of the sequential flame to the outlet of the mixing channel is evaluated, with and without plasma actuation. The effect of fuel reactivity on plasma effectiveness is also studied by varying the fraction of hydrogen in the fuel blend of the second stage of the combustor. The results show that the glow NRPDs regime allows strengthening the flame anchoring for the most reactive blends considered in this work, while the spark NRPDs is required for the ignition and prevention of lean blow-out of the flame for the least reactive fuel blends which exhibit low fractions of hydrogen.
In this work we perform an experimental study of the combustion of pure hydrogen in the sequential stage of a generic combustor. This academic test rig is a simplified model of an industrial sequential combustor. The sequential fuel is injected using different injector geometries. The composition and temperature of the hot stream at the inlet of the sequential burner are defined by the mass flows of the hot combustion products from the first stage and of the dilution air. This temperature is varied between 1100 K and 850 K by modifying the dilution air mass flow in order to study the different combustion regimes of the sequential hydrogen flame. High-speed imaging of OH radicals chemiluminescence is performed with optical emission spectroscopy to measure vitiated gas temperatures. We investigate the transition from a flame anchored in the sequential combustion chamber, to one stabilized upstream into the mixing section, when the inlet flow temperature is increased. Of particular interest is the increasing rate of formation of autoignition kernels in this transition process. The underlying combustion regime change is analyzed with 0D reactor simulations, and the limitations of such a simplified low-order model of the flame location are discussed. The effects and importance of the mixing process between fresh fuel and the hot vitiated co-flow is examined. Two different injectors are compared under the same operating conditions that provide different degrees of mixing and allow to demonstrate the impact of mixing quality on the flame morphology.
The effect of hydrogen enrichment of a premixed hydrogen-methane-air jet in hot vitiated crossflow was studied at atmospheric condition. The hot turbulent vitiated crossflow is generated by a symmetric array of 4 × 4 jet flames burning a lean mixture of natural gas and air in fully premixed condition at equivalence ratio φcf = 0.7 and total thermal power of 50 kW. This crossflow is then used to ignite the premixed perpendicular jet of hydrogen-methane-air at ambient temperature. Three jet parameters are varied to study the effect of hydrogen addition on the flame morphology and stabilization mechanism: the hydrogen mass fraction of the H2/CH4 fuel blend (ξ = 0 – 100%), the jet equivalence ratio (φ = 0.8 – 2.0) and the jet-to-crossflow momentum ratio (J = 3 – 12). High-speed hydroxyl (OH) chemiluminescence is used to obtain the time-resolved imaging of the reactive jet and to compute its time averaged morphology. OH planar laser induced fluorescence (OH-PLIF) is used to acquire OH concentration fields at the jet center plane. The jet morphology is analyzed by considering its mean trajectory, extracted from the experimental data and fitted with empirical correlations available from the literature. New correlations are proposed for the flame length, width and center of gravity as function of the hydrogen content. It is shown that with increasing hydrogen fraction, the flame is shortened and more compact, and it stabilizes close to the jet root. Another finding of this work is the reattachment of the flame at the base of the windward jet shear layer when hydrogen fraction is increased. Robust flame anchoring is observed for H2 mass fractions of the CH4/H2 fuel blend that exceed 50%. Moreover, it is shown using instantaneous OH-PLIF images that for these conditions of increasing hydrogen concentration, the windward shear layer features larger-scale coherent structures that govern the aerodynamics of the reactive premixed jet in turbulent vitiated crossflow.
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