Experimentation and simulation of a swirled burner featuring cross-flow hydrogen injection with a focus on the OH* chemiluminescence
Résumé
Large eddy simulations (LES) and experimental measurements were conducted on a swirled partially-premixed hydrogen–air flame with cross-flow hydrogen injection at two different lean operating points, to have two types of flames. This study represents the first simulation of this configuration using hydrogen as the sole fuel. Flame chemiluminescence is used to observe the OH* emission and the light of burnt gases in the visible range. Velocity profile measurements are conducted to validate the results of Large Eddy Simulation (LES). In cold conditions, the burner is analyzed for the two operating conditions in terms of local equivalence ratio distribution and flow field. The analysis demonstrates that the mixing at the burner outlet is relatively efficient, despite the late cross-flow injection of hydrogen and the low inertia of this gas. This efficiency is attributed to the recirculating zone induced by the swirl flow and to the small convergent nozzle at the burner outlet. The reactive flow is then studied at the two operating conditions by analyzing velocity, local equivalence ratio, OH* emission and heat release rate (HRR) distribution. Numerical simulations are in good agreement with the experiments. Compared to images of visible burnt gases, the simulations have successfully reproduced the flame shapes observed experimentally. Although the two operating conditions appear to be stabilized in a different manner from the OH* images, one giving a lifted flame and the other is attached, the LES revealed that both flames are attached to the injector tip. This was observed from the presence of significant heat release rate inside of the injector for both operating conditions. The differences between the OH* emission and the heat release rate localization obtained by the LES are then discussed. 1-D calculations carried out in a strained counterflow flame configuration are used to interpret them. 1-D calculations indicate that the maximum OH* level is not at the same equivalence ratio as the maximum HRR level, and that the OH* level decreases much faster than that of HRR on the rich flame side. 1-D calculations also show that a non-negligible OH* emission also exists in the burnt gases of hydrogen–air flames close to stoichiometry. For highly preheated mixtures, the OH* concentration in burnt gas can be even higher than in the flame front.
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