LES Investigation of the Hysteresis Regime in the Cold Model of a Rotating-Pipe Swirl Burner
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We investigate numerically the hydrodynamics of regime transitions of flow in a cold replica of a non-premixed swirl burner in which hysteresis was detected when transiting from an attached long flame to a short lifted flame and vice versa (Hübner et al. Exper. Thermal and Fluid Scie. 27, 481–489 2003 Tummers et al. Comb. Flame 156(2), 447–459 2009. The unconfined highly swirling annular jet is generated by rotating the outer pipe of the annular air supply at 4000 rpm, while gas is fed through an inner annulus. The imposed rotation number N=Uwall/U0 , controlled by the flow rate, ranged from 2.8 for the stable flame to 4.9 for the unstable flame. The large-eddy simulations (LES) results for two regimes, N = 2.8 and 3.26, agree well with the available experimental data, reproducing notably different sizes and strengths of the central recirculation bubbles. Just as in the experiment, the transients were simulated by a sudden imposition of the inflow mass flux that corresponds to the target hysteretic state at an intermediate rotation number. Contrary to the experimental findings in flame, where both the stable and unstable flames were observed at the same bulk flow parameters in the hysteresis region (N = 3.26), the LES of cold flow resulted in indistinguishable time-averaged flow patterns differing from the both initial states, indicating that in the configuration considered the hysteresis is associated with flame lift-off and reattachment. The analysis of the transients dynamics showed, however, that the flow undergoes different adjustments when approaching from the stable and unstable initial states, suggesting that the hydrodynamics is indeed the precursor of hysteresis. The sudden hysteretic change of the regime observed depends on whether the thermal effects will overrule the inertia of the strong near-nozzle vortex structures.
KeywordsJets Vortex dynamics Turbulence simulation
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