| Swirling flow is commonly used in the practical burner to stabilize the flame, due to the recirculation zone induced by swirl flow and ability to enhance fuel/oxidizer mixing. Swirl flow contains a variety of large-scale coherent structures, such as vortex breakdown (VB) and precessing vortex core (PVC), which have complex influence on flame. With the rapid development of computer resource and computational fluid dynamics, numerical simulation has become one of the most important approaches in research of turbulent flow and combustion. Large eddy simulation (LES) can directly solve the large-scale motion, which contains the most energy in turbulent flow fields. Nowadays, the LES approach plays a more and more important role to study the turbulent flow structures, flame structures and the interaction between turbulence and chemical reactions.Stratified swirl burner and Sydney swirl burner, which have the detail experimental data for non-reactive flow and combustion, are suitable to study the characteristics of swirl flow fields and the impact on combustion process. LES were performed to study the non-reactive flow of two types of swirl burners. Combined with the flamelet/progress variables (FPV) subgrid combustion model based on the thermo-chemical table, LES were also performed on two typical combustion process of Sydney swirl burner to investigate the turbulence/chemistry interactions.The flow field of stratified swirl burner contains bluff-body stabilized recirculation zone and swirl shear layers, and exists some kinds of instability phenomena, such as spiral vortex shedding and breakdown, PVC et al. Bluff-body recirculation zone is formed by inner annular jet. Spiral vortex generates in the swirl shear layers, which sheds from the swirling flow exit and then breaks down downstream. With the increasing of swirl number, the vortex breakdown occurs more upstream. PVC exists near the central axis behind bluff-body stabilized recirculation zone. Both of the spiral vortex and PVC are formed as the result of the Kelvin-Helmholtz instability of swirl shear layers. The flow features under four swirl numbers0.00,0.25,0.45and0.79were numerical studied. The results show that, the axial lengths of bluff-body recirculation zone are approximately20mm under four swirl numbers. The vortex shedding occurs under lower swirl number0.25. In swirling flow, PVC exists in downstream region. The power spectrum results show that the strengthen of procession motion increase obviously, while the frequencies keep unchanged under higher swirl numbers, but decays along the flow direction. The terminal of bluff-body stabilized recirculation zone appears precession motion under higher swirl numbers0.45and0.79, which indicates the instability of recirculation zone. The results of proper orthogonal decomposition (POD) show that, the vortex shedding forms the large-scale parallel annular coherent structures, and the PVC forms the spiral coherent structures. The vorticity and Reynolds stress distributions given by triple decomposition (TD) based on the data of POD indicate that the flow stretch is the main reason of vortex breakdown.A extra fuel jet in the central of bluff-body of the Sydney swirl burner will lead to more operating conditions. The flow field contains bluff-body stabilized recirculation zone, second recirculation zone (vortex breakdown bubble, VBB) and "collar-like" structure, and some kinds of instability phenomena such as vortex breakdown, PVC in central jet and "collar-like" structure region, cyclic collapse/contraction and expansion of VBB et al. The LES was carried out under three categories of operating conditions:non-swirling (Sg=0, Res=32400,41900and59000), higher Reynolds number (Res=59000, Sg=0.40,0.45and0.54) and lower Reynolds number (Res=32400, Sg=0.57,0.68,0.91and1.59). The predicted length of bluff-body stabilized recirculation zone is sensitivity to swirl numbers, but not to Reynolds numbers. The large VBB are formed in downstream region under higher Reynolds numbers. Two independent PVCs, leading to the low frequency precession of flow fields, exist in the regions of central jet and "collar-like" structure respectively. Comparing with PVC, the predicted cyclic collapse/contraction and expansion of VBB is a type of weak instability. The POD results of non-reactive flow shows that the PVCs of central jet and "collar-like" structure forms the large-scale columned coherent structures respectively, which divides into several intertwine spiral vortex structures downstream. The instability of VBB forms several spiral coherent structures around the recirculation zone. The coherent vorticity and Reynolds stress given by triple decomposition (TD) using POD data indicate that the flow stretch is main reason of vortex breakdown.Large eddy simulation combined with FPV subgrid combustion model were applied to study the diffusion flame SM1and partial premixed flame SMA2respectively in Sydney swirl burner. Good results of turbulent mixing, mean temperature and species mass fractions can be obtained by using the outlet instantaneous velocity of developed turbulent piped flow as turbulent inlet velocity of central fuel jet. The results show that, much higher scalar dissipation rate, causing the partial quenching, appears in the neck region of SM1and the terminal of bluff-body stabilized recirculation zone of SMA2, which increases the difficulty of numerical calculation in swirling combustion. In the flame SM1, the irregular precession of central jet can strengthen the mixing effect of fuel/air; the VBB presents a weak unstable phenomenon called cyclic contraction/collapse and expansion. In flame SMA2, the combustion heat release causes the expansion and elongation of bluff-body stabilized recirculation zone, which presents the cyclic contraction/collapse and expansion. The bluff-body stabilized recirculation zone and VBB play an important role on flame stabilization. Especially, the existence of VBB can enhance the stability of combustion.The work of this paper can provide some useful reference information for research and design the practical combustor with good performance. |
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