Large eddy simulation study of the effect of pilot swirler blade angle on the coherent structure in a dual-swirl combustor

Large-scale coherent structures in a dual-swirl direct-mix combustor and the effect of the pilot swirler blade angle variations (35°, 40° and 45°) on them are investigated by using the large eddy simulations. The results show that no apparent coherent structure is observed downstream of the main swirler, and four typical helical vortex structures are captured downstream of the pilot swirler, representing the PVC and its three higher harmonic modes, respectively. According to the "decoupling" position from the shear layer, the spiral vortex is divided into two regions in space, the "development zone" and the "dissipation zone", respectively. In the frequency domain, as the pilot swirler blade angle increases, the characteristic frequency of the PVC and its higher harmonic structures first decrease and then increase, the energy contribution of the harmonic mode increase, and the double-helix structure replaces the single-helix structure as the dominant structure in the non-constant flow field. In the space domain, as the pilot swirler blade angle increases, the total length of PVC remains basically constant, the initial radius gradually increases, the expansion angle first increases and then decreases, and the position of "decoupling" between shear layer and spiral vortex gradually advances. 利用大涡模拟方法研究了一种双旋流分区直混燃烧室内的大尺度相干结构以及副模旋流器叶片角度变化(35°, 40°, 45°)对其的影响规律。结果表明: 在主模下游并未观察到明显的相干结构, 在副模下游捕捉到4种典型的螺旋涡结构, 分别代表了PVC及其3种高次谐波模态。根据与剪切层的"解耦"位置, 螺旋涡在空间中被划分为2个区域, 分别为"发展区"和"耗散区"。随着副模旋流器叶片角度增大, 在频域内, PVC及其高次谐波结构特征频率先减小后增大, 谐波模态的能量贡献比例提升, 双螺旋结构取代单螺旋结构成为非定常流场中的主导结构; 在空间内, PVC总长度基本保持不变, 初始半径逐渐增大, 扩张角先增大后减小, 剪切层与螺旋涡"解耦"的位置逐渐提前。