Abstract We examined the flow behaviors and mixing characteristics of double-concentric jets using laser-assisted smoke flow visualization method to analyze typical flow patterns and binary boundary detection technique to investigate jet spread width. Time-averaged velocity vectors, streamline patterns, velocity distributions, turbulence properties, and vorticity contours were analyzed using Particle Image Velocimetry (PIV). Topological flow patterns were analyzed to interpret the vortical flow structures. Mixing properties were investigated using a tracer-gas concentration detection method. Four characteristic modes were observed: annular flow dominated mode, transition mode, central jet dominated mode-low shear, and central jet dominated mode-high shear. The jets’ mixing properties were enhanced by two major phenomena: the merging of annular flow and central jet at the centerline and the large turbulence fluctuations produced in the flow field. The merging of the jets induced stagnation points on the central axis in the annular flow dominated mode, which caused reverse flow on the central axis and drastic turbulence fluctuations of the near field region. When the central jet penetrated the recirculation region in the other three modes, the stagnation points on the central axis and the reverse flow vanished. Therefore, the mixing behaviors were prominently enhanced in the annular flow dominated mode.

中文翻译：

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双同心射流时均流动特性及混合特性

摘要 我们使用激光辅助烟流可视化方法分析典型流型和二元边界检测技术研究射流扩散宽度，研究了双同心射流的流动行为和混合特性。使用粒子图像测速仪 (PIV) 分析了时间平均速度矢量、流线模式、速度分布、湍流特性和涡度等值线。分析了拓扑流动模式以解释涡流结构。使用示踪气体浓度检测方法研究混合特性。观察到四种特征模式：环流主导模式、过渡模式、中心射流主导模式-低剪切和中心射流主导模式-高剪切。两种主要现象增强了射流的混合性能：环流与中心射流在中心线汇合，流场产生大的湍流波动。在环流主导模式下，射流的合并在中心轴上引起了停滞点，从而导致中心轴上的逆流和近场区域的剧烈湍流波动。当中心射流穿透其他三种模式的回流区时，中心轴上的停滞点和逆流消失。因此，在环流主导模式下混合行为显着增强。导致中心轴上的逆流和近场区域的剧烈湍流波动。当中心射流穿透其他三种模式的回流区时，中心轴上的停滞点和逆流消失。因此，在环流主导模式下混合行为显着增强。导致中心轴上的逆流和近场区域的剧烈湍流波动。当中心射流穿透其他三种模式的回流区时，中心轴上的停滞点和逆流消失。因此，在环流主导模式下混合行为显着增强。