Abstract:

The dynamic mode decomposition method was used to study the pressure and vorticity fields obtained from numerical simulation of thermal mixing in a T-junction with a rotating impeller. By comparing the dynamic mode decomposition results of different blade numbers (2—4), the characteristic modes of the coherent structure of the flow field in a T-junction were obtained at a speed of 20 r/min under the condition of deflecting jet flow (M_R=0.49). The diameter ratio of the main duct to branch duct is 2. By observing the spatial structure of the characteristic modes of pressure distribution, it is found that coherent structures mainly appear in the internal region of the impeller, the spatial structure of the characteristic mode of the vorticity field is mainly distributed at the blade tip, and a small amount of strip coherent structures appear near the duct wall. The second-order to fourth-order mode frequencies of the dominant pressure field and vorticity field are equal, corresponding to the 1st, 2nd, and 3rd harmonic frequencies of the blade passing frequency. Their fifth order mode frequencies tend to be equal as the number of blades increases. In addition, when the mode frequency is doubled by the blade passing frequency, “radial interference fringes” appear near the blade. The research results can provide theoretical guidance for flow control in T-junction channels.

Key words: rotating impeller, T-junction, numerical simulation, mixing, dynamic mode decomposition

摘要：

采用动态模态分解方法对加装旋转叶轮T型通道内热混合数值模拟获得的压力场和涡量场进行了研究。通过对比不同叶片数（2~4片）的动态模态分解结果，获得了偏转射流条件（M_R=0.49）下，主支管管径比为2的T型通道在转速为20 r/min时流场相干结构的特征模态。通过观察压力分布特征模态的空间结构，发现相干结构主要出现在叶轮内部区域；涡量场特征模态的空间结构主要分布在叶尖处，同时在管壁附近也出现了少量条状相干结构。主导压力场和涡量场的2阶~4阶模态频率相等，对应叶片通过频率的1倍频、2倍频和3倍频。二者的第5阶模态频率随着叶片数的增加而趋于相等。此外，当模态频率为叶片通过频率倍频时，在叶片附近出现了“辐射状干涉条纹”。研究结果可为T型通道中的流动控制提供理论指导。

关键词: 旋转叶轮, T型通道, 数值模拟, 混合, 动态模态分解