半封闭圆管冲击射流紊动特性PIV试验研究

PIV experimental study on the turbulence characteristics of a semi-confined impinging jet issuing from a circular pipe

  • 摘要: 冲击射流作为一种高效的质量、动量及能量传递技术,广泛应用于水利工程、热工设备及清洗系统等众多工程领域。在实际应用中,射流常受到结构空间的限制,形成半封闭冲击射流,其特征为喷嘴出口存在与冲击板平行的半封闭板,使射流流场结构更加复杂,湍流特性显著变化。本文采用二维粒子图像测速技术,对不同冲击距条件下的半封闭圆管冲击射流流场进行了试验研究。通过获取射流中心面上的瞬时速度场,重点分析了紊动能、脉动速度概率密度、偏斜因子、平坦因子及两点相关函数的沿程演化规律。结果表明,随着冲击距的增大,自由射流剪切层内紊动能峰值及高紊动能区域均显著增大,而壁面射流区紊动能峰值呈现先增后减趋势;其中,壁面紊动能局部峰值沿径向呈指数衰减,其半值宽随径向线性增长,且扩展率与衰减指数对冲击距变化不敏感。脉动速度概率密度分析显示,射流中心线上轴向速度分布由瘦高逐渐趋于扁平,壁面射流区的径向速度分布则由扁平转向瘦高;进一步的偏斜因子与平坦因子分析表明,射流中心线上轴向速度分布呈左偏态且陡峭特性,而壁面射流区径向速度分布则呈右偏态且同样陡峭特性,表明射流发展过程中存在非高斯性与间歇性特征。两点相关函数分析显示,积分长度尺度随射流发展沿程增大,表明湍流结构在沿程发展中不断合并、扩大。本研究深化了对半封闭冲击射流紊动特性的理解,为其在质量、动量及能量传递中的机理分析提供了试验支撑,并为相关工程应用中的结构优化与性能提升提供了理论依据。

     

    Abstract: Impinging jets are widely recognized for their efficiency in mass, momentum, and energy transfer, making them essential in various engineering domains, including hydraulic engineering, thermal systems, and industrial cleaning technologies. In many practical applications, jet flows are constrained by spatial limitations, leading to the formation of semi-confined impinging jets. These jets feature a semi-confined plate positioned parallel to the impingement surface at the nozzle exit, generating a more complex flow structure and significantly modifying turbulence behavior. This study experimentally examines the turbulence characteristics of a semi-confined impinging jet produced by a circular pipe using two-dimensional particle image velocimetry (2D-PIV). Experiments are conducted for four impingement distances (H = 3d, 4d, 5d, and 6d), where H denotes the distance between the nozzle exit and the impingement plate, and d is the nozzle diameter. The inlet Reynolds number, based on nozzle diameter and bulk jet velocity, is maintained at a constant value of 11,052. Instantaneous velocity fields are captured along the jet center plane to investigate the streamwise development of turbulent kinetic energy, probability density functions (PDFs) of fluctuating velocity, skewness and flatness factors, and two-point correlation functions. The results reveal that, with increasing impingement distance, both the peak turbulent kinetic energy and the spatial extent of the high-turbulent-kinetic-energy region in the free jet shear layer grow markedly. In contrast, the peak turbulent kinetic energy in the wall jet region initially rises and then diminishes as the impingement distance increases further. Notably, when H ≤ 4d, the peak turbulent kinetic energy in the wall jet region exceeds that in the free jet region, whereas for H ≥ 5d, the free jet region displays higher peak values. The local peak turbulent kinetic energy in the wall jet region exhibits exponential decay in the radial direction, with its half-width increasing linearly. Importantly, both the growth rate and decay exponent remain largely unaffected by changes in impingement distance. Analysis of velocity fluctuation statistics shows that the axial fluctuating velocity distribution along the jet centerline transitions from leptokurtic to platykurtic. Conversely, the radial fluctuating velocity distribution in the wall jet region shifts from platykurtic to leptokurtic. Further examination of skewness and flatness factors reveals that the axial fluctuating velocity along the centerline is left-skewed and leptokurtic, while the radial fluctuating velocity in the wall jet region is right-skewed and likewise leptokurtic. These observations underscore the non-Gaussian and intermittent nature of turbulence during jet development. Two-point correlation analysis demonstrates that increasing the impingement distance broadens the spatial extent of velocity correlations in both the free jet and impingement regions. Furthermore, the integral length scale gradually increases along the jet's development, indicating the continuous growth and amalgamation of turbulent structures downstream. This study enhances the understanding of turbulence in semi-confined impinging jets. By providing comprehensive experimental data on the mechanisms of mass, momentum, and energy transfer, it offers valuable insights for optimizing structural design and improving performance in engineering applications. These insights are particularly applicable to technologies involving heat transfer, fluid flow, and industrial cleaning, where efficient jet configurations are critical.

     

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