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蔡国辉, 张晓光, 吴二军, 刘志豪, 王博, 陈晓东. 微射流撞击形成液膜的形态演变特征. 力学学报, 2024, 56(5): 1-10. DOI: 10.6052/0459-1879-24-071
引用本文: 蔡国辉, 张晓光, 吴二军, 刘志豪, 王博, 陈晓东. 微射流撞击形成液膜的形态演变特征. 力学学报, 2024, 56(5): 1-10. DOI: 10.6052/0459-1879-24-071
Cai Guohui, Zhang Xiaoguang, Wu Erjun, Liu Zhihao, Wang Bo, Chen Xiaodong. Morphological evolution characteristics of liquid sheets formed by microjets impingement. Chinese Journal of Theoretical and Applied Mechanics, 2024, 56(5): 1-10. DOI: 10.6052/0459-1879-24-071
Citation: Cai Guohui, Zhang Xiaoguang, Wu Erjun, Liu Zhihao, Wang Bo, Chen Xiaodong. Morphological evolution characteristics of liquid sheets formed by microjets impingement. Chinese Journal of Theoretical and Applied Mechanics, 2024, 56(5): 1-10. DOI: 10.6052/0459-1879-24-071

微射流撞击形成液膜的形态演变特征

MORPHOLOGICAL EVOLUTION CHARACTERISTICS OF LIQUID SHEETS FORMED BY MICROJETS IMPINGEMENT

  • 摘要: 利用高速相机从正面和侧面两个视角拍摄了不同速度微射流撞击形成液膜的流动形态, 以深入理解液膜形态演变的物理机制. 实验中的射流韦伯数在6.3 ~ 404.5之间变化, 结果表明: 随着微射流速度的增加, 射流的流动形态经历了从层流到湍流的变化, 所形成的液膜经历了液体链条、封闭液膜、边缘失稳和波动液膜等多种流动形态. 侧面视角观察到了撞击点附近射流表面存在静止(stationary)毛细波, 其波长随韦伯数增加呈现快速减小后缓慢减小的趋势. 还观察到液膜顶端存在界面不稳定状态, 其摆动导致沿液膜边缘产生向下传播的界面扰动, 进而在液膜边缘形成液珠, 液珠又可发展成指状液丝. 指状液丝的生成带走了液膜中的部分流体, 导致液膜在指状液丝根部上方撕裂. 当射流中出现速度脉动后, 会激励起液膜表面波动. 若速度脉动间歇出现, 液膜波动会在扰动消失后很快衰减; 而持续的速度脉动会激励产生剧烈且持续的液膜摆动. 这一发现证实了速度脉动引入的有限大小的扰动是激发液膜波动的必要条件. 此外, 液膜波动加速了液珠形成以及液珠向指状液丝的转变过程, 还影响了指状液丝的空间分布. 液膜摆动使两侧的撕裂点向液膜中心汇聚, 形成下游的长液丝, 完成了从液膜到液丝再到液滴的雾化过程. 研究还利用纹影法获得了射流从层流向过渡流演变过程中液滴直径数据, 发现液滴大小概率密度分布由多峰分布逐渐转变为符合Gamma函数的单峰分布. 本研究得到的微射流撞击形成液膜的演变规律和机制, 为相关应用研究提供了理论支持和定量认识.

     

    Abstract: This paper employs high-speed cameras to capture the flow morphology of liquid sheets formed by the impingement of microjets from both frontal and lateral perspectives, aiming to gain an in-depth understanding of the physical mechanisms governing the morphology evolution of liquid sheet. The Weber number of the jet varied between 6.3 and 404.5. Experimental results indicate that as the velocity of microjets increases, the flow morphology transitions from laminar to turbulent. The resulting liquid sheet undergoes various flow morphologies, including liquid chain, closed liquid sheet, destabilized liquid rim, and undulating liquid sheet. From a lateral perspective, stationary capillary waves are observed near the jet impact point, with the wavelength exhibiting a trend of rapid decrease followed by a slow reduction with increasing Weber number. An interface instability state is observed at the top of the liquid sheet, where oscillations induce interface disturbances propagating downward along the sheet edge, forming liquid beads that can develop into liquid threads. The generation of liquid threads removes some fluid from the liquid sheet, causing tearing above the base of the thread. The appearance of turbulent disturbances in the jet excites surface oscillations in the liquid sheet. Intermittent disturbances result in rapid attenuation of sheet oscillations after the disturbance disappears, while continuous turbulent disturbances stimulate vigorous and sustained oscillations in the liquid sheet. This finding confirms that introducing finite-sized disturbances by turbulence is necessary for exciting liquid sheet oscillations. Furthermore, sheet oscillations accelerate the formation of liquid droplets from liquid sheet and influence the spatial distribution of filamentous liquid threads. The swinging of the liquid sheet causes the convergence of tearing points on both sides towards the sheet's center, forming downstream elongated liquid threads, completing the atomization process from sheet to threads and finally to droplets. The study also employs shadowgraphy to obtain droplet diameter data during the transition from laminar to turbulent flow, revealing a shift from a multimodal distribution to an unimodal distribution conforming to the Gamma function. The observed evolution patterns and mechanisms of liquid sheet formation due to microjet impact contribute theoretical support and quantitative insights for related applications.

     

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