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Study on the collapse length of compressible rectangular microjets
Experiments in Fluids ( IF 2.4 ) Pub Date : 2020-08-12 , DOI: 10.1007/s00348-020-03030-5
Taro Handa

The present study focuses on the collapse length of compressible rectangular microjets whose Mach numbers range between 0.3 and 1.5. The microjets are created from a convergent rectangular nozzle whose height is 500 μm at its exit. The techniques of planar laser-induced fluorescence (PLIF) and molecular tagging velocimetry (MTV) are used to visualize the microjets. The PLIF images reveal that each microjet spreads abruptly at a certain location. It is confirmed from the instantaneous MTV images that this location corresponds to the location where the jet starts to collapse. The jet collapse length, which is defined as a distance between the nozzle exit and jet collapse location, is estimated from the PLIF image. The plot of the collapse length versus the jet Reynolds number reveals that the collapse length is inversely proportional to the Reynolds number for subsonic and ideally expanded microjets. On the other hand, the collapse length for underexpanded microjets is almost uniform when the jet Reynolds number is higher than a certain value (~ 103) although the length is inversely proportional to the Reynolds number for the lower Reynolds numbers. To clarify the reason why such peculiarities appear in the underexpanded microjets, the numerical flow simulations are carried out. The results reveal that the collapse length remains constant as long as a jet screech occurs. Consequently, the collapse length of the screeching jet is related to the feedback length in the jet. The critical Reynolds numbers at which laminar-turbulent transition occurs are estimated from the collapse lengths of the microjets without screeching and plotted against the convective Mach number. It is found that the critical Reynolds number increases with the convective Mach number.

中文翻译:

可压缩矩形微射流坍塌长度研究

本研究的重点是马赫数在 0.3 到 1.5 之间的可压缩矩形微射流的坍塌长度。微射流由一个收敛的矩形喷嘴产生,其出口处的高度为 500 μm。平面激光诱导荧光 (PLIF) 和分子标记测速 (MTV) 技术用于可视化微射流。PLIF 图像显示每个微射流在某个位置突然扩散。从瞬时 MTV 图像中确认,该位置对应于射流开始坍塌的位置。射流塌陷长度定义为喷嘴出口和射流塌陷位置之间的距离,是根据 PLIF 图像估计的。坍缩长度与射流雷诺数的关系图表明,坍缩长度与亚音速和理想膨胀微射流的雷诺数成反比。另一方面,当射流雷诺数高于某个值 (~ 103) 时,膨胀不足的微射流的坍塌长度几乎是一致的,尽管长度与较低雷诺数的雷诺数成反比。为了阐明在未充分膨胀的微射流中出现这种特性的原因,进行了数值流动模拟。结果表明,只要发生喷射尖叫,坍塌长度就会保持恒定。因此,尖叫射流的坍缩长度与射流中的反馈长度有关。发生层流-湍流转变的临界雷诺数是根据没有尖叫的微射流的坍塌长度估计的,并根据对流马赫数绘制。发现临界雷诺数随着对流马赫数的增加而增加。
更新日期:2020-08-12
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