Measurement of breakdown length of supersonic micro jets usinglaser-nduced fluorescence and phosphorescence

近年来，随着推进、发电、MEMS元件冷却等工程应用的增加，超声速微射流的研究日益受到重视。为了提高每个工程应用的超音速微射流的性能，必须了解这种流动的详细特征。尤其重要的是要知道射流的击穿长度，因为湍流强度和射流的增长率在击穿点附近突然变得很高。在本研究中，利用丙酮分子的激光诱导荧光改变雷诺数来测量击穿长度。结果表明，随着雷诺数的增加，击穿长度变短。这与亚音速微射流的实验结果一致。然而，已知亚音速射流的击穿长度与雷诺数成反比，而超音速射流的击穿长度与雷诺数不成反比。由此推测，除了雷诺数之外，还有一些参数对超声速微射流的击穿长度起主导作用。

Recently, study on supersonic micro jets has been important due to increase of engineering applications such as propulsion, power generation, cooling of MEMS components, and so on. In order to improve the performance of a supersonic micro jet for each engineering application, the detailed characteristics of this flow has to be understood. Especially, it is important to know the breakdown length of the jet because turbulence intensity and the growth rate of jet become suddenly high near the breakdown point. In this research, the breakdown length is measured changing Reynolds number by using laserinduced fluorescence from acetone molecules. As a result, it is found that the breakdown length becomes short with increase of Reynolds number. This agrees with the experimental results of subsonic micro jets. However, the breakdown length of the subsonic jet is known to be inversely proportional to Reynolds number, in contrast, that of the supersonic jet is found not to be inversely proportional to Reynolds number. It is expected from this result that there are some parameters other than Reynolds number that dominates the breakdown length of the supersonic micro jet.