The boundary layer is formed on micro-channel walls and its thickness becomes 0 at the exit of the channel. And, it plays a role of a wall of a converging and diverging nozzle and the flow becomes supersonic at the micro-channel outlet. Then outlet Mach number is beyond unity. This fact is not widely known. Therefore, experimental investigations on behavior of super sonic flow at the outlet of straight micro-tubes whose diameter ranges from 150 to 500 μm are conducted. The stagnation pressure ranges 379 from to 812 kPa. The successive expansion and recompression waves of under-expanded state were visualized by Schlieren method and a high-speed camera. The numerical investigations are also performed for straight micro-tubes with diameter ranging from 50 to 400 μm. Numerical methodology is based on the aribitary-Langrangian-Eulerian (ALE) method. The stagnation pressure was chosen in such a way that the Mach number at the tube outlet ranges from 1.0 to 1.6. The ambient back pressure is fixed at the atmospheric pressure. The flow at the tube outlet change from the over-expanded to the under-expanded state. It is observed that the recompression and expansion waves are alternately formed in downstream of the micro-tube outlet in both experiments and numerical computations. The experimental correlation for the distance from the micro-tube outlet to the Mach disk as a function of pressure at the outlet was proposed for the prediction of outlet pressure of micro-tube in under-expanded.

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