Abstract
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.
Published Version
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