Phenomena peculiar to underexpanded flows in supersonic micronozzles

In the present study, the characteristics of supersonic flows in micronozzles are experimentally and computationally investigated for Reynolds numbers ranging from 618 to 5560. In the experiments, the flows are created in a rectangular contoured nozzle whose heights at its throat and exit are 286 and 500 μm, respectively. The number-density distribution along the nozzle centerline is measured using the laser-induced fluorescence technique under an underexpanded condition for each Reynolds number. The experimental results reveal that the underexpanded flow expands along the streamwise direction in a range where the cross-sectional area of the nozzle is constant although the flow in such a range has been believed to be compressed owing to friction. The results also reveal that the unexpected range where the flow expands extends with a decrease in Reynolds number. In the computations, the Navier–Stokes equations are solved numerically. The computational results agree very well with the experimental results; i.e., the computational code used in the present study is validated by the experiments. By using the computational results, the reason for the appearance of the phenomena peculiar to supersonic micronozzle flows is discussed. As a result, it is found that information about the back pressure under which the flow is underexpanded can reach into the inside of a micronozzle. Such a property induces the unexpected phenomena observed in the experiments.