The performance of the micro propulsion system is determined primarily by the performance of the micro nozzles. 3D rectangular cross-section straight-convergent-divergent-straight (SCDS) micro nozzle, with throat width 16 μm and throat depth 20 μm, was fabricated and studied based on the experiment and numerical simulation. The results indicate that the first sonic point position moves away from the throat to the outlet of the microchannels, and the critical pressure ratio (defined as the local static pressure to inlet total pressure ratio when the first sonic point occurs in the internal flow of SCDS microchannel) decreases with the increasing of the S/V (surface-to-volume ratio) of micro nozzle. These behaviors might be attributed to the increased surface-to-volume ratio leads to high viscosity dissipation in micro nozzle. The relationship of critical pressure ratio and the S/V ratio is further discussed based on numerical simulation.

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