Phenomenon of gas flow choking in micro-nozzles

With micro fabrication technology, a micro nozzle with a throat width of 20 pirn, depth of 240 μm and expansion ratio 1.7 was fabricated on a silicon wafer. Experiments and numerical simulation were done for a study of flow characteristics. The mass flow rate and pressure distributions were measured under different inlet-outlet pressure difference. Results of numerical simulation showed that when the throat's Mach number came near to 0.8, the choking of mass flow rate appeared and the corresponding choking critical-pressure-ratio was about 0.650. Moreover, in the divergent area of the micro nozzle, a closed supersonic region occured when the inlet-outlet pressure difference were near 48 kPa and 66 kPa. These particular phenomena as were different from conventional scale could be attributed to the higher viscous dissipation owned by the micro nozzles.