Phenomenon of Gas Flow Choking in Micro-Nozzles

With micro fabrication technology,a micro nozzle with a throat width of 20?μm,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 chracteristics.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.