Compressible gas flows in microchannel and microjets

An experimental investigation of microscale gas flows is presented. The research consists of two parts: the fundamental aspects of compressible gas flow in microchannels and jet flows emitted from micronozzles. The compressible flow of nitrogen, helium, and argon was examined in channels that were microfabricated on silicon wafers. The channels were typically 100 $\mu$m wide, 10$\sp4\ \mu$m long, and ranged in depth from 0.5 to 20 $\mu$m. Based on the channel Knudsen numbers (10$\sp{-3}$ to 0.4), some degree of rarefaction was encountered despite the relatively high inlet pressures (1 to 4 MPa). Flows with low Reynolds numbers but high (subsonic) Mach numbers were obtained. The experimentally measured friction constant is within 8% of theoretical predictions based on isothermal, locally fully developed flow incorporating a single coefficient wall slip model. Jets of nitrogen emitted from non-axisymmetric nozzles with smallest dimensions of the order of 10 $\mu$m were examined experimentally using pitot tube pressure measurements and a novel mass deposition technique. The jets were discharged into a vacuum chamber maintained at various pressures. Significant off-centerline stagnation pressure peaks were observed even for jets emitted from very large aspect ratio nozzles, resulting in three-dimensional flows. These off-centerline peaks are often more than four times the size of those observed by other researchers, who attribute the effect to vortex motion occurring in the jet. Both subsonic and supersonic jets were examined. In supersonic jets, the shock structure was readily apparent. Despite the small jet dimensions, the shock structure is similar to that observed experimentally by other researchers for macroscale jets, and reasonable agreement is obtained between experimental observations and predictions based on a simple first order perturbation solution