Investigation on choking behavior of gas flow in microducts

In continuum regime, the large surface-to-volume ratio in microscale flow indicates stronger influence of boundary layer on internal flow, which is confirmed by the present study through quantitatively analyzing the in-duct choking and profile of boundary layer in a series of straight rectangular microducts with convergent entrances. The exit height and width of the microducts are 500 µm and 2500 µm. The number density distribution along the duct centerline is measured using a laser-induced fluorescence technique in underexpanded conditions for Reynolds numbers ranging from 745 to 6710. The experimental results show that an unexpected drop in number density emerges upstream of the duct exits. By numerically solving the 3-D Navier–Stokes equations, the computational results reveal that the build-up of boundary layer forms a virtual throat upstream of the duct exit, thus turns the straight duct into a convergent–divergent micronozzle. The location of Mach-number unity (choking) and the boundary-layer thickness are found affected by both duct configuration and Reynolds number at choking. In addition, location of the farthest in-duct choking from duct exit is found corresponding to a certain range of transition from laminar to turbulent at Re ∼ 2000. The 1-D analysis confirms that the in-duct choking phenomenon is related to the boundary-layer blockage rather than friction. The results of the present survey indicate the significance of reckoning boundary-layer blockage in micronozzle or microduct design.