Flow separation in constriction microchannels

Constriction devices contain an element inserted into the fluid stream, which changes the local streamwise distribution of the flow area. One such element is the orifice-like obstruction with sharp corners, basically a back-to-back abrupt contraction and expansion, which could trigger flow separation. A series of microchannels, 40μm×1μm×4000μm in dimensions, with orifice element at the centers of the channels has been fabricated using standard micromachining techniques. The channel width at the orifice section varied from 10μm to 34μm, and each channel was integrated with a set of pressure sensors. Nitrogen gas was passed through the microdevices under inlet pressure up to 50psi. Mass flow rate was first measured for all the devices as a function of the pressure drop, showing a monotonic decrease of the flow rate with decreasing orifice width. Then, the pressure distributions along the microchannel with the narrowest orifice were recorded, showing a pressure jump across the 10μm-wide orifice. The mass flow rate and pressure measurements indicate the existence of flow separation in the microchannels.