Size-dependent flow separation in microchannels with cavities

Pressure driven gas flow in microchannels with cavities is studied experimentally and numerically. Several microchannels, 1-2 μm in height, have been fabricated using surface micromachining with varying number of cavities. Streamwise pressure distributions along microchannels with and without cavities are found to be the same within experimental error. The mass flow rate increases slightly with the number of cavities, less than 10% increase for the microchannel with the maximum number of cavities. Numerical simulations reveal the development of several flow modes depending on the channel height. The flow pattern inside the cavities is determined not only by the reduced Reynolds number, Re, but also by the cavity aspect ratio, A_c. Fully attached flow in the cavity is obtained for Re<5 and A_c<2/3, while flow separation occurs if either Re>10 or A_c>2/3. The normalized cavity circulation as a function of the reduced Reynolds number collapsed onto a single curve only for A_c<0.2.