| Experiments have been conducted to identify some of the most likely causes for the large inconsistencies in published pressure drop data for microchannel flows. Using MEMS microfabrication technologies, straight channel test sections designed to permit internal pressure measurements were fabricated. The microfabricated test-sections consist of a system of three silicon chips that incorporate the microchannel to be tested, static pressure tap lines, and pressure sensing membranes.; The MEMS based system is held together by a brace that connects the microchannel to an external fluid handling system. The internal pressure sensors operate on the principle of the optical lever and utilize an off-chip optical system. Two fluids were tested; water in order to obtain incompressible flow data, and air, to obtain compressible flow data. The mass flow rate of water was measured by a weighing tank method and the gas flow rates were measured by a volume displacement method.; Five different channels were tested. The channel hydraulic diameters ranged from 24.9 to 99.8 microns. The height-to-width ratio ranged from 0.26 to 0.97, while the length-to-hydraulic diameter ratio ranged from 220.4 to 533.6. The gas data spans 6.8 < Re < 18,814 and the water data spans 4.9 < Re < 2068.; The measured internal pressure distributions were consistent with the expected incompressible linear pressure distribution for water and showed a distribution consistent with compressible flow for the gas experiments. Results suggest that friction factors for microchannels with hydraulic diameters in the range of 25 to 100 microns can be accurately determined from data for standard large channels. The large inconsistencies in previously published data are probably due to instrumentation errors and improper accounting for compressibility effects. The results do not indicate an early transition to turbulent flow as reported in previous publications. It is believed that the observations of early transition are a result of improperly interpreting the effects of developing flow in the entrance region of the microchannel for incompressible fluids and the effects of compressibility in compressible fluids. |
