We concentrate in this contribution onto the pressure-drop in rectangular stainless steel microchannels with a hydraulic diameter of dh133 μm. Three aspect ratios are engaged, namely 1:1, 1:2, 1:5, whereas the hydraulic diameter is maintained constant. The roughness of the channel walls is around r ≈ 1.3 μm, the Reynolds number is up to Re ≈ 4000. We investigate all microchannels in two different lengths to infer highly-accurate correlations for fully-developed flow conditions from pressure difference measurements between the inlet- and outlet plenum. We find with this new technique pressure drop correlations for the fully-developed flow, which agree, both in the laminar and the turbulent regime, reasonably with conventional (macroscopic) correlations. In all cases the laminar/turbulent transition is in the range Rec ≈ 1800–2300, consistent with findings in macroscopic channels. The influence of roughness is found to be particularly strong for the microchannel of aspect ratio 1:5. This raises the question, whether the dimensionless group (r/dh) remains the responsible parameter for roughness at extreme aspect ratios.

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