The advances of micro-fabrication techniques allow for the manufacturing of micro heat exchangers or micro reactors. These micro devices are characterized by a large surface-to-volume ratio and, hence, allow for the transfer of large heat fluxes or offer large catalytic surfaces for reactions. The design and optimization of such micro devices heavily relies on correlations for pressure drop and heat transfer, as well as on information on the laminar/turbulent transition. As these questions are still discussed controversially in literature, a careful investigation appears highly desirable. We concentrate onto rectangular stainless steel micro channels with a hydraulic diameter of dh ≅ 133 μm. Three aspect ratios are engaged, namely 1:1, 1:2, 1:5, whereas the hydraulic diameter is kept constant. The average roughness depth of the channel walls is r ≅ 1–2 μm in general, specific channels are of roughness r ≅ 25 μm. Filtered and degassed deionized water is driven at pressure differences up to 20 bar through the channels, allowing for Reynolds numbers up to Re = 4000. The measuring techniques allow for a highly-accurate determination (i) of the mass flow rate (precision weighting), (ii) of the temperatures at inlet and outlet, (iii) of the pressure drop, and (iv) of the time-resolved velocity field (μPIV). We find from all measured quantities — consistently — the laminar/turbulent transition for smooth channels in the range Rec = 1900–2200, in agreement with findings for macroscopic channels. The influence of rough channel walls appears particularly strong for the micro channels of aspect ratio 1:5 (Rec = 1000). This raises the question, whether the dimensionless group (r/dh) remains the relevant parameter at extreme aspect ratios. In this article we focus on the μPIV results.

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