Experiments on the Laminar/Turbulent Transition of Liquid Flows in Rectangular Microchannels

ISSN： 1521-0537

Source： Heat Transfer Engineering, Vol.30, Iss.1-2, 2009-01, pp. : 70-77

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Abstract

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. This paper concentrates on rectangular stainless steel micro-channels with a hydraulic diameter of about 133 m. Three aspect ratios of 1:1, 1:2, 1:5 are studied, whereas the hydraulic diameter is kept constant. The average roughness depth of the channel walls is about 1-2 m in general, and specific channels are of roughness depth of about 25 m. Filtered and degassed de-ionized water is driven at pressure differences up to 20 bar through the channels, allowing for Reynolds numbers up to 4000. The measuring techniques allow for a highly accurate determination of the mass flow rate (precision weighting), the temperatures at inlet and outlet, the pressure drop, and the time-resolved velocity field (PIV). The measured quantities consistently show that the laminar/turbulent transition for smooth channels is in the Reynolds number range of 1900-2200, which is 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 (Reynolds number of about 1000). This raises the question of whether the relative roughness remains the relevant parameter at extreme aspect ratios. In this article, we focus on the PIV results.