(65g) Experimental Investigation of the Laminar/turbulent Transition of Liquid Flows in Rectangular Micro Channels Using µpiv

The advances of micro-fabrication techniques allow the manufacturing of micro heat exchangers or micro reactors. These micro devices are characterized by a large surface-to-volume ratio and, hence, provide 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 the laminar/turbulent transition. This presentation concentrates onto rectangular stainless steel micro channels with a hydraulic diameter of dh = 133 μm. Three aspect ratios are engaged, namely 1:1, 1:2 and 1:5, whereas the hydraulic diameter is kept constant. The roughness depth of the channel walls is k ≈ 1 - 2 μm in general, specific channels show a roughness depth of k ≈ 20 μm. Filtered and degassed de-ionized water is driven at pressure differences up to 20 bar through the channels to achieve turbulent micro flows at Reynolds numbers up to Re = 4000. The measuring techniques allow a highly-accurate determination of several parameters like mass flow as well as of the time-resolved velocity field (μPIV). For all measured quantities - consistently - the laminar/turbulent transition for smooth channels is found in the range Re ≈ 1900 - 2200, in agreement with findings for macroscopic channels. The influences of channel walls with increased roughness result in a transition to turbulent flow at a lower Reynolds number (Re ≈ 900). Integral measurements, as e.g. the total pressure-difference between inlet and outlet plenum, suffer from a superposition of developing, laminar and turbulent flow regions along the micro channel. Local pressure measurements within small micro channels appear likewise difficult without major disturbances. Even though the pressure-difference measurements between inlet and outlet plenum can be performed at identical micro channels of different length to isolate the pressure drop in fully-developed regions, the measurement of local quantities remains superior to determine the precise site and parameter range of the laminar/turbulent transition. For this reason this presentation concentrates on the results of the μPIV measurements within micro channels. As an example, figure 1 presents the measured local turbulence intensities in the centre of an 1:1 aspect ratio micro channel, close to the channel outlet. The turbulent fluctuations u′ and v′ are in the axial and wall-normal direction. Starting at Re ≈ 1500 a characteristic increase of the axial fluctuations to a maximum of about 12 % is visible. With further increasing Reynolds numbers the axial fluctuations drop to a constant value of about 4 % beyond Re ≈ 2200. This maximum of the axial fluctuations is characteristic for the laminar/turbulent transition range, where an intermittency of laminar and turbulent states occurs. In contrast, the wall-normal fluctuations in the channel centre do not exhibit such large amplitudes, but a smooth transition (without maximum) is found for increasing Re. The third-direction (likewise wall-normal) fluctuations cannot be measured by the μPIV technique, though due to symmetry they should behave like the given wall-normal fluctuations. Further investigation results given in the presentation include the evaluation of the intermittency factor and the probability density function (PDF) of the velocities at specific local positions. Figure 1 (attachement): Turbulence intensities in the centre of the 1:1 aspect ratio micro channel as function of the Reynolds number.