Flow induced mixing in high aspect ratio microchannels Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5712m897w

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  • Micro-fluid mixing is an important aspect of many of the various micro-fluidic systems used in biochemical production, biomedical industries, micro-energy systems and some electronic devices. Typically, because of size constraints and laminar flow conditions, different fluids may only have the opportunity to mix by diffusion, which is extremely rate limited. Therefore, active or highly effective passive mixing techniques are often required. In this study, two pulsed injectors are used to actively enhance mixing in a high aspect ratio microchannel (125 μm deep and 1 mm wide). The main channel has two adjacent flowing streams with 100% dye and 0% dye concentrations, respectively. Two injectors (125 μm deep and 250 μm wide) are located on separate sides of the channel, with one downstream 2 mm (equivalent to two main channel widths or eight injector widths) from the other. This results in an asymmetric mixing as the flow proceeds downstream. A dye solution is used to map local mixing throughout the channel by measuring concentration variations as a function of both space and time. The primary flow rates are varied from 0.01 to 0.20 ml/min (Reynolds numbers of 0.3 to 26.6), the injector flow rate ratios are varied from 0.125 to 2, and the pulsing frequencies are varied from 5 to 15 Hz. Images of the concentration variations within the channel are used to quantify mixing by calibrating the intensity of the image with the concentration of the dye solution. The degree of mixing (DoM) is used as a measure of quality and is defined based on the integration across the channel of the difference between the local concentration and the 50% concentration values. DoM is normalized by the 50% concentration value and subtracted from one to yield a parameter that varies from 0 (no mixing) to 1 (perfect mixing). It is shown that there is a high degree of repeatability of concentration distribution as a function of phase of the pulsing cycle. A mixing map is constructed over the range of variables tested which indicates an optimum set of flow and pulsing conditions needed to achieve maximum mixing in the main channel flow. The flow rate ratio between the injectors and main channel is found to be the most influential parameter on overall mixing. The highest DoM in the main channel was found to be 0.89. It is also noticed that improved mixing can occur at very low flow ratios under a unique set of primary flow and low frequency pulsing conditions. In general, there is an inverse relationship between primary flow rate and pulsing frequency to achieve better overall mixing.
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