Assessment of mixing in passive microchannels with fractal surface patterning

Abstract

Abstract. We explore numerically the feasibility of enhancing the mixing capability of microchannels by employing the Weierstrass fractal function to generate a pattern of V-shaped ridges on the channel floor. Motivated by experimental limitations such as the finite resolution (∼10 µm) associated with rapid prototyping through soft lithography techniques, we study the influence on the quality of mixing of having finite width ridges. The mixing capability of the designs studied is evaluated using an entropic measure and the designs are optimized with respect to: the distances between the ridges and the position range of their tip along the width of the channels. The results are evaluated with respect to the benchmarks established by the very successful staggered herring bone (SHB) design. We find that the use of a non periodic protocol to generate the geometry of the bottom surface of the microchannels can lead to consistently larger entropic mixing indices than in cyclic structures. Furthermore, since the optimization curves (mixing index vs. geometric parameters) are broader at the maximum for fractal microchannels than for their SHB counterparts, the microchannel designs using the Weierstrass fractal function are less sensitive to experimental uncertainties. PACS

Use of Grooved Microchannels to Improve the Performance of Membrane‐Less Fuel Cells

“…This type of flow modification involving the generation of transversal flow components using fluidstructure interactions, is similar with successful strategies used in some micro-scale mixers and reactors, in which the long timescale associated with diffusion-based transport imposes limitations on the efficiency of mixing and on the reaction rates [25][26][27]. A system of angled groove/ridges integrated on the bottom or/and top surface(s) of the cell's microchannel is used to induce transversal flows and promote increased mass transfer toward the electrodes.…”

“…While the flow through the ridged channel is still laminar in nature, the addition of the ridges and the consequent introduction of a transversal component to the otherwise longitudinal flow, increases the amount of reactant that comes in contact with the catalytic electrode surfaces, overcoming the limitations imposed by the slow diffusion-based transport. This type of flow modification involving the generation of transversal flow components using fluidstructure interactions, is similar with successful strategies used in some micro-scale mixers and reactors, in which the long timescale associated with diffusion-based transport imposes limitations on the efficiency of mixing and on the reaction rates [25][26][27]. In our work, the length and orientation of the ridges structures used is optimized to maximize the transversal flow close to the electrodes and thus the fuel consumption efficiency, while maintaining the integrity of the virtual membrane at the liquid-liquid interface between the reactants.…”

“…One of the strategies employed in promoting reactant transfer in diffusion limited microchannel reactors, that is transferable to the case of membrane-less fuel cells, is channel surface modification [25,26,29]. The challenge in achieving this is similar with that encountered in designing microreactors where the laminar nature of the fluid flow and the slow diffusional transport hinder the volume mixing of chemical reactants.…”

“…The challenge in achieving this is similar with that encountered in designing microreactors where the laminar nature of the fluid flow and the slow diffusional transport hinder the volume mixing of chemical reactants. One of the strategies employed in promoting reactant transfer in diffusion limited microchannel reactors, that is transferable to the case of membrane-less fuel cells, is channel surface modification [25,26,29]. Essentially, this method relies on the addition of a system of ridges/grooves to the bottom and/or top surface of the microfluidic channel ( Figure 6).…”

“…Given that the oxygen reduction rate is the limiting step [5,8], the reaction rate for the fuel is adjusted to maintain global charge neutrality, and the discussion below focuses on analyzing the performance of the fuel cells in terms of oxygen utilization. Similar computational fluid dynamics models have been previously used to model flows and chemical species distribution in channel based micromixers [26,29]. In regard to the addition of the surface reactions modeling the electrochemical reactions occurring at the electrodes, the model used was validated against the experimental results reported by Jayashree et al [8] who used the same type of fuel, i.e., formic acid.…”