A symmetric Y-shaped Micromixers with Spherical Mixing Chamber for Enhanced Mixing Efficiency and Reduced Flow Impedance

Microfluidic devices have many attractive qualities such as low cost, small size, and in-field use. Micromixers are very important components of these devices because affect their efficiency. In a passive mixer, the structural characteristics of the mixer are crucial and must be analyzed. This paper presents a numerical study of the mixing in passive Y-shaped micromixers with a spherical mixing chamber for a volume constrained system. The effect of asymmetric bifurcated ducts, the angle in between the inflow ducts, eccentricity and, obstacles inserted in the mixing sphere, on the mixing efficiency and flow impedance is evaluated. Vortical structures characteristics and the possible occurrence of engulfment are also identified. The results show that flow impedance (pressure drop for unit volumetric flow rate) can be decreased greatly for the same mixing efficiency as the volume of the spherical mixing chamber is 20% of the total volume. Insertion of the obstacles into the sphere mixing chamber decreases the mixing efficiency while they increase the flow impedance. The results also show that spherical mixing chamber enhances mixing efficiency while decreasing flow impedance if the volume reserved for it is greater than a limit value which depends on the diameter and length scale ratios in between the mother and daughter ducts as well as the total volume. Overall, the paper documents the variation of mixing efficiency and flow impedance based on the geometrical parameters of three-dimensional asymmetric passive micromixer with sphere mixing chamber

The constructal-law physics of why swimmers must spread their fingers and toes

Here we show theoretically that swimming animals and athletes gain an advantage in force and speed by spreading their fingers and toes optimally. The spacing between fingers must betwice the thickness of the boundary layer around one finger. This theoretical prediction is confirmed by computational fluid dynamics simulations of flow across two and four cylinders of diameter D. The optimal spacing is in the range 0.2D – 0.4D, and decreases slightly as the Reynolds number (Re) increases from 20 to 100. The total force exerted by optimally spacing two cylinders exceeds by 53 percent the total force of two cylinders with no spacing when Re = 20. These design features hold for both time-dependent and steady-state flows.This research was supported by the National Science Foundation, the Air Force Office of Scientific Research, and the Republic of Turkey.http://www.elsevier.com/locate/yjtbihb2016Mechanical and Aeronautical Engineerin

Constructal Vascular Structures With High-Conductivity Inserts for Self-Cooling

In this paper, we show how a heat-generating domain can be cooled with embedded cooling channels and high-conductivity inserts. The volume of cooling channels and high-conductivity inserts is fixed, so is the volume of the heat-generating domain. The maximum temperature in the domain decreases with high-conductivity inserts even though the coolant volume decreases. The locations and the shapes of high-conductivity inserts corresponding to the smallest peak temperatures for different number of inserts are documented, x ¼ 0.6L and D/B ¼ 0.11 with two rectangular inserts. We also document how the length scales of the inserts should be changed as the volume fraction of the coolant volume over the high-conductivity material volume varies. The high-conductivity inserts should be placed nonequidistantly in order to provide the smallest peak temperature in the heat-generating domain. In addition, increasing the number of the inserts after a limit increases the peak temperature, i.e., this limit is eight number of inserts for the given conditions and assumptions. This paper shows that the overall thermal conductance of a heat-generating domain can be increased by embedding high-conductivity material in the solid domain (inverted fins) when the domain is cooled with forced convection, and the summation of high-conductivity material volume and coolant volume is fixed

Constructal vascularized structures

Smart features such as self-healing and selfcooling require bathing the entire volume with a coolant or/and healing agent. Bathing the entire volume is an example of point to area (or volume) flows. Point to area flows cover all the distributing and collecting kinds of flows, i.e. inhaling and exhaling, mining, river deltas, energy distribution, distribution of products on the landscape and so on. The flow resistances of a point to area flow can be decreased by changing the design with the guidance of the constructal law, which is the law of the design evolution in time. In this paper, how the flow resistances (heat, fluid and stress) can be decreased by using the constructal law is shown with examples. First, the validity of two assumptions is surveyed: using temperature independent Hess-Murray rule and using constant diameter ducts where the duct discharges fluid along its edge. Then, point to area types of flows are explained by illustrating the results of two examples: fluid networks and heating an area. Last, how the structures should be vascularized for cooling and mechanical strength is documented. This paper shows that flow resistances can be decreased by morphing the shape freely without any restrictions or generic algorithms

Constructal branched micromixers with enhanced mixing efficiency: slender design, sphere mixing chamber and obstacles

Here we uncover the passive micromixer designs with the maximum mixing efficiency under a lesser flow impedance. Three different designs of micromixers were considered for volume constrained systems: branched systems of ducts, branching ducts with sphere mixing chamber and branching ducts with obstacles. The best performing designs, with maximum mixing efficiency and minimum flow impedance, are uncovered numerically by considering three degrees of freedom (ratios between diameters, between lengths, and between length and diameter) under total volume constraint. The mixing efficiency, the flow impedance and the mixer performance (or mixer quality) for all the designs are determined based on numerical results. The results uncover that the branched micromixer should have long mother ducts with larger diameter than daughter ducts. Our results also show that branching ducts with sphere mixing chambers and obstacles also enhance the mixing efficiency but with an additional penalty on flow impedance. Besides, systems with a sphere mixing chamber insertion in the junction between mother and daughter ducts have greater mixing efficiency than systems with embedded obstacles into the mother channel. However, for a given flow impedance, the mixing efficiency is greater for branched systems of ducts than for branching ducts with sphere mixing chamber and with obstacles. For mixer systems built in a space with limited size, branching ducts with sphere mixing chamber may be a good option because they require less space than the other systems. Here new analytical models are also proposed to predict the mixing efficiency and mixer performance based on numerical results. In summary, this paper provides important insights for the designers of micromixer based on Constructal law

The natural emergence of vascular design with turbulent flow

<p>Here we show that vascular design emerges naturally when a volume is bathed by a single stream in turbulent flow. The stream enters the volume, spreads itself to bathe the volume, and then reconstitutes itself as a single stream before it exits the volume. We show that in the pursuit of a smaller global flow resistance and larger volumes, the flow architecture changes stepwise from a stack of identical elements bathed in parallel flow (like a deck of cards) to progressively more complex structures configured as trees matched canopy to canopy. The transition from one architecture to the next occurs at a precise volume size, which is identified. Each transition marks a decrease in the rate at which the global flow resistance increases with the volume size. This decrease accelerates as the volume size increases. The emergence of such vasculatures for turbulent flow is compared with the corresponding phenomenon when the flow is laminar. To predict this design generation phenomenon is essential to being able to scale up the designs of complex flow structures, from small scale models to life size models.</p>Thesi