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Mixing within electrokinetic micromixers is studied numerically in this article. The micromixer studied here is simply a heterogeneous parallel plate microchannel which is imposed to the electroosmotic flow field. For the thorough modeling of such flows, the coupled equations of Navier-Stokes, Nernst-Planck, Poisson-Boltzmann and concentration equations are solved for the flow motion, electric charges transport, electric field and species concentrations, respectively. Numerical solution of these set of equations for the heterogeneous microchannels is complicated and difficult. Therefore, a simple and approximate model such as Helmholtz-Smoluchowski has been proposed which is appropriate for the case of microchannels with the homogenous properties on the walls. Validation of Helmholtz-Smoluchowski model has been well-used for the prediction of two dimensional flow fields, yet its application is rarely validated for the prediction of concentration field and mixing performance. In this article mixing due to electroosmotic flow field is investigated using Nernst-Planck equations as well as Helmholtz-Smoluchowski models and the accuracy of the Helmholtz-Smoluchowski model is evaluated. Comparison of the results indicates that in the proper condition the approximate model can predict the mixing performance accurately along the micromixer length. [ABSTRACT FROM AUTHOR]

Viscous micro-pumps have attracted researchers' attention due to their simple design and fabrication and many applications in various industrial and medical fields. Therefore, a wide range of numerical and experimental studies has focused on this topic. In the present paper, a numerical study has been performed to investigate the effect of geometrical parameters of a viscous micro-pump on the flow rate and entropy generation. The governing equations have been solved using the finite volume method. The present research has been carried out for three geometrical parameters of micro-pump including eccentricity (ε), sizes (S) of rotors and also their distance from each other (L) in the range of 0.1 to 0.9, 1.5 to 3.5 and 0.85 to 4.5, respectively. The results show that by increasing ε, the micro-pump flow rate also increases. On size variation effects, it is observed that decreasing the downstream rotor diameter while keeping constant the upstream rotor diameter, decreases the flow rate exponentially. By increasing L, a steep increase in flow rate is initially observed, which becomes almost constant when rotors are sufficiently far apart. With regard to entropy analysis, with increasing z, the ratio of Rs also increases. However, more complex variations of RS are observed for other examined geometrical parameters due to the relatively complex micro-pump flow patterns. Also in this paper, for obtaining the maximum flow rate at the minimum frictional dissipation, optimal geometrical parameters are extracted. In this regard, the values of L = 2, ε = 0.5, S1 = 1.5 and S2 = 2.5 are selected as the optimum geometrical parameters of viscous micro-pump. Finally, it should be noted that the effect of simultaneous variations of the rotors diameters on the flow rate and entropy generation and, moreover, obtaining the optimal geometrical parameters based on the entropy analysis, are among the novelties of the present paper. [ABSTRACT FROM AUTHOR]

Numerical modeling of electro-osmotic flow in heterogeneous micro-channels using two different models is presented in this article. For the thorough modeling of such flows, the coupled equations of Navier-Stokes, Nernst-Planck and the Poisson-Boltzmann are solved for the flow field, electric charges transport and electric field, respectively. Numerical solution of these equations for the heterogeneous micro-channels is complicated and difficult. Therefore, a simple and approximate model such as Helmholtz-Smoluchowski has been proposed. In this model the solutions of Poisson-Boltzmann, Nernst-Planck equations are neglected and the effect of the electric field on the flow field is applied through a prescribed slip boundary condition. The electro-osmotic flow fields within the heterogeneous micro-channels are usually complex and contain the vortex region which is ideal for mixing purposes. Hence, in this paper, the microchannels are designed so that they will be capable of serving as micro-mixers in the mixing applications. For the micro-channels proposed here, the flow fields are obtained both with approximate modeling and the full simulation of electro-osmotic flows so that a comparison can be made to discuss the accuracy of the approximate model. The results of this study can be used to model the electro-osmotic flow field within heterogeneous micro-channels. [ABSTRACT FROM AUTHOR]

Rupture of abdominal aortic aneurysm is a result of the relatively complex hemodynamic forces that are exerted along the arterial walls. In the present study, numerical simulations of blood flow in a patient-specific model are performed employing the fluid-structure interaction method. The aneurysm model is reconstructed from CT angiographic scans from a patient diagnosed with abdominal aortic aneurysm, which also contains an intraluminal thrombus. Both isotropic and anisotropic material models are considered for arterial wall. In particular, the effects of arterial wall heterogeneity with respect to its material model are examined. Results indicate that unlike the computational solid stress method, which exerts the peak systolic pressure to the inner surface of the arterial wall, FSI method predicts the time of peak wall stress between the times of peak systolic velocity and pressure. Results also indicate that the isotropic material model with uniform wall thickness as compared to the anisotropic material model with variable wall thickness significantly underestimate wall stresses. The peak wall stress in all models are located somewhere on the posterior wall near the maximum diameter of AAA and the extent of the region of higher wall stresses are larger in models with variable wall thickness as compared to the uniform wall thickness. This fact along with the higher values of wall stress for variable wall thickness models, increase the rupture risk of the variable wall thickness model as compared to the models with uniform wall thickness. [ABSTRACT FROM AUTHOR]

The effects of nitrogen and argon dilution on NO emission of turbulent propane-air non-premixed flame in a furnace are experimentally investigated. Dilution is an effective process in reducing NO emission, since diluents cause an increase in the mass flow of the fuel, and consequently the temperature in the combustion chamber and NO emission decreases. We measured NO emission and temperature for a wide range of dilution and equivalence ratios. The results show that nitrogen dilution is more effective than argon dilution in reducing NO emission. In addition, both dilutions caused the yellow color of the non-premixed flame to turn blue, which indicates more complete combustion and better mixing of fuel and oxidant. [ABSTRACT FROM AUTHOR]

This empirical study investigates the influence of initial relative humidity (RH) variations (35 % and 95 %) on key fire behavior parameters, specifically focusing on the fully developed stage, occurrences of flashover, and the temperature of the upper gas layer. Wooden cribs weighing 10 kg–40 kg are used as fuel in a compartment with three openings to establish different natural ventilation conditions. The experimental findings reveal that increasing the initial RH within the chamber leads to a delay in the onset of the fire growth phase. Furthermore, it induces two noticeable effects on the fully developed fire stage: a reduction in its duration by up to 50 % and a delay in its initiation by at least 30 s. While heightened initial RH does not prevent flashover, it effectively postpones its occurrence by a minimum of 40 s. Experiments with maximum fuel loads demonstrate negligible effects of increased RH on the maximum temperature, even under varying ventilation conditions. Conversely, lower fuel loads exhibit a significant decline in temperature with rising humidity, notably from 626 °C to 474 °C. The quantitative and qualitative insights derived from this study have considerable potential to inform the development of more effective fire suppression strategies in enclosed compartments. • Augmenting the initial relative humidity (RH) not only retards the initiation of the wood crib fire growth phase by an average duration of 30–60 s but also, while not precluding flashover, significantly defers its onset by an average of 45 s. • Higher initial RH reduces the fully developed stage duration by up to 50 %, delaying its initiation by at least 30 s. • Implementing humidity in firefighting strategies can effectively delay fire stages, offering crucial time for extinguishment in multi-compartment scenarios. [ABSTRACT FROM AUTHOR]