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Abstract- The present study aims to investigate numerically the natural convection of various nanofluids inside a square enclosure with a central heat source at different aspect ratio. Also, some correlations are presented in order to calculate the Nusselt number in terms of Rayleigh number and volume fraction of nanoparticles. The heat source and cavity walls are kept at constant temperatures of Th and Tc, respectively. The nanofluids are considered to be water as the base fluid and different nanoparticles such as Cu, CuO, Ag, Al2O3, or Tio2. To discretize the governing equations, the control volume method and SIMPELER algorithm have been employed. The study has been carried out for aspect ratios from 0.2 to 0.8, Rayleigh numbers from 1e3 to 1e6 and the volume fractions of nanoparticles ranging in 0-0.05. The results indicated that the Nusselt number increases with increasing the volume fraction of nanoparticles as well as the aspect ratio. Furthermore, by increasing the Rayleigh number, some eddies, of kind of Rayleigh-Benard, are developed in the space between the heat source and the upper wall of the enclosure. Based on the obtained results, several correlations with high accuracy have been present in order to evaluate the Nusselt number.

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In this study, hydrodynamic behavior of four-sided wind tower attached to parlor and courtyard of a scaled model form existing historical house with wind incident angel as variable was numerically investigated. Hazire-ei house wind tower, which has six channels with rectangular cross section, integrated with parlor and courtyard is considered among the most typical ones in the vernacular architecture of Yazd city. This article seeks to investigate the performance of four-sided wind tower regarding suction and supply amount of air and the way it was used as a vernacular solution for natural ventilation in order to provide engineers with design guidelines for contemporary use. Numerical study was conducted on a 1:25 scaled model and for 13 wind incident angels with 15 degrees intervals and interested parameters are mass flow rate and flow direction in each channel. A structured mesh was generated and ANSYS Fluent software was used for numerical simulation. Numerical modeling results were validated against experimental tests conducted on the same scaled model and good agreement was observed. Results indicate that in 68.5% of incident angels, four-sided wind tower acts as sucking the air out of building and in other incident angels with approximately equal amount of supply and extract rate, it operates as an air exchange unit. Accordingly it can be concluded that putting aside stack effects, four-sided wind towers in dry regions of Iran are mostly employed for heat dissipation elements rather than inducing outdoor cool breezes.

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Recently shear thickening fluids (STF) are applied more and more to improve the penetration resistance of fabrics. In this research, at first, the performance of the neat and STF impregnated fabric subjected to the impact of 8.7 mm diameter steel spherical projectile is investigated experimentally. Then, the numerical analysis is done to study the effective parameters such as fabric density, static and dynamic coefficients of friction between yarns and between projectile and fabric, boundary conditions and number of layers of fabric by using commercial tool LS-DYNA software. Previous studies expressed that the major factor that improves the energy absorption capacity of STF impregnated fabrics is the friction between the impact projectile, fabric, and yarns within the fabric, however here the investigations showed that in addition to the friction, the mass of added STF is effective in the results. Increasing the mass of the fabric by adding STF, is considered as the increasing density of the fabric. Empirical investigations showed that STF-impregnated fabrics exhibited a significant enhancement in penetration resistance performance as compared to neat fabric such that the projectile penetration subjected to the fabric with 44% wt STF decreased 63% compared to neat fabric. The simulation results showed that, if the STF effects just assign to increased friction, the projectile penetration decreased 43% compared to neat fabric. But if in addition to friction, the mass of the STF is considered as the effective parameter, the penetration decreased 58% which have good agreement with experimental data.

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Thermal energy storing technologies are a new approach in reducing energy costs, managing demand side, pick shaving and increasing portion of renewable energies in energy production. In spite of lots of advantages of thermal energy storage techniques, there are still major challenges in the path of Latent heat thermal storages (LHTS). One of the challenges is the low charge and discharge rate of heat transfer in LHTS. In the current study charging rate of a shell and tube LHTS is numerically studied by enthalpy-porosity numerical technique. Exact positioning of the heat transfer tubes and thermal fins has great impact on the natural convection flows. In this study effect of increasing heat transfer tubes (HTF), lower positioning of tubes in four tubes configuration, changing upper tubes distance and using interconnected axial fins has been studied and compared to each other. Moreover, velocity and temperature contours have been analyzed. Results demonstrated that increasing number of tubes could not solve the slowing rate of charging at the end of process and tubes need to be positioned lower in the tube. In addition, it was observed that heat transfer axial fins can decelerate convection flows and develop stationary areas inside the shell. Prediction results revealed by lowering tubes and closing them to the shell wall, introduced in this article, it is possible to decrease charging time of 0.95 of storage capacity to one fourth of similar time in a one tube LHTS.

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New passive double L-shaped micromixers have been investigated based on the split and recombination flow. Numerical study on micromixers was performed in the Reynolds number range of 50 to 200. The three-dimensional Navier-Stokes equations have been used to analyze flow and mixing behavior. Two different configurations from the positioning of L units have been investigated and two solutions have been proposed to improve the mixing index. If two L units are same shaped, aligned on one plate (design 1), the mixing index is low due to inappropriate split and recombination. The placement of two L units of the same shape on a two-plane parallel and non-aligned (design 2) improve the mixing index and increase to over 95% in Reynolds numbers of 100, 150, and 200. The orthogonal solution to the inputs did not affect the pressure drop and only in design 1, the mixing index could exceed 95% in all Reynolds numbers. Unbalanced micromixer solution improves mixing index by increasing pressure drop. The effect of geometric parameter of asymetric width ratio in both designs was studied and design 1 in asymetric width ratio 2.5 and design 2 in asymetric width ratio 2 and 2.5 have been completely mixed in all Reynolds numbers. Also, the performance of proposed micromixers was better than L-shaped micromixer due to the split and recombination mechanism. In addition, the mixing index was higher in porposed micromixers compared to the split and recombined micromixers of previous researchers due to the use of L-shaped units.
 

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Passive micro-mixers have simpler manufacturing in comparison with active micro-mixers and only require energy for flow pumping. In the present study, non-Newtonian fluids and non-Newtonian power-law fluid’s mixing behavior in passive micro-mixers have been studied. Simulation has been performed, using computational fluid dynamics commecrical code of Ansys fluent and two different approaches of two-component mixing have investigated. The first approach studies fluid’s mixing behavior by changing flow behavior index and flow consistency index in 5 different 3D geometries as multiple T-micromixer with aligned and non-aligned inputs in one and two plane, respectively, multiple T-micromixer, double T-micromixer, and T-micromixer, while the second approach studies mixing behavior by changing flow behavior index while flow consistency index is constant in two multiple 3D geometries with non-aligned inputs. In all studies, water was used as Newtonian fluid and carboxymethyl cellulose solution was used as non-Newtonian fluid. The studied range of Reynolds number was 1 to 100. In both approaches, the results for mixing index and pressure drop for power-law index according to criterion are reverse of each other; it means that in the first approach, with increasing power-law index, the mixing index increased and the pressure drop decreased and in second approach, this procedure is reversed. But, procedure of non-dimensional fully developed velocities in two approaches investigated is similar in comparison to geometries with non-aligned inputs.