Journal of Mechanical Engineering
Volume:4 Issue: 3, Summer 2020

Heat transfer enhancement has been the goal of many researches carried out over the past three decades. Employing the modified fluids, changing the flow geometry, and active methods are among the commonly used techniques. The effects of three referred techniques, employing pulsation flow as active method, nanofluids instead of conventional fluid as modified fluids and helical-coil tube as changing the flow geometry on the fluid flow and heat transfer have been investigated numerically. The results of interests with pulsation flow and non-pulsation flow have been presented. In this research, the results indicated that, the coil pitch tube and coil diameter have a minor effect on the Nusselt number (5 and 7%, respectively). But the Reynolds number has a major effect on the Nusselt number, so that by increasing the Reynolds number from 5000 to 100000, the Nusselt number will be enhanced by more than 200%. The nanofluid pressure drop and heat transfer coefficient increased with nanoparticles volume fraction. In addition, by introducing a concept as the performance evaluation criteria, the mutual effects of the pressure drop and Nusselt number were investigated with referred concept. Performance evaluation criteria revealed that employing the nanofluid in lower Reynolds numbers yields higher effects on the fluid flow and heat transfer.

Many researchers have experimentally and theoretically studied fresh water productivity in solar stills. In this regard, water preheating can play a noticeable role in enhancing the productivity of solar stills. In the present study, in order to study the effect of water preheating at the inlet of desalination system, a cascade solar still is built and integrated with two different solar collector in separated modes; an evacuated solar collector and a conventional flat plate solar collector. The mentioned solar still includes an external condenser, fin, and internal and external reflectors. It is worth noting that the embedded fins in the water passage are applied to induce hot spots and increase the evaporation rate and fresh water. The experiments were performed in August 2015 and summer 2017. The results showed that the combined desalination system with conventional flat plate collector and evacuated tube collector enhace productivity %60 and %13, respectively. In addition, efficiency of solar still in combination with conventional flat plate collector and evacuated tube collector was %81.8 and %59.1, respectively. The price of produced fresh water of solar still with conventional collector obtained 0.035 $/lit and for solar still with evacuated tube collector was 0.045 $/lit.

In this study, the experiments and computational fluid dynamics simulationswere carried out to determine the removal efficiency of the indigo carmine dye from wastewater via graphene nanoadsorbent. The adsorption process was done in a packed bed column. The synthesized nanoporous graphene via chemical vapor deposition method was characterized by Brunauer-Emmett-Teller, scanning microscopy and X-ray Diffraction techniques. Batch adsorption experiments were conducted, and Langmuir isotherm was best fitted to the experimental data. A parallel series of fixed bed column adsorption and desorption tests was done. The breakthrough curves were investigated by varying indigo carmine dye solution flow rate (1–10 ml/min), indigo carmine dye solution concentration (10–100 ppm) and adsorbent bed depth (10–28 cm). The reusability of adsorbent was studied. The computational fluid dynamics simulation was used for the three-dimensionally simulation of flow patterns and dye concentration changes in the packed bed column throughout the adsorption. High bed depth, low flow rate and high initial dye concentration were recommended to be the potential parameters for the high adsorption capacity. The conformity of the experimental test with the computational fluid dynamic calculations was a suitable methodology method to prove the reliability of the dye adsorption process. The removal efficiency of indigo carmine dye was achieved to be 67% as flow rate, bed depth, and concentration of 1 ml/min, 28 cm and 10 ppm, respectively.

Numerical simulation is extensively used in the many industries to investigate transport phenomena inside channels with variant dimensions to save time and cost.  In this paper, novel designs for cylindrical polymer fuel cell in the numerical and three-dimensional form are presented. In the simulation of these models, finite volume method is used. Thereafter, novel designs are used to increase the proficiency of polymer electrolyte fuel cell. In this study at first, the effect of semi-circular prominent gas diffusion layers is studied. By locating these prominences on gas diffusion layers, this fact is observed that the performance of fuel cell is enhanced in same condition. However, the optimum size of the prominences is also investigated in the present work. By reaching the magnitude of the radius to 0.55 mm, the flow velocity exceeds the desired magnitude. By this way the diffusion of species impressed negatively. Then, the effect of gradual changes in cross-section, the geometrical configuration of cell parts on species diffusion, the output current density is put under careful study. The results displayed the optimal performance is belong to the cylindrical fuel cell with elliptical cross-section. Wider transport region and lower drop quantity in identical volume for reactive gases to pass to the reactive domain are the reasons for optimal function.

In the present study, the realistic model of the human trachea with five generations that are obtained from computerized tomography scan images is considered. Due to the complexity of lung geometry, many researchers have used simple models. Therefore in the present study realistic model with all geometrical details are considered. The airflow behavior, particle transport and deposition in various conditions such as steady flow, transient flow, light breathing and heavy breathing condition for various micro-particles diameters are investigated. Governing equations are solved and obtained results show that the flow patterns in the realistic model are much more complicated than those of symmetrical models. Also, the particle deposition pattern in the realistic condition is very different from that of the symmetrical model and the details of the trachea are very important and affect the deposition fractions in the small airways. Also, results show that the turbulent effect should be counted properly since the particle deposition in turbulent flow is 30 percent greater than the laminar flow and the dominant mechanism of micro-particle deposition is impaction and increasing of particles diameter, particle density, and the airflow rate leads to an increase of particle deposition.

In this study, the effect of spinning speed fluctuations along with the twist angle, on the stability and bifurcation of spinning slender twisted beams, with linear twist angle and large transverse deflections, near the primary and parametric resonances have been analyzed using the Euler–Bernoulli model. The spinning speed fluctuation along with the twist angle, asymmetry and imbalance, play an important role on the frequency response of the twisted beam. The equations of motion, in the case of pure single mode motion, are analyzed by using the multiple scales method after discretization by the Galerkin's procedure. The instability of the twisted and untwisted beams is investigated and cases and domains are determined in which bifurcation could occur. Effects of the speed fluctuations, twist angle, damping ratio, asymmetry, eccentricity and mass moment of inertia about the longitudinal axis on the frequency response of the twisted beam are investigated. This is explained that the spinning speed fluctuation effect is weak in lower modes and smaller twist angles while asymmetry effect is dominant. By ascending the mode number and twist angle, spinning speed fluctuation effect amplifies the amplitude of system. The results are compared and validated with the results obtained from Runge-Kutta numerical method in steady state, and confirmed with some previous researches.

In this paper, an interconnection and damping assignment passivity based controller is developed for nonlinear bilateral teleoperation system. The aim is to track the position and force in the teleoperation system in the presence of non-passive external interactions and asymmetric variable time-delay in the communication channel.  For this end, a nonlinear control law is designed based on the notion of time-delay Port-Hamiltonian systems for unforced teleoperator and the Lyapunov-Krasovskii theorem. Sufficient synthesis conditions are derived in terms of linear matrix inequalities to tune the parameters of controller. Then, by Lyapunov redesign scheme, an auxiliary controller is developed to assure the stable position tracking in the presence of non-passive operator and/or environment. The main contribution of the proposed method is that the stability and position tracking of system is attained via a fixed-structure controller in the presence of non-passive interaction forces without need to their dynamical models and force sensor. Since the proposed design conditions include the upper bounds of the varying time-delays and their rates; they are less conservative than some rival methods in literature. Finally, transparency of the proposed scheme is proved. Simulation results on a 2-degree of freedom teleoperation system are compared to rival methods to demonstrate the merits of the proposed strategy.

In this study, strongly nonlinear free vibration behavior of a microbeam considering the structural damping effect is investigated analytically on the basis of modified couple stress theory. Employing Von Karman’s strain-displacement relations and implementing the Galerkin method, the governing nonlinear partial differential equation is reduced to a nonlinear ordinary differential equation which is related to the size effect of the beam. Because of large coefficient of nonlinear term and due to existence of the damping effect, none of the traditional perturbation methods leads to a valid solution. Also, there are many difficulties encountered in applying homotopy techniques when the damping effect is taken in to account in the strongly nonlinear damped system. To overcome these limitations, here, a new analytical method is presented which is based on classical perturbation methods and fundamentals of Fourier expansion with an embedding nondimensional parameter. To solve the equation, the nonlinear frequency is assumed to be time dependent. The comparison between time responses of the system obtained by the presented approach and numerical method indicates the high accuracy of the new method. To validate the results of the presented method with those available in the literatures which are obtained for a special case of an undamped system, the damping coefficient is set to zero. The comparison shows a good agreement between the results for a wide range of vibration amplitudes.

In this study, the formation mechanism of iron aluminide phases in the Fe-Al system with different raw material ratios was investigated. In aluminide systems, full consumption of aluminium or its presence have a major impact on the reaction process, the type of products and the mechanism. However these have not studied and only a defined stoichiometric ratio has been established. Therefore, the current study objective is to determine the effect of raw materials proportion on the reaction mechanism. To achieve this goal, the samples with ratios of 1:3, 1:1 and 3:1 of iron and aluminum were heat treated at 700, 800 and 900ºC. It was found that the first phase formed is Fe2Al5. It also proved that the following reaction trend depends on the primary content of iron and aluminum. In the ratio of 3:1 of iron and aluminum, the system tends to achieve the Fe3Al phase at higher temperatures while in the ratio of 1:1 and 1:3, FeAl is the final product. It also was found that more Al postpones the FeAl formation. In the other words, FeAl can be produced at the lower temperature in the ratio of 1:1 of iron and aluminum in the comparison with the ratio of 1:3.

Flux assisted tungsten inert gas welding process known as activated tungsten inert gas welding process is being extensively used in order to improve the performance of tungsten inert gas  welding process. In this paper, welding current, welding speed and welding gap have been considered as process input variables in fabricating of AISI316L austenitic stainless steel parts. Depth of penetration and weld bead width have been taken in to account as process response parameters. In this paper SiO2, Nano-particles have been considered as an activating flux. To gather required data for modeling and optimization purposes, Taguchi method has been employed. Then, process response parameters have been measured and their corresponding signal to noise ratios have been calculated. Next, different regression equations have been applied on signal to noise ratio values and the most fitted ones have been selected. Furthermore, welding current has been determined as the most important parameter affects depth of penetration and weld bead width with 68% and 88% percent contribution respectively. Next, signal to noise analysis, in such a way that weld bead width minimized and depth of penetration is maximized has been used. Finally, experimental performance evaluation tests have been carried out, based on which it can be concluded that the proposed procedure is quite efficient (with less than 7% error) in modeling and optimization of the process.