مجله مهندسی مکانیک
سال چهل و سوم شماره 2 (پیاپی 66، پاییز و زمستان 1392)

In this paper, a polymer membrane fuel cell using a one-dimensional, steady and homogeneous model has been studied. The governing equations of bipolar plate and gas diffusion layer are derived initially and ohmic losses and the rate of oxygen transfer are modeled. The oxygen transfer rate in the cathode layer is modeled using a homogeneous method. Differential governing equations of cathode catalyst layer are solved using a MATLAB code. The ohmic loss in the membrane polymer and also the concentration loss are modeled using some engineering assumptions. Finally, using all losses, the total voltage of fuel cell is calculated and voltage curves are plotted. comparison of obtained results with the experimental results show the validity of the model. Also, parametric studies to consider the effect of parameters such as, porosity coefficient of gas diffusion layer, saturation of gas layer, gas diffusion layer thickness, the amount of platinum mass on the surface of cathode layer, the degree of membrane penetration in catalyst layer, and the degree of saturation at the cathode catalyst layer on the fuel cell performance and its efficiency have been conducted. The results show that the degree of saturation in the cathode catalyst layer has the most effect on the activation losses of fuel cell and its performance.

Internal-force-driven flows in which the force acting on channel cross sections have a perfect uniform distribution create a fully-developed velocity field. Even if the driving force is inserted on a part of channel, the velocity profiles are parabolic. In this situation, firstly the driving force with non-uniform axial distribution can be removed temporarily and then one can use an equivalent pressure gradient alternatively throughout the channel. In this case, although the distribution of pressure gradient and the driving force change but the resulting velocity profiles remain unchanged. The main advantage of this replacement is that the solution of the equations in the 3-D geometries can be converted to a 2-D solution using Poisson equation in the channel cross section. After determining the velocity distribution in the cross section, one can inversely calculate the actual pressure distribution easily. This will be done by resuming the real axial force. One of the applications of this simplification is that the simulation of MHD channel flows can be carried out easily. Good agreement between the results of the new solution method and the results of the perfect solutions shows that the present method with enough accuracy can be used for prediction of velocity and pressure fields in microfluidic networks and can be reduced the heavy costs of 3-D analysis.

Using of smart materials in hydrofoils increases the efficiency of high-speed boats. The aim of this paper is development and simulation of flow around the smart hydrofoil and comparison of it with the conventional ones. Then, the effect of submerge distance (h/c), angle of flap (AOF) and Froude number (Fr) is studied. The free surface waves, generated by two types of hydrofoils, will be compared with together and the smart martial influence on the wavelength and amplitude is investigated for different submerge distance. To simulate the smart flap behavior, a parametric bending profile of a cantilever beam under a non-uniform load is considered so that it can simulate the smart martial treat very well. Moreover, a pressure-based implicit procedure is applied to solve the Navier-Stokes equations and Volume of Fluid (VOF) model is also utilized to simulate air and water. Phases. Results demonstrate that the smart hydrofoils increase the lift to drag ratio (L/D) when compares with the conventional hydrofoils; finally, they have a suitable efficieney against the conventional ones.

Axial turbines can be applied in our country for low heads and high flows, especially in off- grid low capacities. In this research, an axial turbine was designed for flow ranged between50 l/s to 150l/s and for 2 to 5 meters of head. Firstly, primer design software was established to calculate all required geometrical parameters of turbine. Indeed, this software is able to show the 3D shape of runner in CATIA. In the next step for evaluation of design, turbine was simulated by Numeca using a verified numerical method by experimental data for a similar geometry. Simulation results did not show a suitable efficiency for axial turbine. Therefore, to improve the performance of machine, runner shape was optimized by genetic algorithm and neural network. Final geometry had 15% more efficiency than the initial one.

In this research, 13 passes of accumulative roll bonding (ARB) process is performed on A 1050 sheets. The microstructural evolution to access of grains with nanometer size during ARB process is investigated. Moreover, the change in strength and elongation of the sheets during the cycles of process is studied by uniaxial tensile test in 3 directions (rolling direction (RD), transverse direction (TD) and at an angle of 4 owith respect to RD); and then the inhomogeneity of mechanical properties in these directions is investigated.The tensile strength of sheet is increased by increasing the number of ARB cycles.The elongation of specimen is decreased rapidly at the first cycle of ARB; and it is subsequently increased gradually.The inhomogeneity of mechanical properties of specimen is increased by performing the ARB process.

In this study, a number of tests are conducted on a four-cylinder spark ignition engine with natural gas and the cycle to cycle variations of peak pressure max P and peak pressure angle Pmax  in the engine are analyzed for nine different equivalence ratios. By using multi scale entropy technique, it is shown that the complexity of peak pressure and peak pressure angle depends on the equivalence ratio variations. Thereby, a limitation is found for optimum equivalence ratios that can be used for controlling the combustion process and improving engine performance.

Using Lorentz forces under effects of electro-magnetic fields for flow control is a suitable procedure. Lorentz forces may be added as a source term in the governing fluid flow equations. For this phenomenon, the numerical modeling of subsonic flow with the Mach number of 0.2 for turbulent flow regime using a finite-volume TVD scheme with implicit time marching for solving the two-dimensional compressible Navier Stokes flows and the Baldwin-Lomax turbulence model around aerofoils are investigated to control undesirable effects of flow separation. By using Lorentz forces, it was shown that the separation was delayed or avoided over NACA0015 aerofoil and hydrodynamic performance was enhanced. Employing flow control using the Lorentz force, the lift coefficient was increased and the drag coefficient was not changed, considerably. Hence, the stall angle was increased.