جستجوی مقالات مرتبط با کلیدواژه « توزیع جریان » در نشریات گروه « فنی و مهندسی »

This paper studied the oxygen flow distribution in a PEM fuel cell stack numerically. At first, the flow in the stack was simulated as a single phase, and the effect of gradual reduction of manifold height (tapered manifold) on improving the flow distribution between fuel cells was investigated. Standard deviation and non-uniformity coefficient have been used to measure the flow maldistribution. Gradual reduction of manifold height up to 70% in the tapered manifold improved standard deviation and non-uniformity coefficient by 6.9% and 8.4%, respectively. The results of the two-phase simulation showed that the condensation of water vapor in the saturated oxygen could cause the accumulation of liquid water at the end of the manifold. As the smaller droplets merged, the droplet radius formed at the end of the manifold increased until it detached from the manifold wall and entered the last cells. Less liquid water entered the last cell at shorter intervals in the tapered manifold. It was suggested that by creating a water chamber at the end of the manifold, condensed water be collected from the manifold and discharged from the stack. Increasing the mass flow at the manifold inlet and discharging the excess gas through the water collection chamber can push the liquid water into the chamber without causing significant changes in pressure drop.

In a PEMFC, the proton conductivity of the membrane depends on its water content and using saturated reactants can improve the performance of the fuel cell. The heat transfer between saturated reactants and the walls causes condensation of part of the water vapor in the inlet gas. Phase change affects the distribution of gas flow among the cells in the stack. In this paper, CFD is used to study the effect of water vapor phase change on the distribution of oxygen flow in the cathode side of a polymer electrolyte membrane fuel cell stack with 26 cells. For this purpose, a code is developed in OpenFOAM software and validated with experimental data for the single-phase flow distribution. Three different boundary conditions are applied to the walls of the manifold: constant temperature, free and forced heat convection. The results indicate that water generated from condensation on the lower wall of the inlet manifold enters the first cell. Also, the accumulation of water in this area reduces the flow velocity at the entrance of the first cell. The condensed water vapor on the upper wall of the inlet manifold moves to the end of the stack. Part of the water enters into the four last cells, and the other part returns to the manifold due to the vortex. Therefore, the first cell and the last four cells receive less reactant than other cells. The non-uniform flow distribution parameter increases by up to 1425% on using saturated oxygen and under forced convection conditions.

One of the most important factors in decreasing the lifetime and inappropriate performance of PEM electrolyzers is the non-uniform current distribution on membrane surface. Since the smoothest distribution of species and water leads to optimal current distribution, in this research, a 1D- 1D model has been developed that explores the distribution of species and water, and finally the current distribution in layers and determines the optimal performance conditions of the high PEM membrane electrolyzers. In this model, the pressure is assumed constant throughout the channel, the cell temperature is constant, and the membrane is fully hydrated. The length of the anode and cathode channels is divided into 20 equal parts. By simultaneously solving the equations along the channel and perpendicular to it in each section, the distribution of species and current are obtained. The result showed that by increasing the average flow density, the flow distribution is smoother along the channel and, with increasing water flow, the current distribution is smoothed, but it has little effect on the polarization curve. Fick's effect on the distribution of species at the interface between the membrane and the gas diffusion layer has been investigated. Finally, the effect of thickness on the polarization curve is determined. By increasing the thickness of the membrane and the electrodes, the function of the system decreases.